Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 202333

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
733 PM EDT Fri Apr 20 2018

High pressure atop the region to start off the weekend will give way
to increasing moisture on Sunday.  Low pressure will then track east
across the area Monday and Tuesday triggering rounds of rainfall.


As of 730 PM...The forecast is on track so far. Only minor
tweaks to the hourly temp and dewpt grids to line up with latest
obs. Otherwise, no changes needed with this update.

A quiet near term is in store as strong sfc hipres is advanced
across the mid-atl region ahead of an h5 s/w ridge. The column
will remain quite dry and no chance of precip let alone clouds
will be had across the FA. Winds will weaken within the BL as
well thus there will be no sigfnt gust potential as well. Mins
will cool off efficiently tonight within very good rad cooling
conds and patchy frost is possible outside the mtns over the NC
piedmont. This frost shud not be widespread enuf for a FR.Y and
will be mentioned in the HWO. However...a FR.Y will be issued
for the srn NC mtns and extreme NE GA where sfc tdd/s will be
low enuf for areas of frost formation.  Mins will drop to or just
below normal most locales and max temps Sat will likely be held
a couple degrees below normal within a reinforcing cP airmass.


As of 230 PM EDT Friday: Fair weather will come to an end Saturday
night. It still looks like our weekend weather will go downhill
steadily on Sunday as an upper low moves gradually east from the srn
Plains to the MS Valley with a classic split flow pattern aloft. A
nrn stream upper ridge will support slow moving sfc high pressure
over the lower Gt Lakes/srn Ontario on Sunday that will ridge down
east of the Blue Ridge. Meanwhile, moisture will return from the
Gulf/Atlantic ahead of the approaching upper low, with isentropic
lift and warm advection commencing over the top of the sfc ridge
east of the mtns. The increasing moisture and forcing should allow
light precip to break out from SW to NE, initially close to the Blue
Ridge Escarpment thru Sunday afternoon, but there remains timing
differences between the model solutions as to when the precip will
begin, with the NAM being the fastest and the GFS/ECMWF hanging back
to the west thru the end of the day. SREF plume diagrams suggest the
operational NAM might be too quick, so no big changes were made to
the precip probabilities at onset Sunday. By the end of the day,
once precip breaks out, all the ingredients should be in place
for a cold air damming event to begin by Sunday evening. Once
established, the CAD wedge will remain in place for the early
part of the week. The upper pattern slows down Sunday night and
Monday as the nrn stream ridge moves out ahead and phases with a
srn stream ridge along the east coast, so the upper low progresses
only slowly to the TN Valley where it essentially grinds to a halt
by late Monday. The result will be a prolonged period of excellent
isentropic lift that develops/overspreads the region Sunday night
and persists through Tuesday that should bring widespread precip
to the western Carolinas and northeast Georgia. The strong and
relatively deep southeast flow will probably focus the higher
amounts over northeast GA, Upstate SC, and the upper French Broad
Valley mainly late Sunday night through Monday night, with the
potential for a widespread 2-3 inch rainfall. It is too early to
speculate on flood potential, though, as the ensemble guidance
shows a great deal of spread. A lack of sfc-based convection
and PW holding somewhere in the neighborhood of 1.0 to 1.25 inch
might be limiting factors. The upper low should eventually open
up late Tuesday and get kicked out to the northeast by the next
strong upper system diving down over the Plains. As that happens,
precip chances will slowly diminish from SW to NE late Tuesday
thru daybreak on Wednesday. Temps will be tricky throughout with
a low diurnal range expected. If the precip begins too early
Sunday, the max temps will easily be a category or two on the
warm side. Monday the temps will struggle to get out of the 50s,
and perhaps the same can be said about Tuesday.


As of 225 PM EDT Friday: The second of a series of upper lows
approaches the area Wednesday and crosses the area Thursday. The
guidance is in some disagreement on the third in the series during
the Friday time frame. The ECMWF is slower and farther south while
the GFS is faster and farther north. At the surface, the damming
high erodes Wednesday as the low pressure center moves up the east
coast and away from the area. This brings a brief end to precip as
isentropic upglide ends. However, precip returns Wednesday night and
Thursday as the surface low and cold front associated with the
second upper low crosses the area. However forcing and low level
inflow is not that strong, so precip will be light. Precip
diminishes Thursday night as that system moves out of the area but
returns once again for Friday as either a cold front, GFS, or
developing low pressure system, ECMWF, moves into the area. Too
early to tell if any of this QPF will be heavy or if thunderstorms
develop. Lows bounce around near normal each morning while highs
bounce around from just below to 5 degrees below normal each day.


At KCLT and elsewhere: A very quiet TAF period in store, as a large,
dry sfc high crosses the mid-Atlantic region, ridging down into
our area. So no flight restrictions are expected thru at least
06z Sunday. Winds should become light this evening, and remain
fairly light thru the day on Saturday. The winds will favor a NE
direction overnight, then veer to ESE/SE by around the 17-19z time
frame at all sites, as the high shifts east. A few cirrus will start
to stream by from the west from time to time starting Saturday aftn.

Outlook: Flight restrictions in clouds and precip are expected
from late Sunday thru at least Tuesday as a moist low pressure
system crosses the Southeast states.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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:


Dry conditions will continue across the area Saturday. Low RH values
combined with dry fuels across northeast Georgia will create
increased fire danger conditions during the afternoon and evening,
thus a Fire Danger Statement may be issued. Elsewhere, while winds
will remain below SPS thresholds, RH values are expected to fall
below 30 percent across Upstate SC and the NC Piedmont/Mountains
Saturday afternoon.


GA...Fire Danger Statement until 8 PM EDT this evening for GAZ010-
     Frost Advisory from 4 AM to 9 AM EDT Saturday for GAZ010.
NC...Frost Advisory from 4 AM to 9 AM EDT Saturday for NCZ048-


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