Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 260901
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
501 AM EDT Mon Sep 26 2016

.SYNOPSIS...
A cold front will approach the area from the west today, and slowly
cross the region tonight through Tuesday. An upper level low will
drop south into the Ohio Valley by Wednesday and linger over the
Central Appalachians through the end of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM EDT Monday: Portions of the I-77 corridor/northern
Piedmont and adjacent northern foothills have seen prolonged
convection overnight with locally heavy rainfall. With the front
stalled and slowly trying to wash out over the area, weak upslope
surface flow combined with little to no steering winds aloft have
resulted in a small bullseye of moisture convergence in these areas.
Current QPEs since 11pm especially in southern Caldwell County are
upwards of 4". Luckily (or unluckily, depending on how you look at
it) it has been pretty dry lately so very little response on small
basins in the area.

As for the rest of the near term, surface high pressure ridging down
the Eastern Seaboard has allowed this back door cold front to move
into the region. In the meantime a strong upper low is diving out of
Canada and over the Great Lakes this morning, and will sweep an
occluded cold front into the Ohio Valley and Appalachians today.
Downstream ridging aloft ahead of this upper low will briefly
increase today, and with low-level WAA picking up as flow shifts
around to southerly, the back door front should retreat to the
northeast (does that make it a front door front?) as the Great Lakes
system approaches. With this comes a surge in instability across the
area today with near-term guidance generally agreeing on a max of
about 1500 J/kg, though the NAM as usual is more aggressive. Expect
an increase in convection across the mountains late morning into
early afternoon, spreading east across the area this evening. CAMs
disagree on exact timing but the general look is the same. Lack of
deep-layer shear and poor mid-level lapse rates will limit severe
potential, but with strong mechanical lift plus increasing surface
buoyancy, an isolated strong or low-end severe storm cannot be ruled
out. Other concern is for isolated heavy rainfall, with PW values
increasing to 1.6-1.7" - well above the 90% threshold and pretty
close to the daily max. The limiting factor here will be increasing
winds aloft which will allow storms to actually start moving.
Nonetheless, locally heavy rainfall will be possible with storm
total basin-average QPF approaching 1.5" for the Day1 period.

Back to the increasing WAA today and the retreating back door cold
front...as this lifts northeast, despite the increasing moisture and
cloud cover, expect temperatures to rise again today especially with
the increasing amplitude of the downstream ridging aloft over us and
corresponding increase in thicknesses. MET guidance was warmest of
the pack yesterday and still too cool. It is again on the warmer
side today so leaned that direction and brought in a little bias-
correction. Might see a little cooling of overnight lows tonight in
the northern mountains as the front starts to push through, but the
main pattern change will start in the short term.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 300 AM EDT Monday...There is still a lot of uncertainty on the
short term forecast, as a large upper low will drop south across the
Great Lakes to the Ohio Valley. From there, models disagree on
how far south the low will get, and how quickly it will lift back
north. Even a slightly more westward position of the low could
result in lingering moisture and precip chances thru Wednesday. The
00Z ECMWF continues to show this type of solution. Meanwhile, the
GFS has the low slightly east and also keeps it drifting east across
the central Appalachians into the medium range, while the ECMWF
stalls it out around Louisville, KY.

So with all that said...it looks like the front will be slower to
push through on Tuesday into Tuesday night, resulting in solid
chance PoPs across most of the area. There will be some instability
for TSTMS, but shear looks weak, so severe threat should be low.
While an isolated excessive rain threat may exist with the
slow-moving front, generally most areas will recent much needed
rainfall. For Wednesday into Wednesday night, I kept relatively low
PoPs that are more in line with the GFS, as the EC still more of
an outlier.

Max temps will continue to be above normal Tuesday and Wednesday,
with lows near normal in the Piedmont and slightly below normal in
the mountains.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 315 AM EDT Monday...An upper low is expected to be near the
area Thursday, then wobble toward the north or northeast Friday thru
the weekend. A weak surface low under the mid and upper low will
spin north of the area, either in the Upper Ohio Valley or the
Central Appalachians. So much lower thicknesses and generally dry
air will work into the area from the west.

For Thursday and Friday...other than some westerly to northwesterly
upslope showers possible along the TN/NC border, the forecast
looks dry. Temps will be below normal for the first time in a while,
with highs in the 50s and 60s in the mountains and lower to mid 70s
across the piedmont. Lows in the 40s in the mountains and 50s
piedmont. The weekend looks nice with mostly clear skies and a
slight rebound in temps to around normal.

&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
At KCLT: Added TEMPO TSRA to KCLT based on current convection.
Cig/vsby trends still look good. Should see a slow lifting trend
later this morning as the back door front begins to wash out, but
stronger cold front approaching from the west will approach toward
the end of the period. Continued PROB30 TSRA for this afternoon,
transitioning to prevailing -SHRA after 00z. TSRA still possible at
that point but am already over TAF length. Winds should remain
mainly SE transitioning to S through the period, though early
overnight could see some brief oscillation to ENE.

Elsewhere:  Trends similar to that of KCLT, cigs will develop
earlier and be lower at KHKY, and later and not as low (generally
MVFR) at the other terminals, with only some brief MVFR fog possible
at KAND. With the approach of the cold front this afternoon, kept
VCTS through the end of the period at KHKY.

Outlook: Cold front will slowly cross the region tonight through
Tuesday. This should result in better chances for convection. Then
dry high pressure builds in behind the front on Wednesday.

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       Low   56%     High  81%     High 100%     High 100%
KGSP       High  89%     High  87%     High 100%     High 100%
KAVL       Med   78%     High 100%     High 100%     Med   63%
KHKY       Med   75%     High  87%     High 100%     Med   73%
KGMU       High  89%     High  87%     High 100%     High 100%
KAND       High  83%     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:

www.weather.gov/gsp/aviation

&&

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

&&

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


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