284
FXUS63 KLOT 210548
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1248 AM CDT Tue May 21 2024

.KEY MESSAGES...

- Scattered severe storms with large hail and damaging winds
  ending by evening, favored across far eastern IL and NW
  Indiana.

- Lower chance for thunderstorms late tonight northwest of the
  Chicago metro

- Another round of thunderstorms is expected as early as mid to
  late Tuesday afternoon, but more likely Tuesday evening into
  the nighttime hours. Severe thunderstorms with damaging winds
  and brief spin-up type tornadoes are the main hazards, though
  large hail may occur as well.

&&

.DISCUSSION...
Issued at 327 PM CDT Mon May 20 2024

Through tonight:

The MCV has now shifted into southern Wisconsin, east of
Janesville and west of Racine. With an unstable atmosphere ahead
of the MCV, arcs of convection have shift mostly over Lake
Michigan this afternoon. Additional showers and storms may
occur across northern Illinois and even into northwest Indiana
into early evening where the atmosphere is unstable, but this
window will also be fairly short lived into the evening, and
areas farther south are a bit more removed from the MCV
influence. The severe threat is low (<10-15%), with hail to dime
sized and isolated instances of minor tree damage remain
possible. There is another subtle wave across SW Iowa that could
trigger a few more showers or storms atop a shallow boundary
layer through mid to late evening south of I-80, though
confidence on this is only low-medium.

There should be a period of ridging behind the MCV which should
lead to a dry period later this evening and into the overnight
hours, and the warm front that has shifted north today will
likely get shunted back south, with some uncertainty as to where
that will stall. This front in conjunction with additional
upstream waves that may be embedded in the flow aloft may
trigger some additional thunderstorms late tonight. Tough to
say right now as these are not apparent on upstream water vapor
satellite just yet.

Tuesday and Tuesday Night...

A complicated storm evolution is setting up for Tuesday. While
there are two sharp upper troughs upstream, our feature of
interest is the sharpening vort max nearing the desert
Southwest. This feature is well agreed upon in guidance to
strengthen as evidenced by becoming negatively tilted through
the afternoon hours. With a fairly potent southern stream jet
axis progged to shift into the central Plains, in addition to a
secondary northern stream jet core, the surface cyclogenesis
process will lead to deeply anomalous low pressure core that
will likely drop below 990 mb by late afternoon and even deeper
in the evening hours. The low strength will approach the bottom
of the NAEFS climatology for this time of year. These all point
to a significant severe weather event in the region.

From a sensible weather perspective, there are several periods of
concern. The first is along the advancing warm front ahead of the
low on Tuesday morning on increasing low level southwest flow.
The boundary will still be across northern Illinois during the
morning. The aforementioned system will be advancing upon an
elevated mixed layer, and thus any storms have the potential to
generate mostly sub- severe hail.

It will be a warm and breezy day following any morning activity, and
a rapidly destabilizing warm sector through the day portends a
concern for discrete supercells, with all hazards possible in
the mid to late afternoon as this larger system approaches. This
is currently favored across Iowa and into NW Illinois, but does
extend to the I-39 corridor. By the evening hours the deep
layer shear vector will become oriented more perpendicular to
the surging cold front across the plains as the surface low will
continue to shift north into Minnesota. Upscale growth into a
mature wind producing system (MCS) appears the more likely mode
in the evening, making damaging winds the highest concern,
although some lingering low level shear will still support a
tornado threat along the line.

There is still some uncertainty as to how far south and east the
severe threat will be into the evening, thus the highest severe
probabilities do still remain favored west, but with the potential
for a fairly organized MCS these hazards will extend across much of
northeast Illinois and possibly into northwest Indiana as well.

Wednesday and Wednesday night:

There is still some signal for showers later Wednesday into
Wednesday evening south and east of I-57, but most areas will
have cooler conditions and a dry day.

KMD

Rest of the forecast discussion unchanged...

Thursday through Sunday:

Brief upper level ridging should allow for better height rises
on Thursday for much drier conditions setting up. Winds should
be out of the west and light with the occasional afternoon burst
to 15 mph. Afternoon temperatures should warm into the 70s,
with slightly cooler conditions in Northern Indiana along the
immediate shoreline.

The next upper level trough ejects out of the Rockies Thursday
night and starts tracking toward southeastern Manitoba. Despite
the system`s track being farther to the northwest than earlier
in the week. As the reflected surface low passes through the
Dakotas and Minnesota, a warm front will slowly enter Illinois
on Friday. Southerly winds will slowly advect precipitable water
amounts over an inch back into the area. The best instability
will be in the warm sector behind the front in a Rockford to
Fowler, IN line, but even then forcing does not look impressive.
So PoPs for scattered showers were kept at the chance
threshold.

Overnight and into Saturday, the low exits into Canada as a
secondary surface low strengthens over the Southern Plains.
Friday`s surface front will be come a NW/SE oriented cold front
and move over the region on Saturday. With it comes better
forcing and instability to allow widely scattered thunderstorms
to develop through the day. This far out, timing can be adjusted
closer to the event. At this juncture, model soundings have
weaker mid level lapse rates and deep layer shear to keep the
word severe out of the forecast. There may be isolated showers
that stick around through Monday, but there should be breaks in
the rain. However, confidence remains to low this far out for
specifics at this time.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1248 AM CDT Tue May 21 2024

Key Aviation Impacts for the current TAF period:

- Slight chance for some low end MVFR cig around daybreak, but
  confidence remains low

- Strong southerly winds develop late morning. Gusts will
  gradually ramp up from 20 to 35 knots through the afternoon,
  potentially occasional gusts over 40 knots during the evening.

- A strong and potentially severe line of storms is likely to
  arrive in the RFD airspace around 00Z and to the Chicago
  terminals just after 02Z

The TAFs will remain dry and VFR through 00Z. There is a cloud
deck around 1000-1500 near Grand Rapids, MI that will be
monitored through the night. Therefore, a SCT008-SCT015 group
was left in area terminals for the slim chance for some low end
MVFR cig development overnight, but confidence is low.

Winds will be out of the SE through Tuesday morning and early
afternoon, but as a low level jet aloft increases, the direction
is expected to become more true south. Gusts will slowly
increase around noon to 20-25 knots, but by the late afternoon,
gusts around 35 knots are expected with the potential for gusts
greater than or equal to 40 knots during the evening.

By Tuesday evening, a strong squall line will exit Iowa and move
east over all the area terminals. This line of storms will be
strong to potentially severe, where strong localized westerly
winds gusts, hail, and maybe a brief tornado will be possible as
it passes. Current models have dry conditions before and after
it, so there is no VCTS/VCSH prevailing, and the main line is
represented in the TEMPO. The line is expected to move quickly
through the area and not stall, so the timing of the TEMPO was
reduced to give moderate confidence in the projected time
window, though it can be shifted if needed at later TAF
packages.

Low confidence in any lingering showers behind the line. Some
models have it but kept the TAFs dry for now. Main impacts after
the line will be from cigs and winds. MVFR conditions from cigs
2000 feet or higher are expected behind the line. Additionally,
west southwest winds are expected to remain strong. Kept gusts
around or just under 25 knots at this issuance, but it would not
be surprising if there was a secondary "pop" of winds behind the
line between 04Z-12Z where gusts went back over 30 knots.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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