917
FXUS63 KLOT 020832
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
332 AM CDT Wed Apr 2 2025

.KEY MESSAGES...

- Waves of showers and storms will parade over the area through
  early evening, some may be severe.

- Primary threats this morning are large hail and gusty winds.
  All hazards are possible this afternoon (contingent on
  morning storms clearing).

- Gusty southeast winds continue this morning with gusts to
  35-40 mph. Strong south winds develop this afternoon,
  especially south of I-80 where gusts in excess of 45 mph are
  possible.

- Periods of rain, possibly heavy, late Friday afternoon into
  Saturday morning, with possible localized flooding south of
  I-80.

&&

.DISCUSSION...
Issued at 330 AM CDT Wed Apr 2 2025

Through Thursday:

The main focus remains the potential for waves of thunderstorms
through early evening, some of which could be severe with all
hazards possible.

Low pressure continues to consolidate early this morning across
central Kansas. Meanwhile a line of mainly pre-frontal strong
to severe thunderstorms currently extends from Kansas City
southwest into north central Oklahoma and is drifting northeast
toward the region. Embedded within this line appears to be one
or two developing embedded MCVs that we will have to keep an eye
on as they approach. Well ahead of the low, warm advective
showers and thunderstorms have begun to increase in coverage
across the area, with more robust clusters of storms currently
in southern IA/northeast MO and central IL. This will continue
to consolidate and lift across the local area between now and
daybreak as a mid-level lapse rate plume moves overhead
allowing storms to tap into better elevated instability. Main
hazards this morning are frequent lightning, large hail up to 1"
in diameter and localized flooding in poor drainage areas. We
will also continue to watch for the potential for a gusty wind
threat if any gravity wave associated convection develops (some
guidance continues hint at this potential and storms over Kansas
City are beginning to take on this appearance) which could
allow for strong downbursts to punch through the stable surface
layer in spite of storms remaining elevated.

Unfortunately confidence in how the rest of the day unfolds
remains quite low so expect frequent updates from us throughout
the day. The surface low is expected to gradually deepen as it
lifts north northeast toward the Upper Mississippi Valley
through the day today. The aforementioned line of pre-frontal
convection is expected to lift across the area through the
morning hours. While there continue to be notable model
differences in handling the various waves/lines of convection,
particularly around midday, if some clearing can occur during
this time, strong low-level flow would support rapid recovery
of the low-level thermal environment, especially south and east
of I-55. Areas further north we continue to have the lowest
confidence. Will also have to keep an eye on the cold front
which could serve as another focus for renewed development in
the late afternoon into early evening with the 6Z NAMNest still
showing this scenario.

Thus we plan to continue messaging the conditional potential
for severe weather across the entire area, though with greater
confidence (albeit still relatively low) in areas mainly south
and east of I-55 this afternoon into early evening. All hazards
remain possible with the afternoon/early evening thunderstorms,
including damaging winds, large hail, and tornadoes. Lastly,
given the expected multiple rounds of convection, localized
flooding will be possible in low-lying, urban, and poor drainage
areas.

Non-Thunderstorm Winds:

There is a window where if storms are able to clear out even
briefly in the afternoon, with a strong low-level jet overhead,
slightly deeper mixing would tap into these stronger winds. Due
to this potential, opted to issue a Wind Advisory for our
southwestern counties (mainly south of a Mendota to Kankakee to
Fowler line where non-thunderstorm wind gusts could exceed 45
mph in the afternoon. Further north, wind gusts to 35-40 mph
will remain possible with perhaps a short 1-2 hour period where
sporadic gusts to 45 may still occur.

Tonight through Thursday:

After storms clear the area, expect winds to ease and
temperatures to drop into the 40s across the area. Heading into
Thursday we warm back into the 50s with our next weather system
beginning to lift toward the area toward the end of the day with
increasing rain chances south of I-80.

