675
FXUS63 KLOT 170112
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
812 PM CDT Fri May 16 2025

.KEY MESSAGES...

- Breeze southwest to west winds will continue tonight and
  tomorrow.

- Otherwise, a quiet weekend is expected.

&&

.DISCUSSION...
Issued at 308 PM CDT Fri May 16 2025

Through tonight:

A large upper-level low over Minnesota has brought continued SW
flow aloft into our region, ultimately resulting a relatively
uncommon High-Plains pattern across northern Illinois today.
Earlier today, a very dry airmass upstream per WV 12Z KSGF RAOB
and WV imagery over Missouri has advected over the northwest
half of Illinois amid deep diurnal mixing. This has pushed RH
values as low as the upper teens over the western CWA as S/SSW
winds gust 35 to 40 mph. This prompted an earlier issuance of a
Red Flag Warning for critical fire wx conditions across portions
of the forecast area where little to no rain fell on Thursday,
though elevated fire wx conditions are occurring elsewhere
(covered by a Special Weather Statement) this afternoon.

Meanwhile, deep mixing into 40kt+ flow aloft should begin to
generate sporadic surface gusts to 45 mph or briefly higher mid to
late afternoon for areas around and northwest of I-55. Then, as a
surface trough crosses the area late this afternoon into this
evening, a favorable isallobaric component to an already strong
synoptic gradient combined with modest residual mixing with CAA
could yield SW gusts to 50 mph for a period this evening. Have
therefore maintained a Wind Advisory for the aforementioned
locations through midnight. While there is no headline specifically
in effect for blowing dust, observations from gusty winds in rural
areas on Thursday support maintaining a mention of patchy blowing
dust in the forecast grids and Wind Advisory through this
evening.

No substantial changes have been made regarding the severe
thunderstorm potential late this afternoon into early evening (see
the latest Mesoscale discussion update for more detailed
information). However, observational trends as of 3pm as well as
near-term CAM guidance continue to favor areas south of a Pontiac to
DeMotte line for the highest chances of severe convection as the
northern edge of the discrete cells over southwest Illinois ride a
northward-advacing plume of modest low-level moisture. Farther
north, a gradual increase in high-based cumulus coverage will be
a focus for at least isolated storm development across the
remainder of the forecast area.

Kluber


Saturday through Friday:

The big, vertically stacked, low pressure will be over northern
Lake Michigan early Saturday morning with continued gusty
westerly winds and extensive stratocumulus deck across our area
to the south of the upper low. This system will move eastward
away from the area during the day Saturday, which should allow
cloudiness to scatter out some during the afternoon. Pressure
gradient will begin to ease some Saturday afternoon, though
deepening mixing of the boundary layer may offset the decreasing
gradient and slow the decrease in winds during the afternoon. By
sunset winds will quick subside as boundary layer decouples.

A sprawling ~1030 mb high will build east across Ontario and the
Hudson Bay on Sunday with mid-level shortwave ridging forecast to
build in across the local forecast area. This high to our north
with send a robust afternoon and evening lake breeze/backdoor
front through the entire region. This will end up holding
lakeside highs in the mid/upper 50s, while inland locales will
likely warm into the 70s prior to the lake breeze push. While
some guidance hints at the potential for some intermittent lower
cloud cover on Sunday, most indications are that any morning
cloud cover should scatter into the afternoon.

On Sunday night into Monday, a sharpening warm frontal zone well
to our south will slowly advance northward into southern Iowa,
northeast Missouri, and southern Illinois. Increasing low-level
warm advection in response to strengthening 925-850 mb
southwesterlies overtopping this frontal zone will lead to the
development of showers and thunderstorms, but given the current
location of the aforementioned boundary and MUCAPE gradient,
most/all of this activity through Sunday night looks to remain
well to our south. Even into the day on Monday any activity
locally currently looks like it should remain isolated to
scattered, with deeper moisture still relegated to our south.
Forecast soundings off the GFS in particular look like they`d be
more supportive of intermittent drizzle/showers as opposed to
thunderstorms.

On Monday night into Tuesday, guidance pushes a modest surface
low (near 1000 mb) towards northern Missouri/southern Iowa as a
robust shortwave pivots across the central plains. During this
time, the northward progress of the returning warm front is
forecast to slow, with the boundary overall becoming
quasistationary across central to downstate Illinois. Isentropic
upglide will increase once again, likely leading to a nocturnal
expansion of showers and storms which--depending on the precise
location of the stalled front--may begin to push into our
forecast area, particularly south of about I-80. Could end up
being a locally heavy rainfall threat somewhere in the vicinity
with this setup.

Thereafter, guidance generally depicts a complex upper pattern
developing, with a series of upper lows/gyres pinwheeling around one
another into the middle of the week. This general evolution will
keep the threat for intermittent periods of rain with embedded
thunderstorms going in the vicinity, at least into Wednesday.
There`s a signal for things to dry out into the end of the week.

Izzi/Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 627 PM CDT Fri May 16 2025

A dust storm will move over GYY/DPA/ORD/MDW with visibility
below 1 mile and southerly wind gusts up to 45kt. A thunderstorm
may occur at the Chicago terminals during the dust storm, as
well. Thereafter, visibility should gradually improve
especially after a westerly wind shift in the 02-03Z timeframe.
Gusty west winds at or above 30 kt should continue through
tomorrow morning before easing in the afternoon.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory until midnight CDT tonight for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

     Blowing Dust Advisory until 9 PM CDT this evening for ILZ003-
     ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ107.

IN...Wind Advisory until midnight CDT /1 AM EDT/ tonight for INZ001-
     INZ002-INZ010-INZ011-INZ019.

     Blowing Dust Advisory until 9 PM CDT /10 PM EDT/ this evening
     for INZ010-INZ011-INZ019.

LM...Gale Warning until 10 PM CDT this evening for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until 10 PM CDT this evening for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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