316
FXUS63 KLOT 162343
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
643 PM CDT Tue Jul 16 2024

.KEY MESSAGES...

- Few early shower/thunderstorm chances this evening

- Bit cooler and less humid Wednesday...may some late day
  showers along lake breeze?

- Dry pattern along with cooler than normal temperatures settle
  in for Thursday through Saturday.

&&

.DISCUSSION...
Issued at 331 PM CDT Tue Jul 16 2024

Large scale pattern early this afternoon features an upper low
spinning east across northwest Ontario...with troughing extending
into the upper Midwest/upper Great Lakes.  Expansive upper ridge
encompasses the southern third of the CONUS.  17z surface analysis
shows at 1002mb low over northwest Quebec with a cold front trailing
back to the southwest across Lower Michigan/northern Illinois/
northern Missouri.  Drier air is...slowly...filtering southeast
behind the front with dew points in the upper 50s/lower 60s across
Wisconsin compared to the lower 70s lingering across Illinois.

Upper trough will swing across the Great Lakes Wednesday with some
weak height falls into Illinois...but more importantly will nudge
some cooler temperatures into the area as high pressure settles into
the Dakotas/Minnesota for Wednesday...and eventually into the
Midwest for Thursday/Friday as a basic ridge west/trough east
pattern sets up for the end of the week.

Through Wednesday Night:

At mid afternoon morning St deck has bubbled up into a
scattered-broken Cu field...with mid and high clouds approaching
from the west/southwest. Heating and lingering moisture has
generated 1000-1500J/kg MLCAPE across northeast Illinois...a few
showers have popped in the vicinity of the Wisconsin border
(and finally some lightning over Rock County, Wisconsin as of
1911Z) with visual reports of funnel clouds with some of these
cells (presumably of the landspout type). As the cold front
slowly sinks across the forecast area this evening along with
the influence of the lake breeze can`t rule out some convection
developing across northeast Illinois especially given current
activity. Radar shows several east-west oriented fine lines that
could provide a focus as well. Beyond that expect a mid cloud
overcast for most of the night...and Wednesday right now looks
dry with the only exception possibly being some lake breeze
induced convection during the afternoon/evening. Tail end of PV
anomaly associated with upper trough crossing the Great Lakes
may provide support for this. Should be a bit cooler but
noticeably less humid.

Thursday through Tuesday:

High pressure settling into the region Thursday is expected to
provide northern Illinois with a break from the summer heat and
humidity. Normal mid July highs are in the mid 80s...but
Thursday highs expected to be in the mid 70s give or take a
couple of degrees. A gradual air mass moderation is expected
heading into the week...but highs Friday in the upper 70s-around
80 expected and probably a couple degrees either side of 80 for
Saturday. Dew points should be in the comfortable range as well
(50s-lower 60s) so should be a nice stretch of weather.

A weakness in the upper level pattern is expected to develop by the
end of the weekend over the central Plains.  This feature may spin
some moisture in our direction to allow for the next chance of
widespread rain in the Monday-Tuesday time frame.

JPB

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 640 PM CDT Tue Jul 16 2024

Key Messages:

- Lingering spotty showers with ~1hr window for VCTS at ORD

Spotty showers have developed across the area with a cluster of
more robust showers that developed along the lake breeze in the
vicinity of ORD that have produced a few lightning strikes.
These will gradually weaken over the next hour or so though a
few showers can`t be ruled out through 3Z.

Winds have turned northeast at ORD and will be shortly at
MDW/GYY. Winds then become light and variable overnight
eventually settling into a prevailing northwest by daybreak
Wednesday. Winds will turn northeast behind the lake breeze
Wednesday afternoon at GYY and could reach ORD/MDW toward early
evening.

Petr

&&

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

&&

$$

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