NCHurricane2009's Blog

2013 Atlantic Hurricane Season Birdseye Discussion #88A (Special Update)

By: NCHurricane2009, 2:47 AM GMT on September 11, 2013

...TUESDAY SEPTEMBER 10 2013 10:47 PM EDT...
Please visit www.nhc.noaa.gov and www.wunderground.com/hurricane for latest info on the tropical activity in the Atlantic basin.

I am sorry to announce that as we peak into the 2013 Atlantic hurricane season that I have made a decision to cease my daily discussions. Due to too many personal developments in my life....many within the past month...I am unable to find the time or energy to continue these posts. Sometimes it is important to take care of one's self before getting spread to thin...

God bless to all and stay safe for the remainder of the hurricane season....

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2013 Atlantic Hurricane Season Birdseye Discussion #88

By: NCHurricane2009, 7:25 AM GMT on September 10, 2013

...TUESDAY SEPTEMBER 10 2013 3:25 AM EDT...
Remnant of Gabrielle...currently located south of Bermuda...has become better organized in the last day. Re-genesis of this system appears extremely likely...especially as upper winds become more favorable by Wednesday. Interests in Bermuda should monitor this system for potential impacts within the next 36 hours...and interests in Atlantic Canada (Nova Scotia and Newfoundland) should monitor this system for potential impacts by 96 hours (4 days). See Gabrielle special feature section below for details.

Tropical depression nine has strengthened to Tropical Storm Humberto since the release of special update #87A last night...see Humberto special feature section below for additional details. The tropical storm is showing signs of strengthening while pulling away from the Cape Verde Islands into the open eastern Atlantic and could become the first hurricane of the season. Of note...if this system does not become a hurricane by the morning of September 11...this will be the longest we have gone into an Atlantic hurricane season without a hurricane since the reconaissance era began in 1944.

Computer model support for tropical cyclone development in 72 hours (3 days out) in the Bay of Campeche and or souwthwestern Gulf of Mexico has increased. See paragraph P5 for details. However I am looking for more cohesive organization within the thunderstorm activity discussed in paragraph P5 before considering this area a special feature on this blog.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_09_2013_2045Z_zpsd0246ea9.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1920Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_09_2013_2045Z_zps8cf05989.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...REMNANTS OF GABRIELLE...
Remnant surface low of Gabrielle is currently south of Bermuda and is moving north due to low-level ridge weakness associated with 981 mb frontal cyclone mentioned in paragraph P2. Still expecting Gabrielle to get left behind by the frontal cyclone such that the northward track should bend west in the next 24 hrs as paragraph P2 1024 mb ridge passes to the north...which should put Bermuda in the impressively organized east half of the system within the next 36 hours. Based on how well Gabrielle has re-organized...I suspect it will soon be a tropical storm that dumps heavy rain and gusty winds across Bermuda. Unfavorable westerly shear that is currently pushing the heavy thunderstorms to the east of Gabrielle's remnant surface low is expected to diminish by Wednesday...which could allow a re-generated tropical storm Gabrielle to strengthen briskly. This is because the paragraph P1 upper trough will have pushed paragraph P2 central US upper ridge toward western Atlantic...which will squeeze out the shear-inducing paragraph P2 upper vortex offshore of the SE US between the incoming upper ridge and the low-latitude upper ridging mentioned in paragraph P4. After passing by Bermuda...Gabrielle is expected to curve more north-northeast toward Atlantic Canada (Nova Scotia and Newfoundland) while getting caught in ridge weakness associated with paragraph P1 upper trough and associated surface frontal cyclone. As Gabrielle interacts with the eastern divergence of the upper trough...I suspect she will be transitioning into a vigorous non-tropical low while moving into Atlantic Canada...potentially gettting absorbed by the frontal cyclone.

...SPECIAL FEATURE...TROPICAL STORM HUMBERTO...
As expected...Tropical Depression Nine in the eastern tropical Atlantic has intensified into Tropcial Storm Humberto since special update #87A was written last night. My forecast for the newly-named tropical cyclone versus the NHC is shown in Figure 1 below. The northern half of the storm has overspread the Cape Verde Islands with some gusty winds and rainfall...but as the impact swath in Figure 1 shows the tropical storm force winds (40+ mph) have passed south of the islands. Tropical storm warnings have been removed from the Cape Verde Islands. Visit www.nhc.noaa.gov for up to the minute latest information on Humberto.

 photo Sep_10_2013_TS_Humberto_Forecast_zpsd2f3f852.png
Figure 1: My forecast versus the NHC for Tropical Storm Humberto

As the tropical cyclone continues to intensifies briskly...it will soon become tall enough to be steered by features in all layers of the atmosphere...which will include northeastern Atlantic deep-layered ridge mentioned in paragraph P3...a cut-off upper vortex north of the Cape Verde Islands mentioned in paragraph P3...and finally a SW-NE cut-off upper trough that will span from mid-ocean upper vortex mentioned in paragraph P4 to another upper vortex that develops over the northern Canary Islands from the vorticity of upper trough NE of the Azores mentioned in paragraph P3.

In the next 24 hrs my forecast track agrees with NHC that Humberto will turn northwest while becoming pulled to the right by upper vortex north of the Cape Verdes as Humberto becomes tall enough to feel influence from the vortex. This upper vortex then de-amplifies and exits stage right after 24 hrs...leaving tropical cyclone to thrive under a large eastern Atlantic upper ridge cell (mentioned in paragraph P4) and become steered by the deep-layered ridge and SW-NE upper trough. The NHC has shifted their track to the right since special update #87A due to computer models wanting to show Humberto tracking straight north after 24 hrs. Likewise I have shifted my forecast track to the right....but I am still left of NHC track after 24 hrs as the first upper vortex exits stage right hence no longer pulls the tropical cyclone to the right...plus the deep-layered ridge and mid-ocean upper vortex are two things that would imply a more leftward track versus the northern Canary Islands upper vortex being the only thing favoring a more rightward track. The SW-NE upper trough eventually breaks up in the models under the strength of the deep-layered ridge...and I turn the tropical cyclone to left based on when 18Z GFS model run breaks up the trough which so happens to allow me to re-align with the NHC forecast track by the end of the forecast period. My forecast intensity from discussion #87A has done very well and the NHC has matched that intensity forecast tonight...and therefore I am still forecasting a category 2 at 100 mph sustained winds. Not comfortable at this time forecasting a major hurricane of 115+ mph as the forecast track keeps it close to the 26 deg C isotherm rather than abundantly warm waters...and because we've had an unusual lack of Atlantic hurricanes this year for reasons not yet fully grasped. Humberto will be encountering westerly shear on the north side of paragraph P4 eastern Atlantic upper ridge cell by Friday and Saturday...but I weaken Humberto later and slower than the NHC showed at 11 PM EDT because in my estimation (based on my forecast track and 18Z GFS model run of upper winds) the shear will not be so bad on Thursday.

Impact swath is based on extrapolating 11 PM EDT tropical storm wind field along my forecast track. The swath is widened a little to simulate a strengthening tropical cyclone...then narrowed at the end of the forecast period to simulate a weakening tropical cyclone.

...MID-LATITUDES DISCUSSION...
P1...Next mid-latitude weather system is working its way into the upper-left of the above atmo chart with a 1003 mb frontal depression over the eastern Dakotas/western Minnesota and western Canada frontal cyclone not yet in the scope of the above charts. In the next 48 hrs the western Canada frontal cyclone and associated upper trough will become the dominant feature of this weather system.

P2...Upper trough and associated surface frontal cyclone previously in the Atlantic high seas in the vicinity of Greenland...mentioned in paragraph P1 of discussion #86...has since exited the picture from the upper-right of the above atmo chart...although what is left of the lengthy front extending from the cyclone is presently in the upper-right of the above atmo chart over western Europe and waters just offshore. Upper troughing persists in the western Atlantic with a cut-off upper vortex offshore of the SE US and with an amplifying shortwave upper trough over eastern Canada and NW Atlantic. Eastern divergence of the eastern Canada upper trough supports frontal cyclone that has rapidly deepend from 999 mb to 981 mb while moving from Atlantic Canada and into the waters between Canada and Greenland. Western conergence of the eastern Canada upper trough suppors 1024 mb ridge that has rapidly moved from Onatrio and into the NW Atlantic from the NE US shore...and western convergence of aforementioned cut-off upper vortex offshore of the SE US supports surface ridging over the SE US. To the west of all the upper troughing discussed in this paragraph....western US upper ridge is shifting into the central US in advance of paragraph P1 mid-latitude weather system.

