FXUS62 KGSP 230428

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1228 AM EDT Thu Mar 23 2023

Southerly winds will continue across the area as high pressure moves
east of the Carolinas, resulting in a sharp warming trend through
Friday. On Saturday, a cold front arrives, bringing rain and a
potential for some thunder, but failing to cool temperatures very
much. Another front may bring rain to the region early next week.


As of 1240 AM...For this update, visibility continues to drop across
the Piedmont as low clouds settle in a bit sooner than previously
forecasted. Fog is starting to develop throughout the area at this
time. Winds remain light and should contribute to even more fog
development overnight. At this time, will not issue a Dense Fog
Advisory with this update, but one may be needed for portions of the
area at some point overnight through Thursday morning.

Otherwise, the atmosphere will remain quite moist overnight, and low
clouds will persists thru the morning hours. Not a great chance of
dense fog tonight, but with a highly saturated sfc-layer, any areas
that decouple from increasing sw/ly winds will be suspect. This
situation will have to be monitored over the next couple shifts to
see if a DFA will be needed. Winds increase out of the southwest
mid-morning Thu while a shallow, yet energetic, mixed layer allows
for low-end to moderate gusts across most non/mtn locales. Expect
mins held a few degrees abv normal overnight in weak rad cooling,
while highs jump 10-12 degrees abv normal due to llvl WAA flow
and increasing insol during the afternoon.


As of 205 PM EDT Wednesday: After a period of quiet weather on
Friday, deep troughing over the Great Plains will begin to nudge
the subtropical ridge eastward.  This won`t put much of a dent in
Friday`s temperatures, given anomalously high thickness values
across the forecast area and mostly sunny skies, and afternoon
highs are still expected to creep into the low or mid 80s for most
locations outside the high elevations.  Deterioration in conditions
won`t come until Friday night, as heights aloft begin to fall, and
strengthening SW flow moistens profiles through the overnight hours.
A robust surface cold front will move into the Ohio Valley by Friday
night, and as the supporting trough continues its eastward march,
enough synoptic forcing should be present for an onset to rainfall
by midnight or so for the western zones.  In what has been the
ongoing trend for days, guidance has slowed down compared to 24
hours ago, with the front only slowly making its way across the
Ohio Valley and supporting PoPs that peak at or after daybreak.

There remains a significant amount of uncertainty in Saturday`s
severe risk.  Models have struggled to get a handle on the evolution
of this system for days, but the general slowing trend is indicative
of increasing potential for instability...with all but two members
of the 12z GEFS / 00z ECE cycles supporting at least 100-200 J/kg
sbCAPE across the extreme southern and eastern zones, and high
ball estimates producing 500+ J/kg.  Furthermore...while earlier
model runs depicted a fairly stout subsidence inversion, limiting
CAPE and suppressing the amount of available shear...the latest
round of guidance supports both a less-pronounced inversion over
the Upstate (perhaps as a result of better synoptic forcing) and a
moister PBL, which in addition to enhancing instability would open
up potential storms to deeper shear and increased severe risk.
Current effective shear estimates by the operational GFS and 09z
SREF cycle are supportive of at least some convective organization,
on the condition that enough instability develops over the southern
Upstate.  And yet, a solid third of GEFS and SREF members still
favor a less- unstable, lower-shear environment as the front
arrives in the Carolinas, so confidence remains only moderate on
thunder chances.  Until tomorrow evening`s run of the HREF when CAMs
become available, it`ll be tough to pin down much more than that.


As of 153 PM EDT Wednesday: Saturday night, lingering showers over
the eastern tier will give way to gradually-drying air behind the
advancing cold front.  Winds will veer around to NW...and guidance
now hints at some gusty winds developing across the NC mountains,
particularly the northernmost zones of the Blue Ridge, where the
extreme elevations may tap directly into stout SW winds along
the frontal boundary.  Even the normally-unimpressed raw guidance
sources depict unusually fast low-level winds...likely resulting in
strong to gusty winds in the mountains throughout the overnight.
By daybreak Sunday, winds will die down as CAA pulls north, and
the last of any lingering showers should have pushed east of the
forecast area.  Dry conditions will prevail the remainder of Sunday
and into the first part of Monday, as a surface high scoots across
the Great Lakes / upper Mid-Atlantic and off the Atlantic coast.

The next surface low arrives early next week...but the pattern
remains complex and uncertain.  Some guidance (mostly the GEFS)
depicts a weak shortwave over the Mississippi Valley producing
cyclogenesis there, resulting in rain chances again as early as
Monday morning...while other guidance (mostly the ECE) holds off
on any rain until the arrival of a more robust 500mb trough on
Tuesday.  Either way...instability associated with the system(s)
appears minimal presently...and should give way to clearer, drier,
and cooler conditions in the mid-60s by Wednesday.


At KCLT and elsewhere: A low-confidence forecast for tonight, as
light rain has moistened the low-levels, but plenty of lingering
mid and high clouds will limit raditional cooling. Guidance seems
to be trending toward more widespread fog development across the
Piedmont during the pre-dawn hours, but confidence is below-average,
due to the mid clouds and some potential for increased mixing before
daybreak. Even if fog isn`t dense/widespread, LIFR cigs look pretty
likely, so will continue with that, but lower vsby some, based on
trends. Light SW wind will increase and become gusty by midday,
and continue thru the aftn. The fog and stratus should scatter
out with the increased mixing.

Outlook: Dry high pressure and VFR conditions expected thru
Friday. A cold front is expected to bring precipitation and
associated restrictions back Friday night into Saturday. Dry air
returns for Sunday.





NWS GSP Office Area Forecast Discussion