688
FXUS63 KLOT 072348
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 PM CDT Sat Jun 7 2025

.KEY MESSAGES...

- Showers and a few thunderstorms will develop this afternoon
  and last through the evening. A few localized instances of
  ponding of water may occur where rain is heaviest.

- Isolated to scattered showers will move through the area on
  Sunday and Monday afternoons. A few instances of pea to nickel
  size hail as well as 40 to 50 mph winds may occur with the
  strongest storms.

- A period of dry and warmer weather is expected Tuesday and
  Wednesday. However, there is an emerging signal for wet
  weather to close next week.

&&

.DISCUSSION...
Issued at 300 PM CDT Sat Jun 7 2025

Through Sunday Night:

Recent regional radar and GOES-19 water vapor imagery augmented
by surface observations depict a convectively-enhanced low
pressure system moving across northern Missouri to the south of
a warm front stretching along the I-88 to I-80 corridors. A
secondary, weaker, circulation is present in central Iowa, and a
broader surface low is passing through southern Missouri.
Temperatures locally are in the mid to upper 70s, though low-
level moisture is notably higher along and south of the front
where dew points are in the lower 60s. Areas of haze continue
across the area this afternoon as lingering wildfire smoke
remains trapped in the boundary layer.

While the surface low will pass south this afternoon and
evening, increasing cyclonic convergence along the front will
lead to the gradual development of showers over the next few
hours mainly along and south of Interstate 88. Coverage of
showers should maximize in the 4 to 10 PM timeframe as the
surface low makes its closest approach (maximizing low-level
confluence along the front), with showers then slowly tapering
form west to east overnight. While this afternoon will not be a
washout for most locations, nearly pseudo- moist adiabatic mid-
level lapse rates, slow cell motions (mean flow less than 25kt
through the convective layer), and respectable PWATs (nearing
1.5") will support slow-moving (if not stationary) low- centroid
torrential-rainfall producing cells. These are the types of
scenarios where one neighborhood quickly picks up 2 to 4 inches
of rain while surrounding neighborhoods escape with just a few
tenths. As confirmation, did note radar indicated a rainfall
rate of 2 inches per hour with a stationary cell west of
Lincoln, IL at press time. So, if any of the highly localized
pockets of heavy rain were to occur over an urban area,
instances of flooding would become a concern (around a 5% chance
at any given urban location). A lightning strike or two cannot
be ruled out this afternoon as well, though effective
equilibrium levels near the -20C and low-centroid instability
profiles suggest most cells should just be (very) efficient rain
producers.

Even as the convectively-enhanced surface low pulls away,
showers may fester across our area as the aforementioned weaker
circulation currently over Iowa slides over northern Illinois
overnight. If showers and clouds do clear overnight, do think
fog may develop given the residually moist boundary layer in
spite of the seasonally short nighttime period. Did opt to
introduce patchy fog along and west of I-39 where there are at
least some prospects for clearing by daybreak. Lows will fall
into the lower 60s (upper 50s if/where clouds clear).

Tomorrow, a sharp upper-level shortwave and associated cold
front (currently diving from Saskatchewan into North Dakota)
will race into the Great Lakes. An ensemble of high resolution
model guidance suggests that an narrow ribbon of moisture-driven
and uncapped MLCAPE of 750 to 1250 J/kg will develop ahead of
the front. However, weak convergence along the front as well as
only a glancing blow from the forcing associated with the
shortwave (which will be more directed into Wisconsin than here)
suggest coverage of showers and storms locally may end up
isolated to scattered at best. In terms of the threat for severe
weather, forecast kinematic profiles defined by meager low-
level shear/storm relative inflow but modestly better mid-level
shear suggest relatively small storm sizes with occasional
periods of mid-level rotation. Such a regime is typically
supportive of a few bouts of pea to nickel size hail and gusty
downburst winds of 40 to 50 mph, with perhaps a single storm
briefly reaching severe caliber (roughly a 5% at any given
location). In all, tomorrow should be neither a washout nor too
big of a deal.

Showers may linger across the area after sunset tomorrow as the
upper-level vorticity ribbon associated with the shortwave
drifts southeastward toward northern IL. Easing temperatures
behind the cold front will lead to overnight lows in the mid
50s.

Borchardt


Monday through Saturday:

An expansive closed upper low is forecast to shift toward the
region during the day on Monday. Guidance remains consistent in
developing showers and thunderstorms during the late morning
through early evening period as an impulse rounds the upper low
across northern Illinois. Steepening mid-level lapse rates
along in response to the cooler airmass will support a few
stronger storms capable of gusty winds, especially closer to the
WI/IL stateline during the daytime hours. Non-thunderstorm
winds could also be rather breezy outside of any thunderstorms
as steepening low-level lapse rates begin to tap into a 30-40kt
low-level jet overhead which may lead to a period of gusts over
30 mph in the afternoon. Forecast highs are generally in the low
to mid 70s.

While a stray shower can`t be ruled out on Tuesday, decreasing
moisture in the wake of the departing closed upper low will likely
limit most shower development. Temperatures trend slightly
warmer with highs in the mid-upper 70s. Upper level ridging
then builds back into the region mid week allowing more summer-
like temperatures and dry conditions to return with forecast
highs on Wednesday back in the mid 80s.

Confidence in forecast trends late week decreases markedly due to
model differences in the strength of the upper ridge as well as how
quickly a closed low over the south central Plains lifts toward the
region. The 12Z EPS has trended toward a slightly more
southerly track of a low-amplitude wave that rounds a flatter
upper ridge which could bring a return to shower and storm
chances to southern WI/northern IL as early as Thursday. The
latest blended guidance of 20-30% chances seems appropriate at
this range. While further refinements are expected, at least
periodic shower and thunderstorm chances continue late week
into the upcoming weekend associated with the closed low as it
merges back into the upper level flow.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 647 PM CDT Sat Jun 7 2025

Aviation forecast concerns for the 00Z TAFs:

- SHRA spreading north into the terminals this evening. No TS
  expected.

- Brief period of TSRA possible Sunday afternoon, with SHRA
  potentially lingering into Sunday evening.

An area of rain/showers was developing eastward across the
region early this evening, with embedded convective showers
were moving slowly to the north at 5-10 mph. These showers will
likely affect the RFD area within the next hour or so, and may
lift as far north as the ORD area by mid-late evening. Showers
will persist through late evening before slowly moving off to
the east overnight. While there has not been any TS noted with
this activity so far, there is some deeper instability generally
south of about a KVYS (Peru IL)-KRZL (Rensselaer IN) line which
could support some isolated TS to the south of the terminals
this evening. After the showers exit to the east of the area
overnight, there is some potential for some patchy shallow fog
development mainly along/west of the I-39 corridor where some
clearing may occur prior to sunrise. Have included a mention of
MVFR BR for KRFD for this.

On Sunday, another disturbance and accompanying cold front will
move across the terminals. A brief window of TSRA appears
possible ahead of the cold front by midday/early afternoon at
KRFD and mid-late afternoon for the Chicago terminals.
Additional scattered shower chances may linger into the evening
after the thunder threat ends.

As for surface winds, east to northeast directions (around 10 kt
for the Chicago metro sites early off the lake) are favored
this evening, before becoming light and variable later tonight.
Winds are expected to turn west-southwesterly ahead of the cold
front Sunday, becoming breezy with gusts near 20 kt.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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