P3...Eastern convergence of north Atlantic mid-latitude upper ridge continues to support what is now a strong 1034 mb surface ridge center over the Azores. The upper and surface ridges continue to effectively make a deep-layered NE Atlantic ridge. To the east of the deep-layered ridge...a cut-off upper trough is located NE of the Azores...a cut-off upper vortex is located over northeastern Morocco...and a cut-off upper vortex is just north of Tropical Storm Humberto and the Cape Verde Islands. These three cut-off upper vorticity features are leftovers from upper trough mentioned in paragraph P2 of discussion #86 retrograding southward around the east side of the deep-layered ridge. The south side of this deep-layered ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated by the sprawling moisture field of Tropical Storm Humberto.

...TROPICAL BELT DISCUSSION...
P4...Low-latitude upper ridging covering the Atlantic tropical latitudes continues to have a cell over the Bay of Campeche and Caribbean Sea and another cell over the eastern Atlantic...with mid-ocean upper vortex persisting between the two cells. This upper vortex has recently split into one retrograding westward into the Lesser Antilles and another still located in the mid-ocean.

P5...Surface trough with thunderstorms near the east coast of Mexico persists due to spit flow upper divergence between westerlies on the north side of paragraph P4 upper ridging and southerlies rounding the west side of paragraph P2 central US upper ridge. Tropical wave previously in the western Caribbean is moving across southeastern Mexico and the Bay of Campeche while merging with the surface trough. An area of thunderstorms extends from the tropical wave...across southeastern Mexico...and into the western Caribbean while supported by outflow of Caribbean upper ridging mentioned in paragraph P4. The upper winds for all the activity mentioned in this paragraph are currently unfavorable for development due to the paragraph P4 low-latitude upper ridging axis staying too far south such that shearing upper westerlies are pre-dominant on the north side of the axis. However an animation of high-res upper wind forecast from tonight's 18Z GFS...available at mag.ncep.noaa.gov...suggests the upper ridge axis shifting north by 72 hours as the central US upper ridge mentioned in paragraph P2 (which is keeping the axis suppressed to the south) becomes replaced by upper trough of paragraph P1 weather system. The GFS...CMC...and Euro (ECMWF) agree tonight on tropical cyclogenesis in 72 hours in the Bay of Campeche/SW Gulf of Mexico due to favorable outflow beneath the northward-shifting low-latitude upper ridge axis and therefore this area bears watching...especially with this region being tropical cyclone prolific this year (Barry in June...Fernand in late August...and tropical depression eight last week). Models currently agree that whatever tropical cyclone spins up will be generally steered west to west-northwest into east-central Mexico by SW quad of a surface ridge that builds under western convergent side of paragraph P1 upper trough.

P6...Tropical wave formerly classified as Invest 98-L is currently midway between the Cape Verde Islands and Lesser Antilles. The system remains inactive in environment of southerly vertical shear ahead of paragraph P4 upper vortex moving into the Lesser Antilles which is allowing entrainment of paragraph P3 dry Saharan air from the south. As the Lesser Antilles upper vortex continues moving westward and away while mid-ocean upper vortex mentioned in paragraph P4 dives southward...this system may get caught in favorable low shear and upper outflow between the two upper vortices which could allow for it to make a comeback in the days ahead. However none of the reliable computer models develop this system at this time.

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2013 Atlantic Hurricane Season Birdseye Discussion #87A (Special Update)

By: NCHurricane2009, 10:57 PM GMT on September 08, 2013

...SUNDAY SEPTEMBER 8 2013 7:00 PM EDT...
This special update is released due to recent developments with the remnants of Gabrielle located northeast of the Bahamas and strong tropical wave Invest 91-L approaching the Cape Verde Islands.

Based on improving satellite organization seen with the remnants of Gabrielle during this afternoon's visible satellite animation...I am now re-considering this system a special feature prior to the release of my next full blog update tomorrow. Although upper winds are expected to become more favorable for Gabrielle to re-generate by Wednesday...the already-improving organization suggests that Gabrielle could re-generate into a tropical cyclone before that time. Outlook on path of Gabrielle should she re-generate was discussed in paragraph P5 of discussion #87. Interests in Bermuda should continue to monitor the progress of this system. In addition...interests in Newfoundland should also monitor the progress of this system in case this system takes a more westward path.

Tropical wave Invest 91-L in the eastern tropical Atlantic has intensified into tropical depression nine while centered 275 miles east-southeast of the southern Cape Verde Islands. My forecast for the new tropical cyclone versus the NHC is shown in Figure 1 below. Tropical storm warnings are in effect for the Cape Verde Islands. Visit www.nhc.noaa.gov for up to the minute latest information on the warnings and status of the tropical cyclone.

 photo Sep_08_2013_TD_Nine_Forecast_zpsded2a392.png
Figure 1: My forecast versus the NHC for newly-formed Tropical Depression Nine in the eastern tropical Atlantic.

Based on the quickly improving satellite apperance featuring a tight storm core and outer spiral bands...forecast philosophy is for a small to medium sized tropical cyclone that will intensify quickly in an environment of sufficiently warm waters and enhanced outflow benath eastern Atlantic upper ridge cell mentioned in paragraph P3 of discussion #87. As the tropical cyclone intensifies briskly...it will soon become tall enough to be steered by features in all layers of the atmosphere...which will include deep-layered ridge over the Azores mentioned in paragraph P2 of discussion #87...a cut-off upper vortex to dive south toward the Cape Verde Islands for the first 48 hrs coming from upper trough mentioned in paragraph P2 of discussion #86...and finally an SW-NE cut-off upper trough that will span from mid-ocean upper vortex mentioned in paragraph P3 of discussion #87 to another upper vortex that cuts-off over the northern Canary Islands orginating from upper trough mentioned in paragraph P2 of discussion #86.

As far as the first cut-off upper vortex to dive south toward the Cape Verdes in the first 48 hrs...I will assume the tropical cyclone is compact enough such that the vortex enhances the tropical cyclone's northern outflow rather than shears it...even more reason that I am more aggressive than the current NHC intensity forecast. I also have a rightward bias at 48 hrs relative to NHC as my aggressive intensity forecast would allow the tropical cyclone to grow taller faster and hence feel a more rightward tug from the upper vortex. The upper vortex de-amplifies and exits stage right after 48 hrs...leaving tropical cyclone to thrive under a large eastern Atlantic upper ridge cell and become steered by the deep-layered ridge and SW-NE upper trough. I become left of NHC track after 48 hrs as the first upper vortex exits stage right hence no longer pulling the tropical cyclone to the right...plus the deep-layered ridge and mid-ocean upper vortex are two things that would imply a more leftward track versus the northern Canary Islands upper vortex being the only thing favoring a more rightward track. I suggest a peak strength of category 2 at 100 mph sustained winds as the forecast track keeps it close to the 26 deg C isotherm rather than abundantly warm waters that would allow it to become a major hurricane of 115+ mph. I am also not confident in predicting a major hurricane at this time since we've had an unusual lack of Atlantic hurricanes this year for reasons not yet fully grasped. Of note...if this system does not become a hurricane by the morning of September 11...this will be the longest we have gone into an Atlantic hurricane season without a hurricane since the satellit era began in the 1960s.

Impact swath is based on extrapolating the size of the circular storm core seen in infrared satellite imagery along my forecast track. This is currently where I believe the tropical cyclone will establish its tropical storm and hurricane force winds....although the true size fluctuations of a tropical cyclone thru any forecast period are hard to predict. This impact swath implies the southwesternmost Cape Verde Islands are likely to see tropical storm force gusts...but the remainder of the islands could still get some squally weather (but not as intense) as infrared also shows outer spiral bands extending well away from the core.

Return to full discussion #87 for an assessment on the rest of the Atlantic tropics.

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2013 Atlantic Hurricane Season Birdseye Discussion #87

By: NCHurricane2009, 7:34 PM GMT on September 08, 2013

...SUNDAY SEPTEMBER 8 2013 3:34 PM EDT...
Remnant of Gabrielle...currently located northeast of the Bahamas...has potential for re-generation when the upper winds become favorable again in 3 days time. If current organization of the remnants continues...I believe it will have a high chance of re-genesis when the upper winds improve and therefore plan on upgrading it to a special feature if the current organization continues by my next blog post. Interests in Bermuda should monitor this system. See paragraph P5 for update statement on remnants of Gabrielle.

