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2014 Atlantic Hurricane Season Birdseye Discussion #70

By: NCHurricane2009 , 3:35 AM GMT on August 22, 2014

...THURSDAY AUGUST 21 2014 11:36 PM EDT...
Tropical disturbance Invest 96-L currently located east of the northern Lesser Antilles continues to track further north than expected. Therefore this system is likely to pass north of the northeastern Caribbean Islands and Bahamas...afterwards stalling in the western Atlantic for a period due to weak steering currents. Despite the more northern track...the southwestern outer squalls of this large system have arrived to the northern Lesser Antilles and Virgin Islands...and will spread into Puerto Rico...the Dominican Republic...and eastern Bahamas over the next day or so. See special feature section below for an updated assessment on this disturbance.

 photo Aug_21_2014_2245Z_zps88127069.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z and the 1930Z-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_21_2014_2245Z_Thermo_zps7d3562bc.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...Between 1800Z and 0000Z...satellite animation suggests this system is now on a northwestward track while moving from 16.5N-57.5W to 17.5N-59.5W. This track is further north than expected...perhaps as the current ciruculation is being steered by the northeastern quadrant of what is a shadow of the original southwestern circulation. Therefore my updated forecast track below and the latest suite of computer model runs have made another northward shift relative to yesterday. Based on reconnaissance observations announced in a 4 PM EDT NHC Special Tropical Weather Outlook this past afternoon of winds at tropical storm force...this system is almost a tropical storm. Satellite has been showing a cloud swirl center with some outer spiral bands of thunderstorms especially to the south and west. The feeble level of storm activity perhaps is due to this system ingesting a patch of dry northeastern Caribbean air and some of the bulk of the dry Saharan air layer to the northeast seen in the above thermodynamics chart. Recently a small storm burst has begun directly over the cloud swirl center...and it remains to be seen if this burst expands such that this system finally gets upgraded to a tropical storm.

Atmospheric Outlook for the Forecast Period...Upper trough over the western Atlantic as marked by a blue-dashed line in the atmospheric features chart above will become re-enforced while merging with the 1013 mb frontal cyclone and associated upper trough currently currently marching offshore from the northeastern US. This upper trough will weaken the west side of the low-level 1022 mb Atlantic subtropical ridge such that the track of this system will bend more northward while steered around the west side of the ridge. By the end of the forecast period...this system is likely to stall while becoming jammed between the west side of the Atlantic subtropical ridge and a building high-latitude ridge nosing in from the north supported by upper convergence on the west side of the upper trough (this high-latitude ridge is in its formative stages as marked by the 1028 mb center over northeastern Canada in the above atmo chart). My updated intensity forecast below is lowered from my previous as the more northward forecast track now means this system will run into westerly and northerly shear on the south and west sides of the upper trough. By 120 hours however the upper trough should be moving eastward and away...which could allow the system to resume stregnthening by that time.

Thermodynamic Outlook for the Forecast Period...This system will be tracking over favorable 29 to 30 deg C waters thru the forecast period. Currently this system is sandwiched between the bulk of the dry Saharan air to layer to the northeast and a patch of northeastern Caribbean dry air to the west as noted in the above current prognosis section. However as the above thermo chart shows...a continued northwestward track as forecast below will take this system to a more favorable moist environment currently covering eastern North America and waters offshore of the southeastern US.

24 Hr Forecast (1800Z August 22)...45 mph max sustained wind tropical storm centered near 20.5N-64W in the western Atlantic just north of the Virgin Islands.

48 Hr Forecast (1800Z August 23)...50 mph max sustained wind tropical storm under westerly shear centered near 26N-70W...or northeast of the eastern Bahamas.

72 Hr Forecast (1800Z August 24)...50 mph max sustained wind tropical storm under northwesterly shear centered near 29N-72.5W...or in the western Atlantic midway between the Bahamas and Bermuda.

96 Hr Forecast (1800Z August 25)...50 mph max sustained wind tropical storm under light northerly shear centered near 30N-75W

120 Hr Forecast (1800Z August 26)...65 mph max sustained wind tropical storm centered near 30N-75W

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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