105
FXUS63 KLOT 041803
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
103 PM 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.

&&

.UPDATE...
Issued at 921 AM CDT Sat May 4 2024

A recent hand surface analysis reveals a 1009mb surface low
pressure system centered in central Iowa ahead of a cold front
arcing southward across far eastern Kansas and central Oklahoma.
An MCV is evident ahead of the front just to the northeast of
Kansas City along the nose of a developing instability plume
arcing northward along the backside of a surface high pressure
system centered over southern Ontario. A narrow line of showers
and storms approaching the Mississippi River continues to
outpace the developing instability gradient and cold front to
the west, and accordingly, should continue to decay. With that
said, we can`t rule out a shower survives as far east as I-39 by
early afternoon.

The forecast for this afternoon and evening appears to be on
track. Pockets of clearing ahead of the cold front and
northward-progressing instability plume will allow for
steepening low-level lapse rates and eroding inhibition. As a
result, showers and thunderstorms appear poised to erupt along
and ahead of the front this afternoon near the Mississippi River
and spread eastward into northern Illinois and eventually
northern Indiana this evening (reaching I-39 in the 5 to 7 pm
timeframe, Chicago metro in the 6 to 8 pm timeframe, and
northern Indiana in the 7-9 pm timeframe). With the upper-level
jet displaced to our north, thunderstorms will be limited to
"pulse" like single-cell behavior. However, somewhat linear
forcing ahead of the cold front, as well as relatively dry low-
level moisture profiles, will afford efficient amalgamation of
surface outflow/cold pools and support "upscale growth" of
initial single-cell thunderstorms into gusty multicell
clusters.

While the environment doesn`t appear supportive of widespread
severe weather, prospects for a wall of 45-55 mph wind gusts
along the leading edge of any consolidated thunderstorm
outflows appears to be increasing this evening. Where
thunderstorm pulses are most intense, locally damaging wind
gusts of 60-65 mph may occur, as well. We`ll have to keep a
close eye on the integrity of the aforementioned MCV as it
tracks toward central Illinois this afternoon, as it may provide
just the oomph needed for a more focused threat area for
damaging winds.

Updated products have been sent.

Borchardt

&&

.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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 101 PM CDT Sat May 4 2024

The primary aviation weather concerns through the 18Z TAF period
include:

* A line of gusty showers and thunderstorms this evening

* MVFR cigs expected late this evening into Sunday morning

A line of showers and thunderstorms will move into northern IL later
this afternoon and into the Chicago metro around early evening. It`s
expected to reach RFD mid-late afternoon and the Chicago sites
closer to 00Z. The rain potential over any given area will last only
a few hours or so clearing the Chicago sites sometime near 03Z. It`s
possible that vsbys could get pulled down to MVFR for a brief period
or two underneath any heavier rainfall. Additionally, gusty westerly
winds will likely accompany the rain and storms. Gusts into the 30-
35 kt range look attainable while the rain moves in and it`s not out
of the question that we could see an isolated higher gust or two.

Cigs are expected to remain VFR ahead and during the bulk of the
rain event. However, MVFR cigs should take hold this evening as the
rain begins to move away. MVFR will persist through the night and
much of Sunday morning over the Chicagoland sites. A period of IFR
is possible overnight, through best guess is that we`ll remain MVFR.
At RFD, the MVFR looks to lift much sooner, likely late this evening
or early in the overnight. The MVFR will clear over the metro
during the late morning and early afternoon.

Meanwhile, SSW winds will continue to occasionally gust into the
teens to near 20 kt this afternoon. Winds are expected to veer to
westerly with the arrival of the rain. A couple of forecast models
suggest we may see a backing in the wind field to SSE for an hour or
two as the system approaches, through a lack of confidence left this
potential wind shift out of the TAF. Winds at the time will be
moving at near 10 kt with gusts as high as 20 kt. Any of those
notably higher gusts just ahead of the system that were discussed
will be out of the west. NW winds just over 10 kt will follow the
rain out of the area this evening before flopping over to NNE early-
mid morning. The majority of Sunday will see NE winds below 10
kt.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 7 PM CDT this evening for Winthrop
     Harbor to Northerly Is. IL.

&&

$$

Visit us at weather.gov/chicago