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618
FXUS63 KLOT 110524
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1224 AM CDT Tue Jun 11 2024

.KEY MESSAGES...

- Dangerous swimming conditions will continue at Lake Michigan
  beaches through this evening.

- Slight chance for thunderstorms Tuesday evening and early
  Tuesday night, mainly north of I-80.

- Scattered thunderstorms possible Thursday, some of which may
  be severe in the afternoon and evening.

- Building heat appearing probable early next week.

&&

.DISCUSSION...
Issued at 307 PM CDT Mon Jun 10 2024

Through Tuesday Night:

Sunny and breezy conditions persist this afternoon with relatively
cool temperatures in the mid 60s to around 70. High pressure is
spreading in from the west which will keep skies clear tonight
and allow winds to diminish with sunset which will also allow
waves to subside and dangerous beach conditions to improve.
Expect calm to light and variable winds overnight which will
support decent radiational cooling. Have nudged overnight lows
down just a bit with values in the low to mid 40s for most areas
outside the Chicago metro and upper 40s to low 50s in the
suburbs and near the lake. Would not be surprised if a few of
the typically colder spots in the Fox Valley cool to a few
degrees lower than forecast.

A weak upper ridge axis will crest the area tomorrow while the
surface high shifts east. Winds will turn southwesterly but be
on the light side. Warm advection and a mostly sunny start will
bring high temps into the upper 70s, though a lake breeze will
keep temps in the mid/upper 60s along the lake. A mid/upper
shortwave trough will reach the area late tomorrow afternoon and
pass overhead through the evening. There will be a modest influx
of moisture but forcing will be on the weak side, given the
parent surface and upper low will be well to the north across
northern Ontario. Not surprisingly, ensemble guidance is mixed
on whether we see much in the way of precipitation activity.
There is enough support to continue with the 20-30% chances,
mainly for areas north of I-88, with a lower chance to the south
toward I-80, from the previous shift. There is enough of a
signal for instability to include a slight chance for
thunderstorms as well. Overall, coverage looks isolated to
scattered at best. Otherwise a much milder night will be in
store, with lows in the low to mid 60s.

MDB

Wednesday through Monday:

On Wednesday, strengthening warm air advection, lingering
relatively low dew point temps, and plenty of sunshine will
result in the warmest temperatures for the area as a whole since
May 21st. 850 mb and 925 mb temps are progged to rise into the
upper teens Celsius and low-mid 20s C, respectively, which
supports highs generally in the 85-90F range. The lakeshore
should also get in on the warmth unless low-level winds are a
bit weaker than expected and enable a lake breeze to push just
inland.

For Wednesday night into early Thursday, the main question mark
is the extent of upstream convection able to overcome antecedent
capping and push southeastward into portions of the area. Given
the expected more hostile conditions for convective maintenance
and unfavorable overnight timing, only low (~20-30%) PoPs for
the northwest 1/3 or so of the CWA appear appropriate. Chance
(25-30%) PoPs are confined to our far northwest CWA. Aside from
the lower end chance for a few showers/storms to survive into
the CWA, it will be a much warmer night than the past few, with
lows in the upper 60s to lower 70s.

Thursday remains the "day to watch" for severe weather potential
in the CWA during the afternoon and evening in favorably strong
northwest mid-upper level flow pattern. There are, however, key
points of uncertainty that make it a conditional scenario. A
cold front is forecast to be draped to our northwest in the
early morning, from northern Iowa to the UP of Michigan.
Upstream overnight convection, even if it mostly falls apart
prior to reaching into the CWA, may have outflow that forces the
effective cold front position a bit farther south to start the
day. Even without the above occurrence, moisture depth may be
quite shallow with northward extent, and a combination of
veering winds and mixing out of dew points as the cold front
approaches in the afternoon may force the PM convective
initiation (CI) farther southward.

High temperatures in the upper 80s to lower 90s and dew points
in the mid to upper 60s would yield 2k+ J/kg of MLCAPE, with
little MLCINh. Where boundary layer convergence and modest
large scale ascent are sufficient for CI, scattered thunderstorms
amidst ~40-50 kt of effective northwesterly deep layer bulk shear
have the potential to become severe with a damaging wind and
hail threat. Our PoPs peak in the 40-50% range, which generally
implies scattered type coverage, though again note that location
of afternoon development is uncertain enough to lower confidence
in how things will play out. The SPC day 4 15% probs appear
reasonable from a pattern recognition perspective, though we
can`t discount a scenario in which portions of the area see
little to no thunderstorms (ie. recent ECMWF runs favor southern
CWA and points south).

The cold front will clear the area well to our south to bring a
quiet, dry (including low humidity), and slightly cooler stretch
Friday-Saturday (coolest near the lake due to onshore flow).
Strong 500 mb ridging nosing northward over the weekend will
quickly bring a return to summer heat by Sunday (low-mid 90s
highs forecast). Dew points will likely remain low enough to
keep heat indices in check, though. While any PoPs are low in
the Sunday-Monday period, the zone of active convection typical
to the type of pattern progged may not be far to our northwest.
Temps well into the 90s are possible on Monday *if* convection
doesn`t end up having more of an effect.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1224 AM CDT Tue Jun 11 2024

Only forecast concern this period is a chance for showers this
afternoon into this evening. The low levels will remain fairly
dry as a weak disturbance moves across the area. There will
likely be some virga aloft and possibly an isolated shower, but
coverage is expected to remain low. Opted to remain dry with
this forecast. Its possible some short tempo mention will be
needed for showers, or possibly vicinity mention as trends
emerge later today.

Light/calm winds early this morning will become southwesterly
generally under 10kts later this morning and then turn back to
southerly this evening. cms
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
IL...Beach Hazards Statement until 1 AM CDT early this morning for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 1 AM CDT early this morning for
     INZ001-INZ002.

LM...Small Craft Advisory until 1 AM CDT early this morning for the
     IN nearshore waters.
-- End Changed Discussion --

&&

$$

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