Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at 

  • NCHurricane2009


Updated: Sep 15

*******Note that forecasts and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********


Over the last twelve hours... the central Atlantic tropical wave of low pressure tagged as area of interest #28 in full post #100 made consistent progress in organizing further and is now tropical depression seven. See full post #100 available on the home page of this site for more information regarding the rest of the Atlantic tropics.

TROPICAL DEPRESSION SEVEN...True-color visible satellite image taken at 1450Z of newly-formed Tropical Depression Seven. The islands in the forecast path of the newly-formed depression are highlighted in yellow text:

Over the last twelve hours... the low pressure circulation located along the central Atlantic tropical wave continued to feature an expanding thunderstorm cluster on its east side. The system has become better organized as the thunderstorm cluster is making attempts at covering the center of circulation... and the NHC has classified this system as tropical depression Seven as of 11 AM EDT. Downstream and well to the west... a pair of upper troughs over the eastern United States and Canada have recently merged... however the merged upper trough will later leave behind a cut-off upper trough over the southeastern United States whose eastern divergence zone will support an offshore cold front/inverted surface trough. The Atlantic surface ridge will also be intact to the north of this system... therefore the track going forward is expected to be west to west-northwest while funneled in the flow between the Atlantic surface ridge to the north and inverted surface trough to materialize to the west. My updated forecast track below still keeps the center of this system just north of the northeastern Caribbean Islands... especially as the the 11 AM EDT center fix is a little north of my previous forecast track from full update #100. However out of respect for the latest average of the computer models and the NHC official forecast... I have made a slight southward adjustment to my forecast track in the longer range... but is not as far south as the NHC which as of this writing brings the center of circulation across the northeastern Caribbean Islands and right into the Dominican Republic/Haiti landmass by day 5.

Regarding forecast intensity... the short-term strength of this system is threatened by a chunk of the upper vorticity tied to Earl's remnant system... which will soon become cut-off and sent southwestward toward this system by an upper ridge to amplify in the western Atlantic (the amplification of the upper ridge is expected due to the warm sector of a strong eastern Canada/northwest Atlantic frontal system soon to materialize from the above-mentioned merging Canada/US upper troughs). With the thunderstorm activity remaining lopsided to the east side of the circulation... the tropical depression is already showing signs of being sheared by this upper vorticity. However the thunderstorm activity is showing signs of still becoming better organized and therefore I project that this system becomes a tropical storm of 50 mph maximum sustained winds in the short-term... before the shear increases further. Also noting that by studying the upper wind pattern in the 0600Z GFS model run from this morning... I do not anticipate at this time the shear will be strong enough to weaken the tropical storm through 48 hours. At 72 hours it appears likely the tropical storm will escape the upper vorticity and slide underneath a lower shear and upper outflow environment directly below the western Atlantic upper ridge... and I project some strengthening during that timeframe. However I only show a slight amount of strengthening... followed by a weaker intensity at days 4 and 5 as new sources of shear could negatively affect this system. For example today's 0600Z GFS model run raises the possibility that some of the current Caribbean upper vorticity gets sent northeastward into this system by the forecast southeastern US cut-off upper trough and just after 72 hours. The cut-off southeastern US upper trough itself may approach as a not-so-amplified feature which will induce upper southwesterly shearing flow over this system in the longer range. The long range intensity forecast however is uncertain as it is not set in stone yet whether or not the center of this system moves into the Dominican Republic and Haiti. It is also possible the longer-term shear may become lower than expected if the Caribbean upper vorticity... which is weakening while remaining cut-off from high-latitude cold air... ends up dissipating before getting a chance to shear this system. And if the cut-off upper trough over the southeastern United States becomes more amplified than previously thought... due to a slightly more amplified upper ridge building over the United States... this would also result in less wind shear. Thus it is no surprise that the long range intensity guidance is all over the place... with the NAVGEM being at the top end and the GFS being at the bottom end while not developing this system. Other models such as the ECMWF are intermittent from run-to-run... for example at 1200Z yesterday not developing this system and now as of 0000Z earlier today quickly ramping up this system in the longer range.

Regarding impact to land areas:

(1) There is a higher chance of tropical storm conditions (gusty winds... heavy rains with flash flooding potential... and some coastal sea swells) for the northern Lesser Antilles... Virgin Islands... and Puerto Rico late this week and into the weekend. I recommend interests here begin reviewing tropical storm preparation plans... and begin executing those plans should tropical storm watches/warnings be raised later today. Check (hurricanes dot gov) for up to the minute latest information on any watches/warnings associated with this system.

(2) I recommend that interests in the Dominican Republic and eastern Bahamas should be aware of this system due to longer range uncertainty in the intensity forecast.

****** forecast. Visit (hurricanes dot gov) for official forecast***********

0 Hr Position (1200Z Sep 14)...35 mph maximum sustained wind tropical depression centered at 16.6N-49.6W

IOH 24 Hr Forecast (1200Z Sep 15)...50 mph maximum sustained wind tropical storm centered at 17.5N-54.5W

IOH 48 Hr Forecast (1200Z Sep 16)...50 mph maximum sustained wind tropical storm centered just east of the northern Lesser Antilles at 18.5N-59.5W

IOH 72 Hr Forecast (1200Z Sep 17)...60 mph maximum sustained wind tropical storm centered just north of the Virgin Islands at 19N-64.5W

IOH 96 Hr Forecast (1200Z Sep 18)...50 mph maximum sustained wind tropical storm centered north of the eastern Dominican Republic at 20N-68.5W

IOH 120 Hr Forecast (1200Z Sep 19)...50 mph maximum sustained wind tropical storm centered north of Haiti and just south of the eastern Bahamas at 21.5N-72.5W

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