345
FXUS63 KLOT 221149
AFDLOT

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

.KEY MESSAGES...

- Beneficial rainfall today with a 30-50% 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.

&&

.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 /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 649 AM CDT Sun Sep 22 2024

- SHRA likely today with a chance for isolated TSRA

- Wind shift to northerly late this afternoon/very early evening
  with cold front

Waves of showers will move across the terminals today with a few
embedded thunderstorms at times. The chance of a thunderstorm
occurring at one of the terminals is only about 20-30%, so TAFs
do not include TSRA due to the low probability of occurrence.
Showers should end later this afternoon in the wake of a cold
front that will result in a wind shift to north to
northwesterly. Some gustiness is expected, particularly for the
terminals closer to Lake Michigan through early evening.

Initially VFR CIGS this morning should trend down to MVFR, first
at RFD then eventually at the immediate Chicago area terminals
later this morning. While most guidance continues to suggest IFR
CIGS will occur by late morning, general lack of any IFR
upstream and conceptual model really doesn`t suggest a high
probability of IFR CIGS until perhaps later this afternoon with
the approaching cold front. CIGS should lift to MVFR several
hours after frontal passage and likely scatter out this evening,
except closer to the lake where some lake effect MVFR CIGS could
linger through the night.

- Izzi

&&

.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 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.

&&

$$

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