813
FXUS63 KLOT 062018
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
318 PM CDT Fri Jun 6 2025

.KEY MESSAGES...

- Showers will develop on Saturday afternoon into Saturday
  evening, mainly south of I-80 where a few isolated
  thunderstorms may occur.

- Sunday afternoon will have a better chance of scattered
  thunderstorms, particularly near and northwest of I-55, with a
  corresponding threat for strong to damaging winds.

&&

.DISCUSSION...
Issued at 318 PM CDT Fri Jun 6 2025

Through Saturday Night:

Weak surface high pressure over the Great Lakes region will
bring quiet conditions tonight with seasonably cool lows in the
50s. Some hazy conditions may persist due to lingering but
diminishing effects from the Canadian wildfire smoke plume. IDEM
did extend the Air Quality Action Day through Saturday evening
for our northwest Indiana counties, though air quality should
generally be improved from the past few days.

Saturday will start out dry and mostly sunny/partly cloudy,
followed by increasing mid-level cloud cover. Forecast highs on
Saturday are in the mid-upper 70s away from Lake Michigan (near
to slightly below normal) and 60s lakeside. A fairly robust
mid-level impulse will approach and crossing the mid-upper MS
River Valley from the mid afternoon into the evening. A dry
antecedent air mass below increasing mid-level moisture will be
initially hostile to any rain reaching the surface, especially
with north and northeastward extent. After trending northward
through Thursday night`s model cycles, there was a nudge back
southward in the trajectory of the short-wave in the 12z cycle.
Given the decent large scale forcing, scattered showers (30-50%
PoPs) should be able to gradually percolate into the southwest
1/3 or so of the CWA in the mid to late afternoon. Meager mid-
level lapse rates <6C/km and cloud cover limiting insolation
will greatly curtail instability and resulting thunder
chances/coverage.

Saturday evening, while somewhat considerable variance remains
amongst the guidance in terms of specific details, the farther
south short-wave path and lingering dryness in the lower to mid
levels farther north should continue to focus the highest rain
coverage south of I-80, where likely (~60%) PoPs were indicated.
Fairly healthy PWATs approaching 1.5" (near 150% of normal)
could plausibly yield some localized downpours and perhaps
isolated embedded lightning. With that said, chances of a
soaking rain (0.5" or more) trended back downward as well, even
for south of I-80 locales. Mostly cloudy skies will keep low
temps Saturday night propped up a few to several degrees from
those of tonight.

Castro

Sunday through Friday:

Following Saturday night`s shower and storm chances, our
attention turns to the digging mid-level trough/low across the
northern Plains and Upper Midwest by early Sunday. This feature
is expected to send a surface cold front southeast across our
area Sunday afternoon and evening. Temperatures in advance of
this approaching front should reach the 75-80F range with dew
points from the upper 50s to lower 60s.

Seasonably strong mid and upper-level flow will translate to
otherwise supportive effective deep layer bulk shear for
organized late day convection. However, some key limiting
factors at this time include: the presence of rather lackluster
mid-level lapse rates; only modest large scale forcing until
evening; and veering winds in the cold frontal trough reducing
low-level convergence. Nevertheless, we`ll need to continue to
monitor this for a low-end/conditional severe threat (primarily
damaging winds), particularly near/northwest of I-55. Convection
would likely be on a weakening trend through the evening as it
moves southeastward towards and past the I-55 corridor. PoPs
peak in the 60-70% range northwest of I-55 in the afternoon,
tapering to low-mid chance (20-50%) off to the southeast, and
then only in the 30-40% range Sunday evening I-55 and southeast.

The upper-level low will settle over the Great Lakes region by
Monday. As it does, a series of embedded compact waves rotating
around it`s southern periphery could drive additional diurnally-
enhanced showers (and maybe a few isolated thunderstorms over
the northwest CWA), particularly Monday afternoon, and to a
lesser extent on Tuesday. Monday should also be rather breezy
with WSW to west winds gusting up to 30 mph. Thereafter, a
deamplifying mid-level ridge is expected to cross the region
for the second half of next week. Accordingly, this will support
a period of drier and warmer weather . Thus, after a cooler
start to the work week, with highs in the low to mid 70s by
Tuesday, temperatures are expected to rebound back into the 80s
mid to late week away from any lake cooling. Shower and some
thunderstorm chances may then return to close out next work week.

KJB/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 103 PM CDT Fri Jun 6 2025

There are no major avn weather concerns for the 18Z TAF period.

NE winds will remain largely below 10 kt for the rest of today
before going northerly and subsiding to near 5 kt for tonight.
Expect NE winds below 10 kt at the start of Saturday going
easterly by mid-morning. Expect VFR throughout the period.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
     ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT Saturday night for INZ001-
     INZ002-INZ010-INZ011.

LM...None.

&&

$$

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