NWS Chicago Decision Support Briefing - Forecast

409
FXUS63 KLOT 191131
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 AM CDT Tue May 19 2026

.KEY MESSAGES...

- Scattered thunderstorms diminish early this morning across
northern IL. Thunderstorm chances then increase again this
afternoon, mainly southeast of a Peru IL to Valparaiso IN
line. A few strong storms are possible.

- Summer-like warmth and breezy winds today will give way to
cooler and dry conditions for the middle of the week.

- Intermittent shower/thunderstorm chances return late in the
week and into the weekend, though with a return of warmer
weather by Memorial Day.

&&

.DISCUSSION...
Issued at 345 AM CDT Tue May 19 2026

Early morning surface analysis places 1005 mb low over
northwest Wisconsin, with a cold front trailing through central
Iowa into southeast Kansas. Strong/severe thunderstorms that had
developed Monday afternoon ahead of the front across the mid-
Missouri Valley and elsewhere ahead of the boundary, have since
evolved into another linear MCS, which currently stretches all
the way from the central upper peninsula of Michigan to
northwest/western Illinois, then southwest to Oklahoma. Aloft,
GOES vapor imagery indicates a deepening mid-level short wave
trough propagating northeastward from the northern/central
Plains toward the northern Great Lakes. Strong large-scale
ascent, depicted by mid-level height falls of 60-80 meters
across the upper Mississippi Valley, is aiding in maintaining a
strong southwesterly low-level jet across our region, which
latest RAP soundings/mesoanalysis depict at 60 kts across
western IL.

While low-levels have diurnally stabilized, the strong low
level jet and associated warm/moist advection has maintained an
elevated MUCAPE axis of 1500-2000 J/kg per RAP soundings. This,
combined with low-level forcing provided by the eastward-
propagating outflow boundary along the leading edge of the
aforementioned MCS has allowed scattered convective cells to
continue to initiate atop the outflow boundary across western
and northwest Illinois. Forecast soundings do indicate a
weakening and veering of the low-level jet over the next few
hours however, along with a decrease in elevated instability.
Combined with the stable boundary layer conditions, this should
continue to result in weakening trend to the convection as the
outflow continues to shift east across northern IL this morning,
which is already evident in recent radar and lightning data.
Stable outflow and lingering scattered morning showers, combined
with a stream of high-level cloud cover off of stronger
convection well to our southwest should limit local convective
redevelopment for the balance of the morning.

The surface cold front is progged move into the forecast area
early this afternoon, pushing southeast across the area into
this evening. Forecast soundings are generally capped across the
area initially, though become conditionally unstable with
diurnal warming later this afternoon ahead of the front mainly
across the southeast half of the forecast area (and especially
southeast of a Pontiac-Valparaiso line). Guidance has generally
backed-off with coverage of afternoon convection across our
area, though frontal forcing and favorable diurnal timing
especially in our far southeast should support an increasing
thunderstorm threat mid-late afternoon. Forecast 1000-1500 J/kg
MLCAPEs and up to 35 kts of deep-layer shear (generally
parallel to the frontal zone) would support some storm
organization with some wind/hail potential. Note SPC has moved
earlier slight risk out of the WFO Chicago area with the new
Day 1 outlook, leaving a marginal (level 1 of 5) risk in our far
southeast.

A few showers may linger across our southeastern tier of
counties this evening until the front exits the region, with a
period of dry and cooler weather then expected Wednesday through
Thursday. After several recent days of highs in the 70s and
80s, daytime temperatures in the 60s are expected mid-late week
with 50s closer to Lake Michigan where breezy northeasterly
winds will persist around an area of high pressure building
across the Great Lakes region. Nighttime temps both Wednesday
and Thursday night will dip into the 40s in most areas.

Rain chances then return Thursday night into Friday, as
guidance depicts an area of low pressure tracking from Missouri
into Illinois and Indiana in association with an upper level
trough transiting the region into the weekend. Some detail and
timing differences remain within the global guidance suite,
though rain chances appear to be highest on Friday, then more
spotty from Saturday into Monday. It does appear that warmer
temperatures will return later in the weekend, but likely with
cooler conditions near Lake Michigan.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 631 AM CDT Tue May 19 2026

Forecast concerns for the terminals...

- Low chance for an isolated shower or storm this afternoon,
mainly south of a PNT to VPZ line.

- Breezy west-southwest winds through this afternoon with 20-30
kt gusts.

- Period of MVFR ceilings this afternoon with otherwise VFR
conditions.


A line of scattered showers continues to meander across northern
IL and will continue to do for the next 2-3 hours. While the
showers may dissipate prior to reaching northwest IN, a period
of light showers/sprinkles cannot be ruled out here. Otherwise,
mostly dry conditions are expected for today at the terminals.
However, there is still a low chance (15-20%) for an isolated
shower or storm to develop this afternoon ahead of the cold
front but the better coverage of any showers/storms should be
south and east of a PNT to VPZ line. That said, did opt to
introduce a PROB30 at GYY for SHRA this afternoon given their
closer proximity to the better forcing and instability.

Winds will continue to increase out of the southwest this
morning with 25-30 kt gusts expected through early afternoon. A
brief period (1-2 hours max) of 180-170 wind directions may
occur this morning in the wake of the showers but overall
southwest winds should prevail. Gusts are forecast to ease a
bit into the 20-25 kt range this afternoon as a plume of MVFR
ceilings drifts overhead behind the aforementioned cold front
with directions becoming westerly as well. The MVFR ceilings are
only expected to last a few hours before the scatter back to
VFR this evening where conditions will remain for the rest of
the TAF period. Gusts will also diminish further after sunset
with generally 10-12 kt winds expected overnight out of the
northwest. Though, some occasional upper teen to lower 20 kt
gusts could linger through the night. Finally, winds will turn
northeasterly early Wednesday morning and remain as such for the
rest of the period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 PM CDT this afternoon for the IL
and IN nearshore waters.

&&

$$

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