Tropical Storm Isaias Forecast Discussion

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991 
WTNT44 KNHC 300301
TCDAT4

Tropical Storm Isaias Discussion Number   7
NWS National Hurricane Center Miami FL       AL092020
1100 PM AST Wed Jul 29 2020

Observations from recent scatterometer passes over the system show 
that it now has a sufficiently well-defined center to be 
designated as a tropical cyclone.  The current intensity is 
estimated to be 45 kt, but these winds are currently occurring over 
the Atlantic waters well to the north and northeast of the center.  
Since the cyclone is expected to move over Hispaniola on Thursday 
some weakening is likely within the next 24 hours.  However since 
Isaias has such a broad wind field, the weakening will probably not 
be as significant as in a typical tropical cyclone with a small 
radius of maximum winds.  Also, a re-formation of the center to the 
north of Hispaniola may occur.  Later in the forecast period some 
strengthening is likely, although this may be offset by 
southwesterly wind shear on the order of 20 kt in 2-3 days.  The 
official forecast is close to the intensity model consensus but 
well below the latest LGEM guidance.

The scatterometer data show that the center of the system is south 
of the previously estimated track, so there is a lot of uncertainty 
in the initial motion estimate of 285/17 kt.  Isaias should move on 
a west-northwestward to northwestward track on the southern and 
southwestern side of a mid-tropospheric ridge.  In 2-3 days, 
the system is expected to turn north-northwestward due to a 
weakness in the ridge and an approaching trough.  Later in the 
forecast period, the trough should cause Isaias to turn toward the 
northeast.  The official track forecast is a little to the east of 
the previous one and a little west of the simple and corrected 
dynamical model consensus tracks.  It should be noted that further 
adjustments to the forecast tracks are indeed possible, especially 
after Isaias moves north of Hispaniola. 


Key Messages:

1. Isaias will produce heavy rains and potentially life-threatening 
flash flooding and mudslides across the Virgin Islands, Puerto Rico, 
the Dominican Republic, northern Haiti, and over the southeastern 
Bahamas.

2. Tropical storm conditions are likely across portions of the
the Virgin Islands, and Puerto Rico through tonight and will spread 
westward to portions of the Dominican Republic, Haiti, and the 
southeastern Bahamas and Turks and Caicos and the Central Bahamas
on Thursday and Friday.  Tropical Storm Warnings are in effect for 
these areas. Do not focus on the details of the track forecast, as 
rainfall and wind hazards will extend far from the center of the 
system.

3. While this system could bring some rainfall and wind impacts to 
portions of Cuba, the northwestern Bahamas, and Florida later this 
week and this weekend, it is too soon to determine the location or 
magnitude of those impacts.  Interests there should monitor the 
progress of this system and updates to the forecast over the next 
few days.


FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 15.8N  67.0W   45 KT  50 MPH
 12H  30/1200Z 17.6N  69.3W   45 KT  50 MPH
 24H  31/0000Z 19.4N  72.2W   40 KT  45 MPH...INLAND
 36H  31/1200Z 21.4N  74.8W   45 KT  50 MPH...OVER WATER
 48H  01/0000Z 22.9N  77.3W   50 KT  60 MPH
 60H  01/1200Z 24.8N  79.2W   55 KT  65 MPH
 72H  02/0000Z 26.6N  80.2W   55 KT  65 MPH...INLAND
 96H  03/0000Z 30.0N  80.5W   55 KT  65 MPH...INLAND
120H  04/0000Z 35.0N  77.0W   50 KT  60 MPH...INLAND

$$
Forecaster Pasch




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