NWS Chicago Decision Support Briefing - Forecast

249
FXUS63 KLOT 161908
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
208 PM CDT Sat May 16 2026

.KEY MESSAGES...

- A better chance for showers and storms this evening-overnight,
especially with westward extent, and then a few showers and
storms may linger past daybreak Sunday, mainly north of I-88.

- Expect more widespread showers and at least scattered storms
at times Monday-Tuesday, some of which may be severe.

- Summer-like warmth and humidity along with frequently gusty
south to southwest winds will continue through early next
workweek. Cooler conditions then return midweek.

&&

.DISCUSSION...
Issued at 208 PM CDT Sat May 16 2026

Through Tonight:

Early afternoon visible satellite imagery trends have cast some
doubt on convective initiation (CI) prior to sunset over
northern Illinois north of I-80. There`s a few axes of clumpier
(but still mostly flat) Cumulus growth, one over far northern
IL, and the other just north of the I-80 corridor. To the south,
thick cirrus debris is greatly curtailing insolation and
resulting destabilization north of ongoing showers and storms
over central IL and IN.

Recent KDVN radar loops show a diffuse remnant boundary lifting
northward over far western IL, coincident with the area of Cu
growth. Dew points have ticked up slightly in this corridor
(low-mid 60s) as well. In an overall subsident regime, it
remains possible/probable that convective coverage, if any, is
sparse/isolated. That said, north of I-80 and near/west of I-39
is looking to be the main area to watch until sunset for t-storm
development. As noted in the previous update, *if* a few storms
occur, the strongest would be capable of producing localized
downburst wind damage.

While large scale forcing will remain modest at best tonight,
there has been run to run consistency in short-range guidance in
(likely convectively modulated) 700 to 500 mb impulses lifting
northeast across the mid MS Valley. Particularly if convective
coverage remains at a minimum into/through this evening, an
untapped MUCAPE reservoir possibly upwards of 1k J/kg may be in
place. The mid-level short-wave forcing now appears sufficient
for fairly widespread showers (50-70% PoPs) and scattered
embedded storms focused near/west of the Fox Valley down into
west central IL. Decreasing mid-level lapse rates, marginal
effective bulk shear, and an unfavorable diurnal timing will
likely limit the severe threat. However, we can`t completely
rule out a strong to severe storm or two (localized wind damage
and perhaps isolated large hail).

Castro


Sunday through Saturday:

Guidance continues to indicate breezy (30-35 mph gusts) south
winds will develop Sunday afternoon, after the warm front lifts
north of the area. With any lingering morning showers/storms
(favoring near/north of I-88) departing and decreasing cloud
cover in the warm sector, high temperatures in the mid to
locally upper 80s appear likely. The notable exception to this
will be near the Lake County IL shore (and part of the day along
the Cook Co. shore), as a just east of south component to the
winds will make for notably cooler conditions (60s to 70s0
until flow turns south-southwesterly Sunday evening. Breezy and
mild conditions look to persist Sunday night, with lows
generally in the mid to upper 60s.

Thunderstorm chances then return Monday and Tuesday, as a
Central Plains surface low pressure slowly lifts northeast
through the northern Great Lakes and eventually trails a cold
front through the forecast area. Ahead of the cold front, the
forecast area will reside in breezy warm southerly flow,
characterized by daytime temperatures in the 80s and increasing
low-level moisture return (surface dew points potentially upper
60s-to near 70F) from the western Gulf. Resulting instability,
combined with mid-level flow near 40 kts would potentially
support strong/severe thunderstorms (as well as some threat for
flooding given high column moisture), though forcing mechanisms
remain unclear within the broad warm sector making convective
timing/evolution of low confidence at this distance. Monday`s
overall messy look also lends uncertainty to how much
instability can be realized at peak heating, particularly if
morning activity ends up fairly widespread.

On Tuesday, in the wake of probable continued overnight/early
day convection, the aforementioned cold front will approach.
It`s not uncommon in these setups for renewed storms and any
associated strong to severe threat ahead of the front to focus
primarily near/southeast of I-55, though time will tell in this
regard. For the Monday-Tuesday period, SPC continues to extend
severe probabilities into our area, which appear reasonable.
Quiet and cooler conditions are then expected by mid-week, as
surface high pressure builds across the region in the wake of
the Tuesday cold frontal passage.

Castro/Ratzer

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1207 PM CDT Sat May 16 2026

Southwesterly winds, with some intermittent 15-20 kt gusts, will
continue this afternoon.

An outflow boundary continues to lift northward out of central
Illinois where convection has been fairly widespread this
morning. On its current trajectory, this feature may be nearing
the southern c90 TRACON towards 20z, and then may continue
lifting north through the rest of the afternoon. It remains
unclear if additional showers and thunderstorms will re-develop
on this boundary, or out of currently-developing cumulus
straddling the I-88 corridor, given the presence of modest
subsidence in the wake of this morning`s activity and a lack of
more substantive large scale forcing in the region. Given a
warm, moist, and unstable airmass in place this afternoon, a
potential for isolated to scattered thunderstorms exists, but
chances/confidence still are too low to hop on VCTS or TEMPO TSRA
mentions in the TAFs.

This evening, winds will trend southeasterly. Additional showers
and thunderstorms may push out of eastern Iowa overnight into
early Sunday morning. The greatest chances currently exists near
RFD.

Thereafter, wind trends into Sunday afternoon remain a bit
unclear at the Chicago-area terminals. Initially southeasterly
winds will likely become SSW and gusty, especially at MDW, and
GYY, but there is some potential that a lake breeze remains in
place INVOF ORD. At this time, will show the development of SSW
winds at ORD during the afternoon, but note that these wind
trends remain a bit uncertain at this range.

Carlaw

&&

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

&&

$$

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