009
FXUS63 KLOT 161941
AFDLOT

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

.KEY MESSAGES...

- There is lower than average confidence in the forecast for
  daylight hours tomorrow owing to uncertainty in the coverage
  (if any) of showers and storms.

- Additional showers and storms may move near and north of I-88
  tomorrow night with a threat for hail and gusty winds (call it
  a 20 to 40% chance).

- Friday will offer an early taste of summer-warmth with highs
  near 80.

- Showers and storms may develop Friday evening, especially
  along and south of I-55. A few storms may be severe with
  damaging winds and hail.

- Saturday should be a relative break in the pattern before
  another storm system brings showers and possibly storms to the
  region on Sunday.

&&

.DISCUSSION...
Issued at 241 PM CDT Wed Apr 16 2025

Through Friday Night:

Borchardt

Thanks to a passing surface ridge, it`s a pleasant mid-April day
with mostly sunny skies, light winds, and temperatures in the
low to mid 50s. The lack of a robust surface pressure gradient
has allowed for a strong lake breeze to surge inland this
afternoon, leading to temperatures in the low to mid 40s
lakeside. Taking a step back, GOES-19 water vapor imagery
depicts a scene of change across the CONUS with an elongated
positively-tilted trough positioned over California, a sharp
upper-level trough diving southward into the northwestern US,
and aggregate upper-level ridging developing across the northern
Plains to Midwest.

Tonight, the northwestern US and California troughs will begin
to merge over the southwestern US, enabling broad southwesterly
mid- to upper-level flow to become established across the middle
Mississippi River Valley. The passage of several subtle mid-
level waves embedded within the developing southwesterly flow
should excite a few or perhaps a consolidated low-level jet
overnight, setting the stage for showers and storms to develop
within a relatively broad region from eastern Kansas to west-
central Illinois. With our area so far removed from mid-level
instability (the reservoir of steep mid-level lapse rates will
remain offset toward the Plains), would have to think that our
area will remain completely dry overnight. Lows will range from
the mid 30s lakeside to upper 40s toward central Illinois.

On Thursday, showers and a few thunderstorms across west-central
IL may attempt to lift east-northeastward into northern IL and
northwestern IN along the leading edge of the waning low-level
jet. CAM guidance seems rather bullish in depicting widespread
showers and storms all the way into the afternoon, which seems a
bit too heavy-handed given the instability and moisture axes
will remain well to our southwest for much of the daylight hours.
Unfortunately, such uncertainty in the coverage of showers and
storms during daylight hours on Thursday introduces lower than
average confidence on temperatures. If convective coverage
tomorrow is muted, highs may attempt to surge into the mid to
even upper 60s. However, if convection is widespread,
temperatures may struggle to reach 60. For now, will carry mid-
range chance PoPs (30-50% chances for showers and storms) and
highs in the lower 60s, as a nod to mostly cloudy skies within
the "middle of the road" outcome.

Regardless of shower/storm coverage, there should be a window
tomorrow afternoon and evening for mixing heights to build
upward into the base of a reintensifying low-level jet. Forecast
BUFKIT profiles even from the wetter guidance depicts some 35
to even locally 40kt of flow at the top of the boundary layer
toward sunset tomorrow, leveraging some concern for 35 to
locally 45 mph surface gusts. For now, will continue a "middle
of the road" approach to the forecast with gusts peaking around
40 mph tomorrow evening. Gusts may take time to ease overnight
as mechanical mixing works against surface decoupling.

Tomorrow night, another upper-level shortwave within the persistent
southwesterly flow aloft will kick off another low-level jet
and round of showers and storms, this time from central Iowa to
southern Wisconsin. As the edge of the EML plume will be
advecting northeastward into the Great Lakes, do have to wonder
if coverage of showers and storms will be further south than
depicted in model guidance. In addition, the pattern may support
a mesoscale gravity wave or two, which could force convection
further south than expected. As a result, will advertise low-
end chance PoPs (20-40%) generally along and north of I-88
overnight into early Friday morning. Any storm overnight would
be prone to producing hail and gusty winds, given the
aforementioned steep mid-level lapse rates and increasingly
sheared kinematic profiles.

Barring any morning convection, daylight hours on Friday look
dry owing to neutral to positive mid-level height tendencies
atop an impressively strong low-level cap (effective lapse rates
of -6K/km at the base of the EML). Aggregate troughing across
the western US will encourage gusty southwest winds, which in
tandem with unseasonably warm 850-700mb temperatures of +15 to
10C, respectively, will set the stage for summer-like warmth.
Highs should have no problem soaring toward 80s areawide,
including up to the lakeshore.

Friday evening, a cold front will slip southeastward through
the area within aggregate surface troughing. Increasing upper-
level diffluence within the right entrance region of a jet
streak propogating within the broad southwesterly flow aloft may
provide just the oomph needed to erode the cap to support
scattered showers and storms along/behind the front. Global
guidance is fairly bullish showing such a scenario, so will
continue to advertise relatively high PoPs favoring areas along
and east of I-55 (40% near Rockford, 80% near Valparaiso).
Strongly sheared kinematic profiles will support storm-scale
organization even for cells behind the front, leading to a
threat for damaging winds and hail with the strongest storms.
SPC maintains a Level 2/5 threat level for our area, though do
think the relative highest threat should be along and east of I-
55.

Saturday and Beyond:

Convection Friday night should push the low-level baroclinic
zone south on Saturday, though just how far south is somewhat
unclear (will ultimately depend on the coverage of
showers/storms and any aggregate cold pool). Will hence hold
onto a gradient in PoPs ranging from around 20% near I-80 to
around 60% near US-24. Notwithstanding uncertainty on the
placement of the front, Saturday does look to be a relative calm
day within the energetic pattern. North to northeast winds (off
Lake Michigan) as well as widespread clouds (from convection to
our south) should cap highs in the mid to upper 50s, save for
the mid to upper 40s lakeside.

Attention then turns toward the next wave of showers and storms
on Sunday as the upper-level trough responsible for the
energetic pattern finally ejects northeastward. Continued spread
in the evolution of the trough within model guidance appears to
stem from the evolution of a lead trough passing through the
northern Great Lakes Saturday night, with EPS/CMCE guidance
favoring a faster/less involved evolution compared to the slower
and more involved GEFS. In forecast scenarios where the lead
trough isn`t as much of an influence, the southwestern US trough
would lift east-northeastward while turning sharply negatively-
tilted supporting a rapidly deepening surface low near the
Mississippi River. This is a somewhat concerning forecast
scenario, as it would match conceptual models for notable severe
weather events in our general region. Meanwhile, a more nuanced
interaction between the lead and southwestern US troughs may
lead to a slower, and more suppressed, evolution of the system,
relegating any threat for severe weather well south of our area.
Notwithstanding such spread within model guidance, the
overarching pattern nevertheless supports relatively high PoPs
(50-70%) for, at a minimum, periods of showers on Sunday.

Looking toward the last full week of April, ensemble model
guidance advertises quasi-zonal flow developing across the
northern US. Such a pattern would support near or above-average
temperatures as well as regular opportunities for showers and
storms.

Borchardt

&&

.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...Small Craft Advisory from 7 AM Thursday to 6 PM CDT Friday for
     the IL and IN nearshore waters.

&&

$$

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