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FXUS63 KLOT 250830
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 AM CDT Sat May 25 2024

.KEY MESSAGES...

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- Dry and pleasant today.

- Numerous showers and thunderstorms on parts of Sunday, with an
  attendant afternoon-early evening severe weather threat that
  includes the potential for large hail, damaging winds, and
  tornadoes.

- Widely scattered afternoon and early evening showers (and
  possibly a few non-severe storms) possibly Memorial Day and
  again Tuesday, though many dry hours are anticipated.
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&&

.DISCUSSION...
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Issued at 330 AM CDT Sat May 25 2024

Through Sunday night:

Early this morning, we find a weak cold front pushing across the CWA
with dry conditions out ahead of it. Meanwhile, a center of surface
high pressure trails not far behind riding along the MO/IA state
line. After this boundary moves east of the CWA shortly after
sunrise, the high will center itself over central and northern IL
for the day providing us with a gorgeous May Saturday. A plume of
awfully dry air behind the front is evident on regional water vapor
imagery. This will lead to sunny skies today with light winds and
highs in the middle to upper 70s forecast, while areas near the
lakeshore will likely be kept in the 60s.

The focus in the short term is a potential for a couple of rounds of
thunderstorms on Sunday, including an all-hazards severe potential
focused in the afternoon and early evening. A low-amplitude upper
trough will eject east of the Rockies today and spin up a surface
low pressure center over the central Plains which will work into the
Midwest late tonight. From this point, models have a very different
look to them now than with yesterday`s daytime runs. Guidance can
agree that, early Sunday, the low level circulation tied to the
surface low will become cutoff and shoot northward on the leeward
side of the broader trough which will continue its eastward trek
across the region. The result is two focused areas of surface low
pressure that will move through the region.

Prior to last evening`s 00Z runs, just about all guidance had the
northern-most circulation dominating as it tracks from eastern IA
into far southern WI during the day on Sunday. This had guidance
pulling the effective warm front and surface-based instability
overall farther north into the CWA with the focus for severe
convection being out ahead of the trailing cold front. Now, most
models have the other, southern-most circulation taking much
precedence as it moves somewhere across central IL. This could shunt
the northward progression of higher theta-e air into northern IL. A
couple of models are spitting out sort of a hybrid solution which
consolidates the two circulations into a single low that tracks
across northern IL. Nonetheless, it`s now looking like the more
appreciable severe threat may be focused closer to central IL.

An EML will advect eastward from the central Plains and overspread
the region late tonight into early Sunday. Resultantly, scattered
showers and thunderstorms are expected in the area during the
morning and early afternoon as the system approaches from the west.
While it looks like just about all of the morning activity will be
elevated, noteworthy mid level instability and effective shear could
produce some gusty winds, heavy downpours, or possibly even some
small hail.

It`s still unknown how well the environment will recover following
the morning activity. Efficient low-level warm and moist
advection and continued steep mid-level lapse rates look to
keep a healthy reservoir of MUCAPE built up over the area into
the afternoon. However, low-level instability and the extent of
capping to surface-based and mixed layer parcels is highly
uncertain and will play a big role in the afternoon severe
potential, particularly regarding the tornado threat. Now that
models are honing in on this more southern track, this places
the effective warm front and better surface-based instability
farther south than previously thought, although a great deal of
uncertainty still remains. Most models think that the front will
lift somewhere into the CWA while some pieces of guidance, the
GFS for one, keep the front south of the area.

Behind the front, southerly surface flow beneath modest SW low level
winds will result in long curved low level hodographs favorable for
mesocyclogenesis. While the magnitude of the low-level shear
doesn`t entirely stand out, impressive veering is driving up SRH
values. This will likely be even more true across areas south
and east of the low track where surface winds will likely back
to southeasterly. This presents a threat for some tornadoes with
any surface-based activity that we can get going during the
afternoon. Again though, low-level lapse rates may recover
poorly from the morning activity and could put a damper on the
tornado potential even in areas with no SB capping.

Appreciable mid-level instability and deep layer shear also
present a chance for severe winds or hail. At this point, the
hail and wind look like an area-wide potential, although the
overall severe threat should build with southward extent. The
potential will last into early evening before instability
dissolves and we may be left with just a few lingering showers
or lighter thunderstorms through the night.

Doom


Monday through Friday:

Following Sunday`s storm system, a building upper-level ridge
across the Rockies is expected to setup a cooler, but still
active, northwesterly upper-level flow pattern across the Great
Lakes region early next week. Ensemble guidance suggests that
this pattern will act to steer a few smaller scale impulses
southeastward into our area for Memorial Day and Tuesday.
Accordingly, there are chances (~30%) for some widely scattered
mid-late afternoon and early evening showers (and possibly a few
non-severe storms) both days. However, coverage may remain low
enough that not everyone will get rain. Either way, a majority of
Memorial Day will be dry and conducive to outdoor plans, though
temperatures will be cool, with highs only in the lower 70s
amidst northwest winds gusting up to 30 mph.

A few day period of dry (and rather pleasant) weather is expected
from Wednesday through the later part of the week as surface high
pressure sets up shop over the Great Lakes region. Mostly sunny
skies are expected with daytime temperatures forecast to be in
the 70s Wednesday and Thursday, before moderating back close to
80 by the end of the week.

KJB
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&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1212 AM CDT Sat May 25 2024

A surface cold front will shift across the Chicago area
terminals by 08z. In its wake, winds will turn northwesterly and
clouds will clear. Area observations do suggest that a brief
period (up to an hour) of MVFR CIGs may precede the frontal
boundary before conditions quickly clear out behind the front. I
opted to advertise this potential with a couple hour tempo
through 08z. Otherwise, VFR conditions are anticipated through
the remainder of TAF period.

While northwesterly winds will persist in the wake of the cold
front this morning, an afternoon lake breeze will turn the
winds easterly at the main Chicago terminals after 19z this
afternoon. East to southeasterly winds are then expected to
persist tonight as mid and high level cloud cover increases in
advance of the next approaching weather system slatted to impact
the area on Sunday.

KJB

&&

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

&&

$$

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