Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 251813
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
213 PM EDT Tue Apr 25 2017

.SYNOPSIS...
An area of low pressure will gradually weaken and drift northeast
along the Mid-Atlantic coast today through tonight. Warm and dry
high pressure will filter in behind the low through Wednesday. A
dissipating cold front crosses our region Thursday, then warm and
humid high pressure builds back in for Friday and Saturday.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 150 PM EDT, the departing closed low over the NC Outer Banks
this afternoon will continue to move northeast away from the region
through tonight as mid level ridging builds in from the west. As
insolation improves in southwest sections late this afternoon, the
HRRR manages to pop a few showers, but will confine any isolated
mention to the immediate Blue Ridge. Any shower activity across the
piedmont should remain well east of the area as the low pulls away.

Surface condensation pressure deficits will be less than 10 mb
overnight, and steadily clearing skies should permit some fog
development. It is possible there will be enough boundary layer
moisture that stratus forms, and a weak 850 mb downslope component
could offset fog and stratus development altogether in some areas.
So, dense fog is by no means certain. Will keep a patchy mention for
the overnight hours for now, along with temps in the 50s throughout.

The southeast ridge axis will be solidly atop the area on Wednesday.
Any boundary layer drying will be brief during the day before
southerly return flow starts up under the ridge in the afternoon.
Mid-level temperatures should be too warm to permit any convective
development despite the good heating and temps rebounding into the
80s most areas.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 300 AM EDT Tuesday: Wednesday looks like a quiet, pleasant
day, as sfc high pressure builds in under an East Coast ridge. Any
fog or stratus should lift/scatter out by late morning, leaving
mostly sunny skies and a light southwest wind. Max temps are
expected to be 6 to 9 degrees above normal.

Wednesday night into Thursday, the upper ridge axis will shift, as a
negatively tilted short wave trough lifts up the eastern side of a
broad long wave trough covering 2/3rds of the CONUS. The short wave
looks to brush the NC mountains midday Thursday, but should be
accompanied by decent instability with a plume of moisture advecting
in from the SW. So PoPs ramp up to high-end CHC to likely in the
west, and low-end CHC east. The operational models are in agreement
on decent overlap of 40-50+ kts of 0-6 km shear and 1000-2000 J/kg
of sbCAPE just to our SW. So it will interesting to see how
convection evolves along and ahead of the trough. Temps will remain
about 5-10 degrees above normal.

Convection may linger east of the mountains well after sunset, as
25-35 kt SWLY LLJ continues advecting higher theta-e air into the
CWFA. Min temps will be about 8-12 deg above normal.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 300 AM EDT Tuesday: It is going to feel like summer this
weekend, as an upper ridge amplifies over the eastern CONUS, setting
up a strong Bermuda High pattern over the Southeast. Diurnally
driven convection is expected both Friday and Saturday with temps
well above normal. Saturday looks like the warmest day of the next
seven, where a few spots may hit 90 in the Piedmont.

Sunday and Monday, the broad long wave trough will begin to progress
across the central CONUS, with its axis entering the Ohio and
Tennessee Valley states by early Monday. An attendant cold front
looks to have plenty of moisture to work with, and may be strongly
forced as it approaches the Southern Appalachians. The 00z ECMWF has
come into good agreement with the GFS on the timing of the fropa
being in the 12z Monday to 00z Tuesday. The front looks to lose a
lot of its better buoyancy, as the best mid and upper support
remains in the cold air region behind the sfc front. Depending on
the speed of the front, it could provide a solid soaking rain. At
this point, it doesn`t look like QPF will be excessive. Temps will
be a category or two cooler than Saturday, but still above normal.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
KCLT: Abundant piedmont moisture is keeping MVFR cigs in place
and these will be slow to lift and scatter through the evening
hours. No further shower development is expected. The main concern
will be the potential for overnight fog or low stratus given the
very moist surface layer. There is some potential for stratus to
reduce the fog potential, or for light NW winds to provide some
downslope and limit both stratus and fog. So, will keep developing
low stratus scattered and feature nothing worse than 2SM vsby in fog
at this point. Winds will adjust to SSW through Wed with scattered
cumulus with heating.

Elsewhere: ceilings across the mountains and foothills have lifted
from MVFR to VFR all but KHKY, and they should improve there soon as
well. Convection allowing models depict isolated showers around the
region late this afternoon, but confidence is far too low to mention
it at any TAF sites. Abundant moisture will create the potential for
stratus/fog overnight but will remain conservative in the forecast
for now given MOS uncertainty and potential for weak downsloping.
Cumulus should develop with heating through the late morning hours
along with light SSW flow.

Outlook: Return flow moisture will develop ahead of the next system
Thursday into Friday with the potential for thunderstorms
increasing. Scattered, diurnal thunderstorms will then continue over
the weekend.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  88%     High  94%     High  86%     High  86%
KGSP       High 100%     High 100%     Med   75%     High  95%
KAVL       High 100%     High 100%     Med   79%     High  86%
KHKY       Med   74%     High  87%     High  82%     High  90%
KGMU       High 100%     High 100%     High  85%     High  97%
KAND       High 100%     High 100%     High  88%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...ARK
NEAR TERM...HG
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...HG



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