Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220220
AFDLOT

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

.KEY MESSAGES...

- Accumulating wet snow late tonight into Friday across far
  northern Illinois north of I-90. Travel impacts likely during
  the morning commute, especially near the WI state line.

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

&&

.UPDATE...
Issued at 920 PM CDT Thu Mar 21 2024

Made some minor adjustments to going forecast for overnight
into Friday, most notably to add a little more detail/tighten up
gradients in hourly pops through Friday afternoon. Overall, no
significant changes made and Winter Weather Advisory headlines
for our northern tier of IL counties from midnight tonight
through mid-afternoon Friday still looks good at this time.

Evening upper air analysis and GOES vapor imagery depict a mid-
level short wave tracking from the northern Rockies into the
Northern Plains this evening. This disturbance is progged to
track east- southeast into the western Great Lakes region
through Friday, along a fairly stout lower/middle level
baroclinic zone. Strong warm advection atop and north of this
baroclinic zone and resulting mid- level frontogenesis is
expected to produce a strong but narrow west- east band of
precipitation - with forcing enhanced by upper level divergence
within the right entrance region to a 120+ kt jet streak across
the northern-central Lakes, as well as instability in the
presence of steep (7.0+ C/km) lapse rates above about 700 mb.
This may support a narrow axis of accumulating snow in close
proximity to our northern cwa border later tonight into early
Friday. Most recent runs of various high-res guidance
(HRRR/RAP/3km NAM) still depict the axis of this band to be just
north of the IL/WI state line at this time, though confidence
in these situations is always on the low side until the band
develops and is evident in radar, especially given the
likelihood of a very sharp cutoff to snow amounts on the
southern periphery of this band. Current forecast has amounts
ramping up to potentially 6"+ just north of the WI state line,
with a sharp drop to an around an inch or less south of our
northern tier of IL counties. Not seeing anything supporting
significant changes to this, thus current Winter Weather
Advisory still looks good for that tier of counties.

While some light snow may linger along the IL/WI border into
the morning, much of the area looks to be relatively precip-free
until a southwest-northeast oriented band of precip develops
southeast along and just behind the surface trough axis into
Friday afternoon. This will support period of rain/snow which
may turn over to all snow for a brief time (few hours) as temps
cool in the afternoon. Some additional accums can`t be ruled out
with this afternoon precip band. As noted above, did make some
tweaks to pops for Friday afternoon to tighten up the
band/timing, and did end up with snow amounts north of I-90 or
so just a smidge higher than previous. Otherwise, no significant
changes made with this update.

Ratzer

&&

.DISCUSSION...
Issued at 254 PM CDT Thu Mar 21 2024

Through Friday Night:

Conditions so far today have been rather quiet across the area. A
surface high parked just to our north has kept winds light to calm
while temperatures are seasonable in the lower 40s. Meanwhile, water
vapor imagery showcases an upper disturbance moving from the Pacific
Northwest into the northern High Plains this afternoon. This wave is
what will become tomorrow`s storm system which will bring a
potential for upwards of several inches of snow to our far north.
Lee cyclogenesis will take off this evening and spin up a strong low
level circulation which will push into the region tonight. Snow will
begin to move into far northern IL from the west late tonight. While
the heaviest snow potential exists during the morning, a textbook
environment favorable for low level CSI beneath a layer of potential
instability in the mid-levels could bring some decent snow
rates, mainly north of I-88, during the night. It`s not out of
the question that we could even hear a clap or two of thunder
with this first push of snow. As morning approaches, snow should
transition to a rain/snow mix over areas roughly south of I-88
with much of that going to all rain by late morning. We expect
to stick with snow or at least majority snow closer to the state
line.

During the early morning, an intensifying low-mid level jet atop
the storm`s northern flank will lead to terribly efficient
deformation and, in turn, a robust frontogenetic circulation
primarily focused in the 850-700 mb layer. Atop this circulation,
models are in good agreement on a layer of convective instability,
which we shouldn`t have any problems realizing, coincident with as
many as -20 to -25 ubar/s of vertical momentum. All that said,
confidence is rather high in a several hour period of snow rates in
excess of 1"/hr and significant snow accumulations beneath this f-
gen band. The tricky part is figuring whether the brunt of this
forcing moves over the CWA or stays just to our north in
southern WI.