Petr


Thursday Night through Tuesday:

There remains uncertainty for a period of showers Thursday
night, generally south of I-80. The NAM was the first to show
this potential as it entered the end of its runs and it still
shows showers across the southern half or so of the cwa Thursday
night. The 03z RAP is now out through part of this time period
and it too is showing showers across much of the southern cwa
Thursday night. Otherwise, not much support for this potential.
Chance pops seem reasonable for now.

If those showers develop, a short break is expected from early
Friday morning through early Friday afternoon and then the
models continue to bring the next system further north with each
run with periods of rain, possibly heavy at times from late
Friday afternoon into early Saturday morning. Precipitable water
values increase to an inch or so across the north to as high as
1.5 inches across the south during this time period. With the
possibility of hours of moderate/heavy rain, there could be a
band of qpf amounts in the 2-3 inch range across the southern
cwa. While confidence is increasing with the northern shift to
this rain/qpf axis, still 3 days away and more changes are
possible. If these trends were to continue, hydro concerns may
develop across the southern cwa. Precipitation looks to
dissipate by mid/late Saturday morning and then one more push of
precip Saturday afternoon/evening as the system finally pulls
away. GFS actually turns some of the precip to snow early Sunday
morning, as the system departs and colder air moves into the
area. No snow mention planned with this forecast.

Now appears there will be a stronger cold front moving south
sometime Monday that will bring colder air into the region with
perhaps a low chance for precip, maybe snow showers, as it
passes the area. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1230 AM CDT Wed Apr 2 2025

Primary forecast concerns include...

Multiple hours of scattered thunderstorms through late morning.
Chance of thunderstorms this afternoon.
Ifr/Lifr cigs overnight through early/mid afternoon.
Strong southeast winds shifting southwest mid/late afternoon.
Low level wind shear mid morning into mid afternoon.

Low confidence to make any significant changes to current
forecast though changes to thunder coverage/timing will likely
be needed with later forecasts/updates.

Scattered thunderstorms are expected to develop over the next
few hours and increase in coverage in the predawn hours across
the terminals and persist through at least sunrise, which is
covered by current tempo mention. There is some consensus of a
1-3 hour break mid morning, before another round of
thunderstorms may spread across the terminals in the late
morning/early afternoon. This time period is currently only
covered with vicinity thunder mention and may need tempo
mention with later forecasts. There remains quite a bit of
uncertainty as to how convective trends will evolve after the
early morning activity. There may then be a larger break in the
precip, perhaps 2-4 hours in the afternoon until a possible line
of thunderstorms moves across the terminals in the late
afternoon/early evening. This time period is covered with prob
mention, which looks fine for now. All of these storms may
produce heavy rain and reduce visibility into the 1sm range.

Vfr cigs to start will lower into low mvfr overnight and into
ifr by daybreak with lifr cigs expected for at last some time
period later this morning. Cigs may lift back to ifr or low mvfr
early this afternoon and then are expected to scatter out as a
warm front lifts north across the terminals.

Southeast winds will gust into the 30-35kt range overnight and
may diminish some after sunrise. Low level south/southwest winds
will be steadily increasing after daybreak and low level wind
shear mention looks on track for both speeds and directional
differences. Winds will eventually turn south/southwest this
afternoon as the warm front lifts north. Outside of
thunderstorms, prevailing winds may gust into the 35-40kt range
mid/late this afternoon and then diminish some with sunset and
turn more to the southwest. cms

&&

.MARINE...
Issued at 330 PM CDT Wed Apr 2 2025

- Gale Warning in effect through 7 PM CDT this evening.

Southeast gales gusting to 35 kt are expected to continue
through daybreak this morning. It is possible there is a brief
lull in gales toward midday. However, expect wind gusts to
increase out of the southwest during the afternoon to 35 kt as a
deepening surface low lifts across the Upper Mississippi
Valley. Winds then gradually ease toward sunset behind a
departing cold front.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory from noon today to 7 PM CDT this evening for
     ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039.

IN...Wind Advisory from noon CDT /1 PM EDT/ today to 7 PM CDT /8 PM
     EDT/ this evening for INZ010-INZ019.

LM...Gale Warning until 6 PM CDT this evening for the IL and IN
     nearshore waters.

&&

$$

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