Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 250515
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
115 AM EDT Mon Mar 25 2024

.SYNOPSIS...
Dry and cool high pressure will persist across the area through
early Tuesday before a rainy cold front crosses the area late
Tuesday into Thursday.  Dry and warmer weather returns Friday
through the weekend.

&&

.NEAR TERM /THROUGH TODAY/...
100 AM Update...Upper level clouds continue to become more
widespread and are advancing east this morning. This may have some
impact on sfc temps reaching freezing across the NW NC Piedmont and
fthills where the Freeze Warning is in effect. Latest hires guidance
suggests many areas will remain just abv freezing, yet still suspect
isol areas to reach this level.

As of 951 PM Sunday: Sprawling surface high remains centered over
southeast Canada this evening with ridging extending down the spine
of the Appalachians. A modest surface pressure gradient will support
light winds overnight with persistent dry/cool low-level advection.
High clouds will also stream across the region in advance of a broad
upstream trough across the Southern Plains with several embedded
shortwave troughs. Temperatures are still forecast to drop to around
or just below freezing roughly along and north of I-40 where a
freeze warning remains in place. If winds were to keep the boundary
layer mixed, combined with some degree of increasing cloud cover,
the coverage of sub-freezing temperatures may end up being more
limited in spatial extent. The frost advisory extending down to the
South Carolina state line also remains in place with the same
caveats from light winds and a mixed boundary layer. Sheltered
locations that decouple could still see patchy frost, however. Will
keep the hazard products going as is with only minor forecast
adjustments needed.

Otherwise, a strong ridge amplifies across the southeast as a
strengthening trough digs across the western CONUS. Gradual height
rises will continue into Monday, before flow aloft turns SW from the
digging trough out west, increasing moisture return ahead of the
next system. Meanwhile, observed soundings from surrounding CWAs
show a stout mid-level subsidence inversion, aiding in fire weather
wx concerns this afternoon and early evening. Should the inversion
weak, an abundant amount of dry air could mix down into the BL,
dropping RH values into critical thresholds, especially across the
mountains. Winds continue to remain light with occasional low-end
gusts through the CWA, with gusts expected to increase over the
ridge tops at the TN/NC border after sunset and through Sunday
night. Outside of fire concerns, temperatures tonight are expected
to reach freezing values across the mountains and NC Piedmont. For
the growing season now activated for certain areas, a Freeze Warning
has been issued for parts of the NC Piedmont tonight, as well as a
Frost Advisory where temps hover near freezing.

By Monday, the upper high pressure across the Gulf of Mexico
amplifies and begins to propagate eastward as the trough out west
also amplifies. This should aid in moisture return aloft by Monday
afternoon, priming the area for rain chances in the next period.
With more easterly surface flow, moisture return at the surface will
gradually increase toward the end of the period, but not before
another afternoon of low RH values and fire concerns once again.
Modeled soundings from the NAM and RAP show the stout mid-level dry
air remaining in place and a well mixed BL by Monday afternoon. Wind
conditions will be similar to Sunday, light winds and occasional
gusts, with gusts 30-35 mph at the higher elevations along the TN/NC
border. Temperatures Monday are expected to be warmer, with most of
the area east of the mountains reaching the upper 50s and low 60s.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...
As of 230 PM Sunday...As the forecast period begins Monday night,
the next weather systems to impact the region will be approaching
from the west as weak surface ridging persists across the forecast
area.  A closed upper-low over the central Plains will deamplify
as it becomes absorbed into a more potent upper-level low across
south-central Canada, while a broad longwave trough dominates
the western two-thirds of ConUS.  This system will be driving a
cold front across the Tennessee Valley Tuesday into Wednesday.
Light precip associated with this system will outpace the actual
front and will start to enter the area late Tuesday morning.
Meanwhile, a very weak low-pressure system embedded in the
southern jet will move across the Deep South, driving precip
response there.  Therefore, the upper-level pattern evolves into a
broad southwesterly flow regime by the time these decaying systems
reach the southern Appalachians later Tuesday into early Wednesday
and we may lack the deep-layer forcing and notable moisture flux
necessary for significant rainfall during this time.  This lack
of forcing and ongoing rainfall will allow the weak surface ridge
axis to persist along the foothills through Tuesday, resulting in
a low diurnal range on Tuesday across the far western Piedmont and
points west.  In these areas, Tuesday morning lows will be near
normal but Tuesday`s highs will be 8-10 degrees below normal.
At this time, the wedge boundary is forecast to set up in the
vicinity of the I-85 corridor so the milder airmass east of the
wedge may result in highs just 2-4 degrees below normal, including
for the Charlotte metro.

