Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211100
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
600 AM CDT Thu Mar 21 2024

.KEY MESSAGES...

- Travel impacts expected late Thursday night through Friday
  morning beneath a narrow heavy snow band near the Wisconsin
  state line. Confidence in where this band sets up remains low.

- Rain returns early next week, accompanied by breezy winds.

&&

.DISCUSSION...
Issued at 340 AM CDT Thu Mar 21 2024

Through Friday Night:

All is quiet early this morning with broad surface high
pressure centered north of the area with light and variable
winds early this morning turning easterly during the day,
reinforced by a late AM/early afternoon lake breeze. An axis of
weak mid-level fgen and associated cloudiness may still attempt
to squeeze out a few flurries early this morning, though so far
the lower levels remain quite dry. Expect temperatures to top
out in the low to mid 40s (warmest south of I-80) with mid-upper
30s expected near the lake in Illinois.

Heading into the evening and overnight hours, low-level
moistening is anticipated ahead of a quickly approaching
shortwave and associated surface low. This will bring periods of
precipitation to the area Thursday night and through the day on
Friday, including the potential for snow! There remains a very
persistent robust frontogenetic circulation between 700-850mb
which sets up near or just north of the WI/IL state line and
results in a narrow swath of potentially significant snowfall
amounts (>6"). A vertical cross section through this band
highlights that the environment is favorable for slantwise
convection, especially on the northern end of the band, which is
typically the case for these narrow heavy snow bands. If
parcels manage to lift above this layer it could tap into an
environment supportive of upright convection. This could help
increase snowfall rates to over 1.5" per hour and even produce a
few lightning strikes. The looming question that remains,
however, is where does this band ultimately set up? In these
scenarios there is typically a sharp gradient in amounts on
either side of the band and confidence in its placement can
remain low even just 6-12 hours out. At this time the higher
probabilities remain just north of the state line across
southern Wisconsin.

Due to multiple solutions having this band precariously close
to our area, and some centered right along the state line,
prefer to have a headline in place in the event that a southward
shift in the band occurs. Thus, a Winter Weather Advisory has
been issued for Winnebago, Boone, and McHenry counties Thursday
night through early Friday afternoon (which includes the Friday
morning commute), where confidence is highest in seeing at
least advisory level impacts (possibly warning level!) across
the northern portions of these counties. Note that southern
areas could end up with little to no meaningful accumulations
(including much of Rockford). A consideration was made to
include Lake County, but persistent easterly flow off of the
lake may act to reduce amounts in an already marginal thermal
environment. It may need to be included with later updates if
the southern solutions become the more likely outcome.

As for the rest of the area, expect mostly non-accumulating
snowfall Thursday night and into Friday with a mix of rain and
snow expected along the I-80/I-88 corridor and potentially all
rain further south. Expect precip to end from northwest to
southeast through the afternoon and early evening with the
front.

Petr


Saturday through Wednesday:

In the wake of Friday`s wintry weather across the western Great
Lakes, a period of low-amplitude WNW flow aloft and a passing
surface ridge will yield quiet but seasonably cold conditions on
Saturday with clearing skies and max temps in the mid 30s north
to low 40s south. Focus then turns to an expansive long-wave
trough building across the western CONUS late weekend into early
next week. While a dry forecast is generally favored Saturday
night through Sunday night, a band of WAA feeding off an axis of
Pacific-based moisture may produce a shield of light snow
across the northwest forecast area late Saturday night. Another
surge of Gulf moisture advecting with a modest LLJ west of the
Mississippi River Saturday night into Sunday will then support
the potential for scattered rain/snow showers brushing the
northwest CWA through Sunday night.

Guidance continues to be in general agreement that a more
substantial wave will eject into the central Great Plains by
late Sunday night before lifting NNE to the far northern Great
Lakes by Tuesday. PWATs of over 1" (200% of normal) will surge
ahead a cold front drifting eastward toward the forecast area on
Monday. While higher convective probs are expected to remain
well south of the forecast area where higher rainfall rates
appear more likely with a focused LLJ, ensemble mean QPF values
nearing 1" Monday night into Tuesday morning does offer some
minor hydro concerns given we are only in the early stages of
green-up and higher rainfall amounts have recently occurred
across portions of our area (specifically the eastern CWA).

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 558 AM CDT Thu Mar 21 2024

Key Aviation Messages:
- 30% chance of -SHSN with IFR visibility late tonight into
  mid-morning Friday

- Chance for IFR to LIFR -SN over or just north of RFD late
  tonight into Friday morning.

Winds will remain calm or VRB under 5 knots this morning before
a lake-enhanced wind shift ultimately results in E/ESE winds
around 10 knots early in the afternoon through the remainder of
the period.

VFR conditions are favored through this evening. Overnight
through daybreak Friday, an axis of warm air advection
developing below a layer of steep mid-level lapse rates across
northern Illinois has the potential to produce scattered bursts
of -SHSN. Confidence is high enough to maintain a PROB30 for
-SHSN with IFR visibility at this time. Additionally, a brief
lightning strike or two cannot be ruled out with these showers.
Meanwhile at RFD, a snow band with IFR to possibly LIFR
visibility is expected to develop near the WI state line and may
settle far enough south to affect the terminal late tonight
into Friday morning.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory from 1 AM to 4 PM CDT Friday for
     ILZ003-ILZ004-ILZ005.

IN...None.
LM...None.

&&

$$

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