Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 211149

Area Forecast Discussion
National Weather Service Columbia SC
649 AM EST Sat Jan 21 2017

An active weather pattern is expected to bring several rounds
of showers and thunderstorms to the region over the weekend
with severe weather possible. A WARM FRONT WILL EXTEND THROUGH
followed by high pressure and dry weather Tuesday.


The main forecast issue is the threat of severe thunderstorms
over the weekend. Several rounds of convection will move through
the area.

Early this morning, high low-level moisture and cooling has
helped cause dense fog to develop in the north section of the
forecast area. We have issued a dense fog advisory. The HRRR
suggested some improvement in visibility beginning around 15z.

A warm front will be in the forecast area today and tonight.
There will be increased moisture and instability associated with
a mid-level shortwave trough today. Expect numerous showers and
thunderstorms. The high-resolution models display an organized
convective system in the forecast area this afternoon. The
models depict increasing shear especially in the south part of
the forecast area. Expect h85 wind increasing to 35 to 40 knots.
The NAM and GFS display weak to moderate instability with
surface-based LI values lowering to -2 to -5. The greatest
instability is depicted in the south section. The shear and
instability support possible severe thunderstorms. The SPC has
outlooked the south part in a slight risk and most of the
remaining forecast area in a marginal risk of severe.

There may be a lull in showers and thunderstorms early tonight
in the wake of the mid-level shortwave trough and previous
convection. However, coverage may increase during the early
morning hours ahead of the next upper-level feature. The models
indicate increasing shear with h85 wind rising to near 50 knots.
The GFS shows the most instability with surface-based LI values
lowering to around -5 during the 09z to 12z time frame. The
instability and shear support possible severe thunderstorms
including tornadoes especially during the early morning hours.

Followed the WPC rainfall forecast. There will be a threat of
localized flooding although it appears limited by rapid storm
movement unless training happens to develop.


The greatest severe weather threat may occur Sunday afternoon
as the anomalously deep low and associated instability gets
closer to the forecast area. The NAM and GFS show surface-based
LI values -7 to -8 with continued strong shear. The main severe
weather threats will be damaging winds with bow echoes and
isolated tornadoes along quasi-linear convective lines. There
should be a tight surface pressure gradient supporting strong
and gusty winds especially Sunday afternoon and night. A lake
wind advisory will likely be needed. We followed the WPC
rainfall forecast although trended slightly higher given the
strong southerly moisture flux into the area. There will be a
threat for localized flooding.


The GFS and ECMWF display the upper low beginning to lift
northeast of the region Monday. Wrap-around moisture and
instability associated with the low supports a continued chance
of showers. There will be tight surface pressure gradient
associated with the strong low. A lake wind advisory will
likely be needed. Dry ridging should dominate Tuesday with an
approaching cold front Wednesday. The models show the front in
the area Thursday supporting a chance of showers. The GFS and
ECMWF MOS support above normal temperatures returning to near
normal behind the front for Friday.


Abundant low level moisture supporting development of stratus
and fog during the early morning hours as shown by latest
surface observations. Latest HRRR model showing fog and stratus
to linger for much of the rest of the morning creating MVFR/IFR
or possible LIFR conditions. More uncertainty on when or if
conditions improve after 12Z as an upper disturbance moves into
the region from the southwest with rain overspreading the
terminals after 15Z. Thunder may be possible during the
afternoon...but not included at this time given uncertainty due
to lingering stratus and timing of convective development.
Convection should end later this evening but models indicating
that another round of showers/storms possible towards the end of
the TAF period before sunrise Sunday morning.

EXTENDED AVIATION OUTLOOK...Showers...and associated restrictions...
are likely Sunday through Monday as several waves of low
pressure cross the region.


SC...Dense Fog Advisory until 10 AM EST this morning for SCZ015-016-


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