951
FXUS63 KLOT 261750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Sun May 26 2024

.KEY MESSAGES...

- Conditional strong to severe thunderstorm potential late this
  afternoon into the evening.

- Scattered afternoon and early evening thunderstorms on
  Memorial Day and again Tuesday.

&&

.MESOSCALE...
Issued at 1141 AM CDT Sun May 26 2024

An initial batch of rain and embedded thunderstorms associated
with a lead shortwave interacting with a plume of steep mid-
level lapse rates will be pushing out of our region through noon
to 1 pm or so. This may yield a bit of a lull in activity across
our region. Noting at least modest pressure falls in and behind
the trailing stratiform precipitation region, on the order of 2
to 4 mb/1-2 hour, and there`s also clearly a circulation
pinwheeling across La Salle County (in regional radar mosaics)
which is indicative of subsidence warming occurring in the mid-
levels. This is suggestive of at least a modest wake low (or a
pair of mesolows), and have seen commensurate increases in peak
gusts across our I-39 observations (to about 35 mph). Typically
we need a more robust/severe MCS before we get concerned about
more significant trailing non-thunderstorm wind gusts. That
said, recent HRRR runs are depicting two swaths of strong
southeasterly gusts in excess of 40-45 mph (across our NW and
far SE), and this model tends to sniff out these types of non-
thunderstorm gusts pretty well. We`ll keep an eye on observation
trends.

The main focus remains on what happens later this afternoon.
Model guidance now hints at a fairly swift airmass recovery
given the slightly earlier arrival and departure of this first
batch of precip. Skies look like they may try to clear out
across our west through early this afternoon, and seeing breezy
southwesterly winds developing across SE Iowa where clouds have
eroded with temperatures jumping into the 70s. HRRR/RAP guidance
continues to advect mid and even upper 60 degree dewpoints into
the region this afternoon which seems perhaps a smidge high
based on upstream observations. Even accounting for this,
instability is forecast to build, with MLCAPE climbing back
towards 1500 J/kg late in the afternoon, focusing in a
relative corridor south of I-88 and east of about I-55. The
main surface low (convectively-augmented) will be lifting
northeast into southern Wisconsin, with only a glancing blow in
our area of the more concentrated region of DCVA overlapping the
best instability/kinematic parameter space south of I-80. This
suggests storm coverage will probably become increasingly
isolated the farther south you go through our forecast area,
with the greatest coverage tied to the surface low across our
north and towards the Wisconsin state line.

All of this said, there is some growing concern that a late-day
severe threat may indeed materialize in the 3 to 10 pm time
frame as a result. Low-level shear doesn`t look particularly
impressive, but deep layer shear values nearing 40 knots
suggest a potential for mid-level updraft rotation/organization.
It looks like the main threats would be from large hail and
strong wind gusts, but can`t rule out a brief tornado potential
in the vicinity of lingering backed southeasterly winds.

Carlaw

&&

.DISCUSSION...
Issued at 306 AM CDT Sun May 26 2024

Through Monday:

Early this morning, we find a center of low pressure spinning up out
over the central Plains with the storm`s warm front extending across
central Missouri and into southern Illinois. On either side of the
front and as far northward as central and eastern IA, efficient low-
mid level warm advection is driving a large swath of showers and
thunderstorms. A handful of stronger storms can be found south of
the front beneath an impressive EML plume which will eventually be
working over our area this afternoon. As the low treks
northeastward, these showers and storms will continue to be pushed
across the Midwest and fall on the area this morning. Precip should
begin to approach the I-39 corridor sometime around dawn. The storms
currently out in eastern IA are expected to weaken some after
crossing the Mississippi as they outrun the better instability,
especially north of I-80. Some late CAM guidance suggests that we
could see a mature linear MCS develop ahead of the low prior to
moving across northern Illinois later in the morning. If organized
convection can get going this morning, locally strong to marginally
severe wind gusts may manifest, particularly in the far southern
reaches of the CWA. The HRRR is one such model and even takes it a
step further resolving a mesolow with a tight pressure rise/fall
couplet on the leading edge. Such an artifact would bump the severe
wind potential up a bit and could also bring the potential a little
farther north during the morning, possibly across portions of the
Chicago metro. In either case, the severe wind threat looks notably
better just south of the CWA where we`ll find much more instability
and shear. Embedded thunderstorms should be on the overall lighter
side north of I-80 where far less instability will reside.

