Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150204

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
904 PM EST Sun Jan 14 2018

Cold and dry high pressure will move from New England to eastern
Canada by late Monday as another cold front approaches our region
from the upper Plains states.  Expecting some moisture over our area
Tuesday night ahead of a sharp upper trough. This trough and cold
surface high pressure will bring a quick shot of cold air Wednesday
into Thursday.  Expect a warming trend at the end of the week.


As of 900 PM: Grids look good for the early evening update. Hourly
temps are running a little warmer than forecast, but with mostly
clear skies, the diurnal curve should catch up through the evening.
Made a few additional minor tweaks to hourly trends and updated sky
grids through the night, but no major changes.

Otherwise, high pressure currently centered over the St Lawrence
Valley will drift northeast tonight. A deep upper trough will dig
into the Midwest; a series of weak vort lobes encircling this trough
will pass atop the CWFA between now and Monday afternoon. Sufficient
mid- to upper-level moisture accompanies them to expect patches of
cirrus, but the forcing is too weak and moisture too elevated for
any chance of precip in our area. Mainly northerly winds will be a
tad breezy this afternoon but should become light tonight,
eventually flipping to southerly as the high loses influence in
favor of the Midwest low. Mins tonight will be slightly cooler in
the Piedmont compared to this morning, but slightly warmer in the
mountains and upper Savannah Valley. Max temps should rebound by 1-2
categories Monday, but still remaining 8-11 degrees below normal.
Afternoon RH will be seasonably low but looks to remain above 25


As of 250 PM EST Sunday: Model uncertainty rears its ugly head again
for the 12Z cycle. The GFS has gone much weaker with the upper
system driving the weather system for the middle of the week. This
results in a drier forecast. The ECMWF retains its stronger and
wetter forecast. The Canadian favors the GFS although slightly
stronger and wetter while the NAM favors the ECMWF. The SREF is a mix
of weak and wet while the GEFS mean favors the operational GFS. This
keeps the forecast uncertainty relatively high. However, our
forecast continues lean toward the ECMWF/NAM camp given the better
run to run consistency of the ECWMF. That said, upper lows are
notoriously tricky to forecast correctly. In addition, snow
forecasts when the cold air is not already in place with a strong
damming high are also problematic. Therefore, have capped PoPs in
the good chance range where precip chances are higher.

The overall pattern weather pattern remains the same. An upper low
over the Great Lakes opens up and moves east as a strong short wave
rotates around the low and digs a deep trough and potentially closed
low across the OH and TN valleys which moves east over or near our
forecast area. The associated cold front moves east into the
mountains Tuesday. As the front moves out of the mountains, a wave
of low pressure forms in the lee with an inverted trough developing.
Precip moves into the mountains Tuesday with cold air filtering in
behind the front. This will keep the precip mostly snow but mixing
with rain in the valleys. The precip spreads out of the mountains
and across the area Tuesday night. The precip remains snow across
the mountains with rain changing to snow elsewhere as the cold air
continues to spread in. The precip moves east through the morning
Wednesday. However, low level instability develops along and east of
the I-77 corridor. This will allow convective snow showers to
develop before the precip moves east of the area. The best precip
chances will be over the NC Mountains and I-77 corridor. The western
Upstate and NE GA will see some snow showers, but any accumulations
look to be a dusting at best. Using a guidance blend for QPF and
snow amounts yields up to 2 inches across the higher elevations of
the NC mountains and up to an inch across the I-40 and I-77
corridors. The timing of the snow showers has the potential to
impact the morning commute on Wednesday. Again, this is a highly
uncertain forecast and many things have to come together properly
for this to develop.

Near normal temps Tuesday drop below normal for Wednesday. Breezy to
windy conditions develop behind the front. Very cold wind chill
values are possible across the NC mountains Tuesday night and
Wednesday morning.


As of 130 PM EST Sunday: Strong high pressure will continue to
build across the SE region early Thu as a s/w ridge axis crosses the
OH/TN valleys. The main concern early on will be the possibility of
a cold wave. Right now it doesn/t look too favorable as the trend
has been for warming high temps on Thu. Took a blend of the best
verifying models recently....namely the ADJMEX/ADJECE which gave max
temps a cat or so above the previous fcst. Min temps are more certain
with good clustering noted on the GEFS. Highs were adj down a
bit...but still avg temps look to remain abv cold wave criteria even
with lows projected to drop into the teens. The other issue Wed
night will be wind chills which will drop to adv levels abv 3.5 Kft
as a strong hipres gradient works across the mtns.

The sfc high will continue to build southeast thru the period and by
Fri a warming return flow will develop over the FA. This will allow
max temps to reach normal levels Fri and abv normal maxes over the
weekend. Surface ridging along the gulf coast will shunt out any
good chance of a srn stream disturbance to bring in precip as it/s
associated upper wave weakens and broadens out over the GOM.
Thus...the fcst will remain dry thru the period until Sun when a
phased upper trof develops over the Midwest and advances toward the
region. Expect increasing h92/h85 moisture flux ahead of this system
as the GOM opens up. There is some uncertainty as to the timing of
the will limit PoPs to slight chance early Sun
increasing to mid/high chance across the wrn zones by the afternoon
and overnight period. No issues with p/type outside of isol
sheltered mtn valley -snsh at onset...however precip will quickly
transition to all rain as the llvls warm within warm sw/ly flow and
diurnal heating.


At KCLT and elsewhere: No changes to the KCLT TAF for the 02z
AMD. VFR through the period. High pressure in place at the surface
will continue to work slowly east, with winds favoring the N side
tonight. Lee troughing will develop tomorrow swinging winds around
to SW and S, but generally very light, around 5kt. Passing high
clouds, perhaps some mid-level cloudiness working toward KAVL, will
continue off and on through the period.

Outlook: VFR conditions are expected to prevail at least heading
into Tuesday. Then, a fast-moving system may bring a brief shot of
moisture and possible restrictions Tuesday night into Wednesday.
Temps will be cold enough that SHSN may result.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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