Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 7/30/2020 9:00:00 PM UTC

Copy of official data

Tropical Storm Isaias Discussion Number  10
NWS National Hurricane Center Miami FL       AL092020
500 PM EDT Thu Jul 30 2020

Surface observations in the Dominican Republic indicate that the
poorly defined center of Isaias moved over the southeastern portion
of Hispaniola around 1600 UTC.  There is currently a significant
concentration of convection near the mid-level center that is
located along the northern coast of the island, and recent
surface observations suggest that a new center may be forming
near the area of mid-level rotation seen in satellite imagery. The
advisory position has been placed between the previous estimated
center location and the mid-level center until it becomes more clear
that reformation has occurred.  Earlier ASCAT data that arrived
shortly after the previous advisory and reconnaissance aircraft
data from this morning supported an intensity of 45-50 kt, and since
there has no significant degradation the overall organization since
that time, the initial intensity remains 50 kt. The central 
pressure of 999 mb is based on a surface observation of 1001 mb and 
25 kt of wind at Puerto Plata in the Dominican Republic. 

The initial motion estimate is a somewhat uncertain 310/17 kt. The 
track forecast reasoning has not changed since the previous 
advisory.  The tropical storm should continue on a general northwest 
heading during the next 24 to 36 hours along the southern side of a 
subtropical ridge.  After that time, a trough moving into the 
east-central United States should cause Isaias to turn 
north-northwestward and northward as the western portion of the 
ridge erodes.  By early next week, Isaias is expected to turn 
northeastward and accelerate ahead of the aforementioned trough.  
The 1200 UTC dynamical model guidance has shifted eastward and now 
that Isaias is expected to become a stronger and deeper cyclone, 
that also favors a more eastward track.  The updated official 
forecast is a blend of the various consensus aids.  Since the new 
center is still in its formative stage some additional shifts in 
the track may occur. In addition, strong winds and heavy rainfall 
will extend far from the center.

Visible imagery has shown significant banding near the mid-level
circulation. Assuming that the center reforms near that feature and
moves away from Hispaniola tonight, strengthening is expected
during the next 24-36 hours and Isaias is now forecast to become a
hurricane.  After that time, there are mixed signals regarding the
amount of southwesterly vertical wind shear and the models 
generally do not show much additional strengthening.  Therefore,
the NHC intensity forecast shows a leveling off in intensity after
48 h.  It still needs to be stressed that that there is a higher
than usual amount of uncertainty in the intensity forecast.

Key Messages:

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

2. Hurricane conditions and dangerous storm surge are expected in 
portions of the central and northwest Bahamas late Friday and 
Saturday. Preparations to protect life and property should be 
rushed to completion. Tropical storm conditions will continue to 
spread across portions of the Dominican Republic, Haiti, the Turks 
and Caicos and the southeast Bahamas tonight through Friday, and 
Tropical Storm Warnings are in effect for these areas.

3. Tropical storm conditions are possible along portions of the 
Florida east coast beginning Saturday, and a Tropical Storm Watch 
has been issued. While storm surge watches are not currently needed 
for this area, they may be required tonight or early tomorrow if the 
forecast track shifts closer to the coast. Heavy rains associated 
with Isaias may begin to affect eastern Florida this weekend, 
potentially resulting in isolated flash and urban flooding.

4. There is a risk of impacts from winds, heavy rainfall, and storm
surge this weekend along the Florida east coast and spreading
northward along the remainder of the U.S. east coast through early
next week. The details of the track and intensity forecast remain
uncertain and it is too soon to determine the magnitude and location
of these potential impacts, but interests along the entire U.S. east
coast should monitor the progress of Isaias and updates to the
forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 19.5N  70.6W   50 KT  60 MPH...INLAND
12H  31/0600Z 20.8N  72.7W   55 KT  65 MPH...OVER WATER
24H  31/1800Z 22.7N  75.1W   60 KT  70 MPH
36H  01/0600Z 24.5N  77.1W   65 KT  75 MPH
48H  01/1800Z 26.1N  78.5W   65 KT  75 MPH
60H  02/0600Z 27.7N  79.5W   65 KT  75 MPH
72H  02/1800Z 29.2N  79.7W   65 KT  75 MPH
96H  03/1800Z 33.3N  78.0W   65 KT  75 MPH
120H  04/1800Z 40.0N  71.0W   60 KT  70 MPH

$$
Forecaster Brown