Tropical wave Invest 91-L which has recently emerged from western Africa continues to remain well-organized and has a high chance of becoming a tropical cyclone that affects the Cape Verde Islands in the eastern Atlantic within the next couple of days...see Invest 91-L special feature section for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_08_2013_1145Z_zps53532631.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1322Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_08_2013_1145Z_zpsed744a83.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL WAVE INVEST 91-L...
Tropical wave which has recently emerged from Africa is now just east-southeast of the Cape Verde Islands and continues to remain well-organized. Interests in the Cape Verde Islands should monitor this tropical wave very carefully over the next couple of days as the Euro (ECMWF)...GFS...and CMC still develop this system below the favorable low shear/enhanced outflow of paragraph P3 eastern Atlantic upper ridge cell into a tropical depression or tropical storm crossing over or very near the islands. Even if no additional development occurs...the Cape Verde Islands can expect vigorous squally weather to soon move in. Afterwards these three models suggest that the tropical cyclone will be strong and vertically coupled enough to feel a northward tug from paragraph P3 mid-ocean upper vortex...followed by a westward turn into the open central Atlantic while the tropical cyclone bumps against northeastern Atlantic deep-layered ridge mentioned in paragraph P2.

...MID-LATITUDES DISCUSSION...
P1...Upper trough and associated surface frontal cyclone previously in the Atlantic high seas in the vicinity of Greenland has exited the picture from the upper-right of the above atmo chart...although the lengthy front extending from the cyclone is currently extends into the open western Atlantic from western Europe. Upper troughing persists in the western Atlatnic with an axis along the SE US shore extending into the W Caribbean...and with an amplifying shortwave upper trough that has moved into eastern Canada and NW Atlantic from Hudson Bay. Eastern divergence of the eastern Canada upper trough supports 997 mb frontal cyclone that has moved from the east Canada coast and into southern Greenland in the last 24 hrs...as well as a new 999 mb frontal cyclone over Atlantic Canada. Western conergence of the eastern Canada upper trough suppors 1025 mb ridge over Ontario...and western convergence of aforementioned upper trough axis over the SE US coast supports 1017 mb surface ridge over the southeastern US. To the west of all the upper troughing discussed in this paragraph....western US upper ridge persists.

P2...As forecast in paragraph P4 of discussion #86...north Atlantic mid-latitude upper ridge has become vertically stacked with surface ridge dominating much of the open Atlantic basin such that we currently have a 1031 mb deep-layered ridge centered over the Azores. The south side of this deep-layered ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P3...Low-latitude upper ridging covering the Atlantic tropical latitudes continues to have a cell over the Bay of Campeche and Caribbean Sea and another cell over the eastern Atlantic...with mid-ocean upper vortex persisting between the two cells.

P4...Remnant surface low of tropical depression eight lost its identity over SW Mexico and was removed from NHC TAFB maps as of 0000Z twelve hours ago. Further east...a surface trough with thunderstorms near the east coast of Mexico persists due to supportive outflow of Bay of Campeche upper ridging mentioned in paragraph P3 above. Tropical wave moving across southeastern Mexico...mentioned in paragraph P7 of the preivous discussion...has merged with this surface trough. Tropical wave previously marked in the central Caribbean...mentioned in paragraph P7 of the previous discussion...is now in the western Caribbean producing another region of thunderstorms supported by outflow of paragraph P3 Caribbean upper ridge cell.

P5...Remnant surface low of Gabrielle is currently northeast of the Bahamas and is drifting north-northeast today due to low-level ridge weakness associated with 999 mb frontal cyclone mentioned in paragraph P1. Still expecting Gabrielle to later get left behind during the Monday/Tuesday timeframe as the frontal cyclone and all upper troughing in paragraph P1 lift out to the northeast...but the question is how close will Gabrielle get to Bermuda during this time which is why interests in Bermuda should monitor this system. Unfavorable westerly shear that is currently pushing the heavy thunderstorms to the east of Gabrielle's remnant surface low is expected to continue thru Tuesday as some of the paragraph P1 upper trouhing will get left behind just offshore of the US east coast. By Wednesday...next major mid-latitude upper trough will have pushed paragraph P1 western US upper ridge toward western Atlantic...which will squeeze out the upper troughing offshore of the eastern US between it and the low-latitude upper ridging mentioned in paragraph P3. This will signficantly reduce the westerly shear by Wednesday and onwards...which may allow Gabrielle to re-generate into a tropical cyclone. The only model that currently re-generates Gabrielle is the agressive CMC....which shows Gabrielle taking a path very near Bermuda...then just offshore of the NE US and Atlantic Canada while re-curving into ridge weakness associated with the next major mid-latitude upper trough.

P6...Tropical wave Invest 98-L currently midway between the Cape Verde Islands and Lesser Antilles continues to not be a special feature on this blog and continues to no longer be mentioned in the NHC tropical weather outlook...but remains posted as an active disturbance on the Naval Research Laboratory site of the US Navy which is why it sill has a green box in the above atmo chart. Tropical cyclone formation is no longer possible with this system as southwesterly vertical shear ahaed of paragraph P3 mid-ocean upper vortex allows entrainment of paragraph P2 dry Saharan air from the southwest.

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2013 Atlantic Hurricane Season Birdseye Discussion #86

By: NCHurricane2009, 8:14 PM GMT on September 07, 2013

...SATURDAY SEPTEMBER 7 2013 4:14 PM EDT...
Remnant of Gabrielle...currently located just east of the Bahamas and north of the Dominican Republic...has not in the short-term re-generated a surface center below the heavier thunderstorms on the east side of the system. Therefore I have cancelled it as a special feature on this blog for now...but their is potential for the system to re-generate when the upper winds become favorable again in 4 days time. See paragraph P9 for update statement on remnants of Gabrielle.

Elsewhere...tropical depression eight has dissipated while moving into central Mexico overnight and this morning...see paragraph P6 for update statement on the remnants of eight. Tropical wave emerging from Africa has been upgraded to Invest 91-L while becoming better orgainzed and has a high chance of becoming a tropical cyclone that affects the Cape Verde Islands in the eastern Atlantic within the next three days...see Invest 91-L special feature section for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_07_2013_1445Z_zpsacf792ae.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_07_2013_1445Z_zps88528827.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL WAVE INVEST 91-L...
Tropical wave emerging from western Africa...mentioned in paragraph P10 of discussions #84 and #85 and marked in lower-right of the atmo chart of those two prior discussions...has become well-organized enough such that the NHC TAFB has gone ahead and marked it as a 1008 mb tropical low pressure spin along the west African coast as of 1200Z. Moroever...this organization has warranted its classification as the next Invest on the Naval Research Laboratory (NRL) site of the US Navy. On a side note...I thought the next available Invest number was 90-L...but an inspection of NRL's archives shows this was briefly given to 98-L mentioned in paragraph P10 during the September 5 timeframe. Therefore this tropical wave is Invest 91-L instead of 90-L.

Interests in the Cape Verde Islands should monitor this tropical wave very carefully over the next couple of days as the Euro (ECMWF)...GFS...and CMC quickly develop this system below the favorable low shear/enhanced outflow of paragraph P5 eastern Atlantic upper ridge cell into a strong tropical cyclone crossing over or very near the islands. Afterwards these three models suggest that the tropical cyclone will be strong and vertically coupled enough to feel a northward tug from paragraph P5 mid-ocean upper vortex...followed by a westward turn into the open central Atlantic while the tropical cyclone bumps against the expected northeastern Atlantic deep-layered ridge mentioned in paragraph P4.

...MID-LATITUDES DISCUSSION...
P1...Large upper trough previously over the western Atlantic and eastern Canada is now lifting northeast into the Atlantic high seas in the vicinity of Greenland...and the 981 mb east Canada coast frontal cyclone it previously supported with its eastern divergence has weakened to 988 mb while the cyclone has been stationary below the non-divergent axis of the upper trough (this cyclone now has a secondary 990 mb center just SE of Greenland supported by eastern divergence of the upper trough). Even though this upper trough is lifting out to the northeast...we still have upper troughing persisting in the western Atlantic as the upper trough itself left behind an axis along the SE US shore extending into the W Caribbean...and a shortwave upper trough from Hudson Bay will be re-enforcing that axis. Eastern divergence of the Hudson Bay shortwave supports 992 mb frontal cyclone that has entered the picture from Hudson Bay and has already arrived onto the east Canada coast within the last 36 hrs. Front extending from this 992 mb center extends across the Great Lakes region and northern tier of the US...and as the Hudson Bay upper shortwave amplifies...its eastern divergence will support a new and quickly strengthening cyclone along that front over southeastern Canada in the next 36 hours. Western convergence of aforementioned upper trough axis over the SE US coast supports 1022 mb surface ridge over the eastern US. To the west of all the upper troughing discussed in this paragraph....western US upper ridge persists.

