251
FXUS62 KGSP 202354
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
754 PM EDT Sun Sep 20 2026

.WHAT HAS CHANGED...
Updated the aviation discussion for the 00z TAF issuance.

&&

.KEY MESSAGES...
1. A few severe thunderstorms will be possible Monday and Tuesday,
mainly in the late afternoon and early evening. The main concern
will be damaging wind gusts and excessive rainfall.
2. A cool-down will finally happen during the middle of the week
that should persist into the next weekend, with temperatures
switching to 5-10 degrees below normal.

&&

.DISCUSSION...
Key message 1: A few severe thunderstorms will be possible Monday
and Tuesday, mainly in the late afternoon and early evening. The
main concern will be damaging wind gusts and excessive rainfall.

Over the next 48 to 72 hrs, our main weather player will be a weak
mid/upper low, currently over the eastern Gulf and FL peninsula,
that is expected to move slowly north/northeast through the
weakness in the flow between the old upper anticyclone over the
Arklatex region and the Atlantic Subtropical Ridge. The environment
will continue to have high precipitable water that will fuel the
development of deep convection during the afternoons and evenings
with associated risks for wet microbursts and excessive rainfall.

For today, the risk for a severe storm is relatively low because
of poor lapse rates and a lack of dCAPE in low levels, but that
risk goes up marginally for Monday with better sfc-based CAPE
expected. The 15Z RAP shows most-unstable CAPE climbing above 3000
J/kg across parts of the region Monday afternoon, for example,
along with sufficient dCAPE for strong downdrafts, but weak
shear. Some of the CAMs suggest storm clustering near the Blue
Ridge Escarpment in the afternoon once the weak cap is breached,
then storms moving out to the S and E on cold pools. Tuesday is more
uncertain because it will depend on the amount of cloud cover across
the area in the morning and how quickly and strong an easterly low
level flow develops. A wide discrepancy is seen in fcst soundings
by afternoon in terms of cap strength, basically whether or not we
already have a cold air damming wedge developing east of the mtns
by Tuesday afternoon. The GFS says no. The threat for excessive
rain will climb upward as the threat for severe storms decreases
on Tuesday. Better forcing and improving low level easterly flow
could force the development of slow moving precip near the Blue
Ridge Escarpment starting on Tuesday.

As for temps, Monday looks like a continuation of seasonally hot,
but Tuesday is really up in the air. It could be another hot one
with temps climbing above normal once again like the GFS shows,
or it could be the start of a cool wedge like the NAM shows, with
temps remaining five degrees on the cool side of normal. There`s
a 15 degree difference between the two models. Unfortunately,
the model blend tends to split the difference, meaning it looks
equally likely to be five degrees too warm or too cold, and not
likely to be correct. Needless to say, confidence is low.


Key message 2: A cool-down will finally happen during the middle
of the week that should persist into the next weekend, with
temperatures switching to 5-10 degrees below normal.

Meanwhile...by Tuesday...a blocking pattern will form across the
northern Plains/upper Midwest/upper Great Lakes that is expected
to drift eastward through Wednesday and Thursday. Naturally, when
this sort of pattern change happens, there is a lot of uncertainty
regarding the details of the transition and where the upper and sfc
lows develop, because of poor run-to-run consistency. At any rate,
it appears likely that an upper low will be located somewhere over
the Midwest/OH Valley region Tuesday night into Wednesday that
would support a sfc ridge coming down the east side of the mtns
that may or may not be aided by low pressure off the Mid-Atlantic
coast. Either way, improving moist east northeast flow is expected
to lock in a cold air damming wedge by daybreak Wednesday that
could drop temps 10-20 degrees potentially for highs Tuesday to
Wednesday. If enough light precip develops, temps might not get
out of the 60s on Wednesday. Once the wedge develops, it should
strengthen Thursday and then linger through Friday. Eventually,
we may get into drier northerly flow aloft as the mid/upper low
wobbles off the Northeast Coast over the weekend. If that can
happen, it would set up our region for classic early autumn weather
with a dry cool northerly breeze and bright sun with temps in the
low to mid 70s. How refreshing would that be?

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Convection should wane within the first
couple hours of the 00z TAFs, with just VCSH around KCLT to start
the TAFs. Guidance has backed off any widespread low stratus or
fog development tonight, except for the usual mountain valley
fog. Cannot rule out some MVFR to IFR cigs forming around sunrise
and lingering thru the morning, but confidence is too low to put
anything more than sct030 in the TAFs for Monday morning. Confidence
remains high in LIFR restrictions in the mtn valleys, so will
keep that in KAVL. Another round of scattered to numerous showers
and thunderstorms is expected Monday aftn into early evening, with
perhaps slightly greater coverage than we had today. Have PROB30s at
all sites, highlighting the most likely time storms will be nearby.

Outlook: Another round of afternoon/evening showers and
thunderstorms expected again Tuesday. Mountain valley fog and
low stratus possible again Monday night thru Tuesday morning. A
cold air damming wedge is expected to develop by early Wednesday
morning, likely resulting in widespread ceiling/vis restrictions
that may persist thru Thursday. Conditions should improve on Friday.

&&

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

&&

$$

ARK/PM