Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180801
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
301 AM CDT Mon Mar 18 2024

.KEY MESSAGES...

- Breezy with an elevated risk for the spread of brush fires
  Tuesday afternoon.

- Period of accumulating snow possible Thursday night into
  Friday morning (mainly along/north of I-88)

&&

.DISCUSSION...
Issued at 300 AM CDT Mon Mar 18 2024

Through Tuesday:

Various shields of 4-6kft stratus continue to filter across the
forecast area early this morning as multiple small trough axes
and mid-level vort maxima rotate across the western Great Lakes
region. Though the saturated layer is no thicker than 2kft, it
resides fully within the DGZ and is therefore able to generate
some snowflakes. However, a rather dry and well-mixed sub-cloud
layer has managed to sublimate nearly all of the flurries/snow
showers aloft from reaching the ground. Have maintained only a
slight chance of flurries for much of the area this morning,
after which mid-level forcing and the existing stratus should
begin to exit eastward with time.

A low-amplitude but quick-moving mid-level wave within broader
WNW flow aloft will cross the far northern Great Lakes on
Tuesday, bringing an associated cold front across the forecast
area late morning through early afternoon. Ahead of this wave, a
50kt+ LLJ feeding into the western Great Lakes late tonight
will result in gradually increasing winds and gustiness prior to
sunrise before daytime heating rapidly mixes down WSW gusts to
35 mph by mid-morning. Winds will maintain a similar magnitude
through the day while becoming WNW behind the front. While some
low-level moisture advection is expected with the front,
antecedent dry low-levels and robust mixing will allow RH values
to fall below 40% area-wide and possibly below 30% across east-
central IL and northwest IN. Given several days of windy and
dry conditions since the recent storms, fine fuels have become
increasingly dry. Will continue to message an elevated risk for
the spread of brush fires, with conditions possibly meeting Red
Flag criteria if mixing is more prominent or low-level moisture
is slower to arrive than currently forecast.

Kluber


Tuesday Night through Sunday:

Cold air advection will be well underway Tuesday night with
overnight lows forecast to drop into the mid 20s to lower 30s.
This will make for a blustery start to Wednesday with morning
wind chills in the teens and lower 20s. Afternoon temperatures
then likely struggle to warm into the lower 40s.

No major changes in thinking for late Thursday afternoon
through Friday morning. A northern stream shortwave is progged
to dive southeast across the area bringing with it the potential
for accumulating snowfall for areas along/north of I-88,
particularly along the WI/IL state line Thursday night. The Euro
ensemble continues to lean colder than the GEFS and accordingly
has higher probabilities for 1+ inch amounts (60-80%) in the
areas mentioned above, with the GEFS having lower probabilities
(30-50%). This would then favor a rain/snow mix along the I-80
corridor and all rain further south, though there is plenty of
time to hone in on the specifics as higher-res guidance becomes
available over the next few of days for that period.

The details become much less clear for the weekend and into
early next work week. A high amplitude western CONUS upper
trough will begin to translate east with lee cyclogenesis
expected on the front range. There remains a fair amount of
variability in the timing and overall magnitude of these
features and accordingly lowers confidence in when precipitation
begins and what precipitation type it will be. Confidence is
high, however, in a return to a more active weather pattern with
multiple chances for precipitation into early next week. Stay
tuned.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1225 AM CDT Mon Mar 18 2024

Non-impactful intermittent flurries remain possible overnight and
through this afternoon. Opted to hang onto PROB30 groups for -SHSN
late morning into early afternoon when a slightly better signal
exists for more persistent flurries (30% chance).

Winds will be sporadically gusty early this morning with more
persistent gusts to around 20kt expected during the daytime hours.
Gusts then ease after sunset but quickly ramp back up early Tuesday
morning with gusts already in the mid 20kt range by daybreak
Tuesday. Marginal LLWS may occur early Tuesday morning as the low-
level jet moves overhead. However, that window appears to be
very brief as surface gusts readily increase.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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