P2...Longwave upper trough remains located just west of Europe and NW Africa. Western convergence of the upper trough supports 1023 mb currently offshore of Europe which is currently and northeast extension of paragraph P4 surface ridge. Cut-off upper vortex left behind by this upper trough remains southeast of Newfoundland. Eastern divergence of this upper vortex supports weak surface troughing just west of the paragraph P4 1028 mb surface ridge center.

P3...Mid-latitude upper ridging in the north-central Atlantic has shifted into the northeastern Atlantic toward the Azores but remains amplified due to warm air advection ahead of paragraph P1 988 mb frontal cyclone.

P4...Surface ridge dominating much of the open Atlantic is currently anchored by 1028 mb center over the Azores which is supported by western convergence of paragraph P2 upper trough and eastern convergence of paragraph P3 mid-latitude upper ridge. The mid-latitude upper ridge will soon become vertically stacked with the 1028 mb surface center...which will result in deep-layered ridging in the northeastern Atlantic over the next few days. The south side of this emerging deep-layered ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P5...Low-latitude upper ridging covering the Atlantic tropical latitudes has split into three cells...one over Mexico overhead of the remnants of tropical depression eight...second over the Caribbean Sea...and third in the eastern Atlantic. What is left of adjacent axis of upper vorticity is now a mid-ocean upper vortex wedged between the Caribbean Sea and eastern Atlantic upper ridge cells.

P6...Bay of Campeche/southwestern Gulf disturbance Invest 99-L intensified to tropical depresison eight yesterday afternoon as highlighed in special update #85A...and now the surface center of the tropical cyclone has become diffuse over the mountains of central Mexico such that it is currently a remnant low. This system apperas to have left behind an active surface trough along the east coast of Mexico supported by enhanced outflow of paragraph P5 upper ridging. If the thunderstorm activity and surface trough shift inland...this could prolong the flood/mudslide threat in central Mexico.

P7...Western tropical wave formerly associated with disturbance Invest 97-L (which is now Gabrielle) has moved into Central America and southeastern Mexico. It previously appeared the eastern tropical wave of former 97-L had become absorbed by what is now the remnant surface low of Gabrielle...but it now appears a southern fragment of this tropical wave escaped absorption and is now currently marked in NHC TAFB maps at a location in the central Caribbean. Both tropical waves are largely inactive in an unfavorable environment of upper-level convergence/sinking air between the Mexico upper ridge cell and Caribbean upper ridge cell mentioned in paragraph P5.

P8...While moving into the Lesser Antilles...tropical wave formerly classified as Invest 96-L has dissipated in unfavorable environment of dry Saharan air mentioned in paragraph P4 and southeastern convergence of Caribbean upper anticyclone mentioned in paragraph P5.

P9...While quasi-stationary east of the Bahamas and north of the Dominican Republic...the remnant surface low of Gabrielle is generally on par with Saturday forecast issued for this system during discussion #84. Quasi-stationary motion is due to Gabrielle being caught in armpit between paragraph P1 1022 mb surface ridge over the eastern US and paragraph P4 open Atlantic surface ridge. Still anticipating formation of new southeastern Canada frontal cyclone for the Sunday (tomorrow) timeframe as predicted in Gabrielle's discussion #84 forecast and as mentioned in the latter part of paragraph P1 above...which will re-widen the low-level ridge weakness and pull Gabrielle on a slow northeast track. As I did during discussion #84...I still expect Gabrielle to subsequently get left behind in the waters south of Bermuda during the Monday/Tuesday timeframe as the frontal cyclone and all upper troughing in paragraph P1 lift out to the northeast. Unfavorable westerly shear that is currently pushign the heavy thunderstorms to the east of Gabrielle's remnant surface low is expected to continue thru Tuesday as some of the paragraph P1 upper trouhing will get left behind just offshore of the US east coast. By Wednesday...next major mid-latitude upper trough will have pushed paragraph P1 western US upper ridge toward western Atlantic...which will squeeze out the upper troughing offshore of the eastern US between it and the low-latitude upper ridging mentioned in paragraph P5. This will signficantly reduce the westerly shear by Wednesday and onwards...which may allow Gabrielle to re-generate into a tropical cyclone. The only model that currently re-generates Gabrielle is the agressive CMC....which shows Gabrielle taking a path just offshore of the NE US while re-curving into ridge weakness associated with the next major mid-latitude upper trough.

P10...Tropical wave Invest 98-L currently southwest of the Cape Verde Islands is no longer a special feature on this blog and is no longer mentioned in the NHC tropical weather outlook...but remains posted as an active disturbance on the Naval Research Laboratory site of the US Navy which is why it sill has a green box in the above atmo chart. Tropical cyclone formation is no longer possible with this system as it ingests some of the paragraph P4 dry Saharan air and as it encounters unfavorable upper winds associated with mid-ocean upper vortex mentioned in paragraph P5.

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2013 Atlantic Hurricane Season Birdseye Discussion #85A (Special Update)

By: NCHurricane2009, 10:31 PM GMT on September 06, 2013

...FRIDAY SEPTEMBER 6 2013 6:25 PM EDT...
Tropical disturbance Invest 99-L in the southwestern Gulf of Mexico...mentioned in paragraph P7 of discussion #85...became tropical depression eight early this afternoon while making landfall in east-central Mexico. Primary threat from this system will be potential for flash floods and or mudslides as it moves inland into central Mexico tonight. Due to landfall...I suspect this system will have dissipated as a tropical cyclone by the release time of my next discussion and therefore will be unable to release a track or intensity forecast on this system.

I am no longer considering tropical wave Invest 98-L southwest of the Cape Verde Islands as a special feature as the system appears to have weakened thunderstorm activity while ingesting Saharan dry air mentioned in paragraph P5 of discussion #85. In addition...the tropical wave is about to encounter unfavorable upper winds associated with mid-ocean upper vortex mentioned in the Invest 98-L section of discussion #85.

No updates on any other systems in the Atlantic tropics. Return to full discussion #85 for an assessment on the rest of the Atlantic tropics.

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2013 Atlantic Hurricane Season Birdseye Discussion #85

By: NCHurricane2009, 10:54 AM GMT on September 06, 2013

...FRIDAY SEPTEMBER 6 2013 6:53 AM EDT...
Gabrielle unexpectedly weakens to remnant low but still dumping heavy rains over Puerto Rico and Virgin Islands. See Gabrielle special feature section for additional details.

Elsewhere...still monitoring tropical wave Invest 98-L...see Invest 98-L special feature section for more details. Although a disturbance flared up in the southwestern Gulf of Mexico and became classified as Invest 99-L within the last 24 hours (paragraph P7)....it is making landfall in east-central Mexico this morning and therefore will not become a tropical cyclone. And finally...a tropical wave emerging from Africa (paragraph P10) has the potential for development in the days ahead.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_06_2013_0115Z_zps376249e8.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0131Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_06_2013_0115Z_zps7583192e.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...REMNANTS OF GABRIELLE...
Yesterday late morning for reasons not fully explainable...the surface center of Gabrielle took a westward deviation into the eastern tip of the Dominican Repbulic while the well-organized mid-level center and circular thunderstorm area persist to the east over Puerto Rico and the Virgin Islands where rainfall and mudslide threat continues. As the surface center encountered the rough terrain of the Dominican Republic...it has weakened and opened up such that Gabrielle was no longer a tropical cyclone as of late last night. One scenario is that Gabrielle re-generates a surface center further east below the mid-level center. Another scenario is that the remnants of Gabrielle linger for a few more days in the waters south of Bermuda while supported by eastern divergence of mid-latitude upper troughing soon to dive into the western Atlantic right behind paragraph P1 upper trough. Because of re-generation potential...I have still kept Gabrielle as a special feature on this blog for now.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 98-L...
Tropical wave currently southwest of the Cape Verde Islands has been re-classified as Invest 98-L due to improved organization observed for part of yesterday. This tropical wave is likely to stay within favorable enivronment of low shear and good upper outflow beneath paragraph P6 low-latitude upper ridge in the next 24 hours. Beyond that time...relatively lower pressures east of the upper anticyclone associated with Gabrielle will concentrate the paragraph P6 axis of upper vorticity into one large mid-ocean upper vortex likely to disrupt the development potential. Therefore the chance of tropical cylcogenesis from this system would be in the next 24 hours...followed by a collapse in the system as it approaches the Lesser Antilles after that time.

...MID-LATITUDES DISCUSSION...
P1...Large upper trough persists over eastern Canada and western Atlantic. Associated surface frontal cyclone centered over the east coast of Canada has deepened further to 981 mb in supportive eastern divergence of upper trough. Western convergence of upper trough supports 1026 mb ridge currently centered over the Great Lakes region. This upper trough continues to confine US upper ridge to the southwestern US.

