447
FXUS63 KLOT 081553
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1053 AM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Isolated to scattered thunderstorms this afternoon into the
  evening, with highest coverage favored 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.

&&

.UPDATE...
Issued at 1053 AM CDT Tue Jul 8 2025

Festering convection west of the I-39 corridor early this
morning encountered a more hostile environment with eastward
extent (weak instability and weak mid-level lapse rates). This
resulted in an outflow boundary moving out ahead of the
convection off to the east and southeast with mainly just
isolated to scattered showers and the occasional rogue in cloud
lightning flash with the activity behind the outflow. As the
outflow races farther ahead, new updrafts will likely have a
tougher time developing into showers toward the I-55 corridor
through about noon.

The key feature of note driving this elevated warm advection
decaying convection this morning that we`re also expecting to
drive this afternoon`s scattered thunderstorm threat is an MCV
crossing into far western IL evident on visible satellite and
automated observations. With northward extent, the festering
generally weak convection already present and cooled/stabilized
air behind outflow (ie. RFD 73 deg of this writing) call into
question the extent of deep renewed convection this afternoon.

For locales near and south of I-80, there appears to be a better
overlap of less "mucking up" from existing convection, deeper
moisture sloshing back north, and the MCV trajectory generally
to the south of I-80 to favor higher thunderstorm coverage this
afternoon. In the gridded forecast, we`re highlighting this
(I-80 and south) area for scattered showers and thunderstorms,
with isolated to widely scattered showers and perhaps isolated
thunderstorms north of I-80. Despite the moist adiabatic lapse
rate and weak deep layer shear environment, up to or more than
1500 J/kg of uncapped MLCAPE, and perhaps slightly enhanced
0-3km shear from the MCV may contribute to isolated strong to
marginally severe storms capable of localized downbursts (50-60
mph gusts). Swath of seasonably high 1.5 to 1.7" PWATs also
sloshing eastward may support localized corridors where
torrential rainfall rates could cause some flooding.

The primary threat timing window for scattered gusty storms,
localized torrential rainfall, and occasional to frequent cloud
to ground lightning strikes is roughly in the 2-9pm CDT
timeframe, again favoring areas near and south of I-80. To the
north, with ongoing weak convection and perhaps widely scattered
showers this afternoon (~30% coverage but only isolated/20%
chance of storms), the main threat will be occasional localized
downpours and isolated sporadic lightning, winding down in the
~6-8pm timeframe.

Castro

&&

.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 /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 640 AM CDT Tue Jul 8 2025

Forecast concerns include...

Chance of thunderstorms this morning over northwest IL.
Chance of thunderstorms this afternoon into early this evening.
Potential for fog early Wednesday morning.

Scattered showers and isolated thunderstorms are developing
across far eastern IA and northwest IL currently. This activity
is likely to continue through mid morning and remain north of
the Chicago terminals, but confidence is low. For now, have
included tempo thunder for a few hours at RFD and trends will
need to be monitored.

There remains a chance of thunderstorms this afternoon into
early this evening with uncertainty for coverage. Latest model
trends support the best chances and higher coverage mainly
southwest of the terminals and prob mention still seems
reasonable for all of the terminals. There may be some lingering
showers through the evening, but mainly dry conditions are
expected tonight into Wednesday morning with isolated showers or
isolated thunderstorms possible by early Wednesday afternoon.

Light southerly winds to start will turn southwest this morning
and increase into the 10-12kt range with some higher gusts
possible through early afternoon. Speeds will diminish with
sunset and turn west/southwest but if precip does become more
widespread, a period of light southeast winds is possible this
evening. Winds may turn more west/northwest by Wednesday morning
as a weak frontal boundary moves into northern IL. Light winds
both at the surface and in the low levels, along with this
front, may lead to some fog development, but confidence is low
and no mention with this forecast. cms

&&

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

&&

$$

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