-- Discussion containing changed information from previous version are highlighted. --
890
FXUS63 KLOT 041122
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
622 AM CDT Sat May 4 2024

.KEY MESSAGES...

- A line of scattered thunderstorms is expected to cross the
  area late this afternoon into mid-evening, with locally strong
  to severe wind gusts to 60 mph possible.

- Periodic shower and thunderstorm chances are expected next
  week. There is a potential for strong to severe storms along
  with a locally heavy rainfall threat on Tuesday.

&&

.DISCUSSION...
Issued at 314 AM CDT Sat May 4 2024

Through Sunday:

A retreating surface ridge extending southwestward across the
western Great Lakes will give way to ongoing lee cyclogenesis
ahead of a mid-level trough entering the central Great Plains
early this morning. Robust convection from northern OK into
western IA has likely enhanced the trough, with multiple MCVs
evident along this corridor. An associated cold front just
behind this convection will become the main focus for storm
potential into our area later this afternoon into this evening.

Surface dew point trends today will be an important factor on
the coverage and intensity of storms into the forecast area.
Friday`s weak cold front has since stalled across central
Illinois and will jump northward in response to diurnal warming
and moistening today. However, the magnitude of moistening
primarily from evapotranspiration remains less clear this
afternoon. Current thought is daytime mixing into a very dry
layer above the growing PBL will with offset rising dew points
or at least reduce the depth of moisture sufficient for
development and maintenance of convection. Have opted to use
only scattered thunderstorm wording even as some CAMs depict
higher coverage as a result of higher modeled dew points.

While it is expected that a more prominent MCV currently near
Omaha will lift NE within the broader mid-level steering flow
this morning, the extreme southern influence of this MCV may
clip the northwest CWA late this morning. Capping should remain
far too high to support convection this far southeast, but
isolated attempts at convection may reach as close as far
northwest Illinois.

Overall expectations are for a line of broken showers and
storms to develop/intensify roughly along and around the
Mississippi River by mid-afternoon within a pre-frontal ribbon
of higher theta-e air. Coverage of thunderstorms should then
begin a slow downward trend across the forecast area late
afternoon through mid-evening as the drier pre-storm environment
becomes less favorable with time. However, given an increasing
reservoir of DCAPE with eastward extent across the CWA, the
potential for strong to locally severe wind gusts may actually
maximize across the west half of the CWA up to and through
sunset. The antecedent dry air ahead of these storms combined
with diurnal mixing of low-level moisture speaks to this wind
risk, with the strongest winds likely occurring with any
existing loosely organized multi-cell cluster or collapsing
cores as storm intensity wanes with time.

After the storms and cold front clear the forecast area by
early Sunday morning, dry and cooler conditions are expected
through the remainder of the period.

Kluber


Sunday Night through Friday:

Quiet weather starts the period Sunday night, as surface high
pressure is progged to be drifting east across the Great Lakes
region. Farther south, guidance continues to depict mid-level
short wave approaching IL late, though it is deamplifying as it
moves into the short wave upper level ridge axis. Forecast
soundings depict very dry low and mid-levels initially in place
across the forecast area, though persistent isentropic ascent
and associated warm/moist advection does eventually produce
saturation and precipitation development into central IL/IN and
the LOT/ILX border region after midnight. Mid-level lapse rates
are unimpressive in forecast sounding guidance, suggesting
little/no thunder threat. Showers are expected to persist into
Monday across the southern parts of the cwa as the mid-level
wave continues its transit of the area. NBM blended pop guidance
appears to spread too far north in this scenario, and note that
much of the EPS/GEPS/GFS ensembles are dry along/north of the
I-88 corridor through the day across northern IL.

