694
FXUS63 KLOT 272030
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 PM CDT Sun Apr 27 2025

.KEY MESSAGES...

- Turning very warm and windy Monday, with highs in the lower
  80s and southerly wind gusts of 35 to 45 mph.

- There is a chance for thunderstorms Monday night, some could
  be strong to severe.

- Widespread rain chances return Wednesday through Thursday with
  embedded storms possible at times.

&&

.DISCUSSION...
Issued at 330 PM CDT Sun Apr 27 2025

Through Tuesday:

Quiet conditions will continue through tonight with steady
southerly flow and increasing cloud cover holding overnight lows
in the upper 40s to lower 50s.

Monday will be breezy to windy with summer-like afternoon warmth
and primarily dry conditions for much of if not the entire area.
Focus will then turn to a highly uncertain/conditional
thunderstorm forecast and the potential for strong to severe
storms Monday evening and overnight.

First, on Monday morning, we can`t completely rule out some
isolated elevated convection across interior far northern
Illinois. This will be driven by isentropic ascent as a warm front
associated with 996 mb low pressure over South Dakota lifts
northeast across the region. A very warm EML with steep mid-level
lapse rates will be advecting northeastward during this time, and
prior to the arrival of the more prohibitive MUCIN, we`ll have to
watch for a few isolated thunderstorms near the Wisconsin state line.

Monday afternoon, any morning (mid-level) overcast should give
way to scattered Cu. With increasingly strong warm advection and
southerly winds gusting to 35-45 mph, a good deal of afternoon
sun and 925 mb temps climbing to around +20C should support
highs from near 80F into the lower 80s (a few isolated 84F
readings possible). A slight southeast component to the flow
should keep the Lake County IL shore cooler until later in the
day.

Returning to the convective threat Monday evening and overnight,
the key limiting factors are: the magnitude of the capping
expected to be in place, only modest large scale forcing, and what
appears to be unfavorable diurnal timing mainly after sunset.
700 mb temps pushing +10C in the late afternoon are unlikely to
quickly cool given the lack of any stronger mid-level impulses.
With dew points reaching into the 60s and mid-level lapse rates
>8C/km, MLCAPE is forecast to grow to 1500-2000 J/kg. On the
other hand, the very warm EML and its high base up near/around
700 mb will result in 100 J/kg or more of MLCIN.

The most widespread convective initiation late day Monday is
anticipated to be up closer to the the surface low (central or
northern MN by then), with lower coverage/confidence in CI farther
south in the warm sector. This aspect and southwest flow aloft
really does call into question the extent (if any) of convective
coverage in the CWA late Monday evening and overnight. Contingent
upon scattered storms developing ahead of the system cold front
all the way down into central Iowa, initially discrete storms
would have a better chance of persisting if they congeal into
linear segments/clusters. With that being said, the strong capping
and only modest height falls in the evening near and east of the
MS River tend to argue against upscale growth of whatever
convection develops.

There does appear to be a plausible scenario in which any isolated
to widely scattered convection that approaches the west/northwest
CWA Monday evening really struggles and fizzles out as we lose
insolation. While this is a realistic outcome, opted to hold onto
PoPs peaking at 40-50% in the late evening and overnight
considering the existing global deterministic and ensemble member
support for convective QPF into the CWA.

Any storms that are able to remain intense into the area will do
so in a strongly sheared environment that would be supportive of
organized severe convection capable of causing all hazards
(damaging winds, large hail, and tornadoes). The highest threat
area across northern Illinois continues to be favored across
northwestern portions of the state (highlighted by a level 3 of 5
threat by SPC), including the Rockford vicinity. The level 2 of 5
threat extends into the Chicago metro, but with the already
conditional/uncertain nature of the threat, we`re continuing to
highlight the uncertainty in our graphical messaging.

