478
FXUS63 KLOT 130839
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
339 AM CDT Mon Apr 13 2026

.KEY MESSAGES...

- Late spring to summer-like temperatures will continue through
  Friday with highs in the 70s and 80s and lows in the 60s.

- Several additional rounds of showers and thunderstorms,
  including a threat for severe weather and flooding, may occur
  as early as today-tonight, but especially Tuesday and Wednesday.

- A brief break in the pattern is expected late Thursday into or
  through the day on Friday before another storm system arrives
  in the Friday evening to Saturday timeframe.

- In the wake of the Saturday cold front, a brief, potentially
  sharp, shot of much cooler air could bring frost/freeze
  concerns into next Sunday and Monday.

&&

.DISCUSSION...
Issued at 338 AM CDT Mon Apr 13 2026

Synopsis:

The transition season will live up to its billing and then some
over the next week. The advertised late spring to summer-like
stormy pattern (with some breaks) will be us through Saturday,
followed by much cooler to possibly downright chilly conditions
Saturday night through Sunday night. The timeframe of greatest
concern for severe weather this week is Tuesday afternoon into
the evening, for which SPC introduced a threat level of 3/5
(enhanced risk) in the overnight day 2 outlook.

Over the next 48 hours, aggregate troughing will become
established across the southwestern United States and reinforce
longwave ridging from the Mississippi River Valley toward the
East Coast. Northeasterly shortwave shedding will support
corresponding episodic surface cyclone development from the
Plains through the Upper Great Lakes, altogether reinforcing a
broad warm sector with late spring to summer-like temperatures
into the region for much of the workweek.

Through Tonight:

The daytime hours today will start out dry but mostly cloudy,
with low clouds near and north of I-80 initially limiting the
diurnal temperature rise. This afternoon, there will be
competing factors regarding whether any convection (likely
rooted above the boundary layer) can materialize.

As the aforementioned aggregate troughing strengthens across
the southwestern US today, a renewed push of southwesterly mid-
level flow will advect a stout EML plume featuring very steep
mid-level lapse rates (>8 C/km) into the Great Lakes. The strong
capping at the base of the EML plume and expected mid-level
height rises typically conceptually point toward dry conditions.
However, we can`t ignore the signal on some of the overnight
CAM guidance for scattered thunderstorms on the nose of the EML
this afternoon. Based on an analysis of RAP model fields (which
initialize the HRRR), a possible subtle 700 mb wave with a
defined wind response and appreciable moisture look to be the
key player. Unfortunately, it`s a bit too early to tell how
realistic this depiction is (satellite and observational trends
after sunrise this morning should help).

Should the base of the cap be moistened and cooled enough and
updraft attempts overcome the lack of large scale forcing, the
modeled scenario on some of the high-res guidance may very well
come to fruition. Given the competing factors and inherent
uncertainty, opted to introduce 30-40% PoPs translating eastward
this afternoon. If a few robust thunderstorms are able to
develop, effective shear of 30 to 35kt, weak effective inflow,
and steep mid-level lapse rates would support a threat for
supercells with both damaging winds and hail. The threat level
1/5 of in the initial day 1 SPC outlook appears appropriate for
now in light of the highly conditional nature of this setup.
Outside of the uncertain convective potential, expect highs in
the 70s to around 80F (warmest south of I-80) this afternoon,
with southwest winds gusting up to 25-30 mph.

Tonight, there may be a short window of continued potential for
isolated to widely scattered storms this evening (20-30% PoPs),
possibly related to outflow from previous storms (if any indeed
occur this afternoon). Otherwise, intense thunderstorm
development is favored to occur somewhere from central Minnesota
into central Wisconsin as a subtle upper-level shortwave riding
within broad upper-level southwesterly flow induces a low-level
jet atop the stalled warm frontal zone to our north. In the
absence of any convection this evening, attention will turn to
likely activity to our north.

With the northeasterly terminus of the low-level jet pointing
toward the west-to-east oriented instability axis from Wisconsin
and eventually central Lower Michigan, any upscale growth into
a mesoscale convection system should stay decidedly north of our
area. However, should the warm front end up further south than
currently in our forecast (as hinted by HRDPS/ECMWF output),
part of northern Illinois may get a glancing blow by (likely
weakening) storms. Currently don`t see this as a particularly
likely scenario, so will cap PoPs at 30-40% from the pre-dawn
through daybreak hours.