P2...Longwave upper trough over the high seas is now located just west of Europe. Western convergence of the upper trough supports 1031 mb ridge due north of the Azores. Cut-off upper vortex west left behind by upper trough is currently southeast of Newfoundland while anticylonically orbiting about the paragraph P4 mid-latitude upper ridge. Eastern divergence of this upper vortex supports a weak surface low embedded in the paragraph P5 surface ridge.

P3...Cut-off east-west upper trough has moved into northern Mexico from the western Gulf of Mexico

P4...Mid-latitude upper ridging has amplified in the north-central Atlantic associated with warm air advection ahead of paragraph P1 981 mb frontal cyclone.

P5...Surface ridge dominating much of the open Atlantic is becoming anchored by 1031 mb center north of the Azores in paragraph P2. The south side of this surface ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P6...Low-latitude upper ridge covers much of the Atlantic tropical latitudes. What is left of adjacent axis of upper vorticity...sandwiched between the low-latitude upper ridge and paragraph P4 mid-latitude ridging...is east of Bermuda and well northeast of Gabrielle.

P7...While taking advantage of low shear/good upper outflow of paragraph P6 upper ridging...Bay of Campeche/southwestern Gulf disturbance flared up shortly after the previous discussion was written...prompting an upgrade to Invest 99-L and a recon aicraft investigation yesterday afternoon. Reconnaissance did not find a closed circulation to upgrade to a tropical cyclone...and moreover this system is making landfall in east-central Mexico this morning while steered by paragraph P1 1026 mb ridge.

P8...Western tropical wave formerly associated with disturbance Invest 97-L (which is now Gabrielle) is in the western Caribbean and is inactive while in an unfavorable environment of upper-level convergence/sinking air between upper anticyclone over Gabrielle and upper ridge over paragraph P7 disturbance. What's left of eastern tropical wave formerly associated with Invest 97-L (now Gabrielle) is a surface trough north of Puerto Rico and the Virgin Islands that is merging with the northeast side of the remnant low of Gabrielle.

P9...Tropical wave formerly classified as Invest 96-L is making an approach to the Lesser Antilles and the remnants of Gabrielle. It remains inactive in environment of dry Saharan air mentioned in paragraph P5 and southeastern convergence of upper anticyclone associated with Gabrielle.

P10...Satellite imagery suggests the next tropical wave is currenty inland over west Africa about to emerge into the tropical belt of the Atlantic as marked in the lower-right of the above atmo chart. If this tropical wave becomes better organized as it emerges off the coast...I may upgrade it to a special feature on this blog considering it is entering the same favorable upper winds that adjacent special feature Invest 98-L is under.

Updated: 11:00 AM GMT on September 06, 2013

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2013 Atlantic Hurricane Season Birdseye Discussion #84

By: NCHurricane2009, 1:38 PM GMT on September 05, 2013

...THURSDAY SEPTEMBER 5 2013 9:38 AM EDT...
Tropical disturbance Invest 97-L has evolved into Tropical Depression Seven and then Tropical Storm Gabrielle within the last 36 hours and is currently making a strike on Puerto Rico and the northern Lesser Antilles primarily with heavy cloud cover and rainfall. See Gabrielle special feature section below for additional details on this new tropical storm.

Elsewhere...tropical wave formerly classified as Invest 98-L is currently southwest of the Cape Verde Islands and has become better organized. And due to the favorable upper winds in the shorter term...I now believe this system has a high chance of becoming a tropical cyclone and have upgraded it to a special feature on this blog. See Invest 98-L special feature section below for additional details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_05_2013_0845Z_zpsb4710400.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0720Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_05_2013_0845Z_zps86a4efb2.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM GABRIELLE...
Disturbance Invest 97-L in the norhteastern Caribbean Sea has finally intensified into a tropical cyclone within the last 36 hours. It was upgraded to tropical depression seven yesterday afternoon and then Tropical Storm Gabrielle overnight. Gabrielle is already moving north-northwest into Puerto Rico while producing an impressive circular clump of thunderstorms biased to the east side covering all of Puerto Rico...the Virgin Islands...and northern Lesser Antilles. Given that the winds are not strong in this new tropical storm...primary impact expected is potential for flash floods and mudslides over the aformentioned areas. My forecast versus the NHC's this morning is shown in Figure 1 below.

Because Gabrielle's current NNW track is in agreement with the NHC's short-term forecast in terms of both heading and speed...I agree with the NHC track for the first 24 hours. Gabrielle's NNW track is induced by attraction to low-level ridge weakness associated with paragraph P1 frontal cyclone. The weakness will narrow between paragraph P1 1024 mb ridge and paragraph P2 1029 mb ridge between 24 and 48 hrs...leaving Gabrielle to slow its NNW track while caught in the armpit between the two ridges...which is why I also agree with the NHC's 24 to 48 hr track forecast. Additional upper troughing is expected to dive into the western Atlantic immediately behind the paragraph P1 upper trough...which will generate another frontal cyclone over southeastern Canada by 72 hrs. This will re-widen the low-level ridge weakness and pull Gabrielle more northeastward. But because the western Atlantic upper troughing and frontal cyclone lift out to the north by 120 hrs...I am not confident in accelerating Gabrielle to the northeast as fast as the NHC shows...so I prefer a slower NE track with Gabrille getting left behind as the low-level ridge weakness re-closes by 120 hrs.

Intensity-wise...I show Gabrille strengthening a little more in the next 24 hrs...followed by a steady-state strength as Gabrielle enters increasingly stronger westerly shear on the north side of the upper anticyclone it is currently under. I show Gabrielle transitioning into a non-tropical low that maintains strength by 96 hrs while caught in eastern divegence of aformentioned western Atlantic upper troughing. By 120 hrs as the upper troughing lifts out...I expect Gabrielle to weaken while no longer receiving supported eastern divergence from the upper troughing.

 photo Sep_05_2013_TS_Gabrielle_Forecast_zps83d8eeaa.png
Figure 1: My forecast versus NHC for Tropical Storm Gabrielle

Impact swath in Figure 1 is based on extrapolating the size and location of circular thunderstorm clump seen on infrared satellite imagery along my forecast track. It should be noted that this clump is biased to the east of the center as Gabrielle is already experiencing light westelry shear on the NW quad of the upper anticyclone it is under...and this shear should only increase as Gabrielle's NNW track takes her into the midst of upper westerlies north of the upper anticyclone. This is why my impact swath never shows the clump becoming symmetric about the storm center. I shrink the impact swath by 120 hrs as I currently forecast Gabrielle to be a weak remnant low by that time.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 98-L...
Tropical wave currently southwest of the Cape Verde Islands was previously classified as disturbance Invest 98-L when it was better organized. The tropical wave has re-organized this morning...prompting me to upgrade it to a special feature this morning. Based on animation of upper wind forecasts from computer models...this tropical wave is likely to stay within favorable enivronment of low shear and good upper outflow beneath paragraph P6 low-latitude upper ridge in the next 48 hours. Beyond that time...relatively lower pressures east of the upper anticyclone associated with Gabrielle will concentrate the paragraph P6 axis of upper vorticity into one large mid-ocean upper vortex likely to disrupt the development potential. Therefore expect a good probability of tropical cylcogenesis from this system in the next 48 hours...followed by a collapse in the system as it approaches the Lesser Antilles after that time.

...MID-LATITUDES DISCUSSION...
P1...Upper troughs mentioned in paragraphs P1 and P2 of the previous discussion have merged into one large upper trough currently over eastern Canada and the western Atlantic. 986 mb frontal cyclone in paragraph P1 of the previous discussion has absorbed frontal cyclone mentioned in paragraph P2 of the previous discussion and has deepened to 984 mb while currently centered over the east coast of Canada and while supported by eastern divergence of the upper trough. Western convergence of upper trough supports 1024 mb ridge currently centered over south-central Canada/north-central US. This upper trough continues to confine US upper ridge to the southwestern US.

P2...Longwave upper trough over the high seas is now located just west of Europe. Western convergence of the upper trough supports 1029 mb ridge due southeast of Greenland. The upper trough has absorbed upper vortex west of Portugal mentioned in paragraph P7 of the previous discussion...but leaves behind upper vortex over the open Atlantic whose eastern divergence supports a weak surface low embedded in the paragraph P5 surface ridge.

P3...Cut-off east-west upper trough continues weakening in the western Gulf of Mexico

P4...Mid-latitude upper ridging spans the vicinity of the Bahamas and north-central Atlantic associated with relatively warmer air ahead of the paragraph P1 984 mb frontal cyclone. Bahamas portion of upper ridging is merging with upper anticyclone associated with Tropical Storm Gabrielle.

