Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 121931

Area Forecast Discussion
National Weather Service Columbia SC
231 PM EST Wed Dec 12 2018

High pressure will move farther off the Southeast Coast through
Thursday. A warm front will develop along the Gulf Coast and
Southeast Coast Thursday night. The associated low will reach
the Lower Mississippi River Valley Friday, and Georgia and the
Carolinas Saturday. The system will begin to lift northeast of
the forecast area Sunday. The heaviest rain will occur ahead of
the low late Thursday night and Friday.


Dry weather continues through tonight. Surface high pressure
over the southeastern states will move off the coast tonight.
Weak upper level ridging over the region will amplify late
tonight as an upper low digs into the Southern Plains.

High level clouds will continue across the area as moisture
spills over the upper ridge axis. More dense clouds across the
northern Midlands may hold temperatures down a couple of
degrees. Still expect afternoon highs in the lower 50s north to
upper 50s south. Went with guidance consensus for overnight lows
in the low to mid 30s.


High pressure will be moving farther off the Southeast Coast
Thursday. There will be a moisture increase in a developing
onshore flow. The models have been consistent holding back deep
moisture ahead of the next low pressure system through the day.
Increased cloudiness should help hold down temperatures. We
leaned toward the lower temperature guidance.

Thursday night...
The models show a warm front developing along the Gulf and
Southeast coasts Thursday night. Isentropic lift plus low-level
moisture in an onshore flow will be on the increase. The
moisture will become deep ahead of a mid-level shortwave trough
late. The guidance consensus supports rain amounts 0.2 to 0.3 of
an inch with the greater amounts in the south section. We
leaned toward the lower temperature guidance because of the
evaporative cooling potential.

The models depict the deepest moisture and most lift Friday.
The upper lift should be greatest early ahead of a shortwave
trough. The models maintain strong isentropic lift with a
southerly 40-knot h85 jet. We have forecasted categorical pops.
The models have been consistent with rain Friday 1 to 1.5 of an
inch. The models also have been consistent with significant
surface-based instability remaining well east of the forecast
area so despite the strong shear there is a diminished chance of
severe thunderstorms. There may be enough elevated instability
for a few thunderstorms. The GFS has depicted a showalter index
lowering to 0 to -1 during the afternoon. There has been a high
spread in the temperature guidance. We followed the lower values
because of rain and in situ wedge ridging for at least part of
the day.

Friday night...The models display strong isentropic lift
continuing early Friday night but then diminishing. Mid-level
dry slotting may also occur. We forecasted a lower pop later at
night. The temperature guidance was close.


Forecast confidence is low Saturday and Sunday. The ensemble
guidance has a high spread. Compared to the ECMWF, the GFS has
been farther north with the low pressure system and associated
moisture. We leaned toward the slower ECMWF because of the cut-
off nature of the system and held on the pops into Sunday. The
models were consistent with dry ridging dominating Monday and
Tuesday with near normal temperatures.


High confidence for VFR conditions.

Surface high pressure over the southeastern states will move
offshore tonight. Low levels of the atmosphere remain dry.
Expect widespread high level clouds through the period. Winds
will be variable less than 10 knots. Fog threat tonight appears
low with dry air mass and nocturnal low level jet.

A low pressure system affecting the region Thursday night into
Saturday will bring widespread rain and associated restrictions.


Significant rainfall last weekend has lead to elevated river
levels with several forecast points above flood stage. River
levels will remain elevated over the next few days as increased
hydroelectric generation, rainfall runoff and snow melt works
through the basins.




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