772
FXUS63 KLOT 042357
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
657 PM CDT Sat May 4 2024

.KEY MESSAGES...

- Gusty showers and storms will sweep across the area this
  afternoon and evening. Stay weather aware if you have outdoor
  plans!

- Sunday will be noticeably cooler with onshore flow and highs
  in the upper 50s (lakeside) to upper 60s (inland).

- There is a potential for strong to severe storms along with a
  locally heavy rainfall Tuesday and Wednesday

- Periodic showers are expected Thursday through Saturday

&&

.DISCUSSION...
Issued at 114 PM CDT Sat May 4 2024

Through Sunday Night:

A surface low continues to lift northeastward into central Wisconsin
ahead of an eastward-moving cold front currently in eastern Iowa and
central Missouri. Ahead of the cold front, the airmass continues to
destabilize with mostly sunny skies and southwesterly low-level
moisture advection supporting the development of 1000-1500 J/kg of
MLCAPE along and east of the Mississippi River. The low-level
moisture plume advecting into the region is relatively shallow,
leading to a reservoir of DCAPE in excess of 1000 J/kg along and
ahead of the cold front. Meanwhile, well ahead of the cold front,
shallow cumulus clouds dot the sky and temperatures are in the mid
to upper 70s.

Recent 1-min GOES imagery depicts the very first attempts to
initiate convection are taking place in far northeastern Missouri
along the cold front and ahead of a decaying MCV. With observed
AMDAR and forecast soundings still depicting a shallow cap from 850
to 800 mb, it may take another hour or so of heating and
moistening ahead of the front before sustained convective cells
can take hold. With that in mind, thunderstorms are expected
erupt near or just east of the Mississippi River by 3 PM and
progress eastward into northern Illinois thereafter (reaching
I-39 in the 5 to 7 pm timeframe, Chicago metro in the 6 to 8 pm
timeframe, and northern Indiana in the 7-9 pm timeframe, give or
take an hour).

With our area removed from the best upper-level flow and hence deep-
layer shear across the northern Great Lakes, convective cells will
exhibit pulse-like single-cell behavior before the high DCAPE
environment encourages consolidating outflows and quick upscale
growth into several multi-cell clusters. As a result, our
expectation remains for gusty thunderstorm clusters (gusts of 45-55
mph) to sweep across our area this evening. Wherever initial
thunderstorms are most intense (e.g. initial "push" of outflow),
locally damaging winds (55 to 65 mph) are a distinct threat, as
well. The current SPC threat level 2/5 area near and west of I-39
highlights where the threat for damaging wind gusts is locally
higher, though in reality, most (all?) of our area should see
convective gusty winds at some point this evening. In fact, with
virtually no low-level shear to resist the outward-spreading
outflows, gusty to locally damaging winds may occur well before rain
and lightning arrives at any given location. As a result, for those
with outdoor plans this warm and summer-like evening, stay weather
aware and heed any statements or warnings issued for your area.

The cold front will sweep across the state along or just behind the
gusty thunderstorm clusters after sunset, causing winds to turn
northwesterly. Shallow clouds in the wake of the front may encourage
a few sprinkles through the overnight hours, particularly near Lake
Michigan where low-level convergence will be maximized.

Tomorrow (Sunday) will be noticeably cooler thanks to onshore winds
and filtered sunshine. Highs will top off in the upper 50s lakeside
to upper 60s inland. Tomorrow night will be cool with overnight
lows in the low to mid 40s.

Borchardt

Monday through Saturday:

A short wave trough will be passing over the area Monday
morning and move east of the area by Monday afternoon.
Persistent southerly flow in the mid levels will help keep
moisture available, but with minor ridging taking place through
much of the day allowing for a brief window of height rises and
very limited instability during the peak heating hours, opted to
cap chances for rain at 20 to 30 percent south of I-88.

Those height rises turn to height falls early on Tuesday
morning as a deepening upper level low, a strengthening upper
level jet, and strong surface cyclone develops over the
Northern Plains. Models are still disagreeing on exact timing
and location, but model runs have projected another, slightly
surface low developing to our northwest over the northern
Mississippi river. As a stronger push of moisture rides the
southerly winds aloft northward, the expectation is for over an
inch of precipitable water to surge into the region on Tuesday
(with around a 50% chance for PWATs to be greater than 1.5
inches, which would put it well above the 90th percentile for
the ILX sounding climatology for May 8th (00z)). Both the Euro
Ensemble and GEFS models have the majority of their members
keeping the strongest instability around the Ohio River
corridor, but but have between a 40 to 60 percent chance for, at
minimum, 1000 J/kg of CAPE available in our forecast area.

Deterministic models are suggesting the first boundary that will
stretch all the way back to North Dakota will provide the first
opportunity for showers and potentially storms in the morning.
After that boundary passes, better instability aided by daytime
heating may lead to redevelopment along a second boundary in
the afternoon which could have better forcing to develop
stronger and potentially severe storms over the area. Some cells
may also develop higher rain rates to allow some localized
flooding concerns, though with drier Sunday and Monday forecast
perhaps it will allow some time for soils to recover to be able
to be more efficient absorbing the rainfall. Model runs as the
date approaches can hopefully inform if there are better chances
for training storms to occur (as a few ensemble members
suggest).

On Wednesday, the upper level low at 500 mb begins to break
down and switch from negative to a positively tilted upper level
trough. The reflected surface low starts to phase with the low
over Iowa and Minnesota, slowly moving eastward. With a warm
front likely to develop pseudo-latitudinal over the forecast
area, it will likely lead to a sharp gradient in afternoon
temperatures. Better lapse rates and moisture will likely be to
the south of the front creating a renewed environment for
showers and potentially strong to severe storm development. The
main question will be where exactly does that front set up. With
renewed cyclogenesis during the day (albeit weaker than on
Tuesday, but relative to the day), storm development could occur
right along the front itself. With strong moisture return from
the south, this could lead to a better set up for training
storms and flooding.

The upper level trough will broaden Thursday and Friday as it
moves eastward, and not leave the western Great Lakes until
Saturday afternoon. Even if it is not record setting, there will
still be plenty of lingering moisture through the end of the
week. While the better axis for instability moves east of the
area (taking the stronger to severe threat with it), additional
lobes of vorticity that pass through the upper level trough
could provide periods of showers through Saturday morning. As
a ridge builds into Saturday, better height rises and subsidence
could lead to a better chance for a drier period over next
weekend, but models this far out would struggle to resolve a
small weak wave that passes that could keep the periods of
showers hanging around.

Temperatures will remain above average through most of the
period, with daytime highs in the 70s to low 80s (Tuesday and
Wednesday have the potential to be the warmest days). Cooler,
more average temperatures are expected by Friday, with highs in
the mid-60s.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 657 PM CDT Sat May 4 2024

Line of TSRA moving across Chicago terminals at this time will
be east of the terminals with thunder chances ending by 01Z. In
the wake of the storms winds will turn light westerly, then
shift northwest and pick up a bit later this evening as cold
front moves across the terminals. At least a several hour period
of MVFR CIGS is expected overnight before skies likely clear
out Sunday morning. Northwest winds will veer north late tonight
and then turn northeast Sunday morning.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 7 PM CDT this evening for Winthrop
     Harbor to Wilmette Harbor IL.

&&

$$

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