199
FXUS63 KLOT 060815
AFDLOT

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

.KEY MESSAGES...

- Lingering Canadian wildfire smoke will make for poor air
  quality through this evening.

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

&&

.DISCUSSION...
Issued at 315 AM CDT Fri Jun 6 2025

Through Saturday:

A broad upper trough continues to reside over the central CONUS
with a couple of shortwaves (one moving across IA and the Upper
Midwest and the other moving across the southern Plains)
traversing through the pattern. While both of these shortwaves
will continue to eject east today, their trajectories will cause
them to largely miss our area resulting in generally rain-free
conditions through tonight. However, as the cluster of storms
associated with the southern wave moves across the Mississippi
Valley there is a non-zero chance (15-20%) that an isolated
shower may brush our far southern CWA (areas along and south of
US-24) this afternoon.

Otherwise, the main forecast concern for today will be some
lingering wildfire smoke which will lead to poor air quality
across portions of northern IL and northwest IN through this
evening. As a result, an Air Quality Alert remains in effect
until midnight for the Chicago metro area and northwest IN. That
said, surface smoke concentrations are expected to gradually
diminish through the afternoon so not expecting reduced
visibilities to be much of an issue today. Outside of the smoke,
expect partly to mostly cloudy skies with high temperatures in
the mid-70s inland and lower to mid- 60s near the lake due to
light onshore winds.

Heading into Saturday, any lingering wildfire smoke is expected
to advect east which should allow air quality to improve. While
most of our Saturday should be dry, another shortwave trough
will be ejecting out of the northern Rockies and moving into the
southern Great Lakes by Saturday evening. Thus chances for
showers (and perhaps an isolated storm) will increase Saturday
afternoon especially for locations south of I-80. Regardless,
temperatures will be similar to today with highs in the mid to
upper 70s inland and 60s near the lake.

Yack


Saturday Night through Thursday:

The larger scale weather pattern is expected to turn more
unsettled with a few periods of showers and storms across the
Midwest through the Great Lakes later this weekend through early
next week. Within this evolving pattern, there are two distinct
weather systems of interest, both of which will play major
roles in driving our chances and timing of showers and storms
through this period. The first, is noted in the water vapor
imagery as a positively tilted impulse (southwest to northeast
orientation) over southern ID early this morning, while the
second is currently moving from the Gulf of Alaska onshore
across far northwestern British Columbia Canada.

The southern ID impulse is expected to transverse the central
Rockies later today before sliding eastward across the Corn Belt
on Saturday. As it does, ensemble guidance continues to come
into better agreement in the development of a surface wave of
low pressure along the remnant west-to-east oriented frontal
boundary draped across MO into southern IL late Saturday into
Saturday night as the mid-level system moves across the Mid-
Mississippi Valley. If this occurs paired with PWATs rising to
or just upwards of 1.5" (~up to 150% of normal for early June),
a corridor of enhanced low-to-mid level deformation induced
frontogenesis may foster some rather efficient rainfall rates
(with perhaps isolated embedded t-storms) just north of the
surface low track Saturday night. This potential continues to be
corroborated by an upward trend in the ensemble probabilities
of 24-hour rainfall amounts (QPF) of 0.5" or higher. The past
few runs of the ECMWF ensemble (EPS) and the GFS ensemble (GEFS)
for example, now peak these probabilities around 50-60% south
of I-80 and 30-50% up to the I-88 corridor.

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 across northwestern parts of IL.
Convection would likely be on a weakening trend through the
evening as it moves southeastward towards the I-55 corridor.

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),
particularly Monday afternoon, and possibly again Tuesday
afternoon. 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 week, with highs in
the low to mid 70s, temperatures are expected to rebound back
into the 80s mid to late week.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1158 PM CDT Thu Jun 5 2025

There are no significant aviation weather concerns anticipated
through the period.

The only item of note in the near term will be the continued
hazy conditions overnight, with some associated minor VSBY
restrictions. While some VSBYs may drop as low as 3 or 5 miles
overnight (possibly lower at KGYY), most terminals are expected
to remain VFR. Winds will remain light (around 5 kt or less)
from the north-northwest through the night.

The hazy conditions look to end this morning, so we are
currently not anticipating any VSBY restrictions from mid-
morning onward. Winds will turn northeasterly at most terminals
by mid-morning, with speeds likely to average in the 7 to 11 kt
range through the day.

KJB

&&

.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 tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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