Tropical Storm Isaias Forecast Discussion

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WTNT44 KNHC 021458
TCDAT4

Tropical Storm Isaias Discussion Number  22
NWS National Hurricane Center Miami FL       AL092020
1100 AM EDT Sun Aug 02 2020

Another recent burst of deep convection has recently developed 
northeast of the center, which has resulted in Isaias making a 
slight northward jog. An impressive mid-level circulation has 
developed within the strongest thunderstorm cluster, along with rare 
reflectivity values of 55-60 dBZ for a tropical cyclone. The 
Melbourne, Florida, Doppler weather radar has been indicating 
patches of velocity values of 65-66 kt at around 10,000 ft just 
north of the center, which equates to 58-59 kt surface winds. For 
now, the intensity will remain at 55 kt since previous convective 
bursts have not persisted for more than a couple of hours at best. 
Another Air Force Reserve Hurricane Hunter aircraft is scheduled to 
investigate Isaias later today. 

Radar and aircraft fixes indicate that Isaias is now moving toward 
the north-northwest or 340/07 kt. A slow north-northwestward motion 
should continue for the next 24 hours or so as Isaias moves into a 
weakness that has developed in the Bermuda-Azores ridge over north 
Florida and off the Georgia coast seen in 02/1200Z upper-air data. A 
turn toward the north is forecast to occur by all of the global 
models by Monday morning, followed by a faster forward motion toward 
the northeast by Monday afternoon and evening when the cyclone will 
be influenced by southwesterly steering flow ahead of a strong 
eastward-moving mid- to upper-level trough. Continued northeastward 
acceleration across the mid-Atlantic and New England states is 
expected on days 3 and 4. The global models continue to show little 
cross-track difference, but still have significant along-track 
differences with the GFS being the fastest and the ECMWF being 
the slowest . Since the preponderance of the model guidance is 
slower than the GFS solution, the new official forecast track 
closely follows the consensus models TVCA and HCCA, and lies near 
the previous advisory track after 12 h.

Isaias will remain over warm Gulfstream waters where water 
temperatures are near 30 deg C. Despite very unfavorable vertical 
shear conditions of 25-30 kt the past couple of days, the cyclone 
has managed to hold together, which is an indication that the system 
has a deep, well-formed vertical circulation. While some slight 
intensification is possible if the shear decreases, the official 
forecast calls for the intensity to remain steady until landfall 
occurs in the Carolinas in 36 hours or so. Some baroclinic 
interaction with the right-rear quadrant of an anticyclonically 
curved jet streak is expected to hold the intensity a little above 
what would normally be expected for a post-landfall tropical cyclone 
in 48-72 hours. The NHC intensity forecast is similar to the HFIP 
corrected consensus model and the IVCN intensity consensus model.

Key Messages:

1. Tropical storm conditions will spread northward along the east 
coast of Florida within the warning area through early Monday and 
will reach the coasts of Georgia, South Carolina, and southern North 
Carolina within the warning area Monday and early Tuesday. 

2. Dangerous storm surge is possible from Edisto Beach South 
Carolina to Cape Fear North Carolina where water rises of 2 to 4 
feet above ground level are possible along the immediate coastline 
and adjacent waterways. A Storm Surge Warning may be needed for a 
portion of this area later today, and residents there should follow 
advice given by local emergency officials.

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 coastal Carolinas and Virginia, is expected through midweek 
along and near the path of Isaias along the U.S. East Coast. 
Widespread minor to isolated moderate river flooding is possible 
across portions of the Carolinas and Mid-Atlantic.

4. A Tropical Storm Watch is in effect for portions of the North 
Carolina coast, where tropical storm conditions are possible on 
Tuesday. Additional watches and warnings will likely be issued later 
today as Isaias is expected to move northward near or over the 
mid-Atlantic and northeast coasts Tuesday and Wednesday.

FORECAST POSITIONS AND MAX WINDS

INIT  02/1500Z 26.9N  79.6W   55 KT  65 MPH
 12H  03/0000Z 28.0N  80.1W   55 KT  65 MPH
 24H  03/1200Z 29.6N  80.5W   55 KT  65 MPH
 36H  04/0000Z 32.2N  79.8W   55 KT  65 MPH
 48H  04/1200Z 35.7N  77.7W   50 KT  60 MPH...INLAND
 60H  05/0000Z 40.1N  74.3W   45 KT  50 MPH...INLAND
 72H  05/1200Z 44.6N  70.4W   40 KT  45 MPH...INLAND
 96H  06/1200Z 51.7N  63.2W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  07/1200Z...DISSIPATED

$$
Forecaster Stewart



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