192
FXUS63 KLOT 261936
AFDLOT

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

.KEY MESSAGES...

- Scattered thunderstorms late this afternoon through mid-
  evening, some of which could be strong to severe with large
  hail, gusty winds and possibly a brief tornado.

- Threat for a few strong to locally damaging wind gusts with
  scattered thunderstorms tomorrow afternoon, after about 3 PM.

&&

.DISCUSSION...
Issued at 236 PM CDT Sun May 26 2024

Through Tonight:

Precip from this morning`s convection has cleared the CWA, with
the associated cirrus shield trailing by an hour or two. Low-
level temps have quickly responded where clearing has occurred
in the southwest CWA. Temps there have risen solidly into the
mid 70s while dew points have held steady in the mid 60s. MLCIN
has correspondingly completely eroded for roughly the southwest
third of the CWA. With pockets of cumulus continuing to grow in
this area, convective initiation is probable within the next
hour. An initial area of convergence, likely from a small trough
axis pinwheeling around an upper-low to the west, appears to be
most favorable for initial development from I-39 SSW through
the IL River Valley. Additional axes of convergence are lined up
into eastern Iowa to along a cold front. Deep-layer shear
profiles will be supportive of organized convection across the
vast majority of the forecast area, but especially roughly along
and east of the I-55 corridor very late this afternoon through
early evening. While it is becoming quite likely that storms
will develop, potential coverage of convection is less clear
given the delayed destabilization this afternoon. At this time,
scattered coverage appears to be the most likely scenario versus
a solid line of storms.

Severe convection with large hail and gusty winds are possible. A
gradually veering near-surface wind profile will slowly lower the
tornado threat, but with existing axes of convergence (including a
lake-enhanced outflow through the Chicago metro) and a slightly
slower veering trend farther east could offer localized areas
supportive of a brief tornado from the southern Chicago metro and
roughly east of I-55.

Convection potential should end from west to east late this
evening, with dry conditions expected for much of the area
overnight.

Kluber


Monday and Monday night

Monday looks to start out on a generally cloudy note. Steep low-
level lapse rates will yield west to northwesterly breezes (gusts
to around 30 mph) within a well-mixed PBL.

The main focus is on the likelihood for scattered afternoon and
evening low (ish) topped showers and thunderstorms. While clouds
looks to remain fairly expansive through the day, instability is
expected to nonetheless build through the day as mid-level
temperatures cool in advance of an approach vort max (500 mb
temps will be near the 10th percentile for this time of year).
MLCAPE values won`t be particularly high, perhaps 500-750 J/kg,
but this should be sufficient for shower and storm development as
MLCIN erodes. The upstream vort max looks quite impressive across
Montana, and this feature will impinge on the region late in the
afternoon with attendant height falls and DCVA maximizing after 3
pm.

Have boosted PoPs a bit late in the afternoon and in particular
across our west (closer to the vort max). Deep layer shear will
modestly step up through the day as westerly 500 mb flow
increases and looks supportive of some degree of updraft
organization. Given the presence of steep low-level lapse rates,
this looks like a decent setup for a few strong to perhaps locally
damaging wind gusts out of swiftly-moving convective elements.
With how cold temperatures will be aloft, suspect thunderstorms
will be somewhat slow to diminish through the evening and have
held onto chance PoPs through the late evening hours.

Carlaw


Tuesday through Sunday

This discussion remains unchanged from the overnight shift.

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. Following
quickly the heels of Monday`s weather maker, 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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