Tuesday and Wednesday:

Toward the middle of the week, aggregate troughing will shift
eastward toward the Four Corners Region leading to a
corresponding "nudge" eastward in broad upper-level
southwesterly flow into the Great Lakes. The overlap of the
plume of steep mid-level lapse rates, stalled frontal zone, a
steady feed of low-level moisture advection, and a parade of
shortwaves and the eventual ejection of the aggregate trough
with associated jet maxima to support deep-layer shear will
support a threat for additional waves of severe thunderstorms in
the general region.

Storm modes ranging from convective clusters to supercells with
a threat for all hazards (including very large hail and
tornadoes) will be fair game, particularly on Tuesday. While
there is low confidence in exactly how Tuesday PM will play out
(and how early storms will initiate), the potential is there
for a very favorable setup for damaging to even destructive
hail, which drove the SPC day 2 outlook upgrade (favoring near
and north of I-80). It remains to be seen if locally backed
winds can materialize, with respect to a tornado threat.

Additional, repeated rounds of storms over the same area may
support a threat for flash flooding (possibly even locally
significant) into Tuesday night. Prior rounds of convection into
Wednesday morning may then muddle the picture convectively
speaking for Wednesday PM. Ultimately, pinpointing favored
locations and time windows will remain challenging until there
is better clarity on exact shortwaves and eventual placements of
boundaries.

Thursday through Sunday:

There`s variance regarding the progression of a cold front
across the region, with some of the slower solutions holding
onto some potential for storms into Thursday afternoon.
Thereafter, into Thursday night and at least the start of
Friday, ensemble model guidance advertises a brief break in the
stormy pattern as shortwave ridging moves through the Great
Lakes ahead of a substantial trough digging into the central US.
High temperatures look to rebound into the low to even mid 80s
by Friday accompanied by another threat for strong to severe
thunderstorms Friday night into Saturday. Thereafter, there is a
reasonably strong signal in ensemble guidance for a cooldown to
start next week.

The key aspect for the weekend into early next week cooldown is
a recent trend towards a much cooler air mass due to trough
amplification over the region. If this comes to pass, parts of
the area may flirt with sub-freezing temperatures Saturday night
into early Sunday, and again Sunday night into next Monday
morning. With the summer-like warmth and rain this week
supercharging plant growth and leaf out, the magnitude of the
weekend cold shot will be one to watch for agricultural interests.

Castro/Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1255 AM CDT Mon Apr 13 2026

Key aviation forecast messages:

- MVFR CIGs (IFR at RFD) expected overnight through the AM with
  IFR possible at times.

- A conditional threat for severe TSRA exists this afternoon.
  Confidence in storms forming remains low (30% chance).

- LLWS concerns return late evening through early Tuesday AM.

The back edge of the gusty showers will be exiting the area
within the next hour. A small cluster of showers and isolated
thunderstorms may develop yet early this morning and move across
areas mainly south of I-80 between ~7-11Z. Can`t fully rule out
the northern periphery of this activity approaching MDW/GYY and
will monitor radar trends closely.

MVFR stratus will remain in place through the night and
potentially through much of the morning hours today at the
Chicago area terminals. IFR ceilings will prevail at RFD and may
occur at times overnight at ORD/DPA. Also can`t rule out a
period of MVFR VSBYs in BR outside of Chicago prior to daybreak.

A conditional threat of severe thunderstorms exists this
afternoon into early evening. While substantial capping is
expected across the area, if elevated thunderstorms can initiate
as some of the hi-res guidance are indicating, damaging winds
and hail would be possible in addition to heavy downpours.
Given the lower confidence in storms forming, have elected to
introduce only a PROB30 for TSRA for now in a broader window
from 19-00Z (1 hr earlier at RFD and 1 hr later at GYY).

Winds will remain at least sporadically gusty through the night
out of the southwest with gusts in the 20-25 kt range then ease
slightly after daybreak remaining SW to SSW through the day. The
low-level jet ramps up again late this evening through early
Tuesday morning. Even with gusty surface winds at least marginal
low-level wind shear concerns may develop given 50-55 kt as low
as 2000 ft.

We will also be keeping an eye on any potential upstream
convective elements that may try to dip into northeast Illinois
toward daybreak Tuesday, though confidence remains too low to
include any mention in the TAFs with this update.

Petr

&&

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

&&

$$

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