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2014 Atlantic Hurricane Season Birdseye Discussion #68

By: NCHurricane2009 , 3:36 AM GMT on August 20, 2014

...TUESDAY AUGUST 19 2014 11:40 PM EDT...
The tropical wave midway between the Lesser Antilles and Cape Verde Islands is now producing a new and organizing low pressure spin to the south of the dry Saharan air layer from which the CMC...GFS...and NAVGEM computer models forecast tropical cyclone development. This system has also been upgraded to Invest 96-L this evening. With this system now becoming a threat to develop...I have upgraded it to a special feature on this blog. Interests in the southern Lesser Antilles...Caribbean Sea...and Gulf of Mexico should monitor the progress of this system over the next few days. See special feature section below for additional details.

The tropical wave previously southwest of the Cape Verde Islands is now in the waters midway between the Cape Verde Islands and Lesser Antilles...and has beocome less organized behind Invest 96-L. Therefore I have dropped it as a special feature on this blog...and the National Hurricane Center is likely to cancel it in their Tropical Weather Outlook if current trends continue.

 photo Aug_19_2014_2045Z_zps2c73a02c.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z and the 1925Z-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_19_2014_2045Z_Thermo_zps2d713a76.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...As of 1800Z the National Hurricane Center has marked a tropical wave axis at 50W longitude...with this system producing a new 1009 mb low pressure spin at the south end of the tropical wave and already showing early signs of organized spiral banding in the associated showers and thunderstorms. Between 1800Z and 0000Z satellite animation shows the circulation moving from 10N-50W to 10N-51W...or at a rate of 1W per 6 hours (4W per 24 hours) upon which the early part of my track forecast below is based on. The westward pace of my later forecast track is a bit faster than that to stay in closer agreement with the faster westward progression shown in some of the models (such as the GFS) that develop this system. However my forecast track below is also generally south of many of this evening's model runs as many of the models show this system consolidating further to the north than what current satellite observations suggest.

Atmospheric Outlook for the Forecast Period...A central Atlantic upper trough has moved westward into the Lesser Antilles as marked by a blue-dashed line in the atmospheric features chart above...and will be moving northwestward and away while merging with the 1002 mb frontal cyclone and associated upper trough currently over the central 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 it marches into the Caribbean Sea. Meanwhile over the US...expect a pattern of upper ridging over the eastern US/Gulf of Mexico and upper troughing in the western US to become established during the forecast period. The unfavorable western Caribbean upper vorticity (marked by blue L) 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. From 72 to 120 hrs...the west end of the low-level Atlantic subtropical ridge is forecast to slowly weaken...first by the arrival of the central US upper trough/surface 1002 mb 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. Such a steering pattern is likely to cause this system to continue northwest into the Gulf of Mexico from the Caribbean in the very long range outside the forecast period.

Thermodynamic Outlook for the Forecast Period...This system will be tracking over favorable 28 deg C waters early in the forecast period...increasing to an even more favorable 29 to 30 deg C later in the forecast period. The dry Saharan air layer seen in the thermo chart above no longer appears to be a problem for development as this system is organizing within the moisture field of the ITCZ (Intertropical Convergence Zone) to the south of the dry Saharan air layer.

24 Hr Forecast (1800Z August 20)....Organized tropical low pressure centered near 10.5N-54W.

48 Hr Forecast (1800Z August 21)...35 mph max sustained wind tropical depression centered near 11N-58W moving into the southern Lesser Antilles

72 Hr Forecast (1800Z August 22)...45 mph max sustained wind tropical storm centered near 11.5N-62W departing from the southern Lesser Antilles and entering the southeastern Caribbean Sea

96 Hr Forecast (1800Z August 23)...60 mph max sustained wind tropical storm centered near 12.5N-67W moving west-northwest toward the central Caribbean Sea

120 Hr Forecast (1800Z August 24)...75 mph max sustained wind hurricane centered near 15N-72W moving northwest across the central Caribbean toward Jamaica

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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