Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212348

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 PM CDT Sun Apr 21 2024


- Frost expected tonight for areas outside of the core of the
  Chicago metro.

- Frost may develop again late Wednesday night.

- Warmer by the end of the week with periodic showers and storms
  Friday into next weekend.


Issued at 230 PM CDT Sun Apr 21 2024

Through Monday Night:

Dry and seasonably cool conditions today will give way to
increasing temps and gusty southwest winds for Monday. The
combination of dry conditions, clear skies tonight, and nearby
surface ridge will support another night with good radiational
cooling. Moderation of the airmass over the past day will yield
somewhat warmer conditions than last night, with lows generally
in the mid 30s to locally low 30s away from the core of the
Chicago metro. Max RH values of around 80-85% late tonight along
with 15 knot flow as low as 0.5-1kft should mitigate widespread
frost generation even with temps nearing freezing. While a
Frost Advisory will include most of the forecast area, the set-
up looks rather marginal for notable impacts to vegetation.

We continue to message elevated fire weather concerns for
Monday as WAA and strengthening SW winds promote decent mixing
by the afternoon. Upstream dewpoints in the source region across
the Great Plains are in the upper teens to mid 20s this
afternoon. While some evapotranspiration should slightly moisten
this airmass, afternoon dew points in the mid to upper 20s
appear quite plausible in our area. Following guidance that is
typically more aggressive with mixing, these dew points with
temps in the mid 60s results in minimum RH values around or just
above 25%. However, RH values could be a little lower if
thickening upper-level clouds are delayed later in the day and
mixing overperforms. Overall, the combination of the low RH, SW
gusts over 25 mph (especially northwest of I-57), and fine-fuel
moisture values under 10% will support near critical fire
weather conditions for much of the area Monday afternoon.

Strong upper-level diffluence ahead of a zonal jet streak over
the northern Great Plains will promote decent mid-level ascent
over an antecedent dry airmass Monday night. Modest mid-level
lapse rates around over 7C/km should drag a band of elevated
scattered showers from northwest to southeast during the night.
With continued strong low-level flow and an existing deep
reservoir of sub-cloud dry air, any shower could produce locally
strong gusts.


Tuesday through Sunday:

Surface low pressure is progged to be tracking across the
northern Great Lakes Tuesday morning, in association with a mid-
level short wave trough digging southeast across the region. The
WFO LOT cwa is expected to start the day within the showery and
breezy low-level warm sector, with the highest morning rain
chances likely across the southeast half of the forecast area
within the main warm/moist advection axis. Lapse rates initially
appear rather unfavorable for thunderstorms during the morning
hours, though the combination of mid-level cooling with the
approaching short wave trough and diurnal warming of low levels
makes profiles more suitable for lightning during the afternoon
and early evening hours along with a re-expansion of shower
coverage as a strong cold front pushes into the area from the
north. Low-level moisture may be a limiting factor in
thunderstorm coverage however, with highest forecast surface
dew points only in the mid-upper 40s ahead of the front. Sharply
cooler air then spreads into the area behind the front Tuesday
night, with showers tapering to some spotty drizzle in low
clouds behind the front.

Overnight lows Tuesday night are expected to dip into the low-
mid 30s, though breezy north-northeast winds and lingering cloud
cover in some spots should largely preclude any widespread
frost issues. Wednesday will be cool, with breezy northeast
winds and persistent cold advection limiting highs to the 40s
near Lake Michigan and the low-mid 50s farther inland. Wednesday
night appears to have a greater frost/freeze threat for the
area as surface high pressure settles over the western Great
Lakes region. Light winds and mainly clear skies would likely
support freezing/sub-freezing low temperatures for locations
outside of Chicago and the immediate Lake Michigan shores.

Broad upper level ridging is then progged to develop for the
end of the week, leading to warming temperatures. Warming looks
to initially be attenuated by low level east-southeasterly flow
across the area Thursday, maintaining lake cooling for northeast
Illinois. More solid warming is expected Friday (perhaps as
early as Thursday night) into the weekend however, as low level
flow veers more southerly with time in response to an area of
low pressure which extended guidance and ensembles continue to
track northwest of the forecast area. Not surprisingly, there is
spread in the model timing and track solutions at this
distance, but increasing storm chances and some severe weather
threat will need to be monitored within the region in this



Issued at 645 PM CDT Sun Apr 21 2024

Breezy west-northwest winds ease with sunset this evening
becoming light out of the west southwest around 5kt through the
rest of the overnight hours. The lake breeze is expected to
remain east of ORD/MDW though it may still get close enough to
MDW for the direction to become somewhat variable after sunset
(under 5kt) before returning to a westerly direction shortly
thereafter. Winds pick back up during the day on Monday out of
the west-northwest with gusts in the 20-25+kt range, highest
toward RFD.

VFR conditions are forecast with clear skies overnight into
Monday morning then increasing cirrus during the afternoon.
CIGS gradually lower heading into Monday evening and overnight
as our next weather system approaches. An associated strong low-
level jet will move overhead resulting in increasing gustiness
at the surface Monday night, potentially near 30kt at the end
of the 30-hr TAF period. Dry low levels should limit shower
coverage during this time, though a few spotty showers can`t be
ruled out, currently accounted for with a VCSH mention.



IL...Frost Advisory from 2 AM to 8 AM CDT Monday for ILZ003-ILZ004-

IN...Frost Advisory from 2 AM CDT /3 AM EDT/ to 8 AM CDT /9 AM EDT/
     Monday for INZ001-INZ002-INZ010-INZ011-INZ019.




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