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Tropical Depression Four Discussion Number   1
NWS National Hurricane Center Miami FL       AL042021
1100 AM EDT Mon Jun 28 2021
 
The area of disturbed weather and associated low pressure system 
that the NHC has been tracking the past couple of days has developed 
into a tropical depression off the coast of South Carolina this 
morning. The inner-core cloud structure noted in high-resolution 
visible satellite imagery has continued to tighten up and deep 
convection with cloud top temperatures of -60 deg C have persisted 
northwest through southwest of the center, yielding a Dvorak shear 
pattern intensity estimate of 30 kt. This intensity estimate is 
consistent with overnight scatterometer surface wind data of 31-32 
kt just north of the well-defined center.

The initial motion estimate is 300/14 kt. The small tropical cyclone 
is expected to maintain a west-northwestward to northwestward motion 
for the next couple of days, resulting in landfall along the 
south-central coast of South Carolina later this evening. The small 
cyclone is expected to dissipate by 48 hours, if not sooner, when 
the system will be located over the southern Appalachian Mountains. 
The NHC track forecast lies close to the tightly packed GFS- and 
ECMWF-based Beta-Advection models due to the lack of any significant 
inner-core convection, which is allowing the cyclone to be steered 
more by the low-level flow rather than the deep-layer flow as 
depicted by the global and regional models.

There is a narrow window of opportunity this afternoon for the 
depression to strengthen into a tropical storm before landfall 
occurs. During the next few hours, the small cyclone will be passing 
over the warmer Gulf Stream where sea-surface temperatures are 
around 28 deg C. In addition, as the outer wind field begins to 
interact with land, low-level frictional convergence along and just 
offshore should help to generate deep convection just prior to 
landfall, helping to spin up the wind field. The NHC forecast shows 
the system becoming a tropical storm before landfall, and as a 
result a tropical storm warning has been issued for a portion of 
the South Carolina coast.

An Air Force Reserve Unit reconnaissance aircraft will investigate 
the system this afternoon beginning around 1800 UTC, providing more 
detailed information on the cyclone's intensity.

Key Messages:

1. Heavy rainfall is possible from coastal southern South Carolina 
and Georgia, inland across the Piedmont of Georgia into northeast 
Alabama.  Isolated flooding is possible across urban areas of the 
southern South Carolina and Georgia coasts.

2. Tropical-storm-force winds are expected across portions of
the South Carolina coast late this afternoon and tonight where a 
Tropical Storm Warning is in effect.

3. Swells generated by the depression are expected to affect 
portions of the South Carolina coast this afternoon and tonight. 
These swells could cause life-threatening surf and rip currents.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  28/1500Z 31.9N  78.3W   30 KT  35 MPH
 12H  29/0000Z 32.8N  80.6W   35 KT  40 MPH...INLAND
 24H  29/1200Z 34.2N  83.3W   20 KT  25 MPH...POST-TROP/INLAND
 36H  30/0000Z 35.2N  85.6W   15 KT  15 MPH...POST-TROP/INLAND
 48H  30/1200Z...DISSIPATED INLAND
 
$$
Forecaster Stewart
 
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