Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 181826

Area Forecast Discussion
National Weather Service Columbia SC
126 PM EST Mon Dec 18 2017

High pressure situated off the Southeast Coast with a stalled
front across the southern Midlands will promote a slight chance
of showers this afternoon. There is a better chance of rain, and
possibly thunderstorms, on Wednesday as a stronger weather
system moves through the region.


Zonal flow aloft will continue through tonight with a
subtropical high over the Caribbean, and a stalled boundary
across the southern portion of the area. Surface high pressure
will be offshore promoting southwesterly low level flow this
evening. Models show a tight moisture gradient from north to
south with PWATS around 0.75 inches in the northern portion of
the area and around 1.5 inches south. There have already been a
few isolated showers today, and all indications are that a weak
shortwave will move through the area this afternoon/evening,
bringing isolated to scattered showers. The surface has also
become weakly unstable as breaks in the cloud cover have allowed
for decent daytime heating, which gives more credence to the
chance of showers. High temperatures are expected to be in the
mid 60s north to low 70s central and south.

The low-level flow will shift more westerly overnight, bringing
drier air into the region, and PWATS will fall to around 0.5
inches. Given the drier are, showers should end shortly after
sunset. High clouds are likely to persist, and guidance has been
showing some fog and stratus with very shallow low-level
moisture below the inversion. Have therefore included patchy fog
in the forecast, but there is a lot of uncertainty due to the
presence of a 30 kt low-level jet, and think broken low clouds
advecting in from Georgia is more likely. High clouds and good
mixing will also limit radiational cooling, therefore
temperatures are only forecast to fall into the 40s to low 50s.


Tuesday, weak upper ridging will continue with a surface boundary
south of the forecast area. There is a large model spread in
temperatures as the NAM keeps a cool, stable surface layer through
much of the day while the GFS is more mixed. Given the NAM was too
cool for Monday`s temperatures and that there is expected to be weak
to moderate downslope mid-level flow the forecast reflects the
warmer GFS. Although the range of the GEFS members show that the GFS
is at the upper limits of the forecast guidance. Many of the members
show more mid and upper level clouds than the deterministic run. So
it seems reasonable the high will be a few degrees cooler than the
GFS guidance. Expect dry weather with highs in the low to mid 70s.

Wednesday, an upper level trough will lift into the mid-Atlantic.
The associated surface low will move across the South with a
warm front pushing northward over the forecast area. Warm, moist
advection and strong synoptic scale lift will support showers
throughout much of the day. Showers may be heavy at times along
and north of the boundary, with areas near the NC/SC border
receiving the most rainfall. Generally we are expecting one
quarter to three quarters of an inch of rain with the Catawba
and northern Midlands receiving around one inch of rain during
the period. In the southern forecast area, south of the warm
front, there may be enough instability to support thunderstorms
late Wednesday afternoon. There will be strong low level shear
present, sufficient for a marginal threat of severe weather
limited by the instability and location of the warm front. The
low moves east of the area Wednesday night and drier air will
move in behind the exiting low.


Surface high pressure will move into the Northeast Thursday and
ridge into the forecast area bringing dry weather and cooler daytime
temperatures. Synoptic scale subsidence and upper level ridging
will help lock in the cool low level layer. The subsidence and
shallow moisture should prevent precipitation Thursday. Upper
level troughing to the west will bring a cold front towards the
region over the weekend. The models indicate the front will
stall across the region early next week. The location of the
stalled front will have a large impact on our weather and
therefore lends a good deal of uncertainty at the end of the
extended period.


Restrictions expected at times throughout the TAF period due to
a frontal boundary in the area.

High pressure will remain offshore today with a stalled
boundary in the area. With increased moisture in the southern
portion of the area today, there is slight chance of showers
through this evening at the TAF sites, but chance is still too
low to include in current issuance. Stratus has lifted and most
locations are currently VFR, but still have MVFR ceilings at AGS
and DNL. Models indicate potential for fog/stratus again
Tuesday morning which appears reasonable, so have included
mention of MVFR stratus. Fog may be limited by a 30 knot low
level jet. Surface winds through the period will be
southwesterly at 8 knots or less.

EXTENDED AVIATION OUTLOOK...A series of weather disturbances
will bring rain and associated restrictions at times through
the week.




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