Area Forecast Discussion
Issued by NWS Chicago, IL

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599
FXUS63 KLOT 220802
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
302 AM CDT Mon Jul 22 2024

.KEY MESSAGES...

- Sparse shower/storm coverage today. A bit higher coverage late
  Tuesday north of I-80.

- Showers and scattered thunderstorms expected on Wednesday.

- Pleasant summer weather with low humidity through Saturday,
  then heating up into next week, potentially turning stormy.

&&

.DISCUSSION...
Issued at 300 AM CDT Mon Jul 22 2024

Through Tuesday Night:

Fog development has yet to fully get underway with only patchy
reduced visibility in the 1-3 mile range in our typical fog
prone locales. Overall not expecting widespread fog development
early this morning though some shallow ground fog can`t be ruled
out at times (perhaps even locally dense over farm fields and
grassy areas) and will maintain a "patchy fog" mention in the
forecast through 8am CDT outside of Chicago.

Today will be a near carbon copy of yesterday with high
temperatures in the lower 80s (upper 70s for lake adjacent
areas due to an early lake breeze passage). Guidance continues
to be rather sparse with the shower and thunderstorm coverage.
In contrast to yesterday where the focus was tied to areas north
of I-88 and toward the Illinois-Wisconsin stateline, there
isn`t a more discernible location with the mid-upper low
directly overhead. As such, opted to include precip chances for
areas along and west of the lake breeze this afternoon and kept
them in the 15-20% range aka "isolated" coverage.

Expect a dry night tonight with low temperatures in the 60s and
perhaps another chance of patchy fog, particularly south of
I-80 closer to the departing mid-upper low.

Tuesday will be a smidge warmer than the past couple days with
highs in the low to mid 80s. Similar to today, isolated showers
and storms can`t be ruled out late morning through the afternoon
for much of the area. There is some guidance that develops
storms along a cold front in Wisconsin that surge ahead of the
front into northern Illinois mid-to-late afternoon (NSSL
WRF/ARW) while most other guidance has this axis of showers and
storms moving into the area sometime Tuesday evening closer to
the front ahead of an approaching mid-level shortwave. Due to
this variability, opted to keep precip chances below 45% during
this period and maintain "scattered" coverage wording with this
update. Convective coverage should begin to diminish with
sunset though a few lightning strikes can`t be ruled out
overnight.

Petr


Wednesday through Sunday:

Wednesday`s daytime hours still appear to offer the highest
coverage of showers and scattered thunderstorms until we enter
another quiet period through the weekend. Mid-upper 60s to
locally 70F dew point temperatures will pool near a cold front
as it presses south, and possibly takes on some backdoor/lake
enhanced characteristics as well.

A short-wave impulse on the southwest flank of the the mid-
upper trough centered north of the northern Lakes, plus the
potential enhanced frontal convergence should be sufficient for
scattered to numerous showers and widely scattered
thunderstorms. Have PoPs in the 40-70% range, highest I-55 and
southeast, and chance TS mention for most of the area. Mid-level
lapse rates will remain generally at or below 6C/km, which will
limit how much we destabilize and may somewhat curtail
thunderstorm coverage to below that of showers. This being said,
the most recent model guidance is in good agreement in erosion
of capping and up to 1000-1500 J/kg of MLCAPE, so confidence is
on the higher side in thunderstorm occurrence within the CWA,
especially the farther south and east you go.

Near the WI border may be a relative minima in t-storms thanks
to the earlier cold frontal passage, which will result in
slightly lower dew points and less instability there. If the
front speeds up more, as shown on some recent operational runs,
the focus for late morning and afternoon convection will also
shift farther south. Regarding thunderstorm hazards, deep layer
bulk shear up to 30 kt or so may offer some threat for a few
strong, gusty thunderstorms, though the marginal lapse rates
will otherwise keep the severe threat low. Finally, seasonably
high PWATs up to 1.25-1.75" will result in locally heavy
downpours capable of causing ponding on roads. The threat for
flash flooding appears low due to relatively fast storm motions
of 30-40 mph toward the east-southeast.

The front will clear our area to the south early Wednesday
evening, paired with much drier air moving in aloft, quickly
ending the chance for storms prior to sunset for most of the
area. Thursday will cool only slightly from Wednesday`s lower
80s away from the lake, along with pleasant for late July
humidity levels. High pressure over the Great Lakes will keep us
dry and fairly comfortable into the weekend, though temps away
from the lake will rise to the mid 80s on Saturday due to rising
mid-level heights and warm advection. A very warm to hot and
eventually more humid and active pattern may then return to
close out July and into the start of August. Despite highs
reaching the upper 80s to around 90F on Sunday, humidity values
and heat indices may remain in check, as better moisture return
could hold off until the work week.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1215 AM CDT Mon Jul 22 2024

Main Concerns:

- Some potential for patchy early AM VSBY restrictions in BR

- Patchy lower MVFR clouds this morning, possibly IFR in spots

- Low chance of SHRA/TS this afternoon

Not seeing any signs of the areas of BR/FG and VSBY
restrictions some of the aggressive guidance had been showing.
This will probably be patchy at most, and accordingly, trended
more optimistic in the TAFs. If patchy lower MVFR clouds expand
some, bases may build down to IFR, though confidence is low in
this occurrence. While a few isolated SHRA/TS may develop inland
this afternoon, potential coverage and confidence in occurrence
remain well too low for any mention in the TAFs.

Light northeast to VRB/calm winds will pick up at GYY, MDW, and
ORD early this afternoon from the ENE as a lake breeze pushes
through. Speeds should generally remain below 10 kt, though.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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