P5...Surface ridge dominating much of the open Atlantic is becoming anchored by 1029 mb center southeast of Greenland mentioned in paragraph P2. The south side of this surface ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P6...Low-latitude upper ridge covers much of the Atlantic tropical latitudes. What is left of adjacent axis of upper vorticity...sandwiched between the low-latitude upper ridge and paragraph P4 mid-latitude ridging...is over the waters north and northeast of the Lesser Antilles at a location due north of Tropical Storm Gabrielle.

P7...Yucatan peninsula disturbance is moving into the Bay of Campeche this morning while associated with surface trough and 1012 mb low. Despite staying under favorable paragraph P6 upper ridging while sneaking behind dissipating unfavorable Gulf upper trough mentioned in paragraph P3...this system has become less active in the last 36 hours. Despite this...this system continues to be mentioned in the NHC 48-hour tropical weather outlook. However if anything emerges from this disturbance...expect a general track west to west-northwest across the Bay of Campeche/southwestern Gulf/eastern Mexico region while steered by paragraph P1 1024 mb surface ridge and or southwestern US upper ridge also mentioned in paragraph P1.

P8...Western tropical wave formerly associated with disturbance Invest 97-L (which is now Gabrielle) is moving into the western Caribbean and is inactive while in an unfavorable environment of upper-level convergence/sinking air between upper anticyclone over Gabrielle and upper ridge over paragraph P7 disturbance. Eastern tropical wave formerly associated with Invest 97-L (now Gabrielle) is moving into the Lesser Antilles and producing active weather to the east and northeast of Gabrielle while sharing in favorable upper outflow of Gabrielle's upper anticyclone. North fragment of tropical wave has fractured into a surface trough northeast of the Lesser Antilles that is highlighted in NHC 48-hour tropical weather outlook. This surface trough is supported by eastern divergence of one of the upper vortices associated with axis of upper vorticity mentioned in paragraph P6. I do not expect tropical cyclone development from this surface trough but instead expect Gabrielle to simply absorb it.

P9...Tropical wave formerly classified as Invest 96-L continues west in the open waters between the Cape Verde Islands and Lesser Antilles and remains inactive in environment of dry Saharan air mentioned in paragraph P5 and southeastern convergence of upper anticyclone associated with Tropical Storm Gabrielle.

P10...Satellite imagery suggests the next tropical wave is currenty inland over west Africa about to emerge into the tropical belt of the Atlantic as marked in the lower-right of the above atmo chart. If this tropical wave becomes better organized as it emerges off the coast...I may upgrade it to a special feature on this blog considering it is entering the same favorable upper winds that adjacent special feature Invest 98-L is under.

Updated: 10:27 AM GMT on September 06, 2013

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2013 Atlantic Hurricane Season Birdseye Discussion #83

By: NCHurricane2009, 4:47 AM GMT on September 04, 2013

...WEDNESDAY SEPTEMBER 4 2013 12:47 AM EDT...
Tropical disturbance Invest 97-L currently in the eastern Caribbean Sea remains the most primed area for tropical development in the Atlantic at this time. See special feature section below for details. Elsewhere...tropical wave Invest 98-L off the coast of Africa (paragraph P10) and western Caribbean disturbed weather which has recently moved into the Yucatan (paragraph P8)...both mentioned in special update #82A...are not showing signs of development.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_03_2013_2345Z_zps0837e0c9.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_03_2013_2345Z_zps0b63c236.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DISTURBANCE INVEST 97-L...
The western of the two tropical waves associated with 97-L has moved into the central Caribbean but has left behind 1008 mb low pressure spin and thunderstorms that has moved across the Lesser Antilles into the northeastern Caribbean. The eastern of the two tropical waves remains east of the Lesser Antilles and is producing its own cluster of thunderstorms. Assuming that the eastern of the two tropical waves does not disrupt or steal surface inflow of the 1008 mb surface low...we can expect a high probability of tropical cyclone formation from the surface low based on its increasing organization and consolidating nature on satellite animation...especially due to the favorable evolution of the upper winds taking place. This evolution consists of the unfavorable axis of upper vorticity to the west and north of the system (paragraph P7) breaking up...allowing for an upper anticyclone to build over the system. This upper anticyclone will compose of a merger between a fragment of the low-latitude upper ridge axis overhead (paragraph P7) and mid-latitude upper ridging in the vicinity of the Bahamas (paragraph P5).

Based on the track of the 1008 mb surface low currently seen in satellite animation...it appars the surface low will pass west-northwestward over Puerto Rico and Dominican Republic in the next day or so while drawn toward low-level ridge weakness developing in the western Atlantic associated with paragraph P1 and P2 frontal cyclones. Although some surface ridging will re-build in the wake of the cyclones...computer model runs such as CMC...GFS...and NAVGEM suggest a tropical cyclone strong and vertically coupled enough to turn northward into the open western Atlantic under the influence of additional upper trouhging expected to dive in immediately behind the paragraph P1 and P2 upper troughs. Interests in the Bahamas should still monitor this system in case such a northward turn takes longer than expected to materialize.

In the shorter term...expect squally weather to move across Puerto Rico and the Dominican Repbulic in the next 24 hours in association with the 1008 mb low...then expect potentially another round of squally weather in these areas after that due to the thunderstorm cluster of the eastern tropial wave follwoing behind. The Lesser Antilles which have already journeyed through the 1008 mb low can expect squally weather from the eastern tropical wave in the next 24 hours.

...MID-LATITUDES DISCUSSION...
P1...Next shortwave upper trough is entering the upper-left of the above charts from central Canada. Its eastern divergence supports 986 mb frontal cyclone currently centered over eastern Hudson Bay.

P2...Upper trough previously over the central US and south-central Canada is now over the eastern US and southeastern Canada. Its eastern divergence supports 1003 mb frontal cyclone currently moving through Atlantic Canada. Western convergence of the upper trough supports 1018 mb ridge over the central US. This upper trough continues to confine US upper ridge to the southwestern US.

P3...Longwave upper trough over the high seas is now east and southeast of Greenland. Western convergence of the upper trough supports 1028 mb ridge due south of Greenland. 1020 mb frontal depression east of Newfoundland continues to be associated with this upper trough system. South fragment of this upper trough has merged with portion of paragraph P7 axis of upper vorticity to produce a SW-NE upper trough southeast of Newfoundland. Eastern divergence of this upper trough supports 1018 mb frontal depression in the open central Atlantic which is a consolidation of the 1014 and 1015 mb depressions in the area mentioned in paragraph P2 of discussion #82.

P4...Cut-off upper vortex in the Gulf of Mexico has weakened into an east-west upper trough extending into northeastern Mexico.

P5...Mid-latitude upper ridging spans the vicinity of the Bahamas and western Atlantic associated with relatively warmer air ahead of the paragraph P2 1003 mb frontal cyclone.

P6...Surface ridge dominating much of the open Atlantic is anchored by 1028 mb center due south of the Azores supported by western convergence of upper vortex west of Portugal mentioned in paragraph P7. Remainder of this surface ridge is generally supported by southeastern convergence of mid-latitude upper ridging mentioned in paragraph P5. The south side of this surface ridge is advecting pockets of dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears mitigated due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P7...Low-latitude upper ridge covers much of the Atlantic tropical latitudes. Adjacent axis of upper vorticity...sandwiched between the low-latitude upper ridge and paragraph P5 mid-latitude ridging...stretches from the waters north of the Lesser Antilles all the way to an upper vortex west of Portugal in the northeastern Atlantic. A fragment of this upper vorticity has retrograded southwestward into Jamaica while orbiting around paragraph P5 mid-latitude upper ridging. This upper vortex over Jamaica currenlty splits the aformentioned low-latitude upper ridge into two.

P8...Tropical wave previously moving across southeastern Mexico has exited the picture while moving into the eastern Pacific...but has gained attention in the NHC tropical weather outlook as noted in special update #82A due to disturbed weather it initialized over the western Caribbean supported by outflow of paragraph P7 low-latitude upper ridge. This disturbed weather has since moved into the Yucatan and developed a surface trough that can now sneak in behind the dissipating unfavorable Gulf upper vortex mentioned in paragraph P4...providing me a more favorable outlook on this system. Despite this...I am still not upgrading this system to a special feature as the thunderstorms are not organized enough and as their is still a lack of computer model support. However if anything emerges from this disturbance...expect a general track west to west-northwest into the Bay of Campeche/southwestern Gulf/eastern Mexico region while steered by surface ridge to build behind paragraph P1 weather system and or southwestern US upper ridge mentioned in paragraph P2.

