Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170830

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
330 AM EST Sat Feb 17 2018

Cool high pressure to our north today combined with low pressure
approaching from the west will make for a cool, wet day.  Cool and
dry high pressure will give our region one dry day on Sunday. Expect
a warm up through mid week as another cold front arrives from
Tennessee by Thursday. This front is expected to move north as a
warm front at the end of the week as our temperatures remain well
above normal.


As of 315 AM: Low clouds moving back into the area as low level flow
turns more southerly and isentropic lift develops. This flow is over
the top of a hybrid cold air damming high building in from the NE.
Expect clouds to cover the area early this morning with rain and
drizzle developing through the morning, first across the mountains
and foothills then spreading across the area through the afternoon.
The highest PoP remains across the mountains through the day where
the deepest moisture exists. The best forcing will be there as well,
closer to the short wave energy and upper divergence from the right
entrance region of the upper jet. The surface pattern looks like a
Miller B with a surface low west of the spine of the Appalachians
and a second moving along the wedge boundary to the coast. The
western low is stronger which keeps better low level forcing over
the mountains. Have kept the character of the precip as rain and
drizzle given the pattern and forecast soundings. The one
complicating factor is surface temps. Latest guidance shows temps
remaining just above freezing across Avery County and the northern
Blue Ridge. WWA has already developed with gusty SSW winds and
warming temps across the ridges. Wet bulbs are currently above
freezing and forecast to remain there although they could still fall
to near freezing early this morning. For now, have kept all precip
liquid, but will have to keep an eye on temps and wet bulbs to see
if any freezing rain or drizzle develops. That said, the recent warm
temps would help limit any accretion to elevated surfaces.

QPF around half an inch is expected across the southwestern NC
mountains, with around a quarter of an inch across the rest of the
mountains into the foothills. Amounts will be lower elsewhere. Highs
will be early in the day most locations, but will see more of a
diurnal trend over the mountains outside of the damming high and
along the southern wedge boundary.

Precip tapers off during the evening as the synoptic forcing moves
northeast out of the area, and the low level flow turns west then
northwest ending the isentropic lift. There will be some lingering
moisture across the mountains which could keep a brief period of NW
flow showers across the northern mountains. Even with weak cold
advection developing, temps do not fall very quickly and precip ends
before any wintry precip can develop. Low clouds may diminish
outside of the mountains after midnight as the strong damming
inversion erodes. Should this occur, patchy dense fog would be
possible toward morning. Lows will be 5 to 10 degrees above normal.


As of 300 AM EST Saturday: The short term will feature an upper
ridge building over the western Atlantic, with deep-layer
southwesterly flow setting up atop the region. At the surface, a
transient high will cross the southern and central Appalachians
during the day on Sunday, exiting off the Mid-Atlantic coast Sunday
night. This will result in a mostly sunny day with pleasant, above
normal temps. Moisture returns quickly Sunday evening, however, as a
deep low in the lee of the Colorado Rockies activates a warm front
across the Gulf states. This front will lift north thru the area
Sunday night, bringing a round of clouds and light showers
overnight. The best chances will be in the southwesterly upslope
areas. Temps will remain elevated under the overcast skies and
low-level WAA flow.

Monday`s forecast looks a bit tricky, as the guidance hints at a
weak in-situ CAD holding on thru the day. This would need to be
anchored by precip, but forecast soundings show mid levels dry out
with a subsidence inversion. So precip should pretty much taper off
during the day, except for lingering chances in the upslope areas.
If low clouds can hold thru the day, temps should be held back, but
still above normal. If clouds break out, temps may bust by a couple
categories from the current forecast. Will go with mostly cloudy
skies thru the day and into Monday night. Temps mainly in the 60s
for highs, and 50s for lows (well above normal).


As of 130 AM EST Saturday.  The medium range period begins Tuesday
morning with 500mb pattern having the center of high pressure just
south of Bermuda with the axis of a trough over the Rockies. The
cold front from Texas to the Great Lakes will approach our region
from the NW in mid week. The low level southerly flow ahead of this
front will result in upslope enhancement of showers from NE GA to the
Caesars Head area.  The GFS is faster in bringing the front and bulk
of the rain into our area while the ECMWF holds the front from east
Texas to KY Wednesday evening.  Instability increases and it appears
Wednesday afternoon has the best CAPE values of 600 to 900 from
Elberton to Rock Hill. After the front stalls out over east
Tennessee to VA Wed night into Thursday, the front will drift north
as a warm front Friday becoming stationary well north of our area
over the weekend. Models vary during this time on afternoon CAPE
values depending on smaller scale vort maxes and amount of moisture.
Interesting that the surface high over northern New England late
Friday may push the front south as a backdoor cold front. The
current official front display keeps the front near Maryland.
Regardless...still going with above normal temperatures through the


At KCLT and elsewhere: Low VFR will return through the night with
MVFR developing in some locations by daybreak and others shortly
after. Precip develops during the day dropping vsby to MVFR then IFR
with cigs following a similar trend. NE winds become ENE by
afternoon then ESE by sunset before returning to NE during the
evening then N overnight. Conditions may improve late in the evening
or overnight as precip ends and low clouds dissipate. However,
should this occur, then restrictions from fog would be more likely.

Outlook: Gradual clearing is expected Sunday, with dry/VFR
conditions. Moisture returns early next week, and the weather will
remain periodically unsettled, with occasional restrictions likely
through much of next week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High  94%     Med   72%     High  85%
KGSP       High 100%     High 100%     Med   75%     Med   78%
KAVL       High  83%     High 100%     Med   76%     Med   76%
KHKY       High 100%     High  87%     High  80%     High  83%
KGMU       High  95%     High  98%     Med   72%     Med   74%
KAND       High  80%     High  95%     Med   69%     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:




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