Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170225

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1025 PM EDT Fri Mar 16 2018

Increasing moisture will return from the west later tonight, as
a low pressure system moves east out of the plains. This low will
move southeast across the southern Appalachians through Saturday
night keeping conditions unsettled. Brief high pressure will return
on Sunday before stronger low pressure arrives from the west early
next week.


As of 1025 PM: Quiet across the western Carolinas and northeast
Georgia this evening. Winds have begun to turn N and NE outside of
the mountains where a sfc boundary has moved down from the N. Skies
still mostly clear but clouds should increase from the west

The weather should get a bit more active over the next 24 hours. A
dampening upper ridge will move in from the west overnight, and
with its passage some subtle changes will take place. The main
issues with the fcst are centered around deep convection and
how it will affect the region on Saturday. The models continue
to show an early round of relatively weak convection moving in
from the west in the early morning hours, driven east in part by
a weak short wave that helps to knock down the upper ridge, and a
wind surge at 850mb. The convection-allowing models seem to have
a good handle on this activity and suggest it will be cellular
in nature, with the best probability in the likely range over the
NC/GA mtns in the early morning, but scattered at best east of the
Blue Ridge through the rest of Saturday morning. In this scenario,
we would spend the better part of the late morning and afternoon
hours dealing with residual cloudiness and rain-cooled air with
no meaningful mid/upper forcing, which translates to relatively
poor chances of precip while the air mass tries to recover. With
that in mind, precip chances have been curtailed east of the Blue
Ridge through the afternoon hours with many locations having a
several hour period of dry wx. The situation gets a bit interesting
over the west late in the day with the approach of a stronger
short wave and sfc low. The models indicate a decent chance that
destabilization will occur over east TN late in the afternoon with
enough sfc-based CAPE and for thunderstorm development and enough
deep layer shear for some organization. Note that SPC has the wrn
mountains in the Day 2 Marginal Risk and that seems warranted. If
we can get something to organize over TN, it could make it to the
NC mtns very late in the day. A chance of thunder will be carried
in that part of the fcst area. Otherwise, temps were bumped up
Saturday afternoon with less precip in the fcst.


As of 2:30 PM EDT Friday: The model guide at the start is in good
agreement allowing a low amplitude trough, and bundle of energy, to
zip across our forecast area (FA) Saturday evening. This will also
be accompanied by a front, with several waves of low pressure along
it, to slip through our region Saturday night. The models are
suggesting some semblance of showers and thunderstorms will scoot
across the northern third of our FA Saturday evening. Considering
the overall fast flow aloft, and suspected fast movement of any
storms which develop, gusty winds will certainly be possible.

Once the front pushes south on Sunday, the atmosphere will be in a
state of reconstructing and reloading. It appears we will squeak a
dry day for the end of the weekend.

Ultimately a hybrid wedge will begin to build into the region Sunday
night and Monday. Meanwhile, the start of an extended period of
isentropic lift, and favorable upper level divergence will also
commence. In the early stages Sunday night into Monday morning, it
appears the old front to our south will temporarily intercept
moisture with globs of convection. As a result we slowed the
northeast movement of precipitation Sunday night, then gradually
edged POPS to likely, especially Monday afternoon. There could be
some elevated thunderstorms Monday, above the stable wedge.

As the primary surface low moves from Oklahoma to western Kentucky
by Tuesday morning, the wedge will erode from the south with the
northward advancing warm front. Moisture influx should be maximized
during this phase. This is when we hit the POPS with the highest
values, Monday night. We will have to watch our southern FA for
surface based instability developing.

Temperatures will be on a roller coaster, with warmer values for the
finish of the upcoming weekend, the cooling back down with the wedge
feature in place.


As of 230 PM EDT Friday: I picked up the forecast at 12z Tuesday,
with a low pressure center over the Ohio Valley and a cold/occluded
front extending southeast into western Carolinas. This front will be
associated with a dry slot punching in from the SW, while a second
cold front will still be to our west over TN and the northern part
of the Gulf states. This should result in some clearing skies,
tapering showers (except near the TN border) and a rebound in temps
to near normal, especially in the southwest.

The sfc low to our NW will start to fill, while a secondary low
develops in the Mid-Atlantic coastal waters Tuesday night into
Wednesday. In the upper levels, guidance is still having trouble
with the interactions of several embedded vort maxes within a
deepening longwave trough over the eastern CONUS. The 12z GFS has
come in very deep with an upper low wobbling right thru the CWFA.
Most of the guidance keeps the trough more open or at least closes
the low further north. In any case, steep mid level lapse rates
under this trough along with some moisture should result in another
round of showers, with snow levels falling in the mountains.
Thicknesses almost get cold enough for snow east of the mountains
early Wednesday, but sfc-based warm layer looks too deep. In any
case, Wednesday looks like a cold day, with highs about 10-15
degrees below normal.

By Wednesday evening, the coastal low looks to become the primary
low and likely the next nor`easter, placing our area in a moist NW
flow. So guidance is in decent agreement on a NW flow snow event
continuing thru Wednesday night and tapering off early Thursday. The
longwave upper trough looks quite amplified and blocked along the
East Coast. So temps will remain well below normal for Thursday and
Friday. The coastal storm should exit to the NE and cool, dry
Canadian high pressure will build in from the NW. So dry conditions
expected to return.


At KCLT and elsewhere: Winds have turned to the N or NE over much of
the area as a back door front drops in from the north. Where winds
haven`t turned, they will overnight. KAVL will see the opposite with
NNW winds this Avignon becoming S overnight. Altocu and cirrus will
move by through the evening becoming thicker and lower overnight.

The latest model guidance continues with the idea that a band of
showers will arrive from the west in the pre-dawn hours on Saturday,
then reaching KCLT around daybreak. Whatever remains of the shower
activity would push east across the region in roughly the 15Z west
to 18Z east time frame, after which lingering clouds and a possible
rain-cooled air mass would have to be overcome before more deep
convection could occur. Have decided to handle this with a PROB30
group in that general time window from west to east at all sites.
The showers could bring an MVFR restriction. In their wake, expect a
period of MVFR ceiling for a few hours until heating takes over and
begins to lift/scatter the low clouds to VFR. Wind is expected to
come around to SW behind the morning convection, ahead of the next
low approaching from the west. Gusts are expected as well. The next
round of showers and possible storms would not happen until
Saturday evening.

Outlook: Chances for scattered precipitation and restrictions
continue over the weekend. Chance looks much better for next Monday
and Tuesday, with a stronger low pressure system arriving from the

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High  93%     High 100%
KAVL       High 100%     High 100%     High  92%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     Med   79%     High 100%
KAND       High 100%     High 100%     High  92%     High  97%

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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