Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/19/23

Area Forecast Discussion

National Weather Service Greenville-Spartanburg SC

 1038 PM EST Mon Dec 18 2023

.SYNOPSIS...
Mountain snow showers near the Tennessee border will gradually
end late tonight.  Dry high pressure will build into the area
through mid-week and linger into the weekend with milder
temperatures. Cloudiness will increase by Christmas as the next cold
front approaches from the west.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 1018 PM Monday...Regional water vapor imagery continues to
depict a deep and amplified northern stream trough swinging across
the Appalachians. The synoptic pattern remains quite progressive and
the strongest forcing for ascent has already began to pull away from
the area. As such, regional radars have shown a drop off in
reflectivity with only light returns for any lingering northwest
flow snow showers. IR satellite imagery further reinforces the
downward trend in coverage with the coldest cloud tops confined
north of the area where upper forcing remains. Furthermore, strong
cold/dry advection has mixed out low-level moisture with rather
large 10+ degree upstream dewpoint depressions. Aircraft soundings
from Knoxville also depict this trend nicely with a steady drop off
in moisture. With that being said, the main update to the forecast
was to lower precipitation chances across the mountains along with
additional snow amounts. Scattered snow showers over the next
several hours may result in an additional inch of accumulation
before tapering off altogether. The winter weather advisory was
updated to reflect this with wording switching towards gusty winds,
cold wind chills and black ice through the remainder of the
overnight. Winds have also begun to decrease as the pressure
gradient relaxes slightly. The wind advisory outside of the northern
mountains was subsequently canceled, but the northern mountains will
still see advisory level gusts through the night.

Otherwise, a strong pressure gradient exists across the region
between a 984 mb low off the Mid-Atlantic Coast and a 1030 mb high
over the Central Plains. The resulting CAA NWLY flow atop the region
is producing gusty winds, which will continue into this evening. The
850 mb flow around the coastal low will veer from WNW to due NW,
with a surge in moisture and CAA along the TN/NC border. As for the
winds, guidance is in good agreement on continued 40-50 kt at 850 mb
thru mid-morning Tuesday across the Northern Mountains, while the
French Broad Valley and west should see winds start to weaken
gradually after midnight. So have extended the Wind Advisory
accordingly. One other concern tonight will be wind chills, where
elevations above around 4500 ft could see sub-zero values thru early
Tuesday morning. Will plan to mention the wind chills in the Winter
Weather Advisory product. The rest of the forecast area should see
generally clear skies, with breezy conditions subsiding late this
evening thru the overnight. The strong CAA should allow temps to
drop into the 10s across the most of the mountains and mid to upper
20s across the Piedmont.

The weather conditions should quiet down on Tuesday, as the center
of high pressure draws closer to the forecast area. Winds across the
mountains should be noticeably weaker by the aftn. And any lingering
clouds and isolated snow showers or flurries along the TN border
should dissipate by midday. Highs will be well-below normal, mainly
in the 30s in the mountains and mid 40s in the Piedmont.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 220 PM EST Monday: The short period kicks off a period of
quiet, dry weather ahead of the holiday weekend. Synoptically, a
rather bland pattern aloft sets up with surface high pressure
building in underneath a stubborn ridge off to the west. Wednesday
will start a gradual warm-up with temps a few ticks below climo. By
Thursday, as high pressure gets more persistent, temps will be more
near the climb mark. Overnight lows Tuesday will be quite chilly, as
the entire CWA dips well below freezing. Expect overnight temps to
increase a bit on Wednesday night, but should remain at or just
below freezing throughout the region. No precipitation is expected
and winds should remain fairly light to calm at times.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 225 PM EST Monday: Continuing the forecast through the
extended period mirrors the short term. The non-progressive pattern
of high pressure and stout ridging in the west keeps the southeast
dry and unchanging through the end of the week. By the holiday
weekend, the persistent ridge starts to progress eastward as a kink
in the flow triggers a trough to develop across the SW CONUS.
Guidance from the GFS and EURO hint at this change in pattern
providing another chance for precip Day 7 and past the forecast
period. However, this still remains well out of range for any
confidence in anything precip related, but is an element to continue
monitoring over the next few days. For the forecast period, winds
will remain light to calm as well. Temperatures should be closer to
climo and warm above as the high pressure strengthens. Overnight
temps will gradually warm from freezing into the 40s as moisture
creeps back into the region over the weekend.

