679
FXUS63 KLOT 230753
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 AM CDT Sun Jun 23 2024

.KEY MESSAGES...

- Today and Monday will be relatively pleasant with highs in the
  upper 70s to lower 80s (today) to mid to upper 80s (Monday).
  Lakeshore locations will be cooler.

- Monday night through Tuesday night may feature several rounds
  of thunderstorms including the threat for severe weather and
  flash flooding.

- Tuesday may be hot and humid with heat indices above 100,
  depending on the coverage and timing of thunderstorms.

- Another period of pleasant conditions is expected Wednesday
  and Thursday before chances for thunderstorms return toward
  the end of the week.

&&

.DISCUSSION...
Issued at 253 AM CDT Sun Jun 23 2024

Through Tonight:

Convection is currently clearing the southeast CWA as of 3am,
with the majority of precip exiting to the east by around
sunrise. Some residual flooding impacts are expected to continue
for several hours in areas that experienced flash flooding last
evening, so remain alert for areas of standing water this
morning.

Stratocumulus along and behind a passing cold front will yield
mostly cloudy skies this morning before the cloud deck begins to
break and lift into scattered cumulus through the afternoon.
Drier and cooler air advecting into the area this morning will
result in a substantially more comfortable day today as highs
reach the upper 70s to low 80s. This will mark the coolest
conditions for many in over 10 days.

A compact mid-level low over southern Manitoba early this
morning will slowly unravel while tracking across the Great
Lakes region through tonight. Steepening low-level lapse rates
under a 10-12kft isothermal layer will likely generate isolated
showers and sprinkles across Wisconsin and Lake Michigan. The
light showers may ultimately brush far northeast Illinois this
evening, so have included a slight chance of sprinkles north of
a line from downtown Chicago to Antioch roughly in the 6-10pm
window.

Kluber


Monday through Saturday:

Upper-level shortwave ridging will build into the Great Lakes on
Monday as a surface high pressure system moves overhead. Monday is
accordingly looking pleasant with plenty of sunshine, light winds,
relatively low humidity levels, and highs in the mid to upper 80s. A
lake breeze should move inland during the afternoon leading to
cooler temperatures in the low to mid 70s shore-side.

Monday night through Tuesday night continues to be a time period to
watch for waves of thunderstorms in the general region as a series
of upper-level shortwaves propagate within the jet stream located
near the US/Canadian border and provide glancing blows to an
extensive plume of instability across the Upper Mississippi River
Valley. Determining when and where individual convective
episodes will develop in this kind of pattern can be tricky, as
there is often an underlying influence of each convective event
on the next (e.g. placement of an effective instability
gradient, and so on). In addition, other forecast elements (such
as temperature, sky cover, wind speed and direction, and so on)
will be prone to busting depending on where and when convection
occurs.

With all of that said, deterministic model guidance exhibits a signal
for the first shortwave to swing through Minnesota and northern
Wisconsin Monday evening. With a plume of steep mid-level lapse rates
>8 K/km, and a seasonably moist airmass characterized by PWATs
nearing 2" contributing to a reservoir of MUCAPE >5000 J/kg, any
convection that develops in central Minnesota would be prone to
growing upscale into a large mesoscale convective system (MCS).
Assuming this occurs, our area will be in a precarious position as
the arcing instability gradient and southwest to northeasterly-
oriented LLJ would direct the MCS right into northern Illinois early
Tuesday morning. It is for this reason that the SPC Day 2 outlook
has our entire area in a Level 1/5 threat level for severe weather
(damaging winds being the main threat).

With the threat for an MCS to roll through early Tuesday morning,
the forecast for the rest of the day isn`t clear cut. In scenarios
where there is minimal convective coverage Tuesday morning, the rest
of the day would be poised to be hot and humid with highs in the low
to mid 90s and rising humidity levels with corresponding heat
indices above 100F. Meanwhile, in scenarios where convective
coverage is high in the morning, there could be a play for
regenerating convection over at least part of the area or to our
south which would limit afternoon high temperatures and
humidity levels. Then, there`s the threat for another round of
thunderstorms in the evening along a southward-moving cold front
that would have to be sorted out, with all sorts of questions
ranging from the rate of recovery (if needed at all), where any
effective instability gradient would be located near or in our
area, and to the extent to which forcing will be strong enough
to overcome low-level capping. Interestingly, blended NBM
guidance came in hot with 60-70% PoPs Tuesday evening, which
appears to be supported by a majority of individual ensemble
members from the EPS/GEFS showing measurable QPF (which would
almost certainly be thunderous). And, with PWATs hanging around
2" once again and MUCAPE potentially >4000 J/kg, any
thunderstorm Tuesday evening would be prone to becoming severe
and producing prolific rainfall rates in spite of otherwise
meager deep-layer shear. This is all to say, Tuesday has the
potential to be busy with plays for headline-worth heat, severe
weather, and flash flooding for parts or all of our area. (Note
that SPC and WPC do have our entire area in a threat level 2/5
for severe weather and flash flooding, respectively).

Now, the forecast Wednesday and Thursday is a little more straight
forward as the instability axis will be shunted southward and a
surface high pressure system moves through the Great Lakes.
Daily highs in the low to mid 80s (cooler lakeside due to lake
breezes), relatively low humidity levels, light winds, and
plenty of sunshine would be the norm. Chances for thunderstorms
look to return Friday and into the weekend, however, as a series
of upper-level shortwaves lift northeastward toward the Great
Lakes.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1258 AM CDT Sun Jun 23 2024

TSRA is currently shifting south of the Chicago terminals early
this morning, with remaining TS ending in the next hour or two.
Otherwise, beside widely isolated -SHRA this evening/early
Monday morning, dry conditions are expected through the
remainder of the period.

SW winds around 10 knots will veer NW 10 to 15 knots behind a
cold front prior to sunrise. A lake breeze will reach GYY and
likely MDW mid-afternoon, but may stall just east of ORD. Have
not included a NE wind shift at ORD, but if the lake breeze does
reach the terminal, it would occur in the 21-23Z window.

MVFR ceilings will expand early this morning along and behind
the cold front, with a few brief periods of IFR ceilings
possible prior to sunrise. Ceilings should gradually lift and
SCT by late morning, with VFR conditions through the remainder
of the period.

Kluber

&&

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

&&

$$

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