Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221905

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
305 PM EDT Thu Mar 22 2018

Dry and unseasonably cool high pressure dominates our weather
through Friday. The next low-pressure system arrives for the weekend
with light precipitation and well-below normal temperatures. The
cold weather will persist through Monday as strong high pressure
builds down the Appalachians.  Temperatures rebound to near-normal
by next Wednesday as the high pressure moves offshore in advance
of the next system.


As of 225 PM EDT: the latest visible satellite imagery is showing
mostly clear skies across the region with some lingering stratocumulus
along the NC/TENN border. Winds will remain gusty and out of the NW
into the evening with speeds decreasing after about 00z or so.

Otherwise, the upper trof axis will continue to shift east of the
fcst area tonight and tomorrow as the upper ridge gradually spreads
farther eastward. At the sfc, broad-scale high pressure will persist
over the region thru tomorrow with drier air expected to remain in
place thru the period. The main concern over the near-term will be
patchy to widespread black ice on roadways and elevated surfaces such
as bridges and overpasses, late tonight thru tomorrow morning. Most
of the concern will be relegated to the higher terrain where temps
will fall well below freezing and widespread refreezing of frozen
precip is likely. In these areas motorists should use extra caution.


As of 215 pm Thursday: Precip chances will increase across the area
Fri night, especially across the mtns, as warm front activates
downstream of weakening surface low. A model consensus of forecast
thermal profiles indicate snow, or at least a rain/snow mix will be
the most likely precip-type above roughly 4000 ft across the
southern/central NC mtns, and above 3000 feet or so across the
northern mtns, with at least light accums appearing likely in those
areas before an expected transition to rain occurs later Sat
morning. Confidence  in at least an advisory-level event is
sufficient to go ahead and begin mentioning the accumulating
snowfall threat in the Hazardous Wx Outlook and Decision Support

Otherwise, rain chances will be maximized (i.e., likely to
categorical) across western and northern areas Saturday, as (in
Miller-B fashion) surface low spins down across the TN
Valley/vicinity, and a new surface wave develops off the Carolinas
coast. Meanwhile, a massive/sprawling area of 1040+ mb arctic high
pressure is forecast to settle into the southern half of Quebec by
Sunday morning, with cold air/inverted ridging building strongly
south along the Eastern Seaboard of the Conus, establishing cold air
damming across at least part of the forecast area by late Saturday.
The position of the parent high would suggest more of a hybrid than
classical damming setup, but its strength indicates a pretty darn
strong hybrid event developing by Sunday. That being the case, it
will be very interesting to see if sufficient cold/dry air advection
and maintenance will set up to force a transition back to wintry
precip (FZRA being the most likely p-type if this were to occur)
across our northern Blue Ridge areas Saturday night/early Sunday.
Fortunately, the stronger forcing will be shifting to the coastal
cyclone during (or prior to) this time, so any wintry precip would
be very light, but it will be something to monitor. For the time
being, only a cold rain is forecast over the mtns Sat afternoon into
early Sunday. Pops fall to chance by sunrise Sunday, slowly
dwindling through the day as moisture becomes increasingly shallow,
likely giving way to a day of drizzly/patchy -RA conditions.

With the CAD expected to begin setting up during the day Saturday,
the max temp forecast is very problematic, with a very strong
gradient expected across our area. In fact, we have maxes ranging
from the upper 30s across the northern Blue Ridge to the upper 60s
across the Upper Savannah River Valley and Lakelands (even that
gradient may prove insufficient). By Sunday, CAD should be
well-established across the entire area, with maxes forecast to be
around 20 degrees below climo.


As of 235 PM Thursday: Some controversy remains with regard to
the weather situation in the wake of the Miller-B low passage
scenario on Sunday night. The models are in general agreement
overall with the full-latitude upper ridge building in from the
west, supporting a large and sprawling sfc high over ern Quebec
that wedges its way all the way down across the Carolinas and into
Georgia by 00Z Monday. The problem is the depth of the cooler air
and the amt of leftover moisture available to be pushed up the
east side of the mtns by the easterly flow. I remain skeptical
about precip chances because the high is so large and don`t see
any reason why we would not dry up steadily Sunday evening, so the
fcst continues to lean in the direction of the drier ECMWF. The
drier scenario seems to have some blessing from the GEFS in that
overall the precip chances are less than the operational GFS. So,
we allow the precip chances to taper off to just a slight chance
over northeast GA in the overnight period, while temps drop down
below freezing across a good bit of the NC part of the fcst area,
which has the advantage of limiting the amt of wintry precip fcst
along the NC part of the Blue Ridge. Think anything that falls will
be in the brief nuisance category. On Monday, the GFS re-invigorates
a swath of precip NW to SE basically along the Savannah River
Basin to southeast TN. This is also viewed with some suspicion, as
the model shows the edge of the cold air damming well down across
south GA and curving around into AL, so again the ECMWF depiction
is favored, but the fcst retains a chance over the SW fringe in
deference to the GFS and neighboring offices. That precip chance
is also allowed to wane in the afternoon and evening. Meanwhile,
the cold air damming will continue across the region thru Tuesday as
the upper pattern evolves very slowly and the sfc high migrates to
the Canadian Maritimes, leaving nothing to really help to destroy
it. The fcst was kept dry Monday night thru Tuesday night across
most of the region, with temps remaining at least ten degrees below
normal. Finally, Tuesday night, a piece of the sfc high is broken
off near the Carolina coast, and we start to get some warm advection
in a developing SW flow. This should allow the remaining CAD to
be mixed out from the S during Wednesday, and temps will rebound
quite nicely to something more like 5 degrees above normal. Once the
upper ridge axis finally moves overhead on Wednesday, the wrn part
of the fcst area will be exposed to return moisture from the Gulf,
and precip chances will come back thru the end of the period. Most
of the fcst area east of the foothills should stay dry.


At CLT and elsewhere: Expect VFR conditions to prevail through the
18z TAF period. Skies should remain mostly clear thru the period with
sfc high pressure building in from the west. The pressure gradient
is expected to gradually weaken later this afternoon and into this
evening, allowing wind gusts to diminish across the area. Prevailing
winds will remain NWLY at all sites except KAVL thru tomorrow morning
before becoming more WLY during the late morning/early afternoon.
KAVL will likely remain NWLY thru 18z tomorrow.

Outlook: Increasing precipitation chances and possible flight
restrictions are possible this weekend with another approaching
low pressure system.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
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:


A Fire Danger Statement remains in effect for most of our Georgia
Counties, where RH is expected to hover around 25 percent into early
evening. Winds will also gust to around 20 mph in these areas until
around sunset. Gradual dewpoint recovery is forecast to yield higher
RH values on Friday, despite warmer temps (min RH mainly in the 25-
30 percent range across much of the Piedmont and foothills). Winds
will also be roughly half of what they are today. Thus, no special
fire weather products are anticipated for tomorrow. Wetter and
cooler conditions are expected across much of the area over the
weekend into early next week.


GA...Fire Danger Statement until 8 PM EDT this evening for GAZ018-


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