NWS Chicago Decision Support Briefing - Forecast

627
FXUS63 KLOT 160740
AFDLOT

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

.KEY MESSAGES...

- Isolated storm chances exist today through Sunday morning,
though many areas could remain dry.

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

- Summer-like warmth and humidity is in store this weekend
through early next workweek. Cooler conditions then return
midweek.

&&

.DISCUSSION...
Issued at 240 AM CDT Sat May 16 2026

Early this morning:

The last of the initial band of showers that moved across
southern Wisconsin and far northern Illinois earlier this
morning is moving out over Lake Michigan. The trailing outflow
boundary and residual cold pool extending southwest of these
storms have put a damper on additional shower and storm
coverage. The leading edge of this outflow could lead to
additional isolated shower development but has been unsuccessful
thus far. Meanwhile, a secondary line of showers and storms in
central Iowa continues to progress east toward the region which
may still give the rest of an area a chance to see a period of
showers and isolated (non-severe) storms toward the pre-dawn
hours through mid-morning, ending from northwest to southeast.

Today and tonight:

A remnant outflow boundary/gravity wave from the early AM
storms is expected to stall out over central IL/IN later this
morning where an east to west oriented axis of showers and
embedded storms may attempt to redevelop during the day along
this feature. There remains some uncertainty in where exactly
it stalls, so have maintained 20-30% shower/storm chances south
of a Pontiac to Rensselaer line in case it ends up farther
north. It could very well remain just outside of the local area
though.

As we head into the rest of the day temperatures are expected
to warm into the lower-to-mid 80s (locally cooler near/under any
showers to the south). In tandem, the airmass will become
increasingly unstable with little to no capping in place.
However, the majority of the area is mostly removed from any
notable forcing mechanisms other than a potential weak lake
breeze in the afternoon. As a result, hi-res guidance remains
highly variable from run to run in the the placement and
coverage of potential for spotty thunderstorms during the
day. Accordingly, capped shower/storm chances at 15-20% for the
to lower confidence. The more likely scenario is that the
majority of the area remains dry the rest of the daytime hours.

A mid-level disturbance lifts into the area late this evening
and overnight which could bring renewed shower and storm
chances to portions of the area, highest near and north of I-80
(40%). We will also have to keep an eye on upstream convective
trends in the event that a more vigorous convective complex
happens to hold together across Iowa. Stay tuned.

Petr


Sunday onward:

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 morning showers/storms (favoring
near/north of I-88) departing and decreasing cloud cover in the
warm sector, high temperatures in the mid-upper 80s appear
likely at this distance. The exception may be near the Lake
County IL shore, as a just east of south component to the winds
could cap highs in the 70s later in the day. 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 /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1240 AM CDT Sat May 16 2026

Aviation forecast concerns for the 06Z TAFs:

- Periods of TSRA/SHRA through the pre-dawn hours.

- Somewhat low confidence in late afternoon wind directions,
though a trend from south-southwest to the southeast is
likely by early evening.

- Low chance (<20%) for isolated afternoon SHRA/TSRA.

- Greater (30-40%) chance for TSRA later Saturday evening/night.

An eastward-moving line of TSRA was ongoing from south-central
WI into central IA at issuance time, with a couple of embedded
circulations (MCVs) noted across northwest IL/southwest WI.
Timing of the primary line of storms looks to be around 06Z at
RFD, 0740-0800Z for DPA/ORD/MDW and shortly after for GYY. This
line may be accompanied by a period of west-northwest outflow
winds in excess of 30-35 kts. Complicating matters is the
development of northeastward-moving scattered TSRA over north
central IL (RFD-DKB vicinity) ahead of the aforementioned MCV
over northwest IL. This activity (while visible from ORD per
webcams due to relatively high cloud bases) is expected to lift
north/northeast of DPA/ORD prior to the main line arrival,
though can`t completely rule out TSRA arriving earlier than the
current 07-08Z TAF timing. Farther west, additional TSRA was
occurring over central IA, generally tracking east-southeast.
High-res CAM guidance suggests thunder threat will diminish with
this activity as it propagates into central IL (mainly south of
the terminals, but perhaps not too far south). Currently have
maintained VCSH/PROB30 SHRA for this but it is possible we may
need to add a TSRA mention with this if TS persists (newly
arriving 04Z HRRR run is more bullish with stronger convective
cores). All of this should move off to the east by 12Z for RFD
and 13-14Z for Chicago terminals.

Forecast soundings depict strong drying of the column behind
departing morning MCS. By late afternoon however, soundings do
become conditionally unstable - but with little in the way of
organized support for convective initiation. Some CAMs indicate
the potential for isolated TS development by late afternoon
across northern IL, though the signal is not consistent in the
guidance. A greater potential for another period of TSRA looks
to arrive late in the evening for RFD and after midnight/pre-
dawn Sunday, as a mid-level disturbance lifts northeast across
the region.

Wind-wise, breezy south winds with gusts around 20 kts should
give way to gusty west winds briefly with TS predawn, then
settle light southwest during the day. It appears that an
outflow boundary associated with morning storms will lay out
south of the terminals during the day, with a weaker gradient to
the north making for lower confidence in direction of light
winds this afternoon. Can`t rule out a lake breeze getting as
far west as ORD/MDW late afternoon, though confidence is low.
It does appear that winds will become synoptically southeast by
early evening however, as the outflow boundary begins to lift
back north in response to the approaching mid-level wave.

Ratzer

&&

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

&&

$$

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