Farther west, guidance continues to depict a deep upper level
trough across the western CONUS, within which a strong,
negative-tilt short wave develops a closed upper low over the
northern high plains by Monday night. Along with a series of
short waves propagating around the southeast periphery of the
upper circulation and upper level diffluence ahead of a strong
cyclonically-curved upper jet, this sets the stage for southerly
low level warm/moist advection from the western Gulf into the
mid-Mississippi Valley and upper Midwest Monday night, with a
northward lifting warm front spreading into the area into
Tuesday morning. Thus shower and thunderstorm chances look to
ramp-up into the forecast area from the west especially after
midnight and into Tuesday morning. While the primary
upper/surface lows will be well to our northwest Tuesday,
guidance continues to depict a strong mid-level jet is progged
to spread east across the area during the day, with a surface
wave developing along an occluding cold front approaching the
Mississippi Valley later in the day. Combined with a warm, moist
and conditionally unstable low- level air mass, additional
thunderstorms (potentially strong to severe) are likely within
the region Tuesday afternoon/evening. In addition to severe
weather, high precipitable water values around 1.75 inches will
support locally heavy rainfall will be possible especially if
storms train over a particular area. SPC and WPC outlooks for
Tuesday for both severe and heavy rainfall potential continue to
appear appropriate at this distance.

The upper trough is forecast to drift east across the upper
Midwest and northern Great Lakes region during the mid and late
week period, maintaining an active weather pattern across our
forecast area. While the primary low-level theta-E axis will
shift southeast of the forecast area Wednesday through Friday, a
series of short waves rotating through the southern periphery
of the upper Midwest upper low will likely produce additional
periods of showers and thunderstorms, especially during the
diurnally more favorable afternoon and evening hours.

Temperatures will remain above average through most of the
period, with daytime highs in the 70s to low 80s (Tuesday and
Wednesday have the potential to be the warmest days). Cooler,
more average temperatures are expected by Friday, with highs in
the mid-60s.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
-- Changed Discussion --
Issued at 607 AM CDT Sat May 4 2024

Aviation Key Messages:

- IFR stratus may approach KGYY from the east this morning, but
  should lift/scatter as it does.

- Scattered gusty thunderstorms this afternoon/early evening.

- MVFR ceilings likely behind a cold front late tonight into
  Sunday morning.

- Winds become southeast this morning, then south-southwest
  around midday. Winds may become somewhat variable behind
  storms this evening, before shifting north-northeast behind
  the cold front tonight, and eventually northeast Sunday
  morning.

Weak surface low pressure was over south central IA this
morning, with a cold front trailing into the southern Plains.
This low and cold front will move east-northeast to the western
Great Lakes by early this evening, with high pressure building
eastward into the region Sunday morning.

Early this morning, an area of LIFR stratus/fog had developed
across MI and far northern IN, and was moving west-northwest.
With the sun already up and mixing commencing, expect that this
will lift and scatter especially over northwest IN, though will
approach KGYY as it does so. Can`t completely rule out a brief
period of IFR conditions there, but think the stratus deck will
scatter prior to reaching the airport. Otherwise, southeast
winds will eventually turn south-southwest across the terminals
by midday, under thickening VFR high clouds. Earlier high-res
model runs had moved the lake breeze inland through KORD and
KMDW later this afternoon, but have since backed off of that
solution and it now appears the boundary will remain east of
those airports.

Farther west, an area of showers and thunderstorms was noted
across southern MN/IA and MO. Expectation is that these storms
will weaken, with dissipation across southern IA/MO this
morning. Scattered storms are then expected to redevelop this
afternoon in the vicinity of the MS river, then push east across
northern IL. Storms are expected to reach KRFD around 21Z, and
the metro terminals 23-00Z, albeit in a gradually weakening
state. Storms may produce wind gusts in excess of 30 kts. Winds
may become a bit variable behind the storms later this evening,
though the gradient would support a westerly direction. The
cold front will then bring a wind shift to the north- northwest
overnight, with winds eventually turning northeast Sunday
morning. An extensive area of MVFR ceilings will likely follow
the cold front, lingering into Sunday morning.

Ratzer
-- End Changed Discussion --

&&

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

&&

$$

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