269
FXUS63 KIND 100521
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1221 AM EST Wed Dec 10 2025

.KEY MESSAGES...

- Rain moves in tonight, with patchy rain changing to snow on
  Wednesday

- Wind gusts of 30 to 40 mph tonight into Wednesday

- Accumulating snow possible Thursday night into Friday and again
  Saturday

- Arctic outbreak expected this weekend with low temperatures down
  to near zero and potentially dangerous sub-zero wind chills

&&

.FORECAST UPDATE...
Issued at 950 PM EST Tue Dec 9 2025

Precip has been quick to develop this evening thanks to saturation
and moderate to heavy virga occurring from the WAA mid cloud deck.
Thus pops have been readjusted back south to I-70 for the remainder
of the evening.

Wind gusts have also been increasing partly due to an increasing
pressure gradient but more importantly due to evaporational
downdrafts into the remaining dry sub-cloud layer and increasing low
level jet to near 50kts. Therefore expect some gusts to occasionally
reach 40-45 mph overnight, just below wind advisory criteria. An SPS
has been reissued to account for these strong wind gusts.

&&

.SHORT TERM (This evening through Wednesday)...
Issued at 257 PM EST Tue Dec 9 2025

Rest of This Afternoon...

Mid and high clouds will continue to increase across central Indiana
during the remainder of the afternoon. The decent pressure gradient
along with mixing down of stronger winds aloft will keep breezy
conditions around.

Tonight...

An upper trough will approach the area from the northwest, while a
surface low moves into lower Michigan. A strong low level jet will
move into the area as well.

The system will bring broad forcing, including isentropic lift, to
the area. Moisture will accompany the low level jet. The forcing and
moisture will be enough to bring some precipitation to central
Indiana tonight. Will go likely or higher PoPs all areas at some
point tonight.

Warm advection on the southwesterly winds will keep temperatures
warm tonight, so precipitation will be all rain. Current timing of
the rain is that much of the evening should be dry for most of
central Indiana, with the bulk of the rain falling overnight.

Strong winds will not be far off the surface, with 50-60kt about
2000ft off the surface at times. However, an inversion will keep the
worst of the winds off the surface. Gusts in the 30 to 40 mph range
are possible at the surface.

Wednesday...

The bulk of the broad lift will be east of the area by 12Z
Wednesday, so will keep most areas below likely category PoPs. The
main cold front will move through in the first half of the day. This
front, cyclonic flow, and cold advection will keep some scattered
precipitation around for much of the area during the day.

Cold air will move in aloft early, then as the boundary layer cools
thanks to the cold advection, rain will change over to snow. Some
lake enhanced snow showers may get into the northern forecast area
as well in the afternoon. Maybe a tenth of an inch of snow might
fall under a heavier snow shower, but would expect most areas not to
see any accumulation.

Winds will continue to be gusty with the tight pressure gradient and
cold advection promoting mixing down of stronger winds aloft. Wind
gusts around 40 mph are likely, especially north.

Temperatures in the upper 30s to lower 40s to start the day will end
in the 30 to 35 degree range by the end of the day.

&&

.LONG TERM (Wednesday night through Tuesday)...
Issued at 257 PM EST Tue Dec 9 2025

Northwesterly flow aloft is expected to remain in place over the
CONUS this week into the weekend. A NW to SE oriented baroclinic
zone stretches from eastern Montana to Michigan, which has served as
the active storm track recently. A strong surface low is currently
dropping southeastward out of the Dakotas, with strong warm air
advection ahead of it. Temperatures have warmed into the low 40s
across much of central and southern Indiana.

This low, expected to deepen to about 985mb, will pass over or just
north of Chicago tonight. As such, our forecast period begins with
strong cold air advection in the system`s post-frontal environment.
Model soundings indicate modest low-level instability, especially
Wednesday evening into Wednesday night. Low-level instability,
combined with a secondary shortwave aloft, should be sufficient for
numerous flurries and snow showers during this timeframe.
Accumulations, should they occur, will be light. Even the most
aggressive guidance is generally under an inch with most showing a
dusting to half of an inch.

ACCUMULATING SNOW POTENTIAL

Cold air advection continues through Thursday as surface high
pressure passes closely to our south. Winds should largely diminish
by this point as well, gradually becoming westerly and then
southwesterly ahead of an approaching low. Like the first system,
this one too is riding the baroclinic zone southeastward. However,
by this point the baroclinic zone will have shifted southward
extending over southern Indiana. Guidance has yet to come to a
consensus but has trended towards a period of light snowfall
Thursday night into Friday morning. This system looks to be a
classic clipper with most of the precipitation on its northern
flank. Since the low is riding the baroclinic zone, most of the
precipitation would fall in the form of snow. Given the relatively
weak strength of the system, a swath of 1-3 inches appears the most
likely scenario. The exact low track will be critical for
determining who finds themselves within this swath.

Thursday night`s clipper does not appear strong enough to
significantly alter the orientation of the baroclinic zone, which
likely remains in place as we head into the weekend. Guidance, with
less agreement than the previous system, is hinting at yet another
clipper on Saturday. Another swath of snow is possible, again
dependent on the system`s exact track.

COLD TEMPERATURES

Ensemble guidance is coming into agreement on deep troughing taking
shape following Saturday`s clipper. An associated arctic air mass
plunges southward Saturday night into Sunday as the system departs.
Dangerously cold temperatures are possible, with lows in the single
digits and highs only climbing into the teens. Negative temperatures
are possible as well, but this is contingent on how much of a
snowpack we have in place. The deepest snowpack currently is across
northwestern portions of our CWA. Recent snows have melted
completely south of I-70. Without taking the two possible clippers
into account, the best chance of below-zero lows is across our
northwest. However, should the clippers dump a couple inches of snow
then more of our CWA could see sub-zero lows. Wind chills as low as
negative 15 degrees are possible, especially in scenarios where most
of the CWA gets snow this week.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1221 AM EST Wed Dec 10 2025

Impacts:

- Gusty winds 25 to 30kt tonight through tomorrow afternoon

- LLWS AOB 015 thanks to 40-50 kt Low level jet through tonight.

- MVFR ceilings developing tonight and persisting into Wednesday.

- Rain tonight, then mix of rain and snow midday Wednesday changing
  to light snow with MVFR vsbys during Wednesday afternoon.

 Discussion:

Fast moving clipper system will move through the Great Lakes over
the next 24 hrs. Strengthening low level winds, as high as 50 kts at
1500 ft AGL, will lead to LLWS this evening into the early morning
hours, before the core of the strongest low level winds move off to
the east by 12Z. However, gusty winds from 25-30 kts generated by
both a moderately strong pressure gradient and mixing of winds from
1000 ft AGL to surface will exist through most of the night into the
late afternoon hours.

Although ceilings currently remain fairly high (090-120),
expectation is with the onset of precipitation they will fall fairly
quickly into the MVFR category between 07-12Z from NW-SE. Initially
with the light precip, expectation is for vsbys to remain greater
than 6SM. However with more steady forcing ahead of main cold front,
and transition to a rain/snow mix and then to all snow will lead to
development of MVFR vsbys by tomorrow afternoon.

&&

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

$$

UPDATE...Crosbie
SHORT TERM...50
LONG TERM...Eckhoff
AVIATION...Updike