Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252331
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 PM CDT Mon Mar 25 2024

.KEY MESSAGES...

- Breezy to occasionally windy conditions into Tuesday,
  especially east of the I-55 corridor. Sporadic wind gusts to
  45 mph possible.

- Locally heavy rainfall (amounts 1-2+") and minor flooding
  potential tonight for portions of northern IL west of the Fox
  Valley and I-55 corridors.

- Line of isolated strong/severe thunderstorms possible across
  eastern half of so of the LOT CWA Tuesday morning through
  midday, with gusty winds the main threat but a non-zero
  tornado potential.

- Chances for showers and potentially storms return this weekend
  into early next week.

&&

.DISCUSSION...
Issued at 311 PM CDT Mon Mar 25 2024

Through Tuesday night:

Early afternoon surface analysis places 993 mb low pressure
over far northeast Kansas, downstream of an elongated closed
upper low over the central Plains. A 140+ kt upper level jet
streak is progged to round the base of this feature into
tonight, with the mid-level short wave trough becoming more
compact and acquiring a negative- tilt as it lifts northeast
across the western Great Lakes region through Tuesday. In
response, the surface low is forecast to track just west of the
cwa overnight, deepening to sub-990 mb as it continues to lift
north to Lake Superior by late Tuesday afternoon. Enhanced
southerly flow ahead of the low and a trailing occluding cold
front will pull a relatively narrow ribbon of deeper moisture
north from the Gulf/Arklatex region, with forcing associated
with the short wave resulting in an axis of fairly widespread
1.00"+ rainfall from Missouri into western/northwest Illinois
this evening/overnight before a mid-level dry slot wraps into
the area from the southwest by early Tuesday morning. In our
forecast area, model guidance is in good agreement in the
heaviest rains occurring generally west of the Fox River Valley
and I-55 corridors. Global ensembles generally depict around
1.50" across these areas, though some of the high-res guidance
(HREF, HRRR, RAP) indicate max amounts of 1.75-2.00"+ across
north central IL. This would likely be enough to cause some
ponding in typical low spots and bring some area creeks/rivers
to near bankfull for a time. Our ESF (hydrologic outlook) will
continue to highlight these impacts.

Surface winds have been breezy/windy across parts of the area
today in the tight south-southeasterly gradient east of the low,
with the strongest winds generally along/east of the I-55
corridor. Extensive cloud cover has tended to limit the depth of
mixing however, making the higher gusts in the 40-45 mph range
somewhat sporadic (though showers have at times helped to mix
this higher momentum down to the surface). Expect this to
persist through the remainder of the afternoon and evening
hours, and have elected reissue an SPS to highlight the
occasionally gusty winds. Another period of similarly gusty
winds is possible after midnight into early Tuesday morning as
the occluding cold front and more numerous convective showers
track across the area. Winds remain gusty Tuesday behind the
initial front, though will turn more west-southwesterly in the
afternoon. A second cold front will push through the area
Tuesday evening, with winds gradually diminishing Tuesday night.

In addition to the above, a band of low-topped convective
showers/thunderstorms will potentially move east across the area
Tuesday morning into midday, along the eastward-propagating
occluded cold frontal boundary. Timing of the front precludes
significant low- level instability for much of the forecast
area, though high-res guidance does develop weak surface-based
instability ahead of the boundary across the eastern half of the
forecast area after sunrise. Despite relatively shallow
instability/storm depth, strong low-mid level wind shear will
support a marginal (level 1/5) severe threat with these storms
with strong wind gusts being the primary threat. The strong low
level shear will also present a small but non-zero isolated
tornado threat as well. Threat will likely be higher just east
of our cwa. SPC day 2 marginal outlook across our eastern cwa
appears very reasonable. Scattered showers will likely persist
into early Tuesday evening, as the second cold front and mid-
level vort and trough axis move through. Will likely see morning
highs in the 50s (low-60s east) before temperatures gradually
fall later in the day.

