081
FXUS63 KLOT 221742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 PM CDT Sun Sep 22 2024

.KEY MESSAGES...

- Beneficial rainfall today with a 20-30% chance for isolated
  to widely scattered thunderstorms.

- Second wave of rain and possibly some thunderstorms is
  possible later Monday night through early Tuesday, especially
  southeast of I-55.

&&

.UPDATE...
Issued at 1100 AM CDT Sun Sep 22 2024

Rain has continued to fill in across northern IL and northwest
IN this morning as a subtle shortwave disturbance pivots into
the area. Periods of showers are expected to continue through
this afternoon and into early evening as the disturbance
interacts with the abundant moisture ahead of a cold front which
is currently analyzed from central WI to northeast MO. However,
instability has not materialized as originally forecast which
means that the threat for isolated to widely scattered
thunderstorms appears to be diminishing. That said, have decided
to reduce thunder chances into the 20-30% range with the
morning update and keep the greatest potential south of I-80
where forecast soundings show better instability to develop.
Regardless of the thunder coverage, rainfall totals across most
of the area should still top out in the 0.5 to 0.75 inch range.
Though, there still remains the potential for localized pockets
of higher totals nearing an inch where any convectively enhanced
showers and/or storms can materialize but the threat for
flooding remains very low.

Showers, and any potential storms, are expected to taper rather
quickly behind the aforementioned cold front this evening
meaning dry conditions should prevail overnight into the day on
Monday. While the POP forecast has been adjusted accordingly,
there is still some slight chance (20%) mention for areas south
of the Kankakee River Valley which may get reduced with this
afternoon`s update. Aside from the rain, winds will quickly
increase behind the front this afternoon with gusts in the 15 to
25 mph range, especially over the open waters of Lake Michigan.
These northerly winds will also quickly build waves into the 6
to 10 ft range this evening and overnight which will make for
dangerous conditions in the lake and along the shore. Therefore,
a Beach Hazard Statement will go into effect for Lake and Cook
counties in IL at 4 PM today with Lake and Porter counties in
IN expected to go into effect this evening. Otherwise, expect
temperatures to remain nearly steady in the lower 70s this
afternoon before dipping into the 50s overnight.


Yack

&&

.DISCUSSION...
Issued at 351 AM CDT Sun Sep 22 2024

Through Monday:

An upper-level trough centered over Colorado is in the process
of getting sheared out this morning. Out ahead of it to the
east, a baroclinic zone extends northeastward into the Great
Lakes with multiple convectively-enhanced disturbances noted
within this zone on GOES water vapor imagery. Unsurprisingly,
there are multiple regions of shower and thunderstorm activity
present across this corridor, and some of this activity will
work its way across our forecast area today. While some
locations could see rain prior to daybreak, the more widespread
rainfall likely won`t arrive until after sunrise, when richer,
deeper column moisture (precipitable water values between 1.6
and 1.8 inches -- near-record values for this time of year, per
DVN/ILX sounding climatology) is slated to roll in from the
southwest.

In response to the increasing large-scale ascent, mid-level
lapse rates should steepen enough to support the potential for
at least isolated thunderstorms today. The main window for
today`s thunderstorm potential appears to be this morning with
the nose of an impinging mid-level jet and/or outflow boundaries
from early morning convection in southern Wisconsin serving as
potential foci for convective development. However, if this
morning`s rainfall ends up being less widespread (or just less
detrimental to destabilization efforts) than expected, then
thunderstorms may also develop later this afternoon along an
incoming cold front. The overall degree of available instability
in either case looks to be far too meager to support a threat
for severe weather, especially in the absence of stronger deep-
layer shear. Additionally, despite the high level of moisture
content for this time of year, progged rainfall rates and storm
motion do not appear to be overly supportive of a threat for
flooding (especially when considering the antecedent dryness),
though suppose that some localized ponding could be seen in poor
drainage areas wherever any convective elements and associated
heavier rainfall rates are able to persist for a relatively
longer period of time. For most though, this should be a
beneficial rainfall that puts a dent in our rainfall deficits
for this month. Most of the rain should come to an end after the
passage of the cold front this afternoon/evening, though light
post- frontal showers or drizzle could linger longer into the
night.

