Merrillville, Indiana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

532
FXUS63 KLOT 170527
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1127 PM CST Tue Dec 16 2025

.KEY MESSAGES...

- Mild temperatures through Thursday should melt much of the
  remaining snow cover.

- Rain showers and drizzle will develop late Wednesday night
  with a period of steady rain ahead with a strong cold front
  on Thursday.

- Blustery and briefly colder conditions Thursday night into
  Friday will be followed by variable but generally above normal
  temperatures into next week.

&&

.DISCUSSION...
Issued at 254 PM CST Tue Dec 16 2025

Through Wednesday:

Temperatures overperformed today (upper 30s to around 40F over
the heart of the Chicago metro as of this writing), which will
lead into a milder night tonight, especially relative to recent
nights. A strengthening low-level jet will result in steady to
slowly rising temperatures after any evening dip with southerly
winds occasionally gusting to 20 to 30 mph. A shortwave will
push across the region (also coinciding with the low-level jet
response) this evening and overnight, and still think the sub-
cloud layer will be too dry to support any spotty light precip
being able to reach the surface.

Looking ahead to Wednesday, mostly cloudy skies (mid-high
clouds) in the morning should give way to decreasing afternoon
cloudiness in the wake of a weak cold frontal passage. Not
seeing any low clouds of note upstream that would anticipate to
play havoc with temps, so there would need to be post-frontal
stratus development. The model guidance exhibited a cool bias
today even with extensive snow cover going into the afternoon.
Suspect that will be the case tomorrow as well, barring low
clouds limiting warming. Bumped up temps to primarily upper 30s
and lower 40s as a middle ground for now. Considering that the
MOS guidance is warmer than these values, wouldn`t be surprised
if the core of the Chicago metro in particular outperforms our
forecast highs, if post-frontal stratus doesn`t materialize.

Castro

Wednesday Night through Tuesday:

There are no big changes in forecast thinking from Wednesday
night onward.

A deep mid-level trough currently approaching the Pacific
Northwest will track eastward to the western Great Lakes
Wednesday night into Thursday night. A 980 hPa low over western
North Dakota Wednesday evening is progged to gradually weaken
while crossing Lake Superior on Thursday. An elongated 50+ knot
LLJ will spread across the area late Wednesday evening into
Thursday morning, advecting a narrow ribbon of higher low-level
moisture into northern Illinois overnight. Robust low-level
isentropic ascent will promote quick saturation and the
development of drizzle or light rain across much of the area by
sunrise Thursday. While air temps should be several degrees
above freezing before drizzle develops, cannot rule out some
patches of ice on antecedent sub-freezing surfaces. However, air
temps at or above freezing with some sunshine on Wednesday
should limit much of this concern.

Deep moisture aligned with strong height falls and broad mid-
level diffluence will result in a line/band of rain along an
incoming cold front late Thursday morning into early afternoon.
Very strong ascent and marginal lapse rates throughout much of
the column do suggest a non-zero thunderstorm chance across as
least the far southeast CWA. If anything else we`ll have to
watch for a "pop" of winds with or just behind the strongly
forced band of rain.

Behind the front, temps will begin falling during the
afternoon, with stronger CAA lagging several hours until the
main core of colder air arrives early-to-mid evening. Decent
drying should end the precip behind the front, so it`s likely
that precip quickly ends as rain with the frontal passage. Later
in the evening, steepening low-level lapse rates with the
passing core of cold air combined with marginal moisture depth
will likely result in scattered gusty light snow showers or
flurries for several hours (20-40% PoPs northeast 1/2 to 2/3 of
the CWA). Otherwise, expect blustery conditions with post-
frontal W/WNW winds gusting over 30 mph to perhaps briefly up to
40 mph through the evening.

A weakening mid-level ridge and associated surface ridge will
cross the forecast area on Friday. A low-amplitude wave will
then cross the Boundary Waters of Minnesota on Saturday while
keeping any appreciable forcing well north of the area. Mostly
sunny skies and developing WAA on Friday should allow temps to
rise into the mid to upper 20s. However, temps may continue to
rise through the evening as modest WAA persists amid weak low-
level stability and a 50 knot LLJ spreading over the area. There
is a very low chance (10%) that mid-level snow will saturate to
the surface as far south as the Wisconsin state line during the
evening. Similarly, a narrow ribbon of very light rain may
proceed a passing cold front Saturday afternoon. However, dry
conditions are expected through this period. High pressure will
then cross the region on Sunday, with signals that a mid- level
disturbance embedded in strong WNW flow aloft could bring a
brief period of light snow Sunday night into Monday.

Following the seasonably chilly Friday and Friday evening,
temperatures will quickly moderate into the 40s on Saturday
ahead of the next cold front. There will be another brief, less
pronounced cool-down Sunday-Sunday night, with moderating
temperatures heading into Christmas Week. Well above normal
temperatures look quite possible for Christmas Eve and Christmas
Day, so prospects for a white Christmas are low despite the fast
start to winter that we experienced.

Kluber/Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1126 PM CST Tue Dec 16 2025

Low-level southwesterly flow will gradually weaken in magnitude
as directions turn northwesterly through the pre-dawn hours of
Wednesday as a weak cold front moves through the area. Recent
satellite imagery depicts a solid bank of MVFR stratus (based
just above 1000ft) racing southward across Wisconsin behind the
front. With the southern edge of the stratus deck struggling to
maintain intregity, confidence in whether the terminals will
actually experience stratus is somewhat low (30 to 50% chance).
All things considered, will go ahead and advertise a few hours
of MVFR in TEMPO groups, timed at around daybreak in line with
RAP guidance.

After daybreak, any remaining clouds should clear. As a surface
pressure ridge slides over the terminals, winds will back from
northwesterly to westerly, southwesterly, southerly, and
eventually southerly. With wind magnitudes remaining fairly
light (near 5kt or so), opted to forgo new lines for every 30
degree direction change and simply add a line at 23Z for when
the direction should settle on southeasterly.

Toward the end of the TAF period (and in the latter 6 hours of
the 30-hour TAF windows at ORD/MDW), intensifying warm-air and
low-level moisture advection in advance of an approaching storm
system will facilitate a rapid development and downward
expansion of low-level stratus, with medium to high (50 to 70%
chances) for high-end IFR to low-end MVFR conditions. With time,
drizzle and showers should develop as well, leading to drops in
visibility (3 to 6 miles). Will go ahead and introduce IFR to
MVFR aviation conditions at ORD/MDW, and allow for the same for
the remaining terminals in the upcoming 12Z set of TAFs.
Finally, the tightening low-level pressure gradient should
facilitate a transition back to southwesterly winds with gusts
increasing to the 20 to 25kt range through the overnight hours.
With the expectation that gusts occur, will hold off on
introducing LLWS in spite of strong low-level flow (50 to 60kt
at 2kft).

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CST Wednesday for the IL and
     IN nearshore waters.

&&

$$

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