The brunt of the morning activity will be out of the area by early
afternoon. A lot of big question marks continue to revolve around
how our environment will recover following the morning. The
aforementioned EML will overspread the region during the afternoon
building up our mid level instability. Our afternoon and evening
storm potential, including the severe threat, will be heavily
reliant on how much clearing we can see behind the morning push.
Even then it`s tough to say how the low levels will respond with
some models having similar looks regarding sky cover but very
different ideas for low level thermo profiles. Undoubtedly, the best
potential for seeing surface-based convection resides south of the
effective warm front which should remain well south of the area.
North of the front, guidance is messy but most can agree that we
will struggle to storms based in the lowest 100 mb or so owing to
very poor low level lapse rates if not a low level capping
inversion. The HRRR is an outlier being the only one showing an
uncapped environment with steep low level lapse rates during the
late afternoon. Should we manage to get anything surface-based
going, an impressive 0-1 km hodograph could mean a few tornadoes
attempt to spin up. If storms remain even a bit elevated, which
looks to be the best bet, then severe wind and hail would be the
biggest concern later today. The greatest severe potential later
today will again be across the southern CWA in a few hour window
between late afternoon and early evening, though odds are again
better down across central and southern IL. The storm threat should
pretty much wrap up behind the system`s cold front which will sweep
across during the evening.

Monday will be a bit cooler for the holiday with highs forecast to
top out in the lower 70s. The better part of the day should see a
fair mix of clouds and sunshine with good westerly breeze.
Several models are now resolving scattered showers or even a
couple of thunderstorms during the afternoon. Diurnal heating
should do away with a morning cap and steep low level lapse
rates will extend up to a reservoir of modest mid level
instability. With upwards of around 50 kt of deep layer shear
expected, it`s possible that a few thunderstorms could feature
some locally strong wind gusts. It`s possible that precip
struggles to combat a rather dry profile but could get a boost
from an encroaching 500mb jet max. Storm chances should dissolve
by mid-evening as instability wanes.

Doom


Monday Night through Saturday:

An upper level ridge axis building across the Rockies early in
the week will drive an active belt of enhanced northwesterly flow
aloft across the Great Lakes region through Tuesday. Within this
pattern, two distinct impulses will track southeastward across
our area. The first will track into our area later on Monday, and
that is talked about in the discussion above. Following quickly
on its heals, a stronger impulse (currently seen spinning in the
water vapor imagery across central Manitoba) will eject
southeastward into the western Great Lakes on Tuesday along the
eastern periphery of the building upper ridge axis.

A chilly late May airmass accompanying this impulse is likely to
make Tuesday one of the coolest days of the week. In fact, breezy
northwesterly winds through the day, combined with increased
afternoon cloud cover could keep temperatures in some parts of
northern IL from climbing out of the 60s! There will also be
another good shot for scattered afternoon showers and even a few
storms as lapse rates steepen under the cold mid-level trough
(temps at 500 mb around -20C).

Following the eastward passage of this second impulse on Tuesday,
the ridge axis over the Rockies will shift eastward, likely
becoming centered over the western Great Lakes later in the week.
As this occurs, we are looking to see at least a few day period
of dry (and rather pleasant) weather 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, and while
temperatures will remain in the low to mid 70s for Wednesday and
Thursday, readings are expected to rebound back to near 80 for
the end of the week into next weekend.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1250 PM CDT Sun May 26 2024

Key aviation messages:

- First round of RA ending by 19Z
- Increasing scattered TSRA potential late afternoon into the
  evening, some could be severe
- Westerly wind shift late this evening along with MVFR cigs
- 30-40% chance for scattered TSRA again Monday afternoon

The area of rain will begin to lift north of the terminals over
the next hour or so. A lull in the precipitation may occur for
an hour or two but signs are pointing toward renewed
thunderstorm development later this afternoon and continuing
into the evening hours. Accordingly have introduced VCTS for a
broader time window for the Chicago area terminals from 22-03Z
with targeted shorter TEMPO groups for TSRA. Any storms that
develop will have the potential to become severe.

Wind directions will favor an easterly direction this afternoon
then southeast by late afternoon/early evening. A cold front
will then swing through the area this evening, earliest at RFD,
latest at GYY turning winds west northwest. The timing of this
wind shift will be refined with later updates and may arrive a
bit earlier than currently advertised in the TAFs depending on
how quickly thunderstorms exit to the southeast.

MVFR stratus is expected to fill in behind the front and
persist overnight into Monday morning. Periods of IFR can`t be
ruled out.

Lastly, toward the end of the 30-hr TAF period there is a 30-40%
chance for scattered gusty showers and thunderstorms again
Monday afternoon into the evening.

Petr

&&

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

&&

$$

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