441
FXUS63 KIND 180531
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1231 AM EST Thu Dec 18 2025

.KEY MESSAGES...

- Widespread rain likely Thursday with QPF amounts generally ranging
  from 0.6 to 1.25 inches.

- Wind gusts between 25-35 mph Thursday and Thursday night,
  slightly higher gusts up to 45 mph cannot be ruled out during
  the afternoon.

- Flash Freeze possible Thursday night/early Friday as temperatures
  could fall 20-30 degrees in 12 hrs.

- Above normal temperatures expected next weekend into Christmas.

&&

.FORECAST UPDATE...
Issued at 946 PM EST Wed Dec 17 2025

Most mid and high clouds exited the area earlier this evening. Even
with winds staying up, temperatures fell decently, especially across
areas hanging on to snow cover. Lowered minimum temperatures a bit
where needed.

Lower clouds have returned to portions of the southern forecast
area, and these will continue to spread north as the low level jet
increases across the area. This will also allow temperatures to
rebound into the 40s.

Tweaked PoPs based on latest short term data, with low chances for
rain arriving late tonight in the west.

&&

.SHORT TERM (This evening through Thursday)...
Issued at 319 PM EST Wed Dec 17 2025

A strong shortwave over the Rockies will emerge into the Plains
this evening with increasing pressure falls/gradient developing over
the region. As a result, strengthening southerly low level flow will
advect north-northeast a large reservoir of higher moisture content
(PWATs in the 90th percentile for mid-Demcember) with surface
dewpoints in the upper 40s to lower 50s F from the lower MS valley
into the Ohio Valley by tomorrow morning.

The low level jet will be pushing 60 kts during the morning hours on
Thursday before some decrease in intensity occurs as the core shifts
ewd during the afternoon. Given the degree of low-mid level
forcing/ascent and moistening, expectation is for a strongly forced
squall line to develop along the cold front during the midday hours
and sweep across the forecast area during the afternoon hours. Most
of the CAMs fit this typical cold-season pattern in the Ohio
Valley of a linear convective mode with embedded thunderstorms.
Despite the strongly forced nature for ascent and aforementioned
moisture, forecast soundings show a paltry amount of instability
(MUCAPE 50-75 j/kg) mainly owing to poor lapse rates through most
of the lowest 6 km. Therefore the expectation is for moderate to
heavy rainshowers with a chance of embedded TS. SPC`s Day 2
convective outlook has dropped the marginal risk in our far south
which appears to be the right call given the marginal instability.
However, with the aforementioned low level jet, isolated wind
gusts may exceed 45 mph in the convective line during the 18-23Z
time period. It is a safe bet that wind gusts from 25-30 mph ahead
of the line will likely be augmented into the 30-40 mph range
with downdrafts in the line.

The forecast for other hazards gets a little more tricky, namely the
expected moderate to heavy rainfall and the degree of hydrologic
impacts. Recent river hydrographs show some uptick in gauge heights
the last 36 hrs before a gradually fall, indicative of melting
ice. There is still some uncertainty in the potential for minor
ice jams, especially on the smaller creeks and streams where ice
thickness was greatest. The saving grace for a river/creek greater
flood threat is the ongoing drought conditions and lower than
normal water levels. Due to warmer than forecasted temperatures
yesterday and today, recent frost depth analysis shows anywhere
from the surface in the far south to 1-2 inches below ground level
in the northern parts of the forecast area. So earlier concerns
for more rapid runoff owing to a frozen ground are waning. That
being said there still is plenty of snowmelt that will occur in
northern portions of the forecast area and with QPF ranging from
0.6 to over 1 inch area wide in a short 3-6 hr window. Therefore
there is likely to be some minor ponding of water on area
roadways at the very least.

Temperatures will fall quickly to 32F between 03Z and 06Z Friday
then down to between 16-22F by 12Z Friday. With the amount of QPF
expected the potential for a flash freeze exists late Thursday
night into Friday morning. Even with wind gusts from 20-30 mph
aiding in some evaporation of area roadways, the combination of
the amount of residual water/ponding and ground temperatures still
relatively cold, it won`t take very long for road/pavement temps
to fall below 32F. In addition, forecast soundings do show some
light snowfall likely in the post frontal cyclonic flow regime,
aided by residual mid level height falls/forcing early Friday
morning which may add in some minor snow accumulation`s.

