Gary, Indiana Weather Forecast Discussion

335
FXUS63 KLOT 230536
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 AM CDT Tue Sep 23 2025
.KEY MESSAGES...
- While there will be many dry hours, the threat of periodic
  showers and thunderstorms will continue through Thursday.
&&
.UPDATE...
Issued at 923 PM CDT Mon Sep 22 2025
This evening, a front is draped across north-central and
northeast IA and up across far southern WI. Along the front is a
line of thunderstorms that has recently filled in quite nicely
across the eastern IA, but coverage remains much more broken
across southern WI as storms push southeast. Part of the reason
for the gap is the front has a much stronger presence out in IA
with a tighter density gradient and greater convergence around
the boundary than farther east. Effective shear, while marginal,
is also maximized over NE IA and model soundings depict a good
deal of mid-upper dry air with eastward extent.
The greatest instability and slightly better, but still very
marginal, shear will remain just to our west tonight.
Accordingly, most recent CAM guidance resolves the greatest
storm coverage through the night across Iowa and northwest
Illinois. With all of that said, the RAP maintains some decent
low level upglide across our area which may help parcels reach
their LFCs amid the meager forcing and better parameters to our
west. In all, adjusted the forecast some to reflect this
thinking and pull some low chance PoPs farther south along the
I-39 corridor given that recent HRRR solution. The greatest
thunder chances certainly appear to be out west near I-39.
Anything too organized or severe is looking like a real stretch,
but certainly can`t rule out a deeper core or two capable of
frequent lightning, a brief downpour, and locally gusty winds.
Additionally, patchy fog still looks likely around portions of
our CWA tonight and early tomorrow. Guidance resolves the
highest fog probs over areas south and east of I-55, as well as
up near the WI state line, mainly away from the lake. Patchy fog
is in the forecast for these areas for late tonight into
tomorrow morning. Dense fog doesn`t look like a widespread
concern, aside from maybe in our far southeast CWA, but could be
found in isolated instances elsewhere.
Doom
&&
.DISCUSSION...
Issued at 226 PM CDT Mon Sep 22 2025
Through Tuesday Night:
Main forecast focus is on timing of scattered shower and
thunderstorm chances, mainly this evening/early overnight and
again Tuesday afternoon. Temperatures are a secondary concern,
as a weak cold front sags across the area on Tuesday and turns
winds onshore off of Lake Michigan.
Isolated, weak showers continue to fester roughly north of the
I-88 corridor at mid-afternoon, within a weak low-level
confluent zone south of a slow-moving outflow boundary along the
IL/WI border. SPC/RAP mesoanalysis and RAP forecast soundings
depict modest diurnal instability developing along the
aforementioned outflow boundary and across our western counties,
with conditional MLCAPEs 800-1200 J/kg or so. Soundings
continue to indicate unimpressive lapse rates in the 900-750 mb
layer likely providing some capping at this time, though the
combination of a weak mid-level short wave tracking across the
area and weak but persistent low-level convergence along the
outflow boundary is expected to eventually result in some uptick
of isolated to widely-scattered showers and thunderstorms
along/north of I-88 into this evening. Farther south, also
noting bubbling diurnal cu southeast of the I-55 corridor which
may also lead to a few lower-coverage showers or storms late
this afternoon/early this evening. Effective deep layer shear
(near 25 kts) is strongest north of I-88, and while organized
strong/severe storms are not expected, can`t rule out some
isolated gusty winds with any deeper cores which develop there,
as well as brief heavy downpours. At least an isolated thunder
threat will likely persist through midnight or shortly after.
Guidance continues to hint at patchy fog development toward
morning across our southern cwa, mainly along/southeast of I-55
where cloud cover may thin out a bit more than in areas farther
to the north. Confidence is low on just how extensive this will
become, dependent on cloud cover, but the potential does exist
for some patchy dense fog by sunrise if clearing were to be more
substantial.
