Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 211812
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
212 PM EDT Thu Sep 21 2017

.SYNOPSIS...
Surface high pressure will remain in the region through the
weekend. Upper troughing will be over the area through Friday
night, which combined with strong surface heating should help
support scattered thunderstorms. Upper ridging will begin to
push back into the area Saturday and help diminish the
thunderstorm chance.

&&

.NEAR TERM /THROUGH TONIGHT/...
Weak high pressure at the surface will remain over the region
through tonight along with a weak upper level trough. Models
show shortwave energy in the eastern portion of the area late
this afternoon and tonight. Satellite imagery shows a cumulus
field across the region this afternoon while only a few showers
and thunderstorms have developed. Expect coverage to remain
isolated through this evening given lack of a well defined
surface feature. The highest potential will be over the eastern
Midlands and southern CSRA. Mesoscale analysis shows
precipitable water values around 1.4 inches west to 1.5 inches
east. Moderate instability is in place with LI values around
-4 to -6. The potential for severe weather remains low, but
cannot rule out strong winds given some dry air in the mid
levels. Cannot rule out a hail threat as well given relatively
low wet bulb zero heights. The HRRR shows convection diminishing
across the forecast area by 06Z. High temperatures are on track
this afternoon and will top out in the lower 90s. Overnight
lows in the upper 60s to near 70. Patchy fog development
possible early Friday morning.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
The models display surface ridging continuing in the region.
Aloft, troughing is depicted with h5 temperatures near -10 C
through Friday night. Some weakening of the trough is forecast
Saturday with temperatures near -9 C. Instability associated
with the upper feature combined with strong surface heating
should help support thunderstorms Friday, but expect limited
coverage because of shallow moisture and the surface ridging.
The thunderstorm chance should further diminish Saturday with
less upper support. The guidance consensus supports pops around
20 percent Friday and less than 20 percent Saturday. Expect
moderate instability Friday afternoon because of the cold air
aloft and strong surface heating with highs in the lower 90s.
The NAM displays surface-based LI values -7 to -8. This
instability plus dry air in the mid levels aiding downdrafts
could help support strong wind with any thunderstorms. Forecast
wet bulb zero heights around 10,500 feet indicates possible hail
as well. The temperature guidance has been consistent with
continued above normal values.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
The GFS and ECMWF display surface ridging continuing during
most of the medium-range period. Some weakening is noted toward
the end of the period ahead of an approaching cold front. The
models also exhibit upper ridging during the period. The pattern
supports a diminished thunderstorm chance with above normal
temperatures. The GFS and ECMWF MOS plus ensemble means support
pops 20 percent or less through the period.

A weakness in the ridging north of Hurricane Maria plus upper
troughing over the Southeast States should steer the storm northward
and well east of the forecast area. The models have been consistent
with this solution. Please see the latest advisory on Maria from the
National Hurricane Center.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
VFR conditions outside of possible afternoon/evening convection
and early morning fog.

Visible satellite imagery shows cumulus development across the
forecast area this afternoon. Currently only a few showers and
thunderstorms on radar, but expect isolated coverage across the
area through this evening. The chance is too low to mention in
current issuance, but will monitor radar trends and amend as
necessary. Shower and thunderstorm potential will diminish with
sunset and lack of heating. Fog will be possible again tonight
with dry air aloft, light winds and shallow low-level moisture.
Persistence forecast with MVFR/IFR fog at fog prone, AGS/OGB
terminals developing around 06z and improving by 14Z.

EXTENDED AVIATION OUTLOOK...Late night/early morning
fog/stratus possible each morning, mainly at AGS and OGB.
Slight chance diurnal thunderstorms Friday.

&&

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

&&

$$



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