2013 Atlantic Hurricane Season Birdseye Discussion #82

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

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...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.

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