Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/21/23

Area Forecast Discussion

National Weather Service Columbia SC

 646 PM EST Mon Nov 20 2023

.SYNOPSIS...
Light showers will develop late tonight ahead of a strong cold
front. The front will bring widespread showers and potentially
strong to severe thunderstorms to the area Tuesday afternoon and
evening. Drier air fills in briefly Wednesday before rain
chances return late in the week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Closed upper low, currently over E KS/W MO with trough
extending south into TX, will move slowly east into the Mid Miss
Valley tonight. Associated surface low, currently near
ArkLaTex, progged to shift east into the Lower Miss Valley, then
NE into the lower Ohio Valley, through late tonight. This will
drag a cold front slowly east into the Deep South, while strong
surface high pressure remains anchored over E Can/New England
region. H85 flow progged to veer from SE to S and increase in
speed tonight, leading to increasing isentropic lift and
moisture transport. Plenty of mid level moisture/clouds over the
region currently, but low levels are still dry, evidenced by
large surface dewpoint depressions and dry air noted below H85
on recent ACARS sounding out of GSP. As isentropic ascent
develops and increases tonight, will expect some light
precipitation to develop. Current radar mosaic indicating some
showers offshore, with returns over NC appearing to be mainly
mid level cloud cover/virga. Latest HRRR indicates patchy light
rain developing and shifting north across portions of the
forecast area (FA) tonight, becoming more widespread and
pronounced mainly to our north in response to orographic lift
and surface cold air damming regime setting up over the piedmont
of the W Carolinas. So, latest model guidance indicating higher
POPs northern areas. Cloud cover and wind staying up will lead
to much above normal low temps in the 50`s.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Complex forecast for Tuesday and Wednesday of this Thanksgiving
week. Large, longwave trough over the central US will begin
translating east and northeastward tonight as a 90+ knots 500 hPa
jet streak, currently over west TX, rounds the base of the trough
and ejects towards the OH Valley region. Ridging at 500 hPa will
precede this, and the convergence aloft noted in advance of this
feature will help to establish an intense surface high across New
England by Tuesday morning. This is already having impacts on our
area, with surface ridging noted on the eastern slopes of the
Appalachians and into the central Carolinas this morning.

Intensifying surface low is expected to be pushing into the central
OH Valley by 7a Tuesday morning, with a cold front trailing it into
central Alabama. Expecting that we`ll be firmly locked into an in-
situ wedge situation as this scenario unfolds, as the surface high
over New England will intensify tonight and result in pesky surface
ridging persisting north of a northward advancing cold front. This
will be reinforced by increasing isentropic ascent between 295K and
300K surfaces as low-level flow intensifies as a result of intense
cyclogenesis upstream. So expecting scattered to numerous rain
showers Tuesday morning because of this, and these should last
through midday, with copious low-clouds as well.

The expectations for Tuesday afternoon and evening are becoming more
clear; though, there are some caveats to note in this forecast.
Model guidance continues to suggests that very strong warm air and
moisture advection will overspread the region between 7a and 10a on
Tuesday, helping to push the surface warm front northward and "scour
out" the low-level wedge conditions. I am skeptical in this scenario,

A) actually occurring, and
B) having any significant impact on our severe weather chances.

There will be an impressive low-level mass response to the upstream
cyclone intensifying over the OH Valley, with 850 hPa flow
increasing to 40 to 50 knots by midday. While this is likely intense
enough to result in the warm front lifting northward, the magnitude
of warm air advection may result in another round of widespread
showers developing well ahead of the actual front itself. CAM
guidance is more on board with today`s 12z suite of runs, casting
uncertainty on whether or not we`ll actually be able to recover
enough instability by Tuesday evening ahead of the front to make any
sort of difference in convective strength and potential. This is
highlighted by the most aggressive 00z model (the HRRR) backing on
considerably on MLCAPE in recent runs (most notably, the 12z). So
even if we do actually break out of surface wedging (again, models
do not handle these scenarios well), it seems unlikely that we`ll be
able to destabilize enough to support intense convection during the
evening hours. This is especially due to the intensity of the low-
level shear, which will be on the order of ~40kts of 3km shear by 7p
Tuesday. Shear of this magnitude, without even modest CAPE, is
typically too strong to sustain deep convection.

All of this is complicated by the timing of the front. Guidance is
starting to trend towards a slower approach of the front itself &
any convection along it. This owes to the strongest diffluence aloft
moving well to our north by Tuesday night and early Wednesday, with
another shortwave diving into the lower MS Valley behind it. This is
usually a setup for a front to very slowly move into and through the
area, and I expect that to be the result of this overall setup. As a
result, I am not confident that we`ll see severe weather on Tuesday
evening. The combination of an unfavorable time of day, slow moving
front, showers ahead of the line, and weaker forcing should limit
the overall ability for storms to get going. If a strong storm does
develop, though, it will be capable of damaging winds given the
strength of the shear in the low levels.

There is an increasing likelihood that the second shortwave I
mentioned above will interact with the slow moving front on
Wednesday to create a favorable setup for widespread rainfall during
the day. Right now, it looks like the central and southern FA is in
line for the most precip with this as the front looks to be slowing
down and closest to this area on Wednesday, but confidence isn`t
high in the exact placement of that front. PWs in the 1.2-1.5" range
will certainty suffice for good rainfall, but the IQR range for
totals is very high within ensembles for rain associated with this
wave specifically, with HREF, GEFS, and ECE IQR for 24h precip all
approaching 1". Regardless, it does look like clouds and cooler
temps are likely, especially if we do see the rain that models are
trending towards. Will opt to err on the low side of guidance as a
result, with temps likely staying in the low 60s. The shortwave will
pass through on Wednesday night, pushing rain chances eastward as
well. Temps will fall into the 40s.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Thanksgiving looks really nice, as we`ll have dry surface high
pressure settle over the region beneath convergence aloft. This
should make for a fantastic setting for any backyard football games
or any other outdoor Thanksgiving traditions. Highs will likely be
in the low 60s under sunny skies. Clouds will increase late in the
day as another weak shortwave translates northeastward to bring
some more showers by Friday. These should be light given the
dry air in place. Beyond this, really didn`t change too much
from the NBM given the uncertainty in the forecast between now
and Thursday.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
VFR early this evening then ceilings expected to lower to MVFR
late with some showers near the TAF sites near daybreak.

Considerable VFR strato-cumulus and alto cumulus across the
region early this evening with strong high pressure to the north
extending into the area. Expect clouds to lower to MVFR by
around 06z as low level moisture increases due to increasing
low-level flow 25 to 35 kts. High resolution models suggest
some light showers late tonight/toward morning. Other concern is
low level wind shear so will monitor later this evening. Weak
cold air damming in the morning but the warm front will be near
the area, moving to the north. So ceilings may improve later in
the morning, especially at AGS and DNL. Stronger instability
possible in the CSRA, and a few thunderstorms possible in that
area by afternoon. Showers should be numerous/widespread at
times through the day and visibility may be reduced especially
in the morning if wedge conditions hold. Weak instability in the
afternoon and a few elevated thunderstorms possible but
confidence low. Cold front approaching from the west late in the
period. Will keep MVFR through the period. Winds will be east
near 10 knots early tonight and shift to southeast/south 10 to
20 knots with a few higher gusts by 18z Tuesday.

EXTENDED AVIATION OUTLOOK...A cold front will move through the
area Tuesday night with showers/possible thunderstorms and
associated restrictions expected Tuesday night into early
Wednesday. Improvements occur Wednesday into the first part of
Thursday, with another round of restrictions possible late
Thursday and Friday.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.

&&

$$