Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240230

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
930 PM EST Thu Jan 23 2020

Slowly moderating Canadian high pressure will linger over the area
into Friday. A cold front will then cross the area Friday night bringing
mostly rain as it moves through. Seasonal high pressure slowly builds
over the weekend.


As of 930 PM EST Thursday: High to mid-level cloud cover has
encroached on the region ahead of the anticipated surface
front/upper low, and light precip has begun to spread into the area,
though it`s currently falling into very dry air and is only making
it to the ground at upstate SC/NE GA sites. Despite the unfavorable
position of the parent surface high (quite far north over central
Ontario), the cool precip is aiding in the development of "in-situ"
type cold air damming at the current time. The CAD will end up weak
at best, however, and prospects of it inducing any real cold air
advection at the surface are slim. Upslope flow and diabatic cooling
as precip falls into the wedge are the only remaining options for
temperatures to fall across the mountains, and though both will
occur, it seems that only the highest peaks and farthest north zones
in our forecast area will be able to drop to freezing or below
overnight. The good news is that this limits the areas where precip
and cold air will be able to interact at all, confining any expected
accumulating freezing rain overnight tonight to the far northern
mountain counties.

Slight adjustments to temperature and pop timing trends have brought
expected ice accums up closer to 0.10" for some spots in
southeastern Yancey and Mitchell counties, so those were added to
the original Winter Weather Advisory for Avery county. Impacts
should be isolated and confined mainly to the higher elevations of
those counties, and though freezing rain is expected to end by
daybreak, care should be taken on the morning commute just in case
temperatures are a bit sluggish to warm up. A light glaze on
elevated surfaces is possible southward towards Black Mountain as
well, but those areas shouldn`t see much (if anything) in the way of
ice accums, so they were covered as before with a Special Weather
Statement. All areas should transition over to liquid precip near
daybreak tomorrow morning.

Elsewhere, expect to wake up to a cold rain tomorrow morning. QPF
remains around 1-2" across the area, though there are a few 3"
totals currently anticipated in the normal upslope zones. It still
appears that a small speed max will round the upper low tomorrow
during the late morning and afternoon hours, locally enhancing
rainfall rates near the I-77 corridor for a few hours. Despite this,
any hydro issues, if they materialize, are expected to be isolated.
Another point to note is that as the cool wedge retreats tomorrow
during the day, a small amount of instability may be uncovered in
the southeastern Upstate and near the I-77 corridor. Right now, only
elevated instability is expected to develop with any SBCAPE confined
well to our south, though it should be noted that the chance of a
high shear/low CAPE severe weather event is not absolutely zero. the
most likely scenario, though, is a few rumbles of thunder. Finally,
with decent 850mb warm air advection occurring tonight through
tomorrow afternoon, have kept a high elevation (above 3500`) wind
advisory in place across the NC mountains.


As of 255 PM Thursday: the short-term forecast picks up at 00z on
Saturday with broad and deep upper trofing centered just to our west.
On Sat, the trof will take on a more negative tilt as it lifts up
and over our area. By early Sunday, the trof is expected to be
lifting north of the fcst area with nearly zonal flow setting up
over the southeast in its wake. This will be short-lived as another
upper trof amplifies to our NW and approaches the fcst area to end
the period. At the sfc, a large low pressure system will be lifting
up and over the Ohio River Valley as its associated cold front moves
over our eastern zones. Any lingering precip should be moving east
of the CWFA by roughly 06z with some light return flow snow flurries
expected over the higher peaks and along the North Carolina/Tenn
Border. Otherwise, dry and breezy conditions will continue Saturday
through Sunday. While it looks like a relatively prolonged period of
NW flow snow, the winds are looking more WLY than NWLY and QPF still
looks minimal. Current fcst has storm total snowfall amounts of an
inch or less for most locations and wind speeds should remain below
Advisory levels. Highs Saturday and Sunday are still expected to be
below normal over the mountains and near normal elsewhere.


As of 235 PM Thursday: the extended fcst picks up at 00z on Monday
with broad upper trofing over most of the Eastern CONUS and upper
ridging over the west. The upper trof will move off the Atlantic
Coast early Tuesday with heights briefly recovering. On Wed, the
long-range models have a southern stream upper trof digging down
across the ArkLaTex region and approaching our area. The trof
appears to deamplify and lift northward as it reaches our CWFA.
At the surface, strong low pressure will be lifting north of New
England as the period begins with relatively weak high pressure
over the fcst area. The models try to develop a weak low over the
Northern Gulf of Mexico on Monday but keep it well to our south
as it moves eastward and out over the Atlantic. On Tues, another
low develops over the far Southern Plains and makes its way eastward.
There is quite a bit of uncertainty regarding this system`s track
and speed. There is a good chance it will bring widespread precip
to the area on Wed, however it`s not clear how quickly things will
dry out and whether the low will take a more NELY or SELY track for
the remainder of the period. Based on that, I kept slight to solid
chance PoPs in the fcst for the rest of the period. Temperatures
are expected to remain just above normal thru the period.


At KCLT and elsewhere: Mid to high level clouds are still filtering
into the area, and all TAF sites are VFR at the current time. Expect
restrictions will develop at most sites by 6-12Z as moisture
increases and cigs drop to IFR and possibly even low IFR ahead of
the approaching cold front. Precip will spread from west to east
overnight tonight, with all sites seeing a cold rain by tomorrow
morning. Rain could be heavy at times, especially in the early
afternoon hours for KCLT tomorrow, and low cigs/vis restrictions
will continue through the end of the valid TAF period at all sites
once they materialize. LLWS may develop for a few hours tomorrow
morning, with surface winds starting NErly tonight through early
tomorrow morning before becoming SErly in the late morning/early

Outlook: Low cigs, restricted vsby and precip will continue into
Friday afternoon ahead of a cold front moving through the southeast.
After the front passes tomorrow night, restrictions will clear
quickly (except possibly lingering into Saturday at KAVL within NWLY
flow). After that, dry conditions will persist through the weekend
and early next week.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     Med   68%     High  94%     High  96%
KGSP       High  97%     High  82%     High  94%     High  87%
KAVL       High  94%     High  95%     High  95%     High  96%
KHKY       High 100%     Med   75%     High 100%     High  89%
KGMU       High  87%     High  82%     High  94%     High  84%
KAND       High  85%     High  82%     High  86%     High  80%

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:


NC...Winter Weather Advisory until 7 AM EST Friday for NCZ033-049-
     Wind Advisory above 3500 feet until 4 PM EST Friday for NCZ033-


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