Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KGSP 240800

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
400 AM EDT Sat Mar 24 2018

Low pressure from the west will move along a warm front draped just
south of the western Carolinas through Sunday bringing a wintry mix
for the northern mountains and cold light rain elsewhere. Well below
normal temperatures will persist through Tuesday as strong high
pressure builds down the Appalachians. Temperatures rebound back to
near normal for late week as high pressure moves offshore and
moisture increases ahead of the next cold front.


As of 345 AM EDT: The main challenge for the Near Term with the
morning forecast package is precip type and snow and ice accums in
portions of the NC mountains today thru tonight.

An area of low pressure continues to develop over the Central
Plains, with a warm front extending to the east-southeast across the
Mid-South. A precip shield north of the sfc boundary continues to
expand across the Ohio Valley, with the eastern edge beginning to
work into the NC mountains. Temps are generally in the 40s across
the mountain valleys and the 30s above 3500 ft attm. The highest
peaks are in the 20s. So I expect some snow accums above 3500 ft
with this first shot of precip early this morning. The valleys look
to start out as all rain.

As the day wears on, a vigorous shortwave will round the ridge axis
over the Plains and dive SE thru the Ohio Valley. The associated sfc
low will also track east to the Mid-South. Forcing will increase
along the frontal zone, and a precip shield should persist across
the northern tier of the CWFA thru most of the day. Meanwhile, the
southern half to two-thirds of the CWFA should see mostly cloudy skies,
but little precip. The 850 mb 0 deg C line looks to set up roughly
along the eastern side of the French Broad Valley, with sub-freezing
temps north and east. Precip type is tricky, but the NAM and GFS
forecast soundings look pretty similar today. Taking a 50/50 blend
and using the top-down approach for p-type results in snow in the
northern mountains this morning (possibly also across the northern
tier of counties to the east from Caldwell to Davie). This changes
over to a sleet/snow mix this aftn then to freezing rain, while
shrinking in coverage, as WAA bringing a strengthening warm nose
into the area. Temps will only warm slightly north of I-40, staying
in the 30s to mid 40s. Elsewhere, highs will warm into the upper 40s
to lower 60s. QPF amounts support 2-4" of snow in the northern
mountains today with a little sleet and ice in spots this aftn. Also
a little snow above about 5000 ft across the central and southern NC
mountains. No accums expected below 3500 ft.

Tonight, the aforementioned shortwave will cross the CWFA during the
evening hours, bringing a band of precip associated with the warm
conveyor belt of the associated low pressure system. PoPs ramp up
into likely to categorical range across the area, highest north and
west. Profiles show a fairly strong warm nose holding on thru the
night. So any wintry precip type concerns turn to freezing rain
wherever sfc temps can manage to dip below 32 F. A strong sfc high
will build in across the Great Lakes and should cause classical cold
air damming across the Mid-Atlantic down thru the Carolinas. Precip
will also diabatically enhance the wedge, and wet-bulb effects will
likely bring temps down below freezing along the eastern slopes of
the Northern Mountains. Using the SuperBlend for temps, I only have
freezing rain in eastern Avery County tonight. And with 0.5-1.0" of
QPF, that could add up to 0.5" of ice accums overnight. Also, if the
CAD is colder/stronger than expected, the area of ice accums may be
larger than currently forecast, extending south along the Escarpment
possibly into the Black Mountains. The possible expanded ice area is
already in the warning and eastern escarpment advisory for snow
accums. So I think the warning looks well placed. We may be able to
drop Madison County from the advisory, if they don`t get much snow
accums this morning, as WAA should change everything over to rain
there by midday. For now, I will keep the Winter Storm Warning and
Advisory as is.


As of 330 AM EDT Saturday: Guidance in good agreement in the overall
pattern through the short term. However, differences remain in the
details. Surface low pressure moves east of the area Sunday with the
trailing surface boundary pushing southward ad the cold air damming
high strengthens. In fact, the damming high is so strong that this
looks like a wedge that grandmother used to make. With the low
moving east, isentropic lift weakens through the period bringing
diminishing precip chances. This is good because the strength of the
damming high keeps cold air across much of the area. Best chances of
precip will be along and west of the Blue Ridge and the Upper
Savannah River valley. Thermal profiles and temps suggest only rain
outside of the mountains, but mixed p-types across the mountains
Sunday morning and again Sunday night. QPF is very light so only
small additional accums are expected during the morning and the
winter weather products cover this. No accums are expected Sunday
night. Temps will be well below normal both periods.

Monday looks to be mainly dry but there may be some lingering light
rain over the Upper Savannah River valley north into the
southwestern NC mountains. Highs will again be well below normal.

A weak warm front moves north across or near the mountains Monday
night and Tuesday. There is disagreement on the timing and how much
precip develops with this feature. Have kept PoP limited to slight
chance across the mountains for now. Temps and thermal profiles are
cold enough that some of this precip could fall as snow across the
higher elevations, but no accums are expected. Lows warm but remain
up to 5 degrees below normal while highs rise only a few degrees and
remain well below normal.


As of 345 AM Saturday: The model guidance still shows a cold front
moving through the area Thursday night or Friday time frame. They
still differ slightly on the timing, but both bring showers and
possibly isolated thunderstorms across the area. They do agree on
keeping the bulk of the precip west of the area through Wednesday
night. The ECMWF is about 12 hours slower with the frontal movement
than the GFS. Either way, it still looks like heavy rain would be
the greater threat than severe storms. However, the ECMWF shows less
threat than the GFS. Precip ends Friday night. Temps rise to up to
10 degrees above normal by Thursday then drop a couple of degrees
for Friday.


At KCLT and elsewhere: VFR condtions to begin the period with a
gradual lowering of CIGS thru the day. Guidance has delayed MVFR to
IFR CIGS until this evening, when cold air damming ramps up and the
greatest coverage of precip is expected. With the CAD wedge becoming
entrenched tonight, winds will increase out of the NE and CIGS will
drop into the IFR range at KCLT aft 03z or so. As usual with CAD,
KAVL winds will turn out of the SE today thru tonight. Across the
Upstate, there is less of a chance of precip, so VCSH/Prob30s are
advertised thru the day. There will likely be a lull in precip
during the afternoon across the SC sites with CIGS remaining in the
3500-6000 ft range. All sites should see lower CIGS after sunset.

Outlook: Elevated precipitation chances and periodic flight
restrictions will continue throughout the weekend. Cool and dry high
pressure will build in from the north early next week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High 100%     High  81%     Med   78%
KGSP       High 100%     High 100%     High 100%     Med   70%
KAVL       High  91%     High  97%     High  88%     High  80%
KHKY       High 100%     High  93%     Med   75%     High  83%
KGMU       High 100%     High 100%     High 100%     Med   65%
KAND       High 100%     High 100%     High 100%     Med   79%

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 noon EDT Sunday for NCZ048-501-
     Winter Storm Warning until noon EDT Sunday for NCZ033-049-050.


AVIATION...ARK is the U.S. government's official web portal to all federal, state and local government web resources and services.