756
FXUS63 KLOT 251812
AFDLOT

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

.KEY MESSAGES...

- 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.

&&

.DISCUSSION...
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

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 112 PM CDT Sat May 25 2024

Forecast concerns for the terminals...

- Lake breeze turning winds easterly at the Chicago terminals
  this afternoon

- Couple rounds of showers and thunderstorms possible Sunday,
  first round in the morning with a second round in the late
  afternoon/evening

High pressure will continue to build into the area this
afternoon which will keep sunny skies and light winds in place.
However, a lake breeze is surging inland across northeast IL and
is expected to reach ORD around 19z and MDW between 18z and
1830z. Behind the lake breeze winds will become easterly with a
slight uptick in wind speeds into the 8 to 10 kt range.
Elsewhere, winds will maintain a north-northwest direction
through this afternoon before turning southeasterly overnight.

Heading into Sunday, a line of showers and thunderstorms is
expected to develop across the central Plains and advance
across northern IL Sunday morning through early afternoon.
While initially these showers and storms should arrive in a
decaying state, instability is forecast to be gradually
increasing through the morning as the showers/storms approach
the Chicago terminals (around 16-17z). Therefore, confidence
remains modestly high that showers and storms will impact the
terminals Sunday morning and have maintained the prevailing SHRA
and VCTS mentions. Though, have left GYY dry as the storms are
not expected to reach the airport until after 18z which beyond
their respective TAF period.

Depending on how quickly this initial line of showers/storms
exits the area Sunday afternoon will determine how much
destabilization occurs and whether or not a second round of
storms develops late Sunday afternoon/evening (similar to
yesterday`s forecast). Guidance is unfortunately showing little
agreement on whether or not the environment will recover and
therefore confidence is lower on any impacts at the terminals.
However, felt there was still a sufficient signal to warrant
the addition of a PROB30 to the 30-hour TAF sites to account for
this potential.

Yack

&&

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

&&

$$

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