Palmer, Indiana Weather Forecast Discussion

187
FXUS63 KLOT 222326
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Mon Sep 22 2025
.KEY MESSAGES...
- While there will be many dry hours, the threat of periodic
  showers and thunderstorms will continue through Thursday.
&&
.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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 626 PM CDT Mon Sep 22 2025
The main aviation concerns are as follows:
- There is a growing threat for convection at RFD and at least
  in the western half of the C90 airspace generally between
  03-09Z tonight.
- Winds overnight may have variable directions in the wake of
  any convection before settling on southwesterly by daybreak
  Tuesday.
- Winds will shift from southwesterly to northeasterly tomorrow
  afternoon behind a hybrid lake breeze-cold front.
Discussion:
Recent radar and satellite imagery depicts continued
thunderstorm development from northern Iowa into southwestern
Wisconsin within a west-to-east oriented instability axis and
ahead of a southward-moving mid-level impulse. While the
current trajectory of convection may favor RFD to DPA for being
impacted, increasing low-level confluence in southeastern
Wisconsin may allow for additional thunderstorms to develop this
evening east of the ongoing thunderstorms. For these reasons, will
go ahead and introduce thunder at all terminals with the
highest probability (50%) at RFD. Thunder should approach RFD by
03Z, and then DPA/ORD/MDW/GYY sometime in the 04 to 06Z time
window. Any storm may bring visibility of 1-2 miles and a
northwesterly wind shift, and last 1-2 hours at any given
terminal. AMDs will be issued as trends evolve.
Winds behind convection may "go around the clock" from northwest
to northeast and southeast, before settling on southwest by
daybreak. Speeds overnight should be at or below 5kt as the wind
direction veers. By mid to late morning, a hybrid lake
breeze/cold front will begin to move inland, reaching
GYY/ORD/MDW by 18 and DPA by 20Z. Winds at RFD may simply veer
northeasterly with time. Increasing low-level confluence ahead
of the southwestward-moving front may encourage a few showers
and storms to develop by mid-afternoon, though the current
expectation is for coverage to be highest southwest of the
terminals. MVFR cigs may fill in behind the front toward or
after the end of the TAF period.
Borchardt
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion