NHC Weather Outlook

Tropical Storm Rose Forecast Discussion

WTNT42 KNHC 201447

Tropical Storm Rose Discussion Number   6
NWS National Hurricane Center Miami FL       AL172021
1100 AM AST Mon Sep 20 2021

While a cursory look at visible satellite images would suggest Rose 
is intensifying, other data show that it remains a sheared storm.  
The low- and mid-level centers remain roughly 90 n mi apart 
according to SSMIS microwave, and a 1038Z ASCAT-A scatterometer 
pass showed no significant change in intensity overnight.  The 
current wind speed is kept at 35 kt, with a much heavier weight on 
the 30-kt scatterometer pass than Dvorak estimates near 55 kt.  This 
is a good example of the value of the scatterometer which can tell 
the forecaster much more about the surface winds that conventional 
satellite estimates can miss (even if the data is possibly too low 
with all the thunderstorm activity).

There are a lot of thorns in the way of Rose blossoming into a 
stronger storm.  Increasing shear and drier mid-level air are on 
the way for tonight, competing against the somewhat warm SSTs.  
Thus Rose has about a day to flower into a moderate tropical storm, 
and no significant change was made to the short term forecast.  At 
longer range, stronger shear and dry air should pull the petals off 
Rose one-by-one, causing the cyclone to slowly weaken.  The new 
forecast is similar to the previous one, with some small 5-kt 
downward adjustments.  Rose could even shrivel up into a remnant 
low by day 5, but that's not shown yet in the forecast.

The initial motion remains northwestward at about 14 kt.  Rose is 
still expected to move generally northwestward around the 
southwestern and western periphery of a strong subtropical ridge 
during the next few days.  Around day 3, however, a mid- to 
upper-level trough over the north-central Atlantic is forecast to 
dig southeastward, causing the cyclone to turn northward and 
eventually northeastward by the weekend.  Guidance is in much 
better agreement on this scenario than the last cycle (though 
there are still some westward model solutions), and the new NHC 
track forecast is shifted northeastward at long range.  


INIT  20/1500Z 17.3N  33.4W   35 KT  40 MPH
 12H  21/0000Z 19.0N  34.8W   40 KT  45 MPH
 24H  21/1200Z 21.0N  36.1W   35 KT  40 MPH
 36H  22/0000Z 22.6N  37.1W   35 KT  40 MPH
 48H  22/1200Z 24.1N  38.1W   35 KT  40 MPH
 60H  23/0000Z 25.4N  39.1W   30 KT  35 MPH
 72H  23/1200Z 26.7N  40.0W   30 KT  35 MPH
 96H  24/1200Z 29.0N  40.0W   25 KT  30 MPH
120H  25/1200Z 31.0N  38.0W   25 KT  30 MPH

Forecaster Blake

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