367
FXUS63 KLOT 260806
AFDLOT

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

.KEY MESSAGES...

- Periods of showers and thunderstorms today, with pockets of
  gusty winds possible in the morning and a highly conditional
  risk for strong to severe storms late afternoon into the
  evening.

- Widely scattered afternoon and early evening showers (and
  possibly a few non-severe storms) on Memorial Day and again
  Tuesday.

- Gradual warming and drying conditions for the end of the week.

&&

.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 /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 108 AM CDT Sun May 26 2024

Key messages/forecast concerns:

- Couple hour period of showers and thunderstorms expected mid
  to late this morning through early afternoon.

- Chance for a few additional widely scattered thunderstorms
  late this afternoon afternoon into early evening.

- Wind trends this afternoon and evening following midday
  storms.

- Low MVFR CIGs Potential tonight.

Thunderstorms continue to flourish across the Plains eastward into
parts of the lower Missouri Valley early this morning. This
activity is expected to continue eastward overnight, likely
reaching areas near the Mississippi around daybreak. Thereafter,
we expected a line of storms to shift eastward across KRFD
during the mid to late morning hours (~14 to 17z) and across the
Chicago area terminals from late morning through early afternoon
(~16 to 19z). The main threats from these storms will be locally
gusty winds and some heavy rainfall. The heavy rainfall may
create some temporarily IFR VSBYs.

After this band of storms shift east of the area after 19z, we
will have to monitor the threat from some renewed widely
scattered storm development later in the afternoon (after 21z).
Confidence is currently low on the extent of this redevelopment,
across northern IL, but with the main weather impulse right
across the region, it is certainly plausible that some activity
will be around later today through early evening. We thus
continue to carry a PROB30 mention for more potential late day
storms.

Winds will start the day from the east-southeast this morning,
then are expected to gradually shift to a west-southwesterly
direction sometime either later this afternoon, or early this
evening. The exact timing of this wind shift remains a bit
unclear at time, and will ultimately be dependent on where the
surface low tracks across northern IL later today. Given the
uncertainty, I did not stray from the going forecast of a west-
southwesterly wind shift early evening around 01z. However, we
will have to monitor the potential for it to occur earlier then
currently forecast.

Ample moisture wrapping around the backside of the surface low
may also foster some lower MVFR CIGs across the terminals
tonight into early Monday morning. Currently looks like the best
timing for these would come later this evening and overnight.

KJB

&&

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

&&

$$

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