Tropical Depression #18 Forecast/Discussion

This forecast/discussion is a 5-day forecast for the tropical element of the large storm modeled over the eastern US. I will post a separate discussion covering what happens past 5 days later this afternoon.

Above is a look at TD 18 as of 11:15am EDT. The depression has organized considerably since yesterday, with a much more consolidated area of convection over the center of circulation. Outflow is fair to the north and south of the system but not great.

An examination of recent microwave imagery shows that there is a hint at a curved but over the northern and eastern semi-circle of the system, with the center within a few miles of 13.5N, 78W in this image per the National Hurricane Center. If the system can continue to organize banding and consolidate, it may become a tropical storm later today and will likely become a tropical storm tonight. Assuming an invest in the open Atlantic does not become named sooner, TD 18 will assume the name “Sandy” when it reaches tropical storm strength.

As discussed yesterday, as 18L drifts farther SW it positions itself under decreasing wind shear, which promotes organization. There is a belt of sub-tropical westerlies to the north of the cyclone which is aiding in venting the system…which can be seen with cirrus clouds moving away from the system in the northern semi-circle. As the system drifts southwest into tomorrow, it will remain under favorable upper level conditions for intensification.

http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller?prevPage=Model&MainPage=index&image=&page=Param&cycle=10%2F22%2F2012+12UTC&rname=UPPER+AIR+PARMS&pname=850vor_500ht_200wd&pdesc=&model=GFS&area=WNATL&cat=MODEL+GUIDANCE&fcast=5+Day+Loop&areaDesc=Western+North+Atlantic+-+Southeast+US+Central+America+Caribbean&prevArea=WNATL&currKey=model&returnToModel=&imageSize=M

This 5-day animation of the GFS shows that upper level winds (barbs on map) remain in the 10-20 knot range over the system until it approaches Cuba, before increasing significantly. Due to the very warm waters under the system, and the expectation that 18L will have about 48 hours of light to moderate wind shear and good upper level divergence, if 18L can consolidate a bit further today and tonight it may overperform and become a strong system before shear begins to take its toll in about two days and Cuba shortly thereafter. Will somewhat blend the idea of a more robust system with current ATCF intensity guidance and show 18L becoming a minimal hurricane before impacting Cuba.

After passing Cuba, the global models show 18L interacting with strong upper level southwesterly winds. However, the models generally show the system maintaining or lower its pressure with an expanding wind field, showing that the system will get a shot of baroclinic energy, and also meaning the system will not be completely tropical:

Due to this, will match the NHC and indicate a sub-tropical storm for the day 5 forecast.

As for the track forecast, the system will drift southwest for the next 12-24 hours due to strong low level ridging to its north. However as a trough over the central US impinges on this ridging it will weaken and slide to the east somewhat, allowing a slow north to north-northeast motion to occur.

Note how on the 0z Euro Ensembles, there is ridging surrounding the depression in all directions, with the weakest ridging being to the N-NE. The system will follow the path of least resistance to the NNE starting in about 24 hours, and this general motion will continue through the 5-day forecast period.

Forecast track/intensity:

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