A majority of models and model ensemble members have this east-west
oriented circulation favoring just north of the IL/WI state line.
However, numerous pieces of guidance suggest that the axis of
heavier snow could very well dip south into our northern CWA.
Guidance could honestly be in much worse agreement, but it`s likely
that we`ll find a tight gradient in snow totals on either side of
the f-gen band with one county probably meaning the difference
between minor and significant impacts. It does appear as though more
pieces of guidance are favoring far southern WI for the heavier snow
than yesterday. It`s also worth noting that there is now far less
support among models for the band to move exclusively across our CWA
as opposed to right on top of the state line or up across southern
WI. Even the Euro and Canadian`s respective ensembles strongly favor
us just missing out on the more significant snow, where the two`s
operational runs have been some of the farthest south among guidance
these past two days. The GFS and NAM have consistently been farther
north leaving our area with a few inches at most. The majority of
CAM guidance has the band straddling the state line, if not missing
us by a hair.

If the heaviest axis does decide to push across WI`s southern-
most counties, a few to several inches of snow will still be
expected across our far north. This is sort of the route we took
with our snowfall forecast leaving the northern-most counties
with anywhere from 1-2 inches across their south to around 5 or
6 inches right up against the state line. But again, even a
small change in the thinking with the position of this band
could mean noteworthy changes to the snowfall forecast in our
north. With this in mind, we decided to stick with an advisory
for our northern-most counties, with the recent addition of Lake
County. The time window for this heavier snow band is early-
late morning which encompasses the morning commute. At least
minor impacts to the morning commute can be expected in our far
north, but prepare for more significant impacts including very
hazardous travel.

During the afternoon, a line of precip will push southeastward
across the CWA as the storm works it`s way off to the east. P-type
with this line is rather tricky with the low level thermal profile
not well agreed upon. Forcing along this line could support some
rather heavy precip or maybe even some elevated thunderstorms while
this moves through. This means that a quick hit of up to an
inch of accumulation would be possible across portions of the
metro if the cooler solutions verify. Didn`t do a ton with this
signal in the forecast yet given the lack of support.

Doom


Saturday through Thursday:

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 /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 638 PM CDT Thu Mar 21 2024

For ORD & MDW:

- Sctd SHSN probably (50-60% chance) will impact terminals at
  times overnight with brief IFR VSBY
- A 2-4 hour period of IFR SN likely (70-80% chance) mid-late
  afternoon, with LIFR visibility possible (30-40% chance) at
  times
- Some snow accumulation possible (40% chance) mid-late Friday
  afternoon

For RFD:

- Sctd SHSN expected in/around RFD overnight, likely to build to
  a steady, more continuous SN early Fri AM, ending mid
  afternoon
- Periodic IFR VSBY possible (30-40% chance) with overnight
  SHSN, IFR to OCNL LIFR VSBY expected with SN Fri AM into early
  Fri afternoon
- Some snow accumulation likely, particularly Friday morning
  into early afternoon

VFR conditions expected this evening before scattered snow
showers likely spread across northern IL overnight into early
Friday morning. Snow showers overnight look to be more periodic
in nature with better chances of steadier/more continuous snow
just north of the terminals, though it will probably be close
at RFD.

An approximately 50 mile wide band of snow will move move
southward across northern Illinois Friday. Initially affecting
RFD during the morning, then moving across the more immediate
Chicago terminals Friday afternoon. Band will initially be
slower moving and therefore last longer at RFD. As the band
moves south across the Chicago metro Friday afternoon, it will
likely be moving more swiftly with snow probably only lasting a
couple/few hours. Within this band, snow could fall rather
heavily at times, possibly LIFR or VLIFR for a time. The chance
of accumulations at RFD are higher due to the expected longer
duration, but even with a short couple/few hour burst of snow
there could be some accumulations at the Chicago terminals
Friday afternoon. Snow should end by around 00z at the Chicago
terminals with just some MVFR CIGS lingering Friday evening.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory from 1 AM to 4 PM CDT Friday for
     ILZ003-ILZ004-ILZ005-ILZ006.

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

&&

$$

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