Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201013
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
613 AM EDT Sun May 20 2018

.SYNOPSIS...
A tropical air mass will generally remain in place through the
week between high pressure offshore and low pressure to the
west.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 610 AM: KCLX detected a few light showers across the
Coastal Empire of GA. Elsewhere, conditions were mostly cloudy
and rain free. I will update the forecast to reduce PoPs through
the mid morning. However, I will indicate a gradually increase
from east to west.

As of 345 AM: GFS1deg indicates that H5 heights around a Bermuda
ridge will gradually rise across the forecast area today as a weak
low forms over the FL Panhandle. Weak Q vector convergence is
expected to continue across the Savannah River Valley today. Deep
moisture band is expected to shift inland, covering much of the
Southeast U.S. The combination of the deep moisture, widespread
instability, and weak forcing should yield scattered to numerous
showers and isolated thunderstorms by late this afternoon. Using a
blend of MOS, I will forecast high temperatures in the low 80s.

Tonight: Near term guidance indicates that deep moisture will
continue to drift inland tonight, with drier air approaching from
the western Atlantic. At the sfc, large high pressure centered over
Bermuda is expected to ridge west across SC. At the mid level,
GFS1deg shows a broad mid level low lingering over the FL Panhandle
with ridging over the Grand Strand. Mid level Q-vector convergence
remains across the forecast area through the overnight hours. Given
the relatively moist environment and weak synoptic scale forcing, I
will indicate SCHC to CHC PoPs overnight. Low temperatures are
forecast to range from the upper 60s inland to around 70 across the
beaches.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Not much change in thinking with the latest forecast. The area will
remain on the western fringes of Atlantic high pressure centered to
the east with weak low pressure nearby to the west. This pattern
will maintain the tropical air mass in place leading to higher than
normal rain chances, especially in GA and inland toward the
Midlands/CSRA where the better moisture and forcing are expected.
Rain amounts are still a bit uncertain as much depends on exactly
how close the low is to the area but some places could easily pick
up several inches of rain through the period. Overall though we
don`t anticipate significant flooding, or severe storms for that
matter. Temperatures will generally be near to above normal,
although highs could easily stay below normal in places that see
abundant rain/cloud coverage during the afternoon.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
Atlantic high pressure should extend west over the area through the
period while low pressure persists to the west. This pattern will
maintain the warm and muggy conditions with some heavy rain at
times. In general it looks like temperatures will be near to
slightly above normal.

&&

.AVIATION /10Z SUNDAY THROUGH THURSDAY/...
Rounds of high and mid debris clouds will drift over the
terminals this morning. KCHS and KSAV should begin the 12Z TAF
period with VFR conditions. However, as debris clouds thin and
push inland, periods of MVFR ceilings may develop, highlighted
with a TEMPO from 6Z-10Z. MOS and forecast soundings support
keeping MVFR at KCHS through daybreak, lifting by 13Z. HRRR
indicates that showers and isolated thunderstorms may develop
between 15Z-21Z, greatest coverage across the inland counties.
Winds should remain steady from the SSE through the daylight
hours, light to calm at night.

Extended Aviation Outlook: A persistent tropical air mass will lead
to higher than normal rain chances through the period, mainly each
afternoon. Thunder chances will be low however given the limited
instability.

&&

.MARINE...
A large high pressure centered over Bermuda will gradually ridge
west over the marine zones today through tonight. South winds today
will back slightly from the southeast tonight, speeds remaining
around 10 kts. After daybreak today, wave heights are forecast to
range from 2-4 ft within 20 NM to around 5 ft beyond 20 NM.

Monday through Friday: The area will generally remain between low
pressure to the west and high pressure to the east. No significant
wind concerns, however seas will be elevated near Advisory levels
close to the Gulf Stream at least through Tuesday.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for AMZ350-
     374.

&&

$$
NEAR TERM...NED
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...NED/RJB
MARINE...NED/RJB



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