Deeper mid-level short wave across the upper Mississippi Valley
and northern Great Lakes is progged to shift slowly eastward
Tuesday, allowing a nearly stationary surface front across
southern MN/WI LM and MI to sag south as a cold front. Current
high-res guidance suggests this will move across northern IL
late-morning/midday, eventually settling south of the I-80
corridor during the later afternoon and evening hours. Winds are
expected to turn northeast off of Lake Michigan behind the
front, which will help to create a gradient in temperatures
across the area Tuesday. Highs are expected to range from the
mid-70s along the lake to the upper 70s farther inland across
northern IL, with low-80s from roughly I-80 southward.
In addition to the impact on temperatures, the frontal zone is
also expected to be a focus for additional shower and
thunderstorm development Tuesday, especially during the midday
and afternoon hours and peak diurnal heating. Blended NBM
guidance 35-45 percent pops appear reasonable at this time, with
the front providing better low-level focus for forcing than
we`ve had today. Forecast soundings indicate MLCAPE near 1500
J/kg and deep layer bulk shear around 25 kts Tuesday afternoon,
again supporting the potential for a few stronger storms with
gusty winds and brief downpours. Better shower/storm focus
should shift south of I-80 Tuesday night.
Ratzer
Wednesday through Monday:
A cold front is expected to be traversing across northern IL
and northwest IN Wednesday morning as an upper-low dives into
Lower MI and begins to phase with a shortwave trough ejecting
out of KS/MO. The broad forcing generated by these features in
combination with ample moisture will allow scattered showers and
thunderstorms to persist through the day on Wednesday with the
highest coverage (40- 50%) focused south of I-80. While the
highest rain chances will gradually shift southward with the
cold front Wednesday night, the cooler air associated with the
upper-low over the Great Lakes does look to develop some modest
lake induced instability. Thus allowing some lake effect showers
(and perhaps a storm or two) to linger into Thursday along the
northeast IL and northwest IN shores before the low pivots east
of the area Thursday night.
Temperatures on Wednesday and Thursday are forecast to be the
coolest of the week due to the expected cloud cover and rain.
Therefore, expect highs during this period to top out in the low
to mid-70s areawide with lows in the lower to mid-50s.
Heading into Friday, upper ridging is forecast to develop back
into the Great Lakes region which will set up another period of
dry and above average temperatures to close out September. While
another cold front is forecast to dive through our area on
Saturday, the cold air reservoir behind the front does not
appear to be sufficient enough to alter temperatures. Thus highs
should remain in the upper 70s to lower 80s through the weekend
with overnight lows in the mid- 50s. However, conditions will
become more favorable for daily lake breezes towards the later
half of the weekend which should keep temperatures cooler (low
to mid-70s) at the lakeshore.
Yack
&&
.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1236 AM CDT Tue Sep 23 2025
Key Aviation Messages for the current TAF period:
- Scattered thunderstorms through 10Z with a brief window of
  breezy northwest winds.
- West-southwest winds switch to the northeast tomorrow
  afternoon
- Chances for showers late in the TAF period, though better
  coverage expected to the south in Central Illinois
- Potential for MVFR cigs developing tomorrow evening
A broken line of thunderstorms in southern Wisconsin is slowly
sinking southward into northern Illinois. Racing out ahead of it
is a gust front that will switch the winds to the northwest with
gusts up to 25 knots. Behind that wind gust will be chances for
thunder to pass by the Chicago terminals. There`s lower
confidence in how far south the thunder develops if the outflow
races out ahead of it, so it was decided to keep only -SHRA
mentioned in the KGYY TAF. Most thunder should have passed
terminals by around 10Z, though there is a chance for lingering
showers to fester through daybreak.
Behind the showers and storms, winds will weaken but gradually
return to the west-southwest. They will increase to around 10
knots through the early afternoon. As a cold front moves south
out of Wisconsin and is reinforced by a lake breeze, winds will
switch around to the northeast but remain around 10 knots.
Stratus will move down behind the front with increasing chances
for MVFR cigs into Tuesday evening.
With northeast winds meeting southwest winds, there will likely
be a convergent axis that develops late tomorrow where new
showers and storms can bloom. Confidence in where that axis
will develop is low, though models are showing decent agreement
in it developing after 00Z and closer to Central Illinois. There
is a chance that showers crop up in the afternoon, but kept the
TAFs dry at this range. With lower confidence in exact location
and timing, MVFR VCSH was added at the end of the TAF, with a
TEMPO at KGYY being closer to the convergent axis.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion