Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/25/20

Area Forecast Discussion

National Weather Service Indianapolis IN

 951 PM EST Thu Dec 24 2020

.UPDATE...

The Near Term and Aviation section has been updated below.

&&

.SYNOPSIS...
Issued at 400 PM EST Thu Dec 24 2020

Current arctic air mass will bring very cold temperatures through
Saturday morning, with wind chill values from -5F to 10F. Today`s
flurries will diminish this evening, with considerable cloudiness
then dissipating late Christmas Day. Seasonal and dry conditions
will return for the remainder of the holiday weekend before
chances of rain Sunday night. Next week will trend from slightly
below to slight above normal temperatures, with chances for
rain/snow mid week.

&&

.NEAR TERM /Rest of Today and Tonight/...
Issued at 951 PM EST Thu Dec 24 2020

Upped coverage to scattered flurries area wide. Possible that some
areas may see a dusting, but measurable precipitation still
appears fairly unlikely. Remainder of forecast in good shape.
Previous discussion follows.

Issued at 400 PM EST Thu Dec 24 2020

A deep and highly-amplified H500 trough has established itself along
the Mississippi Valley...between the parent surface cyclone
departure into southeastern Canada and an arctic high that continues
southward across the Plains.  A second surface cyclone is currently
developing over the southern Appalachians, along the strong cold
front that blew through the region last night.  While this next
storm will bring snow to Ohio, even our far eastern counties should
be far enough west to avoid measurable snow. Closer to home,
scattered flurries and a few briefly robust snow showers have been
busy today adding the holiday spirit to this first arctic
outbreak of the season. Even the relatively more impressive echoes
over Putnam-Montgomery-Boone counties have led to only reports of
a dusting. Flurries will diminish this evening, as the stacked
trough`s axis slowly enters the region. Widespread overcast skies
should break from west to east overnight, although considerable
cloudiness will continue.

The cut-off H500 trough will deepen slightly as it slowly but
steadily crosses the Mid-West...with its small center of sub-510 dm
thickness likely including most of the CWA for a brief period around
9z Christmas morning. Meanwhile the corresponding dome of arctic
high pressure will continue its trek southward across the Plains,
continuing to provide cold advection in the form of occasional light
westerly breezes.  The lowest H850 temperatures in the nation will
follow the overall ~southeasterly progression of these
features...bringing -23C to -20C values down the Illinois-Indiana
border.  All of this will combine for the coldest morning across
central Indiana since this past Valentine`s Day weekend...and the
coldest Christmas since 2004 when temperatures rose from below zero
to near 20F.  Readings will slowly drop from near 20F late this
afternoon...to the low teens by Christmas morning. Wind chill
values will drop as low as -5F to 0F by dawn.

&&

.SHORT TERM /Christmas Day through Saturday Night/...
Issued at 400 PM EST Thu Dec 24 2020

While the deep trough/arctic air mass will be over the region
Christmas morning, the next flat ridge will already be entering
central Indiana by late in the day. Any lake effect snow showers
across the far northern counties will therefore dissipate and mostly
cloudy skies should scatter out from west to east in the
afternoon. Nevertheless readings will be at least 15F below
normal, with wind chill values remaining in the 0F to 10F range
throughout the day.  Christmas night will continue the below
normal temperature trend, while not quite as cold as generally
light winds back to southwesterly. Saturday will be the only
mostly sunny day of the holiday weekend, as near normal and dry
conditions return under a zonal mid-level flow. Clouds will begin
to return Saturday Night ahead of the next system.

Temperatures will climb to only near 20F Christmas Day, and then
rebound to the mid 30`s to low 40`s Saturday. Overnight lows will
be 15-20F Christmas night, and mid to upper 20`s Saturday night.

&&

.LONG TERM /Sunday through Thursday/...
Issued at 232 PM EST Thu Dec 24 2020

A more active weather pattern settles in as we wrap up the last few
days of 2020...with storm systems set to impact the Ohio Valley and
lower Great Lakes for the end of the weekend and again by the middle
of next week.

An amplified upper level flow pattern will persist across the
country through much of the extended period. Some phasing of the
northern and southern jets will take place over the upper Midwest
Sunday and pull a deepening surface wave across the Great Lakes.
With the track of the low remaining to the northwest of central
Indiana...expect mainly rain as the system passes through the region
Sunday night with perhaps a few snowflakes as colder air works in
early Monday behind the cold front.

Upper level ridging will spread across the OHio Valley in tandem
with a strong surface high pressure early next week with dry and
chilly weather. Focus will then turn to a deeper and more extensive
upper level wave making its way east across the country. Extended
models continue to deviate a bit in the degree of phasing of the
energy aloft over the central and northern Plains by late Tuesday
into Wednesday...but there is likely to be a rather deep upper low
over the Upper Midwest and western Great Lakes by New Years Eve.
Surface cyclogenesis will develop in the lee of the Rockies late
Tuesday before ejecting out into the central Plains then lifting
northeast towards the Michigan upper peninsula by Thursday as an
occluding low near 990mb. This is likely to bring an extended
period of precipitation to central Indiana beginning late Tuesday
and especially ramping up Wednesday and Thursday.

At this point...low level thermals and the track of the surface wave
both support rain as the primary precip type through much of the
event. That being said...colder air will wrap back around the
underside of the system behind the frontal passage Wednesday night
into Thursday and may bring a better threat for a rain/snow mix or
even a period of all snow on New Years Eve potentially into New
Years Day. Stay tuned with this system over the next several days.

After a decent day Sunday with highs in the mid and upper 40s...
daytime temperatures will fall back into the 30s Monday and Tuesday
before a brief warmup ahead of the midweek system Wednesday. Colder
air then returns for the last day of 2020 and the first day of
2021.

&&

.AVIATION /Discussion for the 25/03Z TAF Update/...
Issued at 951 PM EST Thu Dec 24 2020

Adjustments made to IND/LAF to bring more in line with recent obs
with minor visibility restrictions due to flurries/snow showers.
Previous discussion follows.

Issued at 620 PM EST Thu Dec 24 2020

*IMPACTS: MVFR ceilings likely to continue much of the first 15-20
 hours of the period, although some time in low end VFR is
 possible. Northwest winds 10-15KT much of the period.

*DISCUSSION: A cold northwest wind of 10-15KT is expected much of
 the period, with widespread ceilings over the area. MVFR is
 likely to be the dominant category, although some periods of low
 end VFR will be possible, particularly at LAF/HUF. These impacts
 will be felt by both traditional aircraft as well as airborne
 sleighs. Expect widespread VFR conditions to gradually return
 during the day tomorrow, in the 17-20Z time frame from west to
 east.

*CONFIDENCE: Moderate-high confidence in MVFR conditions with low-
 moderate confidence in periods of VFR and exact timing. High
 confidence in continued northwest winds 10-15KT.

&&

.IND WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SYNOPSIS...AGM
NEAR TERM...AGM/Nield
SHORT TERM...AGM
LONG TERM...Ryan
AVIATION...Nield