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2014 Atlantic Hurricane Season Birdseye Discussion #69

By: NCHurricane2009 , 3:09 AM GMT on August 21, 2014

...WEDNESDAY AUGUST 20 2014 11:15 PM EDT...
Tropical disturbance Invest 96-L currently located east of the Lesser Antilles has re-organized to the northeast while absorbing with the tropical wave to its east. Therefore this system is on a further north track now threatening the central and norhtern Lesser Antilles...the northeastern Caribbean Islands (Puerto Rico...the Virgin Islands...Haiti and the Dominican Republic)...the Bahamas...and then potentially the southeastern United States coast over the course of the next few days. See special feature section below for additional details on this developing situation.

 photo Aug_20_2014_2245Z_zps6942aa0e.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z and the 1926Z-released WPC analysis.

Features boxed in green...if any...are 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 longer term 5-day outlook.

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.

 photo Aug_20_2014_2245Z_Thermo_zps7fba7dd0.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).

Current prognosis...Over the past 24 hours the circulation of this disturbance became elongated while merging with with the tropical wave previously to its east. Meanwhile the original tropical wave that spawned the circulation continued west and has dissipated over the Lesser Antilles as of 1200Z as marked in the above atmospheric features chart. Recently the circulation appears to have become much better organized and consolidated at a location northeast of where it was previously organizing. Because this rapidly organizing circulation is along the tropical wave axis that came in from the east...the same wave that was a special feature on this blog during discussions #66 and #67...one could argue this system is now a continuation of that special feature. The recent re-organization of the disturbance to the northeast while merging with the tropical wave to the east may have been what the computer models have been picking up on...and because this has materialized my updated forecast track below is now within the model consensus instead of being south of the model consensus like it was previously. Satellite animation suggests the organizing circulation has moved from 12.5N-52W as of 1800Z to 12.75N-53.5W as of 0000Z...or at a pace of 1.5W per 6 hours (or 6W per 24 hours) upon which the early part of the updated forecast track below is based upon. Because of the recent quick organization trend...I have moved the time of forecast tropical storm formation (40+ mph max sustained winds) to be sooner than previously forecast.

Atmospheric Outlook for the Forecast Period...Upper trough previously over the Lesser Antilles has moved northwestward into the western Atlantic as marked by a blue-dashed line in the atmospheric features chart above...and will continue northwestward and away while merging with the 1011 mb frontal cyclone and associated upper trough currently over the eastern US. This will allow the favorable tropical upper ridge (marked by blue-zig-zag line overhead of 96-L) to continue expanding westward such that 96-L remains under favorable low shear/enhanced outflow. As the current eastern US upper trough then shifts into the western Atlantic...a pattern of upper ridging over the eastern US/Gulf of Mexico and upper troughing in the western US will become established during the forecast period. The unfavorable Caribbean upper vorticity (marked by blue dashed line) will slowly retrograde westward around the south side of the eastern US/Gulf of Mexico upper ridge such that it also stays clear of this system. Therefore with low shear/good outflow thru the forecast period...I am encouraged to forecast a hurricane by 120 hours as shown below (the only time I suggest weakening is between 72 and 96 hours when this system is forecast to interact with the landmass of the Dominican Republic/Haiti). From 48 to 96 hours...the west end of the low-level Atlantic subtropical ridge is forecast to slowly weaken...first by the arrival of the current eastern US upper trogh/surface frontal cyclone into the western Atlantic...then by a series of central US frontal cyclones supported by the forecast western US upper trough...which is why the system is predicted to bend increasingly to the northwest in the latter part of the forecast below. By 120 hours the forecast track remains northwest instead of north in spite of the weaknesses associated with the western US upper trough and western Atlantic upper trough as upper convergence on the west side of the western Atlantic upper trough will be supporting a building high-latitude surface ridge that noses in from the north.

Thermodynamic Outlook for the Forecast Period...This system will be tracking over favorable 28 to 29 deg C waters early in the forecast period...increasing to an even more favorable 30 to 31 deg C later in the forecast period. This system is currently moving away from the bulk of the dry Saharan air layer which is to the northeast...but as shown in the thermo chart above this system may have to contend with some small patches of dry air presently seen in the northern half of the Caribbean Sea. Currently it is assumed that the low-level convergence/upper divergence of this system will be vigorous enough to maintain a moisture field that does not become adversely affected by the patches of dry air.

24 Hr Forecast (1800Z August 21)...40 mph max sustained wind tropical storm centered near 13.5N-58W moving into the Lesser Antilles

48 Hr Forecast (1800Z August 22)...60 mph max sustained wind tropical storm centered near 14.5N-64W in the eastern Caribbean Sea

72 Hr Forecast (1800Z August 23)...70 mph max sustained wind tropical storm centered near 16.5N-69W...or just south of and moving toward the Dominican Republic

96 Hr Forecast (1800Z August 24)...65 mph max sustained wind tropical storm centered near 21N-73W...or approaching the southeastern Bahamas

120 Hr Forecast (1800Z August 25)...85 mph max sustained wind hurricane centered near 24.5N-77W...or over the western Bahamas

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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