&&

.AVIATION /04Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: VFR conditions will prevail at all terminals
through the TAF period. The main focus will be gusty winds tonight
as a cold front surges through the area. Frequent gusts of 20-25kts
will be common at all terminals with locally higher gusts possible,
especially at KAVL. A scattered to broken deck of low topped cumulus
clouds is also sliding across the area, but ceilings will remain
VFR. Any clouds will clear overnight with winds weakening through
the morning hours tomorrow as much drier air arrives.

Outlook: VFR conditions and light winds are expected through the
rest of the week, as a large area of dry high pressure settles over
the region.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Winter Weather Advisory until 6 AM EST Tuesday for NCZ033-
     048>052-058.
     Wind Advisory until noon EST Tuesday for NCZ033-049-050-501-
     503-505.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...ARK/TW
SHORT TERM...CP
LONG TERM...CP
AVIATION...TW



Area Forecast Discussion...CORRECTED

National Weather Service Indianapolis IN

 734 PM EST Mon Dec 18 2023

.Key Messages...

- Scattered to numerous snow showers through early evening,
lingering in extreme northeast parts of central Indiana into the
overnight

- Generally very light accumulations with over an inch possible
across Delaware and Randolph Counties

- Travel impacts possible through this evening

- Wind chills in the single digits possible overnight

- Dry and gradually warming from cool start through mid week.

- Rain chances return Friday into Friday night

&&

.Forecast Update...
Issued at 733 PM EST Mon Dec 18 2023

A few heavy snow showers persist across southwestern portions of the
CWA. Currently (730pm), the heaviest band of snow showers is located
from just west of Bloomington to Bedford. This band of snow
showers is generally moving southeastward. Areas impacted have
seen rapid reductions in visibility to as low as half a mile,
along with roadways quickly becoming snow-covered. Traffic
slowdowns and a few crashes have been reported. Since nightfall,
snow has easily been able to overcome marginal road temperatures
with sufficient snowfall rates/intensity.

Expect these snow showers to continue for another 1 to 2 hours, with
a general weakening trend. Periodic/rapid reductions in visibility
and sudden slick/hazardous driving conditions are possible in any
snow shower. Decreasing temperatures through the night along with
clearing skies may lead to areas of black ice where melted/partially
melted snow remains on roadways.

&&

.Short Term...(This evening through Tuesday)
Issued at 220 PM EST Mon Dec 18 2023

Snow showers have diminished in coverage a bit since late morning
across central Indiana but do continue. There appear to be two
reasons for this: faster eastward progression of the strong upper
wave axis and a faster intrusion of drier air from the northwest. At
18Z...temperatures across the region were in the low and mid 30s but
dewpoint depressions have increases to 10 to 15 degrees...indicative
of the dry air advection.

Much of the focus for the short term will be on the snow showers for
the rest of the afternoon...lingering into tonight over the far
northeast as lake enhancement banding develops. The trends
highlighted above are going to facilitate some changes to the
forecast going forward over the next 4 to 6 hours.

Through Tonight...

As mentioned...snow shower coverage has dropped off from earlier for
the reasons above. The KIND ACARS has also changed somewhat from its
more favorable setup earlier...with drier air noted working down
into the boundary layer despite being well mixed up to 700mb. In
theory...this is likely disrupting the deeper moisture into the
dendritic growth zone and subsequently producing smaller flake size
and lower overall snowfall rates.

There remains pockets of stronger convective snow showers...
especially to the north and northeast of the Indy metro but the
overall coverage and intensity is not quite as robust as was
expected for mid afternoon. That being said...the combination of the
gusty winds up to around 40 mph are continues to cause localized and
brief visibility drops in the heaviest bursts. Have nudged back pops
and shifted them east for the rest of the afternoon while pulling
back on light accums. There remains a likely impact from more lake
enhanced snow by early evening as the low level flow veers to a more
favorable 320-330 degree direction but this will become focused
particularly over parts of Delaware and Randolph Counties. Further
south and west...flurries may linger into the evening before
completely diminishing as drier air wins out within the lower levels.

CAMs guidance now hinting at that favorable fetch off Lake Michigan
lasting potentially past 06Z and keeping the dominant snow band in
the vicinity of Delaware and Randolph Counties before pivoting east
as winds back in the predawn hours. Outside of a quick dusting from
any heavier snow showers the rest of the afternoon...there still
remains a chance to see an inch or slightly higher in the far
northeast near the lake band tonight...with accums being more
efficient after dark as air and ground temps finally drop below
freezing. Winds will gradually drop off through the evening with
gusts fully diminishing overnight.

Messaging through Tonight...

We will continue to highlight impacts from snow showers into the
evening via graphics and posts on our social media platforms...but
with the trends noted above...we are starting to back off on any
mention of squalls as snowfall rate intensity does not appear likely
to meet the official definition of a squall. Still though...the
scattered snow showers will cause briefly lower visibilities and as
temps fall below freezing this evening...slick spots may develop as
well.

Tuesday...

Skies will clear from the west late tonight as strong high pressure
builds in. The high will pivot across the Tennessee Valley Tuesday
with winds relaxing and backing further. Broad subsidence will keep
skies mainly clear with just a few mid and high level clouds.

Temps...lows tonight will fall into the teens and lower 20s as skies
clear and the winds will make wind chills even lower. Despite the
dry weather and abundant sunshine expected Tuesday...low level
thermals are not suggestive of temperatures warming much higher than
the mid and upper 30s.

&&

.Long Term...(Tuesday night through Monday)
Issued at 220 PM EST Mon Dec 18 2023

Wednesday through Saturday

The influx of cold air today will help amplify the upcoming wave
pattern. This will create a strong temperature gradient
perpendicular to the upcoming SW flow, forming a robust WAA regime.
In return, heights will expand aloft and high pressure will build
over the Ohio Valley. The combination of building pressure and WAA
will create dry, unseasonable weather for mid December with highs
temperatures in the mid to upper 40s Wednesday through Friday.

Ensemble members are in strong agreement with high pressure over the
Midwest mid week, but start to deviate in agreement towards the end
of the week as a modest upper level low pushes in from the west.
Prolonged subsidence Wednesday through Thursday, will dry out the
surface layer substantially out in front of the low, of which should
delay any precipitation from reaching the ground late on Friday.
Still, by Friday evening into Friday night, continued downward
moisture flux will eventually saturate the lower levels leading to
light rain and minor QPF. Low passage should occur overnight
Friday, with rain chances waning throughout the day on Saturday.

Sunday through Tuesday...

A deep trough over the Western CONUS late week will lead to lee
cyclogenesis, with most ensemble members realizing a deep surface
cyclone over various portions of the Plains Christmas Weekend. There
is still a lot of uncertainty on low track and corresponding
impacts; regardless, there looks to be a greater than 50 percent
chance of precipitation sometime early next week. As the trough is
sampled later this week, model initialization will improve and
confidence in potential Christmas Eve/Day impact should increase.
Stay tuned in the coming days for updates.

&&

.Aviation...(00Z TAF Issuance)
Issued at 651 PM EST Mon Dec 18 2023

Impacts:

- Scattered snow showers this evening with brief restrictions
  (mainly HUF and BMG)

- Occasional MVFR ceilings through 03z-05z, becoming VFR

- Wind gusts between 20 to 30 knots early, then diminishing overnight

Discussion:

Snow showers have been on a downward trend for most of central
Indiana except for an area extending from Terre Haute to
Bloomington. Both of these terminals have reported visibility as low
as 1 mile at times. Recent radar trends suggest that on and off snow
showers may continue for the next 1 to 3 hours (mainly HUF and BMG).
Elsewhere, snow showers have largely diminished with a few lingering
flurries here and there.

As the night goes on, snow showers are expected to end at all
terminals by 03z. Rapidly clear skies and a return to VFR
conditions are then expected. Winds may remain brisk out of the
northwest through 12z, but should become light and variable
Tuesday morning as high pressure settles in.

As the high moves eastward, winds should become
south-southwesterly with some increasing thin high cloud cover.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Update...Eckhoff
Short Term...Ryan
Long Term...Updike
Aviation...Eckhoff