The weak, southern stream low pressure system located over Georgia
Tuesday evening will make little progress eastward Tuesday night
thanks to shortwave energy rounding the base of the longwave
trough, which will amplify the trough over Texas and slow its
eastward propagation.  Therefore, light rainfall and ample cloud
cover will persist over the forecast area into Wednesday morning,
keeping low temperatures about 6-8 degrees above normal area wide.
The northern stream cold front will finally enter the southern
Appalachians Wednesday morning but will have very little moisture
to work with as the low pressure system over Georgia starts to
respond to a new round of height falls over the Deep South and
slides into the Atlantic Coastal Plain, taking the best precip
response with it.  The post-frontal northwest flow should help to
erode what is left of the persistent wedge for much of the area and
this combined with downsloping effects and decreasing cloud cover
should induce a notable warming trend for much of the forecast area.
Highs on Wednesday are currently expected to return to near normal
if not a few degrees above normal, especially across the mountain
valleys and Savannah River Basin.  Highs may remain several degrees
below normal further north and east into the northern NC Piedmont,
where the residence time of the eroding wedge is expected to be
the greatest.

The aforementioned shortwave energy ejecting from Texas into the
Deep South will notably sharpen the bottom of the longwave trough
Wednesday night into Thursday.  The trough will become neutrally
tilted by Thursday morning and possibly assume a negative tilt
overnight Thursday as the shortwave begins to lift north and east
over the Carolinas.  This will cause deeper low pressure to develop
along the existing front, which is expected to be somewhere across
the lower Carolinas Wednesday night into Thursday.  There has been
a lot of run-to-run variability across all model suites regarding
the exact frontal placement as a surface low moves up the front and
across the Carolinas on Thursday.  Therefore, the forecast is of
lower confidence during this time as an axis of briefly moderate
to heavy rainfall is possible wherever the low track occurs.
At this time, however, the general consensus has been that the
best forcing and precip will be from the I-77 corridor and points
east, and we have higher confidence in lower rainfall amounts
the further west one looks, especially across the mountains.
There will be ample bulk shear but instability looks to be nearly
nonexistent at this time so heavy rainfall is the primary threat.
Expect highs to be slightly below normal given weak CAA on the
backside of the low track, cloud cover, and rainfall.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
As of 230 PM Sunday...The long-term period will begin Thursday night
for this forecast package as the low-pressure system that impacts
the area during the short term is expected to rapidly lift off to
the east and north into the western Atlantic overnight Thursday
into Friday.  High pressure over the Rust Belt will combine with the
intensifying coastal low to result in a notable pressure gradient
over the forecast area beginning Thursday night and persisting for
much of Friday.  Gusty northwest winds may result depending on how
deep the low actually becomes.  As a result, developing CAA in
the strengthening NW flow behind the low may allow temperatures
to get into the lower 30s over the mountains and mid- to upper
30s east for early Friday morning.

Nevertheless, the departure of this low will set up a nice
warming trend Friday and especially Saturday and Sunday, as high
pressure along the Gulf Coast slides off the Atlantic coast and
low-level southerly flow sets up underneath a flattened southern
stream ridge.  Highs near- to slightly above normal on Friday will
rise into the lower 70s in the mountain valleys and mid-70s east
of the mountains on Saturday or about 7-9 degrees above normal.
As a fast-moving shortwave drops south into the Upper Midwest,
some guidance is suggesting that a warm front may activate from
the Tennessee Valley into the Ohio Valley later Sunday into early
Monday at the end of the period.  However, confidence is low and
any precip impacting our area would be isolated and very light;
therefore it`s too early to consider introducing PoPs at this time.
Sunday`s highs may approach 80 east of the mountains and will
reach the mid-70s in the mountain valleys.

&&

.AVIATION /05Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: VFR flight conds continue thru the 06z TAF
period across all sites. Strong hipres will continue to ridge in
from the NE and dominate the pattern thru the period. Winds may
become more ne/ly at KCLT thru the morning and early afternoon
before veering south of east as the p/grad relaxes. Have low-end
gusts more likely at KAVL this afternoon, but other sites could see
an occasional low-end gusts due to shallow afternoon mixing.

Outlook: Rain chances and possible restrictions return Tuesday into
Wednesday as a cold front moves in. Lingering showers could keep
restrictions in place through Thursday.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Freeze Warning until 9 AM EDT this morning for NCZ035>037-056-
     057-069-072-502-504-506.
     Frost Advisory until 9 AM EDT this morning for NCZ068-070-071-
     082-507>510.
SC...None.

&&

$$

SYNOPSIS...JMP
NEAR TERM...CP/SBK/TW
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...SBK


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