2013 Atlantic Hurricane Season Birdseye Discussion #86

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

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...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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