&&

.LONG TERM (Thursday night through Wednesday)...
Issued at 319 PM EST Wed Dec 17 2025

Central Indiana has been in the deep freezer for much of December,
averaging nearly 11 degrees below normal for the month so far.
However the weather pattern going into the week of Christmas is
looking quite different featuring above normal temperatures and a
few chances for rain instead of snow.

While the period begins with a deep, yet progressive, trough over
the Great Lakes region, the brief arctic intrusion should only last
24 hours as the northern branch of the jet stream retreats well to
the north this weekend into the following week. Looking at the
current teleconnections, a positive arctic oscillation combined with
a positive north atlantic oscillation and a negative PNA support a
progressive weather pattern with ridging over the Central and
Eastern CONUS and arctic air locked up north. Longer range models
and ensembles show this type of pattern setting up for the Holiday
week with the main storm track along the Canadian border with much
weaker, faster moving systems impacting the region. There are some
signs the southern branch of the jet becomes active mid next week,
which could introduce both warmer air and rain chances to the
extended forecast.

Friday through this weekend...

Arctic air briefly engulfs the Great Lakes region on Friday as a
strong sub-520dm trough moves in overhead. Temperatures at 850mb
plummet from +7C on Thursday toward -20C by Friday morning, showing
just how strong and deep this arctic airmass is. A strong low level
pressure gradient and LLJ behind the front will keep the low levels
mixed under a low/mid level inversion, resulting in windy and cold
conditions during the day with wind chill values in the single
digits/teens and highs struggling to get out of the 20s.

Fortunately, the very progressing weather pattern setting up does
not favor arctic airmasses sticking around for long. As the jet
stream retreats northward, upper heights and temperatures through
the column rapidly increase by Saturday ahead of the next
approaching weak system to the north. The roller coaster ride of
temperatures is expected in this type of pattern with each day
potentially featuring very different conditions.

Temperatures rebounding into the 40s on Saturday may quickly be
replaced by highs in the 30s on Sunday behind the cold front.
Strength and timing of the system remains uncertain, as well as
magnitude of moisture return ahead of it. Right now, guidance
generally shows little in the way of precipitation. Some gusty winds
are possible if the low ends up on the stronger side of possible
scenarios.

Next week...

Going into the week of Christmas, confidence decreases regarding the
exact evolution of low level and mesoscale features. However the
synoptic pattern and teleconnections support well above average
temperatures for this time of year along with the the potential for
light rain chances. Keeping highs in the 40s to near 50 for the
first half of next week with slight chances for rainfall. Timing of
features and placement of precipitation will be fine tuned in the
coming days as confidence increases in smaller details.

&&

.AVIATION (06Z TAF Issuance)...
Issued at 1230 AM EST Thu Dec 18 2025

Impacts:

- MVFR ceilings developing after 06Z, IFR ceilings developing aft 15Z

- Low-level wind shear increasing towards 12Z

- Rain arrives Thursday morning, becoming moderate to heavy during
  afternoon.

- Gusty surface wind developing towards 12Z, strongest (40 kt) with
  the band of convective rain showers from 18-21Z.


Discussion:

A potent system will bring a variety of impacts to aviation through
the period.

A low level jet will bring non-convective LLWS later this evening
and will continue into Thursday morning. Gusty surface winds will
develop Thursday, with the most intense gusts during the afternoon
in showers along a cold front.

Clouds will gradually thicken and lower, reaching IFR at times on
Thursday. There will be some brief improvement late in the period
with a dry slot in the system.

Rain will become widespread Thursday. Embedded thunder is possible,
but expected coverage will be too low to mention. Snow showers are
likely behind the cold front after about 04z Friday, persisting
through the end of the TAF period.

&&

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

$$

UPDATE...50
SHORT TERM...Crosbie
LONG TERM...CM
AVIATION...50/Eckhoff