Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 141752

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1252 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 1245 PM: High pressure currently centered over the St
Lawrence Valley will drift northeast through early Monday. 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 currently look to be similar to those this morning, but
max temps should rebound a category or so Monday.


As of 325 AM EST Sunday: The model guidance provides us with another
lesson on how to remain conservative with making forecast changes
in order to avoid flip/flops. Of primary concern is the potential
for a light snow event Tuesday night/Wednesday morning. But before
we get to that point, we should have a quiet Monday and Monday
night. The new guidance agrees that a slightly deeper, closed
upper system dropping down over the upper Great Lakes will slow
the eastward progress of the sfc front to the point where there
is little chance that it will reach the fcst area before Tuesday
morning. So, that part of the fcst was kept dry, with temps on
the order of ten degrees below normal.

Which brings us to the problem at hand. The day shift noted a
drying trend in the 12Z guidance. However, the latest guidance
from the 00Z model cycle has shifted back toward a slightly higher
probability event with slightly better precip amounts, although
we must stress that we are still looking at what should at worst
be an Advisory-level event. The operational GFS/Canadian continue
to favor a more shallow positively-tilted open trof with most of
the moisture stuck in the TN Valley and brushing across the srn
Appalachians Tuesday afternoon/eve, perhaps spilling out over the
nrn Foothills early Wednesday. It is worth noting, however, the
slight uptick in precip chance seen in the GEFS plume diagrams. The
operational ECMWF, though, keeps a deeper/sharper trof that manages
to weakly tap some moisture sources, so in response we get more
light precip across the area east of the mtns Wednesday morning. We
are now afforded a look at the SREF guidance which goes out thru
18Z Wednesday, which agrees more or less with the previous fcst
in spreading a 30-40 pct precip chance east of the Blue Ridge
early Wednesday. With the reverse trend in the ECMWF and the SREF
also showing some potential, prefer to keep a chance across most
of the region early Wednesday morning. The SREF plume diagrams
suggest we have the potential to get an inch or two over the mtns
near the TN border late Tuesday, and perhaps up to an inch east
of the Blue Ridge early Wednesday. If the ECMWF is correct, there
is some potential for this to be problematic as sfc temps would
be in the mid/upper 20s as the light snow was falling, meaning it
could make a mess of the morning rush. That is still a relatively
long way off, though, so for the time being we will keep the idea
of a high impact/low predictability event on a back burner. Temps
are expected to be well below normal on Wednesday afternoon.


As of 3:35 AM EST Sunday: No significant changes were made to
the medium range. The guidance moves the axis of an upper trof
across the region Wednesday night, and in its wake there is a
small potential for some upslope snow shower activity on the
TN border, but too low to mention at this time. An upper ridge
quickly builds in on Thursday with dry high pressure taking over
across the southeastern United States. Temps will rebound nicely
after a cold start, but probably still about five degrees below
normal. This could be enough to raise the average temp such that
we fail to meet "cold wave" criteria in many places east of the
mtns. Both operational models have a curious little development by
way of a compact upper low moving along quickly from the Plains
to the Appalachians Thursday night and Friday, but with a large
disparity in track. This is mentioned only because the GFS would
bring a small potential for precip across the region late Friday,
but for now, this was discounted and Friday was kept dry. Heading
into the weekend, the guidance suggests a warming trend with an
upper ridge migrating off the east coast and a large upper trof
to the west, providing a SW flow aloft. Moisture would return from
the Gulf, but probably not quick enough to spoil Saturday.


At KCLT and elsewhere: VFR. High pressure centered over the
Northeast states will maintain dry sfc conditions. Winds will
prevail NE this afternoon, veering a bit tonight. KAVL could see
winds flip to SE before dawn, though if they do speeds will be
light. The high begins to drift far enough away to lose influence
over the region tomorrow, allowing the remainder of the sites
to eventually go SW, though this should not occur until after
18z. A lee trough is expected to introduce variability around
midday. Passing disturbances will bring periods of mid to upper
level clouds, though only KAVL is expected to see bases below
10000 ft, those being in the morning.

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...

            18-24Z        00-06Z        06-12Z        12-18Z
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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