010
FXUS63 KLOT 052017
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 PM CDT Thu Jun 5 2025

.KEY MESSAGES...

- Visibility reductions and poor air quality in haze and smoke
  expected through Friday.

- Shower and thunderstorm chances will return over the weekend,
  particularly late Saturday-Saturday night and then on Sunday
  afternoon.

&&

.DISCUSSION...
Issued at 315 PM CDT Thu Jun 5 2025

Through Friday Night:

Hazy skies continue this afternoon due to wildfire smoke from
Canada advecting over the region. Given the low-to-mid level
flow pattern does not change all that much over the next 24
hours or so, suspect this may persist into the day on Friday.
As a result the IL EPA and IDEM have extended the Air Quality
Action Days through Friday night for much of the area.

Due to residual low-level moisture (dewpoints in the 60s) and
light winds tonight, another round of fog may develop across the
southeast CWA, generally along and south of a Gibson City, IL
to Valparaiso, IN line. Some of this could end up locally dense,
though the better potential may lie just outside of the area.
For now have introduced "patchy fog" wording into the forecast
and will let the evening shift get another look at this to
determine if greater coverage wording is needed.

Friday looks fairly similar to today, with highs in the mid to
upper 70s expected across inland areas of Illinois, low to mid
70s across northwest Indiana and 60s near the Lake Michigan
shoreline. Precipitation chances continue to trend down on
Friday with most of if not the entire area expected to remain
dry through Friday night, with the expectation that the low
amplitude wave follows a more southerly track.

Petr


Saturday through Sunday Night:

On Saturday, the most recent guidance has come into better
agreement in a fairly robust mid-level impulse approaching 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 until
toward and after sunset. Given the decent large scale forcing,
scattered showers (30-40% PoPs) should be able to gradually
percolate into the southwest 1/3 to 1/2 of the CWA in the
afternoon and then farther north and east into the evening hours.
Meager mid-level lapse rates <6C/km and cloud cover limiting
insolation will greatly curtail instability and resulting thunder
chances/coverage. The best (but still low) chance for a few
thunderstorms will be south of I-80. Forecast highs on Saturday
are in the mid-upper 70s away from Lake Michigan (near to slightly
below normal) and 60s lakeside.

Later Saturday evening into the pre-dawn hours of Sunday, some
guidance members develop a weak surface low just to our south with
the short-wave moving overhead. If this occurs paired with PWATs
rising to or just upwards of 1.5" (~up to 150% of normal for early
June), there may be a corridor of enhanced convergence and
efficient rainfall rates (with perhaps isolated embedded t-storms)
just north of the surface low. The latest ECMWF ensemble (EPS)
probabilities of 24-hour QPF >0.5" peak at 30-50% south of I-80
and 20-30% up to the I-88 corridor. Considering the variance
amongst model/ensemble solutions, ~30% chance showers in the
southeast half of the CWA into pre-dawn Sunday appear reasonable
for now.

Attention on Sunday will then turn to a seasonably strong mid-
upper low and associated surface low pressure over the Canadian
Prairies and northern Great Lakes. A cold front trailing from the
surface low will approach the area in the afternoon as mid-level
height falls impinge upon northwest IL towards evening.
Temperatures ahead of the front should reach the ~75-80F range
with dew points from the upper 50s to lower and locally mid 60s.

Seasonably strong mid and upper level flow will translate to
otherwise supportive effective deep layer bulk shear (for
organized convection). Some key limiting factors at this vantage
point appear to be: still meager mid-level lapse rates; only
modest large scale forcing until evening; and veering winds in the
cold frontal trough reducing low-level convergence. We`ll need to
monitor for a low-end/conditional severe threat (primarily
damaging winds) as the decent effective shear and sufficiently
steep low-level lapse rates (and DCAPE) counteract the marginal
mid-level lapse rates.

There appears to be a plausible scenario in which any convection
that initiates and/or into northwestern portions of the CWA
fizzles with southeastward extent into the evening. Our graphical
messaging will indicate areas roughly northwest of I-55 as having
a higher chance of mid-late afternoon scattered thunderstorms (and
PoPs peaking in the 60-70% range).

Castro

Monday through Thursday:

The upper-level low will settle over the far northern Great Lakes
region early next week. A series of embedded compact waves
rotating around the low could bring diurnally-enhanced showers
(and maybe an isolated thunderstorm far north) Monday afternoon,
and to a lesser extent on Tuesday. This will be contingent upon
coincident timing of the waves with peak heating. A deamplifying
mid-level ridge is then progged to cross the region midweek,
supporting primarily dry and warmer conditions. Temperatures will
reach into the 80s (warmest Thursday) away from any lake cooling,
though humidity levels should remain in check.

Kluber/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1212 PM CDT Thu Jun 5 2025

The main aviation weather concerns for the 18Z TAF period
include:

* A period of MVFR cigs at MDW and GYY this afternoon

An MVFR cloud deck is sloshing westward across the southern
reaches of the lake as of noon local. MVFR has taken hold at GYY
is expected to move over MDW right around 1730Z. Confidence in
the behavior of the MVFR is low, especially at MDW vs GYY, and
cigs may end up scattering before long. Otherwise, this lower
deck should pull away from the TAF sites during the latter part
of this afternoon. VFR can then be expected throughout the
period.

Meanwhile, NE winds will continue to gust to around 20 kt
through early evening before subsiding to between 5 and 10 kt
overnight. Direction may back to NNW for a time tonight while
remaining light. Expect NE winds during the day tomorrow to
around 10 kt.

Doom

&&

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

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

LM...None.

&&

$$

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