Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 262128

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
428 PM CDT Sun Mar 26 2023

Issued at 428 PM CDT Sun Mar 26 2023

Clusters of showers and thunderstorms continue to develop this
afternoon. Most storms seem to be producing at least small hail,
with occasional instances of near to severe hail. Thus far, storm
motions have been more northeasterly (as opposed to easterly on
the right motion vector), which has been carrying them fairly
swiftly north of the stalled frontal zone, limiting the time
available to produce a wind or tornado threat.

Based on observations and fast looping of our radar, it appears
this boundary is sitting just south of Morris, right along the
southern flanks of Will County, and then towards Kouts, Indiana.
For any cells crossing north of the boundary, the main threat will
be from hail. The main area for any brief tornado potential will
likely be tied to areas near and just south of the aformentioned
stalled boundary. Severe threat will wane after 6-7 PM.



Issued at 153 PM CDT Sun Mar 26 2023

Through Tonight...

Short term focus continues to be on the strong to severe
thunderstorm potential this afternoon.

Surface analysis reveals the warm front is likely approaching La
Salle to Joliet to Hebron or thereabouts. To the north, lake-
enhanced push of cooler air with strengthening northeasterlies is
evident, with temperatures dropping near the lake over the last
hour. Latest thinking is the aforementioned locales represent a
reasonable area for the eventual warm front to stall and set up
shop the next few hours. To the south, surface dewpoints continue
to inch their way into the mid and upper 40s. As mentioned
previously, this is just about as low as we tend to see supportive
of surface-based strong to severe convection and remains an
overall limiting factor in the robustness of the threat today.
That said, fairly cold temperatures aloft (around -9 C at 700 mb)
and steepening low-level lapse rates may somewhat offset this,
combined with the sharp nature of the attendant upper impulse.
RAP forecasts depict a 50 ish knot 700 mb jet beginning to cross
east of the Mississippi River, and this will drive commensurate
increases in effective bulk shear towards 40-45 kts this
afternoon supportive of low topped supercells.

Recent satellite loops show increasing bumpiness/congested Cu
developing with MLCIN continuing to slowly erode. Convection
should be initiating shortly. Based on the latest best guess
where the stationary boundary will set up, the main axis favoring
a brief tornado threat will be essentially about 20 miles or so
either side of I-80. The brief tornado threat may also extend
farther south of the Kankakee River, although more veered surface
winds look to cut down on hodograph curvature to some degree. In
this area, the locally damaging wind threat seems to be primary.
The severe threat should wane after about 6-7 PM CDT with the loss
of heating and instability.

This evening and into the early overnight hours, additional
precipitation should develop with the deformation axis across
northwest Illinois. Thermal profiles will initially support mainly
rain, but a transition to a rain/snow mix or even all snow is
possible after about 7-8 PM. Do have some very minor snow
accumulations in the gridded forecast north of about I-88. Any
accumulations should be relegated to grassy or elevated surfaces,
with precipitation expected to largely come to an end during the
early overnight.



Issued at 153 PM CDT Sun Mar 26 2023

Monday through Sunday...

For the first half of the workweek, we`ll find ourselves in a
zonal/quasi-zonal flow regime with the jet stream and an associated
baroclinic zone positioned near/over the region. After today`s lead
shortwave clears the area, a handful of additional disturbances will
traverse through the Midwest and Great Lakes through mid-week.

The first of these disturbances will be riding the coattails of the
lead shortwave, passing to our south tonight into Monday.
Accordingly, the bulk of the precipitation associated with this
trailing wave should remain south of our CWA. A trailing mid/upper-
level vorticity filament could help induce some spotty showers or
sprinkles along a subtle surface convergence axis in our western CWA
Monday afternoon, though the likelihood of getting any measurable
precipitation out of this appears to be low enough to justify
keeping PoPs capped below slight chance for now. Monday`s highs look
to top out in the 40s across most of our forecast area, though
temperatures may not make it out of the upper 30s in our lake-
adjacent locales with onshore flow prevailing through the entire day.

The next wave is progged to scoot by to our south on Tuesday. The
predominant signal in guidance is currently for the bulk of the
precipitation with this wave to remain to our south once again, so
did not feel compelled to stray from the dry NBM PoPs for Tuesday.
That said, the 15Z RAP and 12Z HRRR do show a slight uptick in UVV
over our CWA right at the tail end of their respective runs, and
this is evidently enough for those models to be outputting some
splotchy QPF in or very near our CWA Tuesday morning, so some
refinements to Tuesday`s PoPs may eventually need to be made if this
signal holds steady and/or gains traction in other guidance.

Then, Tuesday night into Wednesday, a vort lobe will be in the
process of getting dislodged from an upper-level low meandering over
Hudson Bay, and the associated low-level response will likely
involve a cold front getting thrusted southward into the Great
Lakes. Moisture profiles on Wednesday don`t exactly look stellar,
and the bulk of the forcing for ascent should remain displaced to
our northeast, but most available forecast guidance is insistent
that we`ll still get some precipitation as the front swings through
the area, nevertheless. The NBM came in with slight chance and low-
end chance PoPs on Wednesday for much of our CWA, and that seems
adequate for now. Progged thermal profiles show support for
precipitation coming in the form of both rain and/or snow, with the
timing of the cold frontal passage likely to be the biggest factor
in determining which precipitation type is favored and where. Winds
also look to become fairly blustery on Wednesday as the front moves
through the area.

Moderating temperatures are then expected into the latter half of
the week as central/eastern CONUS ridging and western CONUS
troughing establish relatively uninhibited south-to-north thermal
and moisture trajectories. The western trough is eventually expected
to eject northeastward towards the Midwest, guiding a deepening low
pressure system in the same general direction. It`s still too early
to attempt to parse out the finer forecast details for what this may
entail for us locally, but rain showers do look like a good bet late
Thursday into Friday, and thunderstorms could certainly occur as
well if the low takes a more northerly track. Given that this late
week system could be pretty potent and draw up a healthy slug of
moisture to our latitude, we`ll need to keep an eye on potential
threats for heavy rainfall and/or severe weather in the region as
well, particularly on Friday. If the low were to take a more
southerly track, then snow wouldn`t be out of the question either
as the backside of the system passes through, but again, this
system is still too far out to say that this will be the case with
any semblance of confidence.



For the 18Z TAFs...

The main focus in this period is on TS potential late this
afternoon at the Chicago metro terminals.

A mid-level jet streak will interact with a warm front moving
northward through central Illinois to produce increasing shower
and embedded TS potential near and south of the Chicago metro late
this afternoon. Surface-based TS will remain south of the warm
front and the terminals, but steep lapse rates aloft will support
isolated TS north of the warm front. Confidence in TS has
therefore increased enough to introduce VCTS in the TAF at ORD/MDW
in the 21-23Z window. Residual SHRA should end by around 01Z. An
area of -DZSN along the Wisconsin line is expected to affect RFD
through late evening, but remain just north of ORD/DPA.

ENE winds around 10 knots will quickly veer to near N around 15
knots shortly after the TS potential ends. Winds may briefly back
west of north for a couple hours early to mid-evening before
settling NNE through the remainder of the night and into Monday.

Otherwise, MVFR ceilings at times mid-afternoon to early evening
will lower to low-end MVFR to perhaps IFR levels through much of
the night. Ceiling trends will then improve through the day Monday
with skies likely scattering by the afternoon.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...1 AM Monday to 3
     PM Monday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...1 AM Monday to 7
     PM Monday.



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