NWS Chicago Decision Support Briefing - Forecast

046
FXUS63 KLOT 011823
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
123 PM CDT Wed Apr 1 2026

.KEY MESSAGES...

- Showers and a few storms will move across the area tonight. No
severe weather is expected tonight.

- There is a threat for a regional severe weather outbreak
tomorrow, with a threat for strong tornadoes between 4 and 9
PM especially along/west of I-39.

- There is another threat for severe weather Friday and
Saturday, though the threats are conditional on placement of
boundaries and arrival timing of systems.

- A transition toward a quieter, but cooler, pattern is on
track to arrive early next week.

&&

.DISCUSSION...
Issued at 123 PM CDT Wed Apr 1 2026


Tonight through Thursday Morning:

A recent surface analysis placed a low-level frontal zone from
the Kansas/Oklahoma border eastward along the Ohio River Valley
ahead of a vigorous upper-level shortwave quite evident in water
vapor imagery crossing the southern Rocky Mountains. As the
shortwave digs into the Southern Plains late tonight, lee
cyclogenesis will lead to the development of a low pressure
system in western Kansas and induce pressure falls in the Upper
Midwest (on the order of 6 to 9 mb/6hr through the overnight
hours). As a result, the expectation is for a gradual increase
of southwesterly low-level flow across the Mississippi River
Valley tonight (925-850mb flow progged to increase toward
35-40kt by daybreak Thursday). The corresponding increase in a
deep layer of isentropic ascent (35 to locally 50kt of upglide
upon the 280 to 300K theta surfaces) as well as rising thickness
within the warm air advection regime to force a weakly coupled
upper-level jet structure will lead to the blossoming and
northward spread of showers through the overnight and morning
hours of Thursday. Put plainly, the aforementioned frontal zone
will shift northward into our area tonight into tomorrow morning
accompanied by showers and a few thunderstorms.

While upright instability will be relatively weak tonight
through tomorrow morning, relatively strong forcing amidst PWATs
climbing above 1.25" will support "warm-rain" process healthy
downpours within the northward-moving shield of showers,
especially where low-topped convective processes can take
place. The progressive northward movement to the front should
tend to limit the opportunity for the heaviest rain rates to
train through the night, but will have to watch for any
persistent convective echoes south of Interstate 80 and
especially near the US-24 corridor where 1 to locally 3 inches
of rain has fallen in the past 24 hours. Did collaborate the
introduction of a Level 2 threat level for flooding with WPC
valid for tonight south of I-80 to account for any worsening of
ongoing ponding in low lying areas.

While heavy cloud cover and warm-air advection scattered
showers and storms may very well continue across parts of the
area through noon, the intensifying southwesterly low-level
wind field forced by the tightening low-level pressure gradient
ahead of the approaching surface low (surface gusts poking north
of 35 mph) will be able to force temperatures upward into the
upper 60s to lower 70s. Will note that there is a play for
southwesterly winds to gust as high as 40 to 50 mph if mixing is
especially efficient tomorrow particularly southeast of I-55,
though the upward effects of warm-air advection usually limits
the ability for mechanical mixing to bring the strong low-level
wind field down to the surface. This will be something for later
shifts to assess (to inform any decision on whether a Wind
Advisory may be needed for part of the area).


Thursday Afternoon and Evening:

Focus then turns to the threat for severe weather. During the
afternoon and evening hours, the core of the upper-level
shortwave will eject northeastward into the Upper Mississippi
River Valley as the surface low lifts from near Kansas City,
Missouri to La Crosse, Wisconsin. Steepening mid-level lapse
rates by virtue of differential cyclonic vorticity advection
atop the continued moistening boundary layer (surface dew points
rising toward the mid 60s) will support the development and
northeastward advection of an uncapped instability plume
characterized by MLCAPE >1000 J/kg into the Mississippi River
Valley by mid-aternoon. At the same time, a 500mb speed max will
arc northeastward toward the Great Lakes atop the already
strong low-level wind field. Conceptually speaking, the synoptic
pattern including the expected path of the surface low across
eastern Iowa during peak heating matches analogs for significant
severe weather events, including tornado outbreaks, across the
region.

At this point, there appears to be two windows for severe
weather tomorrow afternoon. The first window will be during the
early afternoon hours (call it noon to 4 PM) as any residual
elevated warm-air advection convection, or newly forced
convection along a prefrontal trough, attempts to become rooted
to the eastern edge of northeastward-moving instability axis.
Often, this can lead to supercell structures that are nearly but
not fully surface-based owing to relatively cool, or at least
not primely unstable, storm-relative inflow originating off the
main instability axis. Should a supercell manage to become
surface-based during the early afternoon window, a threat for
tornadoes could emerge. Though in most cases, these kinds of
early-day supercells do struggle to latch onto the surface and
largely present a threat for lightning and downpours. (The hail
threat should be mitigated by strong low-level storm relative
inflow beneath weak shear above 3 km).

The more concerning time window for severe weather will be
during the late afternoon and evening hours (4 PM to 9 PM) ahead
of the cold front where low-level moisture will be greater,
mid-level lapse rates will be steeper, and synoptic-scale
forcing will be strongest. With semi-orthoginal orientations
between deep-layer shear vectors and the cold front, initial
storm mode will favor discrete supercells with strong mean-flow
supporting east-northeasterly storm motions of 45 to 55 mph. The
impressively sheared low-level environment characterized by
0-1km shear/SRH of nearly 40kt/275 J/kg, moisture-laden
instability axis supporting LCLs beneath 1000m, and strong
southwesterly flow to support frictionally-generated turbulent
near-surface vorticity streams will lead to an environment
uniquely favorable for strong and long-track tornadoes. Based
on the current arrival time of the instability axis, the favored
area for such a threat for tornadoes appears highest from
eastern Iowa through northern Illinois extending toward I-39,
though perhaps as far east as I-55 in the most aggressive
destabilization scenarios. Some upscale growth into clusters may
transition the primary threat to damaging winds toward Lake
Michigan and northwestern Indiana with time. And, the strong
low-level storm relative inflow beneath weak shear above 3 km
should tend to limit the threat for damaging, or at least giant,
hail. So, the main threat tomorrow could very well be
tornadoes.

