905
FXUS63 KLOT 161742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 PM CDT Wed Apr 16 2025

.KEY MESSAGES...

- Scattered showers and possibly a few thunderstorms are
  expected during the daytime on Thursday, with the greatest
  coverage expected south and west of the Chicago metro.

- Showers and thunderstorms are expected again Friday afternoon
  and evening. Some of these storms could be strong to severe.

- Rain and periods of thunderstorms are possible Sunday into
  Sunday night, with a conditional risk of strong storms toward
  central Illinois and Indiana Sunday afternoon.

&&

.DISCUSSION...
Issued at 346 AM CDT Wed Apr 16 2025

Through Thursday:

An elongated corridor of surface high pressure extending from
Hudson Bay down through the Bay of Campeche will cross our
longitude today, resulting in fairly mundane weather conditions
across the area. The remnants of a shallow stratus deck may
pass over some locations near Lake Michigan this morning before
being fully eradicated by solar insolation and large-scale
subsidence. Cirrus clouds will also be on the increase this
afternoon, but otherwise, ample sunshine should allow this
morning`s frosty low temperatures to warm into the mid 50s to
mid 60s across most of our forecast area this afternoon. Our
lake-adjacent locales look to be an exception to this, though,
as the light synoptic flow with a slight onshore component will
permit the development of a lake breeze. With lake temperatures
still in the 40s, air temperatures within a few miles of the
lakeshore will likely not make it to 50 degrees today as a
result.

Surface winds will turn southeasterly and gradually start to
pick up tonight in response to a developing surface low in the
central/northern Plains. The nose of a strengthening southwesterly
low-level jet will also slide across the Mississippi River towards
our forecast area late tonight into tomorrow morning, accompanied
by a plume of richer theta-e. Convection will blossom across the
Mid-Mississippi River Valley late tonight as a shortwave impulse
crests the top of a low-amplitude mid/upper-level ridge and
enhances lift across this zone of focused warm air
advection/isentropic ascent, eventually crossing into our
forecast area sometime after sunrise as the low/mid-level theta-e
gradient creeps eastward. The expectation is for convection to
decay/become less widespread with time as the low-level jet wanes
after daybreak and isentropic ascent subsequently ramps down, so
have focused our higher PoPs (55-65%) across the southwestern half
of our forecast area.

There should be enough instability upstairs tomorrow morning to
support charge separation within convective updrafts. Thus, a
few thunderstorms could occur in our forecast area with this
activity before it diminishes, though the overall better chances
for lightning should remain to our southwest across western and
central Illinois. Any deeper convection could also support some
small hail with it given the strong cloud-layer shear that will
be in place beneath the upper-level jet rounding the
aforementioned ridge.

High temperatures tomorrow will be modulated by how widespread
convection and any associated cloud cover is, along with how
long the convection is able to persist. Low 70s temperatures
would be in play if convective coverage remains limited and/or
diminished quickly, but on the other hand, more widespread and
longer lasting rain and cloud cover could make it difficult for
temperatures to climb much above 60F. With this forecast
package, have trended tomorrow`s highs towards the 00Z/06Z HRRR
and 03Z RAP solutions, but didn`t quite have the confidence to
introduce any 70 degree temperatures into the forecast grids just
yet given the existing uncertainties regarding tomorrow`s
convection. Tomorrow otherwise looks like another breezy day with
winds from the south likely gusting in excess of 30 mph during the
afternoon as the low-level jet begins to restrengthen.

Ogorek


Thursday Night through Tuesday:

A warm front will quickly lift northward across the area
Thursday night in response to a robust LLJ and corresponding
moisture transport. This will yield a mild and breezy night,
with perhaps some elevated convection north of the warm front
over northern Illinois conditional on any passing subtle mid-
level perturbations.

A lee cyclone will track northeastward along an existing
baroclinic zone west of the area on Friday, ultimately crossing
eastern Iowa into southern Wisconsin Friday afternoon per
increased clustering of ensemble guidance. With a strong and
very deep EML over the area, prospects for convection through at
least mid-afternoon look to be quite low. This will lead to a
warm and windy day with highs potentially into the low 80s in
the absence of any appreciable stratocumulus trapped under the
strong inversion. It remains to be seen if modest mid-level
forcing with an incoming low-amplitude shortwave trough will be
sufficient to fully erode most of the cap as a cold front
crosses the area late Friday afternoon and evening. The mostly
likely scenario at this time would be for a mostly dry and
capped frontal passage through most of the forecast area, with
added support from the nocturnal LLJ finally initiating severe
convection along the front after dark. This would be confined to
areas southeast of I-55, if not even farther southeast.

The cold front will stall across southern IL/IN Saturday and
Saturday night. A continuous southerly feed of low-level
moisture under a series of weak mid-level disturbances within
broader ENE flow aloft will result in an extended period of rain
with some embedded storms around and north of the stalled
front. This supports maintaining at least chance PoPs as far
north as the I-80 corridor during this time. The southwest
trough is then forecast to quickly eject and become negatively
tilted across the central Great Plains on Sunday. A new,
stronger surface low will shift near or west of the area Sunday
into Sunday night, bringing a period of widespread rain for part
of the day, with the potential strong to severe storms in the
vicinity of the southeast CWA Sunday afternoon. However, a
substantial difference in ensemble camps exists with Sunday`s
solution owing to large differences in the amplification and
orientation of the trough, with less favored scenarios
suggesting a flatter mid-level pattern and suppression of more
active convection to well southeast of the area.

The evolution with Sunday`s trough then has major implications
on the larger-scale flow across the central CONUS next week,
with active weather potentially re-establishing across the
central Great Plains and spilling toward the Mississippi River
Valley. With the differences noted above, will maintain the
blended forecast through this period.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1242 PM CDT Wed Apr 16 2025

The primary weather concerns for the 18Z TAF period include:

* Gusty near-southerly winds on Thursday

Light and variable winds will take on an easterly direction at
the Chicago sites early this afternoon as a lake breeze moves
across and speeds will build to closer to 10 kt. Sustained winds
should be largely below 10 kt overnight out of the SE, although
rather frequent gusts to around 20 kt will be possible with a
low level jet sitting just overhead, particularly after 08Z. SSE
winds Thursday morning will gust to around 25 kt, building to
30 to 35 kt for the afternoon. Direction will favor east of
south during the day, although may very well teeter on either
side of southerly at times, especially during the earlier part
of the afternoon.

Additionally, isolated pockets of light rain are possible around
northern Illinois Thursday afternoon, although the potential is
too low at this time to warrant an inclusion in the TAF. It`s
also possible that a period of MVFR cigs materializes during the
afternoon, especially if the rain comes to fruition, although
VFR is favored.

Doom

&&

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

&&

$$

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