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Tropical Storm Isaias Discussion Number  23
NWS National Hurricane Center Miami FL       AL092020
500 PM EDT Sun Aug 02 2020
 
The earlier intense recent burst has waned since this morning, but 
the large convective cell has persisted. At the peak of the activity 
around 1500Z, several small patches of Doppler velocities of 90-96 
kt were co-located with the pronounced mid-level circulation that 
was evident in the Melbourne radar reflectivity data. However, 
these intense wind speed values were short-lived for only about 20 
minutes and, thus, were not considered to representative of Isaias' 
tangential wind field. Since that time, the cyclone has become more 
steady state with Doppler radar and Air Force Reserve aircraft data 
indicating surface winds in the 56-63 kt range. Therefore, the 
initial intensity of 60 kt is an average of these values.
 
Radar and aircraft fixes indicate that Isaias is still moving  
toward the north-northwest or 345/08 kt. The latest model guidance 
remains in excellent agreement on Isaias moving north-northwestward 
through a break in the subtropical ridge tonight and turning 
northward by Monday morning, all the while remaining offshore of the 
coast from east-central Florida to Georgia. By Monday night, Isaias 
is forecast to turn northeastward and accelerate toward the 
Carolinas, reaching the mid-Atlantic states on Tuesday and New 
England by early Wednesday. The new NHC forecast track during the 
first 24 hours lies a little east of the previous one, but is 
essentially just an extension of the previous advisory track 
thereafter, and lies close to the various consensus models, which 
are lightly packed around the previous NHC foreast.
 
Isaias will continue to move slowly over the warm Gulfstream waters 
for the next 36 h or so. Despite unfavorable vertical shear 
conditions of around 25 kt, Isaias is expected to maintain its 
current intensity until landfall, and could restrengthen to 
hurricane status in 24-36 h when the vertical shear vector is 
forecast to switch from westerly to southwesterly which would align 
the shear along the direction of storm motion. Some baroclinic 
interaction is expected on days 2-3 when Isaias will move into the 
right-rear quadrant of an anticyclonically curved jet streak, which 
is expected to hold the intensity a little above what would normally 
be expected for a post-landfall tropical cyclone. The NHC intensity 
forecast is similar to the HFIP corrected consensus model and the 
IVCN intensity consensus model, which agree well with the GFS and 
ECMWF model intensity forecasts.
 
Key Messages:
 
1. There is the danger of life-threatening storm surge inundation of 
2 to 4 feet above ground level from Edisto Beach South Carolina to 
Cape Fear North Carolina along the immediate coastline and adjacent 
waterways. Life-threatening storm surge is possible along the 
North Carolina coast from Cape Fear to Duck.  Residents in these 
areas should follow advice given by local emergency officials. 

2. Tropical storm conditions will spread northward within the 
Tropical Storm Warning area from Florida to North Carolina through 
Monday night. Isaias is expected to be near hurricane strength when 
it reaches the coast of northern South Carolina and southern North 
Carolina Monday night, and strong tropical storm force winds are 
likely with hurricane conditions possible in the Hurricane Watch 
area. 

3. Heavy rainfall from Isaias will continue to result in potentially 
life-threatening flash flooding in the Northwest Bahamas through 
tonight. Flash and urban flooding, some of which may be significant 
in the eastern Carolinas and the mid-Atlantic, are expected through 
midweek along and near the path of Isaias across the U.S. East 
Coast. Widespread minor to isolated moderate river flooding is 
possible across portions of the Carolinas and the mid-Atlantic.

4. A tropical storm watch is in effect for the U.S. mid-Atlantic 
coast, including the Chesapeake Bay, Delaware Bay, and Long Island 
Sound, as tropical storm force winds are possible in these areas on 
Tuesday and Tuesday night. Additional watches and warnings will 
likely be issued tonight and Monday as Isaias is expected to move 
northward near or over the mid-Atlantic and New England states 
Tuesday and Wednesday. 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  02/2100Z 27.8N  79.8W   60 KT  70 MPH
 12H  03/0600Z 29.0N  80.1W   60 KT  70 MPH
 24H  03/1800Z 30.9N  79.9W   60 KT  70 MPH
 36H  04/0600Z 33.7N  78.8W   60 KT  70 MPH
 48H  04/1800Z 38.1N  75.8W   50 KT  60 MPH...INLAND
 60H  05/0600Z 42.5N  72.2W   45 KT  50 MPH...INLAND
 72H  05/1800Z 46.3N  68.4W   40 KT  45 MPH...INLAND
 96H  06/1800Z 54.2N  58.4W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  07/1800Z...DISSIPATED
 
$$
Forecaster Stewart
 
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