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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.
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