309
FXUS63 KLOT 121715
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1215 PM CDT Sat Jul 12 2025

.KEY MESSAGES...

- Other than a chance of some isolated-scattered showers this
  afternoon, dry weather is expected this weekend.

- After a couple day break Sunday and Monday, humidity returns
  mid-week along with chances of showers and thunderstorms.

&&

.DISCUSSION...
Issued at 242 AM CDT Sat Jul 12 2025

MCS that affected much of northern and western IL Friday evening
is rapidly decaying. Early morning surface analysis shows a weak
low pressure across southern Wisconsin with composite outflow
boundary and perhaps synoptic front extending south across
eastern Illinois, though admittedly the surface pattern over IL
has been left somewhat disheveled in the wake of the MCS. As is
typical with summertime fronts, the cooler and less humid air
mass lags hundreds of miles behind the front, so it will be
another warm and humid day across the area today.

Water vapor imagery shows a compact closed mid-upper level low
tracking east across far southern Manitoba. This upper low is
embedded within a broader upper trough which extends south
across the northern Plains. This upper trough will push east
across the upper Mississippi Valley and into the upper Great
Lakes today. In response, the surface low over southern
Wisconsin will lift quickly northeast into Ontario today.

It seems likely that the strongest forcing and greatest coverage
of convection this afternoon will be east of CWA across Indiana
ahead of the composite outflow/frontal boundary. Air mass across
our area will remain warm and humid in advance of a secondary
cold front that is progged to move across the area this
afternoon. Atmosphere across our CWA this afternoon should
become weakly to moderately unstable (500-1000 J/kg of MLCAPE)
and only weakly capped. Stronger forcing with the upper trough
will remain solidly north of our CWA, with mid-upper level
height rises forecast this afternoon. Convergence along the
secondary front should be weak, but could be enough to support
some isolated to widely scattered showers from mid afternoon
until sunset. Given the weak forcing and drier air aloft
advecting into the area, suspect the lightning threat with this
activity is pretty low, so just going to carry showers in the
forecast, despite the progged weak-moderate instability.

Behind this front, a refreshingly less humid air mass will
filter into the area tonight into Sunday with dewpoints falling
into the upper 50s to lower 60s Sunday. The less humid air mass
will remain in place Monday, though temperatures look to heat up
into the upper 80s/lower 90s Monday.

High pressure will scoot off to the east with southerly flow
transporting the humid, Gulf air mass back northward into the
region Tuesday through Thursday. The main belt of westerlies
should remain to our northwest Tuesday and Wednesday, though
medium range guidance does hint at perhaps a weak southern
stream disturbance meandering northeastward into the region
supporting at least chances of primarily afternoon and evening
scattered showers and thunderstorms Tuesday and especially
Wednesday. By Wednesday night into Thursday, it looks like a
northern stream trough will push eastward across the Great Lakes
dragging a cold front across the area with the threat for more
showers and thunderstorms. Another day or two break from rain
chances and humidity looks possible Friday and perhaps into
Saturday.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1215 PM CDT Sat Jul 12 2025

Key messages for the 18Z TAF period include:

* Periods of MVFR possible early this afternoon

A densely SCT to BKN 2-3 kft deck is moving across the region
early this afternoon. Coverage should trend toward more
scattered moving through the afternoon, but bouts of MVFR cigs
will be possible for the first valid hour or two of the period.
Otherwise, VFR is expected throughout. A brief isolated shower
or storm cannot be ruled out this afternoon and again overnight,
but probs are far too low for a TAF inclusion. Expect
westerlies near or just over 10 kt subsiding to closer to 5 kt
this evening.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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