Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/19/22

Area Forecast Discussion

National Weather Service Indianapolis IN

 953 PM EST Sun Dec 18 2022

.Forecast Update...
Issued at 953 PM EST Sun Dec 18 2022

Surface high pressure, centered from around Vincennes to Memphis,
will park out tonight as an upper ridge approaches from the west.
GOES-16 was showing only a strip of cirrus over central Indiana with
more extensive cirrus canopy well upstream over the Ozarks
Meanwhile, BUFKIT and ACARS soundings were showing a dry column
under a strong inversion. The combo will provide subsidence, mostly
clear skies and light to calm winds, conditions favorable for
temperatures to fall to their current dew points for overnight lows
in the middle teens to 20 degrees. No changes needed. With a dry
boundary layer, fog is not expected.

&&

.Short Term...(This evening through Monday)
Issued at 237 PM EST Sun Dec 18 2022

- Clear and Cold Tonight
- Increasing clouds on Monday

Surface analysis early this afternoon shows a large area of high
pressure stretching through the Mississippi River Valley, nudging
eastward toward Indiana and the Ohio Valley. Westerly surface flow
remains in place across Indiana and temps were climbing through the
20s. GOES16 shows the last of the departing clouds over far eastern
and northeast Indiana while sunshine was found across the western
half of the state. Water vapor imagery shows NW flow in place across
Central Indiana, with subsidence in place. The water vapor imagery
showed Indiana caught between two large low pressure systems, one
over NE Ontario and the other over the American southwest.

Tonight...

Clear skies and cold conditions are expected tonight. Weak ridging
aloft is expected to continue to provide subsidence tonight as the
surface high pressure system builds east across Indiana. Forecast
soundings and time heights continue the trend of a dry column with
subsidence. This will lead to clear skies and light winds, ideal for
radiational cooling. Given this along with the low dew points in the
teens will trend toward lows in the middle teens.

Monday...

Models suggest the surface high will be centered over Indiana as we
start on a cold Monday morning. Aloft the models push some of the
forcing dynamics over the American Southwest toward the Tennessee
River Valley by late Monday, but the best forcing looks to remain
well south of Indiana. The ridging aloft is shown to begin to exit
to the east of Indiana along with the associated surface high
pressure system by aftrenoon. At that point the 290K GFS Isentropic
surface begins to show the beginning of lift arriving on the
backside of the departing ridge amid warm air advection, however
moisture is very limited. Thus will just lead to invading high
clouds as the afternoon progresses. Forecast soundings and time
heights both indicate the beginning of a top down type saturation,
indicative of the arrival of high clouds. Thus we will trend toward
clear in the morning but increasing clouds in the afternoon. Given
the expected warm air advection will trend highs to the lower and
middle 30s.

&&

.Long Term...(Monday night through Sunday)
Issued at 237 PM EST Sun Dec 18 2022

Tuesday through Wednesday...

The beginning of the long term features a quasi-zonal pattern in the
wake the deep low departing to the northeast. Strong 1030mb surface
high pressure builds over the region Monday night, then becomes
centered over the Northeast by midweek maintaining influence over
the region and keeping conditions mainly dry. Quiet yet cold
conditions persist Tuesday as a weak system passes just north of
Indiana. Weak low pressure in the Plains falls apart considerably as
it interacts with the aforementioned strong high, with marginal if
any support aloft. With such a disorganized system and such strong
surface high pressure maintaining influence near the surface, no
impacts are expected other than increased cloud cover.

Wednesday looks to be another quiet day with a weaker upper level
ridging regime behind the trough passage. Moisture levels in the low
to mid levels look to remain without a strong advective push,
leaving cloudy weather over central Indiana. This continued cloud
cover will likely lead to more seasonable temperatures with highs in
the mid 30s and lows in the upper 20s both Tuesday and Wednesday.

*********************************************************************
*********************************************************************

Thursday through Saturday(Potential Winter System)...

Confidence remains high in the potential for an impactful low
pressure system to pass through the Ohio Valley sometime between
Thursday morning and Friday. With that said, there is still
significant variability amongst both ensemble members and various
deterministic runs regarding details for exact track and timing of
the system.

***************************************
Differences in Deterministic Solutions:
***************************************

First, lets discuss the deterministic runs. The last few GFS runs
have done a better job of representing the expected very anomalous
cold sector within this system. This along with a gradual shift
westward of the upper level ridge axis over the western CONUS has
allowed for solutions closer to the expected conceptual model
discussed in previously (see below). This leads to increased
confidence that a deep low pressure system will develop over the
Great Lake or Ohio Valley late next week. There will still be a
significant spread in the snowfall maxima over the next several runs
amongst all deterministic models, of which leads to plenty of
uncertainty on snowfall expectations Thursday and Friday. Current
trends in deterministic guidance point to a track further north and
west, possibly tracking right through Indiana... making the forecast
even more challenging with large gradients in temperature, wind, and
P-type possible through the state.

One trend of note is a slight delay in pressure depletion as the low
tracks west to east. This is likely due to the lack of true lee
cyclogenesis, and with organization of dynamic forcings becoming more
aligned later in its life cycle. This will be something to watch
over the next several days to see if this trend continues.

Previous discussion...

The GFS has continued to be an outlier, mainly due to the position
of the ridge axis over the Western CONUS. The American model has had
the ridge axis over the intermountain west, leading to a shorter
wavelength and therefore a neutrally tilted amplified trough over
the Midwest.This solution would result in more impacts over the
eastern US versus the Midwest. Of note is a slightly weaker
thickness gradient prior to cyclogenesis. With the high likelihood
of a rather anomalous cold sector within this system this leads to
the belief that this solution may be misleading.

On the other hand is the remainder of the deterministic suite
(Canadian, ECMWF, UKMET). These three are much more aligned on the
ridge axis placement further west over the Pacific Coast. In return,
the wave length is larger, allowing for a more negatively tilted
upper trough and less subtropical jet influence. The polar jet in
this instance in piece-wise with a strong jet streak upstream of the
trough and a supergeostrophic jet streak near the ridge over the
Northeast. A coupled jet streak provides an efficient environment
for surface cyclogenesis, as seen near the surface of these various
solutions. All three of these solutions also seem to better
represent the anomalous Arctic air expected in the cold sector,
increasing baroclinicity and the rate of surface pressure depletion.
Another point which increases confidence in these solutions is the
better run to run consistency (especially with the ECMWF) regarding
track of the both surface and upper level features. While the latest
run have trended a few hours slower with the arrival of the storm
system, the track has been consistent showing a track right through
the state.

**********************************
Ensemble and Probabilistic Output:
**********************************

These deterministic solutions can help dissect the probabilistic output
given through various ensemble members of each parent model. With
the conceptual model chosen (longer wave length, negatively tiled
trough, coupled polar jet), we can narrow the expected solutions to
those that align. While there is still a wide range of solutions
even through this process, trends are indicating a deep surface
cyclone over the Midwest or Ohio Valley Thursday into Friday. This
leads to a greater likelihood of an impactful winter storm within
the vicinity of central Indiana. The track of the surface low will
still have a significant influence on the severity and type of
impact central Indiana will witness. Any track south of central
Indiana will lead to a greater chance of significant snow fall,
whereas north or through central Indiana will limit snow, but
increase rain and even thunderstorm chances.

This process also shows a wide array of temperature outcomes for
Thursday into Friday, ranging from low 50s in the warm sector, to
mid 20s in the cold sector. For now, temperature forecasts are more
aligned with the ensemble mean, but the result with much more likely
be on the lower or higher extremes (10th or 90th percentile).

No matter the track of the low, there does look to be very efficient
surface pressure depletion within a highly baroclinic system. This
will allow for a large pressure gradient over the region, pushing
very cold air into central Indiana Friday, and continuing through
the weekend. There is still some disagreement on the extent of this
cold air mass, with an ensemble from -10F to 5F degrees on Saturday
morning. Probabilistic output shows temperatures 3 standard
deviations below normal, with a large shift of tails, indicating a
potentially very anomalous cold outbreak. Snow pack will also have
an influence on low temperature potential, of which would be
impacted by low track as discussed previously. No matter the track,
confidence is increasing in a period of high winds around the time
of the wind shift to W/NW flow as very cold air aloft is advected in
with a strengthening sting jet on the south of the low. High winds
Friday night through Sunday along with the cold temperatures could
lead to very dangerous wind chills.

**********************************************
Winter Storm and Arctic Outbreak Conclusion:
**********************************************

Through a diagnostic synoptic process, discussed above, trends are
leaning towards an impactful low pressure system with a low track
ranging from Chicago to the Ohio Valley. Many potential solutions
and outcomes remain in play including the potential for significant
snowfall sometime between Thursday morning and Friday evening. There
are still many factors that will influence the forecast in the
coming days ans this system will continued to be monitored very
closely.

Despite uncertainties regarding the potential winter storm Thursday
into Friday, there are a few areas where forecast confidence is very
high. No matter the track of the upcoming storm system, confidence
remains very high that some of the coldest air of the year will
arrive by the holiday weekend with temperatures potentially falling
below zero! Any snowpack will further allow temperatures to plummet
into the negatives and possibly into record territory. Along with
the cold air will likely be dangerously cold wind chill with values
possibly below -20F. High winds and/or snow could also lead to power
outages further enhancing the potential risks. Take precautions now
to prepare for the upcoming cold weather this holiday weekend.

Areas of higher confidence:
* Period of high winds with a wind shift to the NW Thurs into Fri
* Extremely cold arctic air this weekend with temperatures falling
  below zero
* Dangerous wind chills possibly down to -20F to -30F this weekend
* Periods of snow showers on the backside of the system Fri into Sat

Areas of lower confidence:
* Exact storm track and where areas of heavy snowfall could set up
* Snowfall amounts remain very uncertain at the moment, higher
  confidence in accumulating snow further north and west
* Temperatures Thursday into Friday, could vary significantly across
  short distances through the state depending on track of the system

&&

.Aviation...(18Z TAF Issuance)
Issued at 439 PM EST Sun Dec 18 2022

Impacts:

- VFR Conditions are expected through the TAF Period.

Discussion:

High pressure and a mostly dry column will provide clear skies
tonight. Then, VFR flying conditions will continue but cirrus will
be increasing as the upper flow flattens.

Weak surface pressure gradient will keeps winds under 10 knots.
Direction will start off WSW but back to S and then SE late Monday.


&&

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

$$

Update...MK
Short Term...Puma
Long Term...CM
Aviation...MK