P9...Tropical wave formerly classified as Invest 96-L has moved into the waters midway between the Cape Verde Islands and Lesser Antilles and remains inactive in a hostile environment of southerly shear associated with upper vortex embedded in paragraph P7 axis of upper vorticity and dry Saharan air mentioned in paragraph P6.

P10...Tropical wave previously exiting the west coast of Africa was recently classified as disturbance Invest 98-L as mentioned in special update #82A. It is no longer mentioned in the NHC tropical weather outlook nor is getting an upgrade to a special feature on this blog as the thunderstorm activity has become less organized. However...based on animation of upper wind forecasts from computer models...this tropical wave is likely to stay within favorable enivronment of low shear and good upper outflow beneath paragraph P7 low-latitude upper ridge and bears a little watching in case it re-organizes within the next 4 days. In the long range beyond 4 days...relatively lower pressures east of the upper anticyclone associated with disturbance 97-L will concentrate the paragraph P7 axis of upper vorticity into one large mid-ocean upper vortex likely to disrupt the longer-term opportunties for this tropical wave.

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2013 Atlantic Hurricane Season Birdseye Discussion #82A (Special Update)

By: NCHurricane2009, 7:19 PM GMT on September 02, 2013

...MONDAY SEPTEMBER 2 2013 3:30 PM EDT...
In addition to disturbance Invest 97-L...the National Hurricane Center has highlighted two new areas of disturbed weather in their traditional 48-hour tropical weather outlook while full discussion #82 was being composed.

The first is an area of western Caribbean thunderstorms north of Honduras and east of Belize mentioned in paragraph P7 of discussion #82. However the surface tropical wave generating these thunderstorms is over southeastern Mexico and moving westward and away. Their appears to be no surface low over the western Caribbean left behind by the tropical wave. The thunderstorm intensity in the region is not strong. Latest check with computer models shows no forecast development. And finally...the unfavorable Gulf of Mexico upper vortex mentioned in paragraph P3 of discussion #82 is expected to persist for a few more days. With all this reasoning...I do not expect tropical cyclone development from this area and therefore am not upgrading it to a special feature.

The second is the organized tropical wave emerging from the west coast of Africa mentioned in paragraph P9 of discussion #82 which was being upgraded to Invest 98-L while I was composing discussion #82. Based on animation of upper wind forecasts from computer models...this tropical wave is likely to stay within favorable enivronment of low shear and good upper outflow beneath low-latitude upper ridge mentioned in paragraph P6 of discussion #82. Therefore I will be likely upgrading this tropical wave to a special feature in my next full discussion if the current organization persists...and may even upgrade it to a special feature in another special update if this system organizes quickly before my next full discussion. In the long range...if special feature Invest 97-L evolves into a strong tropical cyclone with a large upper anticyclone...relatively lower pressures east of the upper anticyclone will concentrate the axis of upper vorticity (paragraph P6 of discussion #82) into one large mid-ocean upper vortex capable of disrupting this tropical wave's chances by 5 days and beyond.

I have updated the atmospheric features chart from discussion #82 as shown below to reflect the recent additions into the NHC traditional 48-hour outlook. Return to full discussion #82 for an assessment on the rest of the Atlantic tropics including a statement on special feature Invest 97-L centered just east of the Lesser Antilles.

...UPDATED ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_02_2013_1145Z_UPDATED_zps806d5846.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1328Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

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2013 Atlantic Hurricane Season Birdseye Discussion #82

By: NCHurricane2009, 6:01 PM GMT on September 02, 2013

...MONDAY SEPTEMBER 2 2013 2:01 PM EDT...
Only area of interest for tropical cyclone formation in the Atlantic basin this Septmeber afternoon is associated with disturbance Invest 97-L centered just east of the Lesser Antilles. See special feature section below for additional details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Sep_02_2013_1145Z_zpsde62c86b.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1328Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Sep_02_2013_1145Z_zpscd5c6c63.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DISTURBANCE INVEST 97-L...
Tropical wave with thunderstorms and defined low pressure spin approaching the Lesser Antilles has become stationary just east of the island chain on visible satellite animation this afternoon. It appears this is due to a secondary tropical wave entering the east side of the disturbance. This secondary tropical wave was added to NHC TAFB maps within the last 36 hours...and I believe emerged from Africa sometime between discussion #79 (August 29 morning) and discussion #78 (August 28 morning). Paragraph P8 of discussion #79 discussed a large area of thunderstorms extending from western Africa to the waters south of the Cape Verde Islands...and it now appears the thunderstorms south of the Cape Verde Islands at the time was a marker for this secondary tropical wave.

I expect this disturbance to remain generally stationary east of the Lesser Antilles for another 24 hours as the primary and secondary tropical waves merge. After that time...I expect the disturbance to resume a westward track into the Caribbean Sea while steered by paragraph P5 surface and low-level ridging. I maintain this system as a special feature due to the highly favorable upper winds forecast to develop. The unfavorable axis of upper vorticity to the west and north of the system (paragraph P6) is in the process of breaking up...allowing for an upper anticyclone to build over the system. This upper anticyclone will compose of a merger between a fragment of the low-latitude upper ridge axis overhead (paragraph P6) and mid-latitude upper ridging in the vicinity of the Bahamas (paragraph P4). Continued squally weather can be expected across the Lesser Antilles at times for the next 48 hours as this large disturbance takes it time consolidating and shifting westward. Interests across the Caribbean Sea...including all of the northern Caribbean Islands as well as the Bahamas...should monitor this situation carefully over the next few days. This is because their is potential for the system to deflect more northward in track if it develops quickly enough to become vertically coupled to influence of upper trough mentioned in paragraph P1 and another upper trough to quickly follow behind that over the next 4 days.

...MID-LATITUDES DISCUSSION...
P1...Shortwave upper trough from SW Canada and NW United States has entered the central US and south-central Canada. Its eastern divergence supports 1002 mb frontal cyclone currently centered just south of Hudson Bay and north of the Great Lakes. Western convergence of the upper trough supports surface ridging over the western US. This upper trough continues to confine US upper ridge (mentioned in paragraph P2 of the previous discussion) to the southwestern US.

P2...Longwave upper trough regime continues covering the high seas in the vicinity of Greenland. Western convergence of the upper trough regime supports 1028 mb ridge over the east coast of Canada. The 1006 mb frontal depression over the NE US (paragraph P2 of the previous discussion) is now weaker at 1012 mb located just east of Newfoundland. The eastern divergence of the upper trough regime continues supports 1014 mb frontal depression moving ENE into the open central Atlantic. The longwave upper trough leaves behind a shortwave east of Bermuda whose eastern divergence supports new 1015 mb frontal depression.

P3...Cut-off upper vortex has moved from the vicinity of south Florida and into the Gulf of Mexico while retrograding around paragraph P1 southwestern US upper ridge.

P4...Mid-latitude upper ridging spans from the vicinity of the Bahamas northeastward all the way to western Europe in association with relatively warmer air ahead of the fronts associated with the paragraph P1 and paragraph P2 upper troughs.

P5...Surface ridge dominating much of the open Atlantic is anchored by 1032 mb center offshore of the British Isles. This surface ridge is generally supported by southeastern convergence of mid-latitude upper ridging mentioned in paragraph P4. This surface ridge extends westward all the way into the Gulf of Mexico while supported by southeastern convergence of paragraph P1 southwestern US upper ridge. The south side of this surface ridge previously advected dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears to be diminished due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P6...Low-latitude upper ridge covers much of the Atlantic tropical latitudes. Adjacent axis of upper vorticity...sandwiched between the low-latitude upper ridge and paragraph P4 mid-latitude ridging...stretches from the waters NE of the Lesser Antilles all the way to the Azores in the northeastern Atlantic. A fragment of this upper vorticity has retrograded southwestward into Hispaniola while orbiting around paragraph P4 mid-latitude upper ridging. This upper vortex over Hispaniola currenlty splits the aformentioned low-latitude upper ridge into two.

P7...Tropical wave previously moving across the western half of the Caribbean is now moving across southeastern Mexico. Its eastern side is producing rounds of thunderstorms over the western Caribbean supported by outflow of paragraph P6 low-latitude upper ridge.

P8...Tropical wave formerly classified as Invest 96-L is no longer an area of interest as it has entered a hostile environment of westerly shear associated with south side of paragraph P6 axis of upper vorticity and dry Saharan air mentioned in paragraph P5. The tropical wave is currently positioned well to the west of the Cape Verde Islands.

