Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 280119
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
919 PM EDT Wed May 27 2020

.SYNOPSIS...
Tropical Depression Bertha will move well north of the region
tonight. Unsettled weather will continue until a cold front
moves through the region early next week, followed by high
pressure with drier and cooler conditions.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Tropical Depression Bertha was located over western North
Carolina at 28/01z per satellite and surface analysis. Evening
shower activity is coming to an end with the last of the showers
moving into portions of Orangeburg and Clarendon Counties.
Convection south of the Altamaha River near Valdosta, GA,
should remain out of the local forecast area.

The region is caught within a fairly nebulous pressure pattern
left in the wake of Tropical Depression Bertha. Showers persist
across the far north and south within regions of weak
confluence. Later tonight, guidance is consistent in
redeveloping convection offshore of the lower South Carolina
coast within a sharpening confluence lingering along the
southeast quadrant of Bertha`s large cyclonic circulation. While
most of this convection should remain offshore, it could easily
brush the Charleston Tri-County area, especially upper
Charleston and eastern Berkeley Counties. It will be warm, humid
night with lows only dropping into the lower 70s inland with
mid 70s at the coast and beaches.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
Thursday: The mid-levels will consist of a cutoff low centered over
Arkansas in the morning and high pressure off the Mid-Atlantic
Coast. By the nighttime period, the low will become absorbed into
longwave troughing over the MS Valley, while the high gradually gets
pushed further offshore. At the surface, high pressure will be
located far off the Mid- Atlantic Coast. Meanwhile, troughing is
expected to develop over the Southeastern U.S. Lots of moisture
originating from the Gulf of Mexico will be ushered into our area.
PWATs are forecasted to approach 2", which is 2 standard deviations
above normal per NAEFs. Models remain in fairly good agreement
indicating the highest probabilities of rainfall across the
Charleston Tri- County in the morning. The focus of the rainfall
shifts in the afternoon more towards inland locations, especially
across GA. We tried to capture this with the hourly POPs, having
them peak in the likely category. POPs decrease in the evening with
the setting sun, only to ramp back up again late at night as forcing
increases. Though, the highest POPs should be along the coast. QPF
still varies significantly from model to model and we generally went
with a blend for the forecast, while leaning away from the extreme
high and low amounts. Regardless, higher amounts are possible in
thunderstorms. There is decent instability during the day. GFS
MLCAPEs could approach 1,500 J/kg with negative Showalter values.
However, DCAPEs aren`t very high and shear is lacking. Hence, the
overall severe threat is low. The main hazards from any storms
should be the locally heavy rainfall, followed by damaging winds.
Temperatures should be near normal.

Friday: Mid-level troughing over the MS Valley in the morning will
shift eastward and slowly weaken, becoming located over the East
Coast overnight. Surface high pressure will be in the Atlantic while
troughing prevails over the Southeast. Abundant moisture will
continue to be in place across the Southeast. PWATs should approach
2", which continues to be about 2 standard deviations above normal
per NAEFs. Models seem to be in fairly good agreement indicating
abundant numerous to widespread showers across our entire area most
of the day. Hence, we have categorical POPs. The showers decrease in
the evening, then ramp back up along the coast overnight. QPF still
varies significantly from model to model and we generally went with
a blend for the forecast, while leaning away from the extreme high
and low amounts. Regardless, higher amounts are possible in
thunderstorms. Instability is forecasted to be comparable or maybe
slightly less than on Thursday. Shear is comparable. Once again, the
overall severe threat is low, with the main hazard being locally
heavy rainfall followed by damaging winds. With increased shower
coverage, high temperatures should be a few degrees cooler than on
Thursday.

Saturday: The mid-levels will consist of troughing over the East
Coast, amplifying slightly with time. At the surface, a cold front
will approach from the west during the day, generating decent lift.
The moisture continues to be in place across the Southeast with the
aforementioned PWATs ~2". Models are in fairly good agreement
indicating numerous convection ahead of the front, and aided partly
by the sea breeze. It`s still too early to determine how much
instability will be in place. But at this point it`s comparable to
Friday. Though, DCAPEs are increasing along with maybe shear. Hence,
the severe threat might be a bit higher than previous days. Again,
the concerns remain the locally heavy rainfall and damaging winds.
High temperatures should be a few degrees below normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
The upper trough should push through Saturday night along with
the initial weaker cold front. The secondary stronger cold front
should then move through Monday as a trailing shortwave trough
finally pushes offshore the Mid- Atlantic coast. Will see some
lower rain chances probably into early Monday with drier and
cooler weather then expected into mid week. High temperatures
should be near normal this weekend before dropping below normal
while low temperatures likely stay above normal through Saturday
night before falling back to near normal levels.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
Cigs will likely waiver between VFR and MVFR this evening
before settling into solid MVFR overnight. Shower activity south
of KSAV could approach the terminal by 01z, but should not
bring any significant impacts if it reaches that far north. MVFR
cigs will improve to VFR by mid-morning Thursday. A band of
showers/tstms is expected to develop off the lower South
Carolina coast early Thursday. Current indications are this
activity will remain east of KCHS, but will need to carefully
monitored. Afternoon showers/tstms should remain west of both
KCHS and KSAV.

Extended Aviation Outlook: Low clouds, showers, and occasional
thunderstorms will bring periodic flight restrictions through
Sunday.

&&

.MARINE...
The Small Craft Advisory for the Charleston County waters has
been cancelled.

Tonight: Southerly flow will persist through tonight as
Atlantic high pressure rebuilds behind departing TD Bertha. Seas
will slowly subside through the night.

Thursday through Monday: The coastal waters will be situated
between high pressure in the Atlantic and surface troughing
inland. The resulting surface pressure gradient and surface
winds should be no more than a moderate breeze through Saturday.
A cold front is forecasted to move through the waters on
Sunday. Other than a change in wind directions, both wind speeds
and seas are expected to stay below Small Craft Advisory
criteria. High pressure follows the front, moving in from the
west.

&&

.EQUIPMENT...
The KCLX radar is out of service. Technicians will diagnose the
problem later this evening. It is unknown at this time when it
will return to service.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$


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