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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...
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...
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.
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.
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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