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Tropical Storm Isaias Discussion Number   9
NWS National Hurricane Center Miami FL       AL092020
1100 AM AST Thu Jul 30 2020
 
The satellite presentation of Isaias has improved over the past 24
hours with a concentrated area of deep convection occurring near
and to the northeast of the low-level center.  Radar imagery from
Puerto Rico has shown a mid-level circulation that moved over the 
Mona passage and is now along the northern coast of Hispaniola. 
Surface observations and data from an Air Force Reserve 
reconnaissance aircraft, however, continue to place the center near 
the southeastern coast of Hispaniola. It is possible that a new 
center will re-form along the northern coast of Hispaniola as the 
system interacts with high terrain of that island later today or 
tonight.  The initial intensity remains 50 kt, and is based on the 
earlier Doppler radar data and observations along the southern coast 
of Puerto Rico.
 
Isaias is moving northwestward or 310/16 kt.  A high pressure ridge
over the western Atlantic should steer Isaias on a west-
northwestward to northwestward heading over the next couple of days,
but the likelihood of a center re-formation during the next 12-18
hours means that some adjustments to the track and motion are
possible.  By late Friday, a mid-latitude trough moving into the 
east-central United States is expected to weaken the western portion 
of the ridge.  This pattern should cause the cyclone to turn 
northwestward to north-northwestward on Saturday when it is near 
the northwestern Bahamas and South Florida.  As the trough slides 
eastward over the United States, this should steer Isaias northward 
and northeastward early next week.  Although the bulk of the track 
guidance agrees on this overall scenario, the confidence in the 
track forecast remains lower than usual due to the expected land 
interaction and possible center reformation in the short term.  The 
new NHC track forecast is a blend of the HFIP corrected consensus 
and the TCVA multi-model consensus, and is similar to the previous 
advisory.
 
The intensity forecast remains challenging. The structure of the 
storm is likely to be disrupted by its passage near or over 
Hispaniola today, and some weakening is likely.  Once the system 
moves away from the Greater Antilles gradual strengthening is 
anticipated.  The global models and the SHIPS guidance suggest that 
Isaias will encounter an area of moderate southwesterly shear over 
the weekend, and the NHC intensity forecast is again leveled off at 
that time.  There are models that continue to suggest Isaias could 
become a hurricane when it is near the U.S., but given the continued 
uncertainty, the NHC intensity forecast remains near the intensity 
consensus.

Key Messages:
 
1. Isaias will produce heavy rains and potentially life-threatening
flash flooding and mudslides across Puerto Rico, the Dominican
Republic, northern Haiti, the Turks and Caicos, and the Bahamas. 
Heavy rains associated with Isaias may begin to affect South Florida
Saturday morning, potentially resulting in isolated flash and urban
flooding.

2. Tropical storm conditions will continue across portions of the 
Virgin Islands and Puerto Rico through early afternoon and will 
spread across portions of the Dominican Republic, Haiti, the Turks 
and Caicos and the Bahamas later today through Friday, and 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. There is a risk of impacts from winds, heavy rainfall, and storm
surge along portions of the U.S. east coast beginning this weekend 
in Florida and spreading northward to the Carolinas and southern
mid-Atlantic states early next week. Due to Isaias interacting with 
Hispaniola, 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. Interests in these areas should monitor 
the progress of Isaias and updates to the forecast. Tropical storm 
or hurricane watches could be issued for portions of South Florida 
later today.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/1500Z 18.1N  68.9W   50 KT  60 MPH
 12H  31/0000Z 19.7N  71.4W   45 KT  50 MPH...INLAND
 24H  31/1200Z 21.4N  74.2W   50 KT  60 MPH...OVER WATER
 36H  01/0000Z 23.0N  76.5W   55 KT  65 MPH
 48H  01/1200Z 24.6N  78.2W   60 KT  70 MPH
 60H  02/0000Z 26.2N  79.4W   60 KT  70 MPH
 72H  02/1200Z 28.0N  79.9W   60 KT  70 MPH
 96H  03/1200Z 31.5N  79.1W   60 KT  70 MPH
120H  04/1200Z 37.5N  73.5W   60 KT  70 MPH
 
$$
Forecaster Brown
 
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