Ratzer


Wednesday through Monday:

A brief cool-down to below normal temperatures is expected on
Wednesday as a colder air mass filters in behind our early week
system. High pressure will continue to build into the region on
Thursday, yielding mostly clear skies that will aid in moderating
temperatures back closer to normal for this time of year. This
warm-up will continue into Friday as winds turn southerly between
the departing surface high and a developing region of low pressure
in the Great Plains, helping to reintroduce 60+ degree temperatures
into at least southern portions of our forecast area. However, a
lake breeze may develop and keep temperatures cooler closer to the
lakeshore, particularly on the Illinois side of the lake, where
temperatures may struggle to climb above 50 degrees.

A baroclinic zone is expected to sharpen over the region going into
this weekend, more or less coincident with the positioning of an
anticyclonically-arcing upper-level jet stream that will become
established over the region at this time. One or two subtle
disturbances are likely to traverse this corridor late this
week/early this weekend and potentially spur a round or two of
showers and potentially storms, though the latest GEFS and EPS
are mixed on whether that will actually occur. The ensemble
members that do output QPF during this time period exhibit quite
a bit of spatiotemporal spread with their QPF footprints, so
have capped PoPs at 50% through Sunday to account for these
existing uncertainties. A more pronounced upper-level trough
then appears set to barrel into the region sometime on Monday or
Tuesday of next week. Pending its exact track/evolution and the
location of the low-level baroclinic zone (which is expected to
sag southward with time), the trough`s arrival could bring
another round of showers and storms to the region early next
week.

Ogorek

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 631 PM CDT Mon Mar 25 2024

Key Aviation Impacts for the current TAF period:

- LLWS potentially though 03Z, despite sporadic higher surface
  wind gusts

- Periods of moderate rainfall later tonight into Tuesday
  morning with VIS/CIG reductions for MVFR conditions

- 30 percent chance for -TSRA for a short window late Tuesday
  morning

- Southeasterly winds become southwesterly Tuesday, with
  stronger 30+ knots gusts

Despite under performing surface wind gusts, winds aloft
observed on radar are still gusting 40 to 50 knots. Therefore
the LLWS that was added at 21Z was maintained through 03Z.
However, sporadic gusts of 25 to 30 knots are still possible
through the evening and overnight hours.

The main swath of rainfall is currently over KRFD and stretching
back to St. Louis, MO. In the next several hours, that band will
move eastward over the Chicago terminals. This band is expected
to provide consistent rainfall, with more moderate rates at
times, and reducing cigs and vis down to MVFR levels. Winds
during this time will remain out of the southeast with gusts
possible around 30 knots.

There is some uncertainty in the exact timing when the rain will
move east Tuesday morning. Lingering showers are expected around
terminals but there could be a period early morning of a couple
hours with no rain yet windy MVFR conditions. However, the
next boundary will move over the area late morning bringing a
renewed chance for showers with better instability which could
create thunder near the terminals. There is still some
uncertainty on the timing, so the PROB30 was maintained, and
only slightly adjusted the expected window for timing.

MVFR conditions will continue through the end of the TAF period,
but winds will slowly drift west of south Tuesday morning, and
gradually become true southwesterly Tuesday afternoon. Gusts
will continue with just around 30+ knots Tuesday afternoon.

DK

&&

.MARINE...
Issued at 303 AM CDT Mon Mar 25 2024

Several periods of gale force winds continue to be possible
through Tuesday evening. The first is later this morning and
towards midday with intermittent 35 kt gales, mainly in the first
few miles of the nearshore waters south of LMZ740. The second is
overnight tonight. Given the expected relatively brief duration
and intermittent/occasional nature to gales during these time
periods, the Small Craft Advisory will continue, mentioning
occasional gales.

Thereafter, there is a better signal for more persistent/regular
35 to 40 kt gales developing Tuesday afternoon as a cold front
sweeps across the lake. Uncertainty remains in gales developing
earlier in the morning, however. As a result, the Gale Watch
remains in effect, but an upgrade to a Gale Warning may be
needed later today.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CDT Tuesday for the IL and IN
     nearshore waters.

     Gale Watch from 3 AM CDT Tuesday through Tuesday evening for
     the IL and IN nearshore waters.

&&

$$

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