P9...Based on satellite imagery...a tropical wave with well-organized thunderstorms is emerging from the west coast of Africa as marked in the lower-right of the above atmo chart. Based on animation of upper wind forecasts from computer models...this tropical wave is likely to stay within favorable enivronment of low shear and good upper outflow beneath paragraph P6 low-latitude upper ridge. Therefore if this tropical wave shows signs of persistent organization...I may upgrade it to a special feature with high risk of eventual tropical cyclone formation. In the long range...if special feature Invest 97-L evolves into a strong tropical cyclone with a large upper anticyclone...relatively lower pressures east of the upper anticyclone will concentrate the paragraph P6 axis of upper vorticity into one large mid-ocean upper vortex capable of disrupting this tropical wave's chances by 5 days and beyond.

Updated: 4:23 AM GMT on September 04, 2013

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2013 Atlantic Hurricane Season Birdseye Discussion #81A (Special Update)

By: NCHurricane2009, 5:03 PM GMT on September 01, 2013

...SUNDAY SEPTEMBER 1 2013 1:05 PM EDT...
Tropical wave Invest 97-L approaching the Lesser Antilles...mentioned in paragraph P9 of discussion #81...has seen an increase in thunderstorm activity and has become better organized over the last 12 hours. As some of the more agressive model solutions were predicting...the upper winds are becoming more favorable as the axis of upper vorticity to the west and north of the system (paragraph P7 of discussion #81) is breaking up...allowing for an upper anticyclone to build over the system. This upper anticyclone will soon compose of a merger between a fragment of the low-latitude upper ridge axis overhead (paragraph P7 of discussion #81) and mid-latitude upper ridging in the vicinity of the Bahamas (paragraph P5 of discussion #81). Therefore I am re-upgrading this tropical wave to a special feature with a high risk of eventual tropical cyclone formation. Squally weather with gusty winds will be overspreading the Lesser Antilles during the next 24 hours. Interests across the Caribbean Sea...including all of the northern Caribbean Islands as well as the Bahamas...should monitor this situation carefully over the next few days. This is because their is potential for the system to deflect more northward in track if it develops quickly enough to become vertically coupled with ridge weaknesses associated with the upper trough mentioned in paragraph P1 of discussion #81 and another upper trough to quickly follow behind that over the next 5 days.

Return to full discussion #81 for an assessment of the rest of the Atlantic tropics.

Updated: 5:05 PM GMT on September 01, 2013

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2013 Atlantic Hurricane Season Birdseye Discussion #81

By: NCHurricane2009, 12:20 AM GMT on September 01, 2013

...SATURDAY AUGUST 31 2013 8:20 PM EDT...
Atlhough their are three tropical waves with thunderstorm activity as mentioned in paragraphs P8....P9...and P10...none are showing signs of tropical cyclone formation.

We are closing the month of August with six Atlantic tropical storms and zero Atlantic hurricanes for the season so far. This now makes the season at an average pace with a below average number of hurricanes when compared to the 1966 to 2009 climatological data available on the National Hurricane Center site (http://www.nhc.noaa.gov/climo/images/cum-average_ Atl_1966-2009.gif).

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Aug_31_2013_1945Z_zps06f60597.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-released WPC analysis.

Any features boxed in green were mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's newer longer term 5-day outlook. Systems that I consider special features have less to do with whether they are boxed in green and more to do with whether I think their is a high risk of eventual tropical cyclone formation from that system.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Aug_31_2013_1945Z_zps59e737da.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...MID-LATITUDES DISCUSSION...
P1...Next shortwave upper trough and associated 1003 mb surface frontal cyclone is entering the top-left of the above charts from SW Canada and the NW United States.

P2...Shortwave upper trough from northern Canada (paragraph P1 of the previous discussion)...shortwave upper trough over the central US (paragraph P2 of the previous discussion)...and NW Atlantic upper trough (paragraph P2 of the previous discussion) have all merged into a longwave upper trough regime covering eastern North America...the NW Atlantic...and the high seas in the vicinity of Greenland. Western convergence of the upper trough regime supports 1017 mb ridge over Hudson Bay. The 1007 mb frontal depression moving through the Great Lakes (paragraph P2 of the previous discussion) is arriving into the NE US at 1006 mb. Surface cyclone from northern Canada (paragraph P1 of the previus discussion) is currently at 980 mb while its center arrives into the waters between Canada and Greenland...although it is developing a secondary center of 996 mb over SE Greenland supported by eastern divergence of the upper trough regime. 996 mb frontal cyclone over Atlantic Canada (paragraph P2 of the previous discussion) has deepend further to 993 mb due to the supportive eastern divergence of the upper trough regime while moving NE into the waters south of Greenland. The eastern divergence of the upper trough regime also supports new western Atlantic 1014 mb frontal depression currently just northeast of Bermuda. The amplfication of this upper trough regime has pushed the central US upper ridge (paragraph P2 of the previous discussion) into the southwestern US where it is currently centered over the 4-corners states. Southeastern convergence of this upper ridge supports 1016 mb surface ridge and sinking dry air over the Gulf of Mexico.

P3...Shortwave upper trough previously moving into the NE Atlantic (paragraph P2 of the previous discussion) has moved into northwestern Europe while its western convergence supports surface ridge center that has intensified from 1031 to 1032 mb while moving from the waters south of Greenland and into the waters just offshore of the British Isles in the last 24 hours.

P4...Cut-off upper vortex persists in the vicinity of southern Florida...and a western fragment of this vortex remains spread across the southern Gulf of Mexico in relatively lower pressures south of paragraph P2 southwestern US upper ridge and north of paragraph P7 low-latitude upper ridge.

P5...NE Atlantic upper ridge has moved into southwestern Europe in advance of paragraph P3 shortwave upper trough. Elsewhere...mid-latitude upper ridging spans from the vicinity of the Bahamas northeastward all the way to the high seas southeast of Greenland while pumped up by low-level warm air advection ahead of the 1014 mb....993 mb...and 996 mb frontal cyclones mentioned in paragraph P2.

P6...Surface ridge dominating much of the open Atlantic is now anchored by 1032 mb center offshore of the British Isles mentioend in paragraph P3. Rest of this surface ridge is generally supported by southeastern convergence of mid-latitude upper ridging mentioned in paragraph P5. The south side of this surface ridge previously advected dry Saharan air from Africa as observed by low-latitude brown shading in the above thermo chart. However as also observed by the white shading in the lower-right of the above thermo chart...the stream of dry Saharan air appears to be diminished due to the tropical wave thunderstorm activity from the southeast.

...TROPICAL BELT DISCUSSION...
P7...Low-latitude upper ridge covers much of the Atlantic tropical latitudes. Adjacent axis of upper vorticity...sandwiched between the low-latitude upper ridge and mid-latitude upper ridging in paragraph P5...stretches from the waters NE of the Lesser Antilles all the way to the Azores in the northeastern Atlantic. A fragment of this upper vorticity has fractured off while escaping into Algeria and the Mediterranean...and a fragment of this upper vorticity has retrograded southwestward into the NE Caribbean region while orbiting around paragraph P5 mid-latitude upper ridging.

P8...Tropical wave previously in the NE Caribbean is now moving across the western half of the Caribbean while developing an area of thunderstorms supported by split flow upper divergence between southwesterlies ahead of paragraph P4 upper vortex over south Florida and northwesterlies streaming into paragraph P7 NE Caribbean upper vorticity.

P9...Tropical wave moving toward the Lesser Antilles is showing signs of attempting to develop in favorable environment of low shear/outflow of paragraph P7 low-latitude upper ridge axis and therefore has been upgraded to Invest 97-L in the last 24 hours. The thunderstorms once biased to the east side are wrapping around the cyclonic spin into the north side...but the southwest side remains inactive in a pocket of dry Saharan dry air mentioned in paragraph P6. I am not re-upgrading this tropical wave to a special feature due to uncertainty in further development in association with southwesterly vertical shear that lies just ahead of the tropical wave associated with NE Caribbean upper vorticity mentioned in paragraph P7. The CMC model remains the only model still showing agressive development as it sees the upper vorticity breaking up enough to allow a healthy upper anticyclone to build over the tropical wave. However taking CMC superficially as it usually over-forecasts tropical cyclone development.

P10...Tropical wave Invest 96-L has moved offshore of Africa and into the weaters east of the Cape Verde Islands. The thunderstorm activity has not become better organized in the last day. Expect an increase in squally weather for the Cape Verde Islands in the next 24 hours...with the potential for development possibly declining by 48 or 72 hours and beyond as the tropical wave could interact with less favorable axis of upper vorticity mentioned in paragraph P7.

Updated: 5:00 PM GMT on September 02, 2013

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