Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201553
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1153 AM EDT Sun May 20 2018

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

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Convective temperatures have been reached and convection is
beginning to develop. Only change in the current pop forecast
was to increase pops slightly along the Altamaha River where
KJAX shows an area of showers moving onshore along the lower
Georgia coast. This activity should brush portions of McIntosh
and Long Counties while weakening. The rest of the forecast is
on track.


The tropical moisture plume is not as defined as the past few
days. PWATs are down by about 0.25 inch per 12z raobs and mid-
level heights are rising suggesting the subtropical ridge is
retrograding west as expected. Today`s convective pattern will
be more typical of early summer with convection primary
developing ahead of the afternoon sea breeze circulation and
lingering convective outflows from previous day`s activity.
Expect scattered showers/tstms to develop away from the coast
this afternoon with the greatest concentration occurring across
interior Southeast Georgia ahead of the sea breeze and what
appears to be a southward moving outflow boundary out the
southern Midlands. Pops today will range from 20-30% coast with
30-50% inland, highest interior Southeast Georgia.

Extensive cloud cover will limit afternoon highs, but latest
mesoscale guidance suggests there could be enough breaks in the
cloud canopy to support highs in the mid 80s in a few locations.
However, most areas should only top out in the lower 80s way
from the beaches. Modified soundings are not overly unstable,
but a few strong tstms can not be ruled out where pockets of
higher instability develop.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
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.

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 /16Z SUNDAY THROUGH THURSDAY/...
VFR. Shower/tstm activity is expected to remain west of the
terminals this afternoon. There is a chance for MVFR cigs
redevelop just before sunrise, but guidance is not in agreement
on this potential. Limited clouds to SCT020 for now 09-13z.

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...None.

&&

$$


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