As is often the case with severe weather set-ups, a regional
tornado outbreak tomorrow is hardly set in stone. One very easy
to envision failure mode is for morning and early afternoon
shower and afternoon coverage to be expansive, limiting the
ability for meaningful destabilization in the northeastward-
moving instability axis by late afternoon. In such a scenario,
new thunderstorm development along/ahead of the cold front could
be kept to a minimum or lead to low-topped "stringy" convective
elements that struggle to deepen within the strongly sheared
low-level environment. Or, thunderstorms that develop upstream
in Iowa could fall apart while moving into northern Illinois if
the northeastward movement of the instability axis is impeded.
This is all to say, the occurrence of severe weather in our
area tomorrow is not a guarantee.

When put altogether, the Storm Prediction Center has upgraded
much of our area to a Level 3/5 threat level for severe weather
(driven by tornado probabilities). Tomorrow will be a day to
stay very weather aware, especially if west of I-39, between 4
and 9 PM.


Friday into Saturday:

Yet another upper-level shortwave is poised to dive into the
central Plains Thursday night into Friday, causing the cold
front Thursday evening to stall somewhere in the Ohio River
Valley Friday. A surface pressure col will move over the area
Friday morning before pressure falls in the Midwest allow for
southerly flow to redevelop and the warm front to lift back
northward. Exactly how far northward the front will retreat is
an item of low confidence, and will be important to the forecast
for temperatures by mid-afternoon. In scenarios where the front
gets hung up somewhere across central Illinois and Indiana,
many areas may remain in the 50s for much of the day. Meanwhile,
any areas south of the front will have the potential to jump
back up into the 70s.

Friday afternoon, forecast soundings depict minimal capping
along the warm front leading to concerns for the development of
convection. With the upper-level wave remaining well to our
west, shear profiles along the front will be modest and largely
driven by low-level directional changes across the front itself.
Taken together, the conceptual pattern appears supportive of
low-topped supercell structures with a threat for funnel clouds
and weak tornadoes along the warm front. At this point, the
front is favored to be somewhere south of I-80, and perhaps even
south of US-24. Pin-pointing the location of the front will be
a priority in future forecast packages.

The upper-level shortwave is expected to shift eastward into
the Great Lake Friday night into Saturday likely accompanied by
a band of showers and thunderstorms. The unfavorable diurnal
arrival time of the peak forcing suggests the severe weather
threat should be low in our area, though any slowing of the wave
would be cause for concern for one last window of severe
thunderstorms in our area (This stretch from mid-March through
now has been relentless!). At any rate, Saturday will feature
nearly steady temperatures in the 50s to around 60 as cold air
advection works into the region behind the cold front.

Borchardt


Sunday Onward:

In the wake of this system, a period of drier weather is
expected to close out the weekend as a surface high moves into
the Mississippi Valley. However, the upper trough is expected to
stall over the Great Lakes which will result in nearly constant
northwest flow resulting in seasonably cool high temperatures
in the 40s to lower 50s for Sunday and again on Monday.

The upper trough is forecast to start pushing east of the Great
Lakes on Monday as an upper ridge builds over the central
CONUS. As a parting gift a final shortwave is progged to round
the backside of the trough Monday night which could bring us one
more period of rain showers during this timeframe, That said,
there is a lot of variability in guidance as to how much
moisture will be available Monday night as the wave moves
through so there is a chance that the rain avoids northern IL
and northwest IN all together. For now though will advertise a
20-40% chance for rain. After Monday night, the upper ridge will
be moving into the Great Lakes which will promote dry
conditions for the middle of next week with temperatures
moderating towards more seasonable values in the mid to upper
50s.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1221 PM CDT Wed Apr 1 2026

- IFR/LIFR conditions develop tonight into Thursday morning
along with waves of rain and embedded TS at times.

- Winds shift out of the south Thursday afternoon. Additional
periods of thunderstorms--some severe--possible into the
evening.

Low-MVFR cigs will remain in place at the Chicago-area terminals
this afternoon, with cigs near 2 kft at RFD. Cigs will then
build down fairly quickly late this evening and overnight with
widespread IFR expected towards or just after midnight. Rain
will also commensurately expand across the region. While
embedded convective tops near 25-30 kft appear probable late
tonight, the threat for TS remains a bit too low to warrant
inclusion of VCTS at this time. Have, however, added a mention
at RFD where forcing appears greater.

A warm front will eventually push north of the Chicago-area
terminals although precise timing remains unclear. Just before
this occurs, cigs could lower to LIFR along with a brief period
of vsbys under 1 mile, sometime in the 12-16z window.
Thereafter, cigs will lift quickly and winds with shift out of
the south and eventually south-southwest and become gusty.

An initial wave of thunderstorms appears possible in the 19-23z
window or thereabouts in the extended ORD/MDW TAFs. Not
confident enough in coverage or timing to warrant TEMPO TSRA but
have introduced VCTS along with a PROB for TSRA given the
latest model trends. At RFD, waves of showers and storms may
occur through most of the morning and afternoon.

Carlaw

&&

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

&&

$$

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