Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 211007
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
607 AM EDT Mon Aug 21 2017

.SYNOPSIS...
A weak pressure trough will linger in the forecast area through
Tuesday. High pressure centered off the coast will cause
increased moisture in a southeast to south flow. However, the
moisture will remain shallow because of upper ridging and limit
thunderstorm coverage. The thunderstorm chance will increase
Wednesday with developing upper troughing and ahead of an
associated cold front. The front will be in the area Wednesday
night and thursday. It will remain hot ahead of the front
through Wednesday with afternoon heat index values peaking
around 100.

&&

.NEAR TERM /THROUGH TONIGHT/...
An increase in low-level moisture in an onshore flow plus
convergence into a lingering weak surface trough should help
cause at least scattered cumulus clouds during much of today.
There will be areas of more cloudiness and possibly
thunderstorms but believe these will be limited because of upper
ridging. Greater cloudiness and thunderstorm chances should be
in the southeast section closer to deeper moisture and a
weakness in the h5 ridge near the coast. The HRRR indicated very
little thunderstorm coverage in the forecast area with
convection mainly just southeast of the area. We followed the
guidance consensus pop of near 20 percent northwest to 30
percent southeast. The temperature guidance has been consistent.
Expect highs in the lower and middle 90s with heat index values
peaking around 100. Temperatures will have a brief drop off
because of the solar eclipse, with the temperature rebound
lagging for about an hour after totality ends.

The thunderstorm chance should further diminish tonight with
the loss of heating and continued upper ridging. The
temperature guidance was close. Nocturnal cooling and high low-
level moisture may lead to fog but coverage may remain limited
because of widespread stratocumulus.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
Tuesday will be similar to the previous day with increasing
low-level moisture in an onshore flow. Strong heating and
convergence into a weak surface trough will produce scattered
thunderstorms in the afternoon. Upper ridging should continue to
limit coverage and updraft strength. The thunderstorm chance
should diminish Tuesday night with the loss of heating and upper
ridging. The thunderstorm chance should increase Wednesday with
developing upper troughing and ahead of an associated cold
front. The models depict more pronounced pre-frontal troughing
in the forecast area. We leaned toward the higher guidance pops
because of this support. It will remain hot ahead of the front
through Wednesday. The guidance has been consistent with highs
in the lower and middle 90s with maximum heat index values
around 100.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
The GFS and ECMWF display the cold front in the forecast area
Wednesday night and Thursday. The guidance consensus supported
pops around 50 percent which was reasonable for now because of
timing uncertainty. The GFS and ECMWF show the front east of
the area Friday through Sunday with a diminished shower and
thunderstorm chance. However, the nearness of the offshore front
adds uncertainty to the weekend forecast. The GFS and ECMWF MOS
plus GFS ensemble mean pops are 20 to 30 percent Friday, and 10
to 20 percent Saturday and Sunday. The MOS has temperatures
near or a little below normal during the period.

&&

.AVIATION /10Z MONDAY THROUGH FRIDAY/...
VFR conditions expected through much of the night. However, low
levels are more moist than at this time yesterday, and a southeast
boundary layer wind may provide some stratus potential late tonight
into early Monday morning. Some fog is possible as well, though
less confidence given continued presence of upper level convective
debris cloud cover streaming up from the south. Most guidance
indicating some restrictions, more likely to the east and south.
For now, will indicate MVFR at CAE/CUB/DNL, with IFR at the farther
south and also most fog prone locations, AGS and OGB. After morning
fog/stratus, expect VFR Monday. A SE wind and a gradually moistening
atmosphere will provide a slight chance of afternoon convection, with
the better chance towards the south and mainly late in the afternoon.
Will continue to lean optimistic and only indicate VCSH at OGB after
18Z.

EXTENDED AVIATION OUTLOOK...Scattered mainly afternoon and evening
showers and thunderstorms Monday through Thursday, with increasing
chances each day. Also, low-level moisture could result in early
morning fog and/or stratus.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.

&&

$$
SYNOPSIS...99
NEAR TERM...99
SHORT TERM...99
LONG TERM...99
AVIATION...99


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