Today`s rain and cloud cover will bring an end to our recent
spell of above normal and near-record warm temperatures, though
today`s high temperatures still look like they`ll end up being
near-normal for this time of year (low to mid 70s). Slightly
cooler, though still similar, temperature readings are expected
on Monday in the wake of today`s cold frontal passage. Behind
the front, winds will turn northerly and become blustery over
and near Lake Michigan late today through Monday morning. These
winds will whip up some decent waves over the lake that will
cause dangerous swimming and boating conditions through Monday
night.

Ogorek

Monday Night through Saturday:

Final slug of vorticity associated with the Plains weakening upper
low is progged to move eastward across the Mississippi Valley
Monday night into Tuesday. Some differences in guidance in how
amplified this wave will be as it moves across the region, meaning
some varying solutions in how far north final wave of rain will
spread. NBM pops did a respectable job representing that
uncertainty with highest pops southeast of I-55 into northwest
Indiana. Northwestern CWA may end up seeing little if any rain
from this second wave if the more suppressed solutions verify.

It looks like there will be at least a couple of days of dry
weather with seasonable temperatures Wed and Thu. After that
confidence is very low in the forecast. As mentioned previously,
medium range guidance continues to develop a very high amplitude
and blocky pattern across North America. Model predictability
tends to be much lower in these type of pattern, particularly with
smaller scale details. The general idea for late this week into
the weekend is for Rex Block type pattern to develop with strong
positive height anomalies over the upper Great Lakes into Canada
with a potential cut off low developing south of this ridge across
the lower or middle Mississippi Valley. Further lowering the model
predictability is a potential large tropical cyclone spinning up
in the Gulf of Mexico later this week before possibly interacting
with the cut off low.

The spectrum of possible outcomes for later this week is huge with
our whether late this week into the weekend ranging from potential
flooding rains to sunshine and unseasonably warm temps, and
everything in between. It will hinge on if and where a cut off
mid-upper level low develops and then if a tropical cyclone
develops, how it interacts with the possible upper low. We
continue to see significant model to model and run to run
variability in the details of these key features and wouldn`t be
surprised to see additional large inter and intra model
variability over the coming few days. Given this, made no changes
to the NBM forecast Thursday night onward, which does now include
some very low pops.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1242 PM CDT Sun Sep 22 2024

Key Aviation Impacts for the current TAF period:

- Continued showers through the afternoon, tapering off this
  evening. Lower vis possible within periods of heavier showers

- Low cigs creating MVFR/IFR conditions through 03Z. Potential
  cig improvement tonight, but MVFR conditions expected through
  Monday morning

- Winds will be out of the NW shifting to the north. Stronger
  gusts around 20 knots possible late this afternoon

A subtle surface boundary is moving over the city of Chicago
providing light east-northeast winds at the time this discussion
was published. However, as the next wave of showers moves in
from the west, the prevailing wind pattern is expected to return
to the northwest.

As this system continues to move east over the airspace,
showers are expected to continue through the afternoon. It has
already been observed that during the more intense showers,
ceilings and visibilities have reduced down into IFR
conditions. Given that the next two bands of showers is headed
for the Chicago area, a TEMPO was added for the risk of SHRA
bringing lower cigs and vis once again. The heaviest
precipitation is moving east of the RFD airspace and though
showers will continue this afternoon, the TEMPO for lower
conditions reflects the earlier departure. There is no thunder
expected with this afternoon`s showers at any TAF sites.

Given the lingering rain in eastern to central Iowa, VCSH was
added to the TAFs extending into early this evening for
lingering light rain longer than originally anticipated. Cigs
will continue to lower for an expected window of prevailing IFR
just before the 00Z push. The northwest winds will increase
later this afternoon with gusts at or just over 20 knots
possible. As they increase and the front moves through, these
winds are expected to drift to the north. Winds will gradually
diminish and cigs should begin to recover back to MVFR levels
through the overnight. Low confidence on the return of VFR
conditions Monday morning, but there was enough of a model
signal to leave MVFR through 18Z.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 4 PM CDT this afternoon through
     Monday evening for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from 7 PM CDT this evening through
     late Monday night for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM this afternoon to 7 PM CDT
     Monday for the IL nearshore waters.

     Small Craft Advisory from 7 PM this evening to 10 PM CDT
     Monday for the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago