990
FXUS63 KLOT 080727
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
227 AM CDT Tue Jul 8 2025

.KEY MESSAGES...

- There is a medium chance (30 to 50%) for scattered showers
  and thunderstorms this afternoon, with perhaps highest
  coverage near and south of Interstate 80

- There is a low chance (20 to 30%) for isolated thunderstorms
  on Wednesday, primarily near the Lake Breeze.

- Showers and thunderstorms are likely Friday into Saturday (50
  to 70% chance), though there is still some timing
  uncertainty.

- High and low temperatures through the end of the week will be
  seasonable and in the mid to upper 80s and low to mid 60s,
  respectively.

&&

.DISCUSSION...
Issued at 226 AM CDT Tue Jul 8 2025

Through Wednesday:

Early morning radar and satellite imagery depict a gradually
decaying MCS tracking east-southeastward along the IA/MO border.
An MCV is evident on the northern side of the MCS, which is
similarly tracking east-southeastward toward west-central IL.
Meanwhile, isolated showers and even a few thunderstorms
continue to percolate within a broad zone stretching from near
Davenport, IA to Milwaukee, WI. Evidently, a modest
northeastward-pointing 15-20kt 925 to 850mb low- level jet has
provided just enough moisture advection to activate an ACCAS
plume based at about 6500ft. Thankfully, the environment across
northern IL is pretty darn hostile anything beyond weak
convection this morning, per paltry mid-level lapse rates less
than 6 K/km and a pocket of dry air above 500mb sampled by the
00Z DVN RAOB. Elsewhere, it`s a fairly quiet morning across our
area with partly cloudy skies and "cool" temperatures in the low
to mid 60s.

Toward sunrise, the aforementioned low-level jet should begin
to wane. As a result, would expect any remaining showers across
northwestern IL to dissipate. Interestingly, most CAM guidance
paint a different picture with renewed shower and even storm
development west of I-39 by 10AM. Such development appears tied
to another, but simulated, MCV emanating out of north-central
IA. At this point, don`t really see any signs of another MCV
north of the aforementioned circulation in southern Iowa in
radar, satellite, or observational data. However, the presence
of ACCAS west of the Mississippi River does provide enough
justification to maintain 20 to 30% chances for showers through
about noon, primarily west of I- 39.

This afternoon, rising boundary layer temperatures and dew
points will render the atmosphere uncapped by early afternoon.
Accordingly, will advertise scattered thunderstorms in our
forecast for this afternoon, first developing west of I-39 just
after noon and then spreading eastward toward Lake Michigan and
northwestern Indiana by late afternoon. Even with convective
temperatures being met, coverage of storms remains an item of
lower than average forecast confidence given nebulous large-
scale forcing. With that said, coverage may end up highest south
of I-80 in relative close proximity to the remnant MCV moving
through central IL. Will cap PoPs at 50% for now and let the day
shift make adjustments upward or downward based on morning
observations.

Where storms to occur today, 1500-1750J/kg of MLCAPE,
500-700J/kg of DCAPE, PWATs near 1.75", and 20-25 kt of deep-
layer shear should relegate cell behavior to the "pulse" variety
with loosly-organized upscale growth into eastward-propgating
clusters behind surging outflow boundaries. Such a regime should
support locally gusty winds (45 to 55 mph) and torrential
downpours (1-2"/hr rainfall rates). Will touch base with SPC
later this morning to see if a northward extension of the Level
1/5 threat area currently positioned to our south is needed to
cover an isolated threat for downed tree limbs. Outside of
thunderstorms, highs today should range from the low to upper
80s.

Coverage of showers and storms should wane after sunset, though
would not be surprised to see a few cells fester near the
Wisconsin State line ahead of a very weak frontal zone through
about midnight. With a surface high pressure directly overhead
to promote nearly calm winds and pockets of wet soil, can
envision pockets of shallow ground fog developing during the
early morning hours of Wednesday. Overnight lows should fall to
the mid to upper 60s.

On Wednesday, residual low-level moisture and the warming
boundary layer may allow for an isolated shower or storm to
develop by early afternoon. However, coverage may be quite
limited owing to increasing mid-level subsidence. Like prior
shifts, am noting a signal for a stronger than typical lake
breeze given a building high pressure system over the northern
Great Lakes. At this point, it appears winds and waves may fall
under the level for concern for rip currents, but this will be
something to monitor closely. Highs will range from the low to
mid 80s inland, but only the mid to upper 70s lakeside.

Borchardt


Wednesday Night through Monday:

Primary forecast concern and challenge is thunderstorm potential
Friday into Saturday.

Weak high pressure is expected to move across the western Great
Lakes Wednesday night and shift east of the area on Thursday.
This should allow for mainly dry weather through Thursday
evening though can`t rule out an isolated shower or storm,
especially across central IL. A lake breeze is expected Thursday
afternoon with temps a few degrees cooler, with mid 80s
expected inland.

Confidence is slowly increasing for showers and thunderstorms
for the end of the week into the first part of the weekend. An
upper trough will be moving across the northern Plains and upper
midwest while a wave ejects out of the central Plains. Models
develop a surface low over the Lakes region Friday night into
Saturday with a cold front moving across the region. Quite a bit
of support among the ensembles during this time period as well.
However, there still remains uncertainty for timing and the
location/track of a surface low. Blended pops have increased a
bit with some likely pops Friday evening for the western cwa.
Blended pops also continue Saturday night into Sunday, which may
end up being too long but no changes for now. If storms were to
be moving across the area Friday afternoon/evening with peak
heating, there could be the potential for strong/severe storms.
Precipitable water values will be in the 2 inch range during
this time, with heavy rain and localized flooding also possible.

If precip trends/frontal boundary noted above pan out, temps
could trend back toward normal values for the weekend, lower to
mid 80s for highs, 60s for lows. Trends then support warmer
temps going into early next week, with highs perhaps back into
the lower 90s. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1231 AM CDT Tue Jul 8 2025

Forecast concerns include...

Chance of thunderstorms this afternoon.
Low chance for showers through mid morning.
Potential for fog early Wednesday morning.

Current isolated showers across northwest IL are expected to
dissipate over the next few hours before reaching the Chicago
terminals, though an isolated shower will remain possible. A few
showers across southeast MN/western WI may continue to fester
through the overnight hours, bringing a chance for showers to
RFD after daybreak. If these were to materialize, they may
persist north of the terminals through the mid/late morning
hours.

There remains a chance of thunderstorms for all of the terminals
this afternoon, though coverage remains uncertain and maintained
prob mention for now. A large complex of thunderstorms is moving
southeast across southwest IA into northern MO. How these storms
evolve over the next 12 hours, will impact storm trends locally
this afternoon. The most favored areas appear to be west central
IL with activity possibly dissipating further to the northeast.
Overall, low confidence for trends, which will need to be
monitored for possible later forecast changes. After this
potential precipitation, the rest of this evening into early
Wednesday morning is looking mainly dry.

Winds are light and variable and will become light southerly by
daybreak then become southwest by mid morning with speeds
increasing into the 10-12kt range. The forecast soundings
suggest there may be a period in the late morning, when gusts
into the 15-20kt range will be possible. Wind directions this
evening are uncertain. If there are storms, winds may favor a
light southeast direction.

Forecast soundings also show hardly any wind aloft early
Wednesday morning as weak frontal boundary sags into northern
IL. This would easily support fog development. However, fog may
be dependent on how widespread any precipitation becomes and the
best location for fog appears to be northwest IL, which would
be after the current 24 hour RFD taf period. But some fog
mention may be needed with later forecasts. cms

&&

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

&&

$$

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