952
FXUS63 KLOT 181716
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1216 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Non-zero shower potential this afternoon/evening as a weak
  cold front moves across the area

- A few scattered shower and thunderstorms possible Sunday PM,
  mainly around and south of I-80

- Unseasonably warm weather expected through Tuesday night,
  interrupted only by lake cooling Sunday; then during any
  showers/storms Monday and Tuesday.

- A couple/few bouts of showers and storms expected Sunday night
  through Tuesday night, with many dry hours in between.

- Locally heavy rainfall possible Monday-Tuesday night, with some
  potential for severe weather Tuesday, mainly Tuesday night

&&

.DISCUSSION...
Issued at 222 AM CDT Sat May 18 2024

Through Sunday Night:

As an upper level ridge grows to the northeast over Illinois on
Saturday, warmer and drier conditions are expected to develop.
With both O`Hare and Midway remaining in the low 60s around 2 AM
today and the knowledge that temps over performed on Friday,
and clearer skies expected through the day to allow for maximum
sunshine, afternoon high temperatures today were knocked up a
couple degrees above guidance.

Meanwhile, a surface low in eastern South Dakota will slowly
travel northeastward into Canada through the day. A cold front
extending southward from the low will move over Iowa and
eventually into northwestern Illinois Saturday evening. The
consensus of high res guidance is suggesting that as the sun
sets, the front will slowly weaken as it approaches Lee and Ogle
counties with the showers slowly fizzling out. However, there
is a non-zero chance that the front holds together a little bit
better and can be just enough forcing along with some
isentropic upglide to provide the occasional isolated shower.
But with the lack of confidence and better forcing closer to the
low up north, PoPs were kept below the mentionable 15 percent
as drier conditions are favored, with the exception being over
the lake where there was slightly better instability favored in
model soundings.

That front will continue to move through the forecast area on
Sunday morning. As it moves east, it is projected to slow down
and set up a boundary extending from southwestern Michigan to
northern Missouri, pretty much along I-80. To the north of area
will be slightly mode stable air and northeasterly winds driven
off the lake to help add a little bit for cooler relief to the
warm temperatures expected. However, along and south of I-80
will be better instability and dew points in the low 60s. The
overall set up does not arouse too much excitement given that
mid level lapse rates do not look overly impressive and deep
layer shear is limiting; however, for areas south of I-80, there
remains a chance for a few scattered showers and thunderstorms
Sunday afternoon and into the evening.

DK

Monday through Friday:

Large scale pattern through early next week will feature western
trough/eastern ridging. A weak front is expected to stall out
over central IL/IN Sunday evening. It will then lift north as a
warm front Sunday night in response to a lead shortwave trough
ejecting out of the western trough and toward the Midwest.
Stronger forcing associated with the shortwave looks to arrive
after the front is progged to have lifted north into WI, so better
precip chances look to largely stay north/west of our CWA Sunday
night.

Getting a pretty consistent signal in the various models that
this lead shortwave will become convectively enhanced as
convection across the central Plains congeals into an MCS and
moves into the Corn Belt Sunday night. Chances for convection
Monday will hinge on the ultimate evolution of this lead (likely
convectively augmented) short wave trough that is expected to
move into the western Great Lakes Monday. Some decaying remnant
activity from this MCS could spill into our CWA Monday morning.

Based on consensus of the latest guidance suite, the strongest
forcing with this MCV looks to remain north of our area Monday.
However, assuming convective debris doesn`t stifle
destabilization, strong heating of moderately moist air mass
could allow for 1-2k J/kg of MLCAPE to be realized Monday
afternoon with minimal inhibition. Given the expected
unstable/uncapped air mass over the area, the glancing blow from
the vort passing to our north and/or remnant outflow boundary
(or even outflow enhanced synoptic boundary) could be enough to
spark renewed or reinvigorated convection Monday afternoon.
Weakening mid-level flow is expected as mid-level speed max
exits the region, so weak shear profiles Monday afternoon should
limit severe threat to just an isolated pulse severe storm or
weakly organized multi-cells.

Another round of potentially severe convection is expected
Monday afternoon across the central Plains as a significant
portion of the western trough breaks off and moves northeast
into the Plains. This Plains convection Monday afternoon and
evening could once again congeal into an MCS and track across
the Corn Belt Monday night and eventually into the western Great
Lakes Tuesday morning. This far out, it is impossible to say
with any degree of confidence where this convection could end up
Tuesday morning (assuming it hold together).

By Tuesday afternoon, strong, negatively upper trough is
expected to track across the northern Plains into the upper
Mississippi Valley. Strong surface cyclone is expected to
support potentially widespread severe thunderstorm development
Tuesday afternoon. Guidance has continued to slow (a very common
theme with medium range guidance with a big system like this)
suggesting the better threat of afternoon severe weather Tuesday
could end up well west of our CWA across Iowa, with out CWA
potentially dry (and VERY warm, but more on that in a bit)
Tuesday afternoon. Eventually this convection would likely move
east across the MS River and into our CWA later Tuesday or
overnight Tuesday night, probably in, or soon to begin, a
weakening phase by the time it arrives in our CWA. Given the
progged strength of the synoptic system, we will need to monitor
the potential for severe wx Tues (mainly at night) closely the
next few days.

Unseasonably warm temperatures (well into the 80s) are expected
Monday, assuming left over convection (or convective debris)
doesn`t survive into our area and impede warming or alter wind
fields resulting in a lake breeze. Tuesday has potential to be
even warmer, again with the same convective caveats as Monday.
In fact, with slower solution suggesting a better chance of dry
or mostly dry conditions Tuesday afternoon, temps could really
get out of hand. Progged 925mb temps at least solidly into the
lower 20s (Celsius), if not mid 20s, means with any amount of
sunshine, some areas could see highs eclipsing 90 degrees
Tuesday. Didn`t stray from current NBM mid- upper 80s for now
given uncertainties, but if trends continue then some areas will
probable see their first 90 of the year Tuesday.

Forecast confidence Wednesday onward decreases. Chance pops
offered up by NBM for Wednesday are contingent on the system
slowing and front stalling out in our CWA. Conceptually, the far
more likely scenario is the front will be pushed safely south
and east of our CWA with dry conditions Wednesday, but didn`t
adjust with lowering NBM pops for now given the uncertainties
and propensity for these systems to slow in later run. Thursday
(at this point) will probably be dry in between systems, but
large degree of model spread from Thursday onward supports
maintains slight chance to chance pops each day (which are close
to climo for this time of year). Temperatures should return to
more seasonable levels (70s) behind the Tuesday night cold front
midweek and probably through the rest of the upcoming work
week.

-Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1216 PM CDT Sat May 18 2024

VFR conditions are expected through the period, with only some
scattered to occasionally broken CIGs around 5,000 feet
expected this afternoon. Winds will remain light (up to around
10 kt) from the the south-southwest through this evening. A weak
cold front will then move across the terminal airspace tonight
causing winds to shift north-northwesterly during the predawn
hours of Sunday. Winds are then expected to settle into an east-
northeasterly direction during the day Sunday as a surface high
builds in over southern WI.

A small chance (10-20% chance) exists for a couple of isolated
showers or storms tonight as a cold front moves into the area.
The best chance for these storms will be west of the area today.
Currently it is anticipated that these storms will weaken as
they shift eastward into northern IL and southern WI through
the evening. The possibility even exists that the storms
diminish altogether before the front reaches the Chicago area
terminals overnight. For this reason, I have left a formal
mention out of the TAFs at this time.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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