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By: NCHurricane2009 , 4:37 AM GMT on August 15, 2013
...THURSDAY AUGUST 15 2013 12:30 AM EDT...
Strong tropical wave in the eastern tropical Atlantic...previously designated as Invest 93-L...has intensified into the fifth tropical depression of the 2013 Atlantic hurricane season. My forecast versus the NHC's is detailed in Figure 1 below. In the meantime...return to full discussion #69 for an assessment on the rest of the Atlantic tropics including the threat of tropical cyclone formation in the northwestern Caribbean and southern Gulf of Mexico regions from Invest 92-L. Visit www.nhc.noaa.gov for up to the minute latest information on tropical depression five and latest developments associated with Invest 92-L.
Figure 1: My forecast versus the NHC for Tropical Depression Five
Concerning tropical depression five...the thunderstorm activity is biased to the south half of the circulation perhaps due to the marginal water temps below 26 deg C just to the north and perhaps due to a tinge of dryness from the Saharan air layer to the northwest. I show an only-gradual strengthening trend to a moderate tropical storm for the next 36 hrs as it will be adjacent to this pool of marginal water temps based on the track forecast. Beginning in 36 hrs...based on the thermo chart shown in discussion #69 and the track forecast...the tropical cyclone should begin encountering higher concentrations of Saharan dry air while simultaneoulsy moving away from adjacent pool of marginal waters. For now I am assuming after 36 hrs that this dry air will win out over the warmer waters and hence gradually weaken the system back to a tropical depression especially since models are not gung-ho about developing this system in the long range. Forward pace of NHC track forecast is in agreement with the 6-hourly motion witnessed on satellite between 5 and 11 PM EDT Wednesday. And with the expectation that the cyclone should strengthen a bit in the short term and hence become vertically coupled enough to feel some poleward tug from developing cut-off upper vortex to its northwest...I agree with the WNW track shown by NHC for the first 48 hours. While I agree with the westward bend in track (as the cyclone becomes shallower/weaker and hence decoupled from upper-level poleward tug)...I go a bit faster than the forward pace of NHC between 48 and 72 hrs as the GFS model shows the paragraph P3 surface ridge maintaining strength in that time. I then agree with the NHC's slowdown in forward pace after 72 hrs as this is when the GFS model shows the paragraph P3 surface ridge weakening a bit.
Impact swath begins with the development of a tropical storm wind field just south of center as the tropical cyclone currently has its thunderstorm activity biased to the south due to the marginal water temps to its north. When studying the GFS upper wind forecast relative to the forecast track in Figure 1...it seems that after 24 hours the system will reach some light southerly vertical shear on the east side of the westward-retrograding upper vortex. Even though the thermodynamics thru the forecast period generally suggest the thunderstorms should stay biased to the south (due to either marginal waters to the north or dry Saharan air to the northwest)...I expect the southerly shear to somewhat counteract that and make the strongest thunderstorms more symmetric about the center...which is why after 24 hrs my impact swath becomes more symmetric about the storm track. Impact swath dwindles late in the forecast to simulate my current projection of the system weakening back to a tropical depression later on. Tropical storm warnings are currently in effect for the southern Cape Verde Islands. Based on my impact swath...I personally predict that the islands have a near-zero percent chance of actually realizing tropical storm sustained winds.
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