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By: NCHurricane2009 , 4:58 AM GMT on August 24, 2014
...SUNDAY AUGUST 24 2014 1:00 AM EDT...
Aircraft reconnaissance has finally found a closed circulation to upgrade Invest 96-L to a tropical cyclone. However because the maximum sustained winds measured were below storm force at the time...this system has been upgraded to tropical depression four. With the center expected to track east of the Bahamas...impacts across the island chain are not expected to be significant. The long range threat to the southeastern United States coast is a bit uncertain at this time...but the latest computer model runs favor a track turning away from the United States just beyond the forecast period. See special feature section below for additional details on this system. Visit www.nhc.noaa.gov for up-to-the-minute the latest information on this system...including watches/warnings.
A tropical wave has recently emerged from Africa into the eastern tropical Atlantic over the last couple of days...and reached a peak in organization as of 0600Z this morning while producing a semi-organized storm cluster southwest of the Cape Verde Islands...after which time the storm cluster has become less organized. Because the dry Saharan air layer is weaker relative to earlier this month and because this system will remain below favorable tropical upper ridging (currently marked by blue Hs in the eastern Atlantic in the atmospheric features chart below)...their is some potential for development as it tracks westward across the Atlantic tropics over the next few days...although current computer model support is weak.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
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.
...THERMODYNAMICS BIRDSEYE CHART...
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 DEPRESSION FOUR...
Current prognosis...As of the 11 PM EDT National Hurricane Center advisory...the center of newly formed tropical depression four was located at 22.3N-72.6W...or over the southeastern Bahamas. The center currently has a small ball of thunderstorms overhead...with most of the outer spiral bands of showers and thunderstorms located to the east of the center. Due to light westerly shear imparted by an inverted upper trough over the western Caribbean (marked by blue dashed line west of the depression in the above atmospheric features chart)...outer spiral bands are limited west of the center.
Atmospheric Outlook for the Forecast Period...Western Caribbean inverted upper trough mentioned in the above current prognosis section is expected to impart additional light westerly shear across this system for another 24 hours...after which time the upper trough will have moved westward and away around the south side of the strong upper ridge over the eastern US (marked by blue H in the above atmo chart). However the western Atlantic upper trough to the north (marked by blue dashed line) will become amplified enough to leave behind a cut-off upper vortex to the northeast expected to induce northerly shear across this system by 48 and 72 hours. Shear is expected to finally reduce by 96 and 120 hours after the cut-off upper vortex shifts east. Tropical depression four is currently moving northwest around the west side of the low-level Atlantic subtropical ridge marked by a red zig-zag line. In the middle of the forecast period...a building high-latitude ridge nosing in from the north...supported by upper convergence on the west side of the western Atlantic upper trough...will cause the track to bend more west-northwest (this high-latitude ridge is currently marked as a 1023 mb center over eastern Canada in the above atmo chart). By the end of the forecast...a fragment of the current western US upper trough will have moved around the strong eastern US upper ridge and into eastern Canada while supporting/carrying the 1005 mb frontal cyclone currently over the northwestern US. Between 96 and 120 hours this frontal cyclone will have weakened the ridge to the north such that the track of this system is expected to accelerate northward. The question is will this system go northeastward while getting dragged into the southwest side of the frontal cyclone...or will it turn northwestward due to the low-level ridge behind the frontal cyclone?
Thermodynamic Outlook for the Forecast Period...This system will be tracking over very favorable 30 to 31 deg C waters thru the forecast period. The above thermo chart also shows this system is located in a moist environemnt. Therefore thermodynamic parameters are very favorable for devlopment during the forecast period.
Track Forecast...The track forecast shown above is a continuation of my track forecast from discussion #71...with the exception of the early part of the forecast line which is displaced a bit westward as the circulation has consolidated further to the west relative to the track forecast I presented in discussion #71. Their is debate as to whether this system will go northwest or northeast at the end of the forecast period as outlined in the above atmospheric outlook section. Therefore as shown above I take a more neutral approach...but lean the track to the north-northeast because the more reliable GFS and ECMWF (Euro) runs from earlier on Saturday go with a northeast track.
Intensity Forecast...I initially keep the intensification rate slow due to forecast shear for the first 72 hours as outlined in the above atmospheric outlook section. With low shear and favorable thermodynamic conditions after that time...I forecast intensification to a hurricane (75+ mph maximum sustained winds).
Impact Forecast...The above-illustrated impact swath suggests that no land areas are forecast to received sustained winds of tropical storm force due to the forecast track keeping the center east of the Bahama island chain and east of the Florida peninsula. However with westerly shear expected to abate after 24 hours...outer feeder bands containing showers and thunderstorms are likely to begin filling the west side and overspread the central and western Bahamas by Monday and Tuesday. Feeder bands could continue overspreading the western Bahamas and potentially move into east-central Florida on Wednesday as northerly wind shear has the potential to push showers and thunderstorms to the south of the center. As this system gathers strength...surf and rip currents will become an increasing threat as noted by impact statement (c) in the above forecast graphic.
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