336
FXUS63 KLOT 190603
AFDLOT

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

.KEY MESSAGES...

- A period of showers and embedded thunderstorms is expected
  after midnight tonight through early Tuesday morning, some of
  which could be locally strong, mainly west of I-55.

- Additional thunderstorms may redevelop Tuesday afternoon,
  mainly east of I-55, some of which could be strong to severe.

- Summer-like warmth and breezy winds will prevail through
  Tuesday, then cooler temperatures and dry conditions will
  return for the middle of the week.

- Milder weather returns heading into the weekend with
  intermittent shower/thunderstorm potential.

&&

.UPDATE...
Issued at 846 PM CDT Mon May 18 2026

A robust line of convection from Wichita, Kansas to Des Moines,
Iowa, which created quite the severe event in the Central
Plaines this evening, is slowly moving east. A strong
instability gradient from northwest Missouri toward St. Louis
should focus most of the strongest parts of the line`s movement
through the overnight south of the forecast area toward central
Illinois. However, a plume of better lapse rates into central
Iowa will slowly meander east ahead of the line of storms.
Outflow from the storms should provide sufficient forcing, but
being in a diurnally unfavorable time of day, the lapse rates
should start to weaken by the time it approaches the Rockford
Metro after midnight, but confidence lowers in coverage toward
the city of Chicago as it starts to fall apart away from the
better forcing.

Most of the forecast area will be dry tomorrow morning, but
lingering showers may crop up. The cold front will finally make
it to the forecast area and turn winds from the southwest to
the west and eventually northwest behind the front. Additional
showers and storms are possible along the front, with the best
time being in the afternoon. But by that time, the front could
already be around and east of Interstate 55. Storms along the
front in the afternoon could be strong to even severe with
damaging wind and hail the primary severe threats.

Forecast updates this evening were focused on the near-term
timing of shower and thunder as well as for tomorrow afternoon.


DK

&&

.DISCUSSION...
Issued at 244 PM CDT Mon May 18 2026

Through Tonight...

Convection and associated MCV responsible for this morning`s
severe weather has moved into Lower Michigan. Air mass in the wake
of this wave is substantially more stable and combined with modest
height rises in its wake should prevent any renewed convective
development this afternoon or evening.

Severe convection currently developing over the central Plains
is likely to congeal into a large QLCS that will move toward the
mid-Mississippi Valley tonight. Most intense convection is likely
to focus farther southwest across Missouri, likely intercepting
the low level jet and orphaning the convection farther north
across Iowa. This should result in convection over Iowa
diminishing as it moves eastward into northern IL with some
guidance suggesting very little shower activity would actually
make it into our CWA. Have reflected this trend in pop grids, but
opted to maintain at least chance pops given the inherent
uncertainties in forecasting convective evolution.

Pressure field is recovering in the wake of the morning convection
which has allowed winds to pick and become rather gusty from the
southwest. Gustiness of the winds will generally tend to abate by
sunset, except perhaps remaining somewhat gusty tonight in the
urban heat island of Chicago.

-Izzi

Tuesday...

Convectively induced vorticity maximum will slide through northeast
Illinois into early Tuesday, well outpacing the cold front, and
therefore we may have some showers hanging on into the area, but
confidence is medium-high that thunderstorm coverage will be on the
lower side. Any lingering MCS cold pool footprint and residual
cloudiness will likely delay destabilization into midday Tuesday,
though additional storm development appears somewhat probable across
the southeast half of the forecast area Tuesday afternoon/evening as
the cold front finally pushes through the forecast area.
Southwesterly flow ahead of the front will maintain mid to possibly
upper 60s dewpoints and support MLCAPE approaching 1500 J/Kg.
Stronger mid-level flow on the southeastern periphery of the  Model
guidance would support some additional vorticity maximum riding
along the front, and flow to the southeast of the mid level trough
across the northern Great Lakes will likely support enough storm
development and some organization for a hail/wind threat during
this time. This would be favored along and southeast of I-55,
with short term ensemble guidance suggesting the window for much
of NE IL and NW IN being fairly limited in the mid to late
afternoon. For the Chicago metro area the picture is also a bit
muddled for precipitation chances. High res guidance is mostly
dry for the area, whereas global guidance and NAM/RAP does bring
some shower activity across Chicagoland along the front in the
early afternoon.

KMD

Wednesday onward...

Following the cold front, surface high pressure is progged to
build east into the western Great Lakes region Wednesday and
Thursday. Dry, but breezy and cooler weather is expected both
days, with temps mainly in the 60s, but only in the 50s near
Lake Michigan with northeast-east flow off the lake. Nighttime
lows in the 40s are expected. The high then drifts off to the
east Thursday night, with the upper level pattern favoring a
series of mid-level disturbances tracking through the area
Friday into the weekend. This will support milder temperatures,
but also some occasional shower/thunderstorm chances into the
holiday weekend.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 103 AM CDT Tue May 19 2026

Forecast concerns for the terminals...

- A line of showers and embedded thunderstorms will move across
  the terminals late tonight into early Tuesday morning.

- Breezy southwest winds to develop Tuesday morning behind the
  rain and persist through the afternoon.

- Period of MVFR ceilings is expected Tuesday afternoon behind a
  cold front, but skies should scatter back to VFR around
  sunset.


A line of showers and embedded thunderstorms is progressing
eastward across the Mississippi River and will be encroaching
upon the terminals over the next 1-3 hours (earliest at RFD,
latest Chicago area sites). While the line is expected to be
gradually weakening as it moves through, recent AMDAR soundings
out of MDW do show some developing mid-level instability which
may allow for a couple of lightning strikes to occur. Despite
confidence in thunder actually reaching the Chicago terminals
being somewhat low (~20% chance) have opted to introduce a
PROB30 for TSRA at DPA, ORD, and MDW as a precaution give the
aforementioned instability and lack of broad weakening in the
line so far. That said, still think that the most likely
scenario at the Chicago sites (especially GYY) is for some
isolated to widely scattered showers to occur.

The line of showers/storms should diminish by daybreak and
result in a mostly dry morning. Winds will be increasing with
southwesterly gusts peaking around 25-30 kts through early
Tuesday afternoon, but locally higher gusts could materialize
if deeper mixing is achieved. As the main cold front approaches
Tuesday afternoon it will bring with it a period of MVFR
ceilings and perhaps an isolated shower or two though the
greatest shower/storm coverage should establish more into
northern and central IN. With the increasing cloud cover, gusts
should ease a bit into the lower 20 kt range through the rest of
the afternoon. Directions during this time will also transition
from southwest to west-northwest before turning northwesterly
behind the cold front early Tuesday evening. While a few upper
teen to lower 20 kt gusts cannot be ruled out Tuesday night, it
seems they frequency of gusts should be more sporadic so have
opted to remove gusts from the TAFs from this point onward.

In the wake of the cold front, ceilings should rise back to VFR
Tuesday evening and remain as such for the rest of the forecast
period. Lastly winds will finally settle into a northeast
direction early Wednesday morning.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 6 AM this morning to 4 PM CDT this
     afternoon for the IL and IN nearshore waters.

&&

$$

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