Outside of the thunderstorm potential, unseasonably warm
overnight temps and continued strong/gusty south-southwest
winds will be the main story Monday night. The cold front will
move through the area Tuesday morning with little fanfare north
of I-80 and only a 20-30% chance for isolated to widely
scatteredshowers and a few thunderstorms south. Once past
sunrise, mixing depths will tap into the exiting stout low-level
jet, so southwest to west winds will remain gusty (35-40 mph)
through early afternoon.

Until mid afternoon, our far southeast CWA may have a short
window for convective development out ahead of the cold front.
However, veering surface winds will limit convergence and dew
points may drop even ahead of the front, which should relegate
higher convective chances to south of our far southern counties
(Ford, Iroquois, Benton IN). Finally, temps on Tuesday will be
above normal but noticeably cooler than Monday, except for our far
southeast. A secondary lake enhanced back-door cold front should
shift winds to north-northeast by early Tuesday evening and more
quickly cool off temps prior to sunset near the lake.

Castro

Tuesday night through Sunday:

In the wake of the Monday-Tuesday system, near seasonable
temperatures are forecast through the end of the week with highs
mainly in the 60s to near 70 (locally cooler near the lake).
Our next weather system is progged to lift across the region
during the Wednesday through Thursday timeframe which will bring
our next chance for widespread rainfall and a chance of
thunderstorms. A trailing wave then dives out of Canada which
could lead to continued showers into Friday, though there
remain differences in timing and coverage.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1230 PM CDT Sun Apr 27 2025

Forecast concerns include...

Chance of isolated thunderstorms Monday morning northwest IL.
Strong/gusty southerly winds Monday.

South/southeast winds may gust as high as 20kts this afternoon
and then quickly diminish with sunset this evening. A lake
breeze is expected to move inland this afternoon, though there
is some uncertainty if it will reach ORD/MDW. By the time it
does, likely around 00z, prevailing winds might already be
turning back to the southeast with speeds likely under 10kts.
Winds will turn more southerly in the predawn hours and may
increase back to 10kts by daybreak. Speeds/gusts will steadily
increase Monday morning with gusts into the lower/mid 20kts by
late morning. Deeper mixing by mid afternoon will allow gusts
into the 30-35kt range with a few higher gusts possible.
Directions will likely start 170-180 Monday morning and then
become 180-190 by early afternoon.

There is a low chance (20%) for a few thunderstorms across
northwest IL Monday morning, centered around daybreak.
Confidence is too low to include any mention at RFD. cms

&&

.MARINE...
Issued at 330 PM CDT Sun Apr 27 2025

- Gale Watch Issued for Lake Michigan nearshore waters between
  Winthrop Harbor, IL and Gary IN Monday afternoon through
  Tuesday morning.

Later Monday afternoon, increasingly strong southerly winds from
on land should extend out into the first mile or two of the
Illinois and far northwest Indiana nearshore. Forecast soundings
are supportive of gale force speeds/gusts. A strengthening low-
level jet and unseasonably mild temps on land will keep the
strong southerly winds going through Monday night ahead of a
cold front.

Winds will veer to southwest and then west with the cold front
on Tuesday. The strength of the winds aloft into Monday morning
and increasing mixing depths on land should translate to a
continue threat for gales through midday/early afternoon into
the first couple miles of the nearshore. Confidence is highest
in gale force speeds/frequent gusts for the nearshore zones
included in the Gale Watch. Farther east, some gale gusts are
possible, but more sporadic nature (if they do indeed occur)
precludes Gale Watch issuance with a longer small craft advisory
there instead.

A lake enhanced wind shift to north-northeast late day Tuesday
into the evening may quickly build waves on southern Lake
Michigan is wind speeds are sufficiently strong behind the wind
shift. Finally, there`s a chance for strong to severe thunderstorms
in the late evening and overnight hours on Monday night.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 AM to 3 PM CDT Monday for Winthrop
     Harbor IL to Gary IN.

     Gale Watch from Monday afternoon through Tuesday afternoon for
     Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 9 AM Monday to 4 PM CDT Tuesday for
     Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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