376
FXUS63 KLOT 232335
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 PM CDT Mon Sep 23 2024

.KEY MESSAGES...

- Waves of showers will move through the region tonight through
  tomorrow.

- An unusually high degree of uncertainty exists in the
  forecast from Friday onwards.

&&

.DISCUSSION...
Issued at 234 PM CDT Mon Sep 23 2024

Through Tuesday night:

Early-afternoon GOES-16 water vapor imagery depicts an elongated
positively-tilted trough over the Lower Mississippi River Valley
downstream of a secondary trough racing southeastward across
the northern Plains. Meanwhile, a recent surface analysis
reveals a broad surface low over the Ozarks within a broad
baroclinic zone that arcs northeastward toward the Lower Great
Lakes. Mid to upper-level clouds continue to spread over our
area, which will continue to limit temperatures through the
afternoon. In all, the table is set for another period of wet
weather for parts of our area.

Over the next 24 hours, the Lower Mississippi River Valley and
incoming northern Plains troughs will phase, causing the surface low
to deepen while lifting northeastward along the baroclinic zone and
into the Lower Great Lakes. While a northwestward and stronger trend
in the path and strength of the low, respectively, was noted
overnight, a reverse trend of a weaker and further southeast
evolution of the low has been exhibited by 12Z/morning guidance.
Regardless of where the low goes, increasing diffluence in
weakly coupled and eventually the left exit region of an upper-
level jet acting upon the broad baroclinic zone should allow for
waxing and waning areas of deformation and frontogenesis,
altogether supporting waves of showers to parade across the
general region tonight through tomorrow.

The one fly in the ointment for tomorrow remains the threat for a
lake-induced warm-core mesolow to develop beneath the phasing
troughs, which may lead to a focused area of low-level confluence
and a persistent band of rain somewhere near the Lake Michigan
shoreline. With PWATs near 1.5" and weak tropical-like lapse rates
through the troposphere, rainfall rates in any lake-induced
confluence bands can quickly get out of hand (that is, >2"/hr),
leading to a very localized but high-impact area of flooding.
Forecasting the development of lake-induced mesolows, let alone
stationary confluence bands, is quite tricky especially when phasing
upper-level troughs are involved. Given the trend in model guidance
is for a weaker synoptic-scale low with a further southeastward
path, think the threat for a stalled mesolow near the shoreline now
looks lower than before. With that said, observational trends in the
track and strength of the surface low will be compared against model
guidance overnight to help inform any increase in messaging for a
localized threat for flooding. Mesolow or not, on-and-off lake
effect showers are still expected along the Lake Michigan shoreline
all the way through daybreak Wednesday.

Borchardt

Wednesday through Monday:

Quieter and seasonably warm (highs in the 70s, warmest away
from the lake) conditions are expected Wednesday and Thursday.
It will be relatively cool Wednesday night, in the upper 40s to
mid 50s outside of Chicago, thanks to favorable radiational
cooling conditions.

Our attention will then turn to the remnants of expected
tropical cyclone Helene (still PTC-9 as of this writing), for
the Friday onward forecast. While there is a good deal of
uncertainty at this lead time, the mid and upper pattern over
the region does exhibit resemblance to the approach of
Francine`s remnants a week and a half ago. A strong upstream
blocking ridge (near 590 DaM 500 mb heights) looks poised to
develop and extend across the northern Great Lakes in the
Friday-Friday night period. For this reason, the farther north
guidance with Helene`s remnants and QPF footprint, namely the
Canadian global and its ensemble, may not be suppressing the
remnants enough given the upstream blocking.

The upper pattern is also a bit complex even in the more
suppressed guidance through Friday night into early Saturday,
with interaction and possible eventual merging between Helene`s
remnant closed low and a larger upper low over the mid South/mid
MS Valley. As this occurs, the remnant moist plume may
eventually yield an occasionally showery regime Saturday through
Monday. With the above thinking in mind, collaboratively
lowered PoPs through Friday night, especially with northward
extent, where primarily dry conditions appear quite likely, and
then maintained 30-50% PoPs Saturday onward for the CWA.

Aside from precip. chances, expect a prolonged period of
breezy northeasterly winds Friday-Saturday as Helene`s remnants
and expansive surface high pressure locked to our north yield a
tight pressure gradient. This will likely bring another round
of dangerous lakeshore conditions. Temperatures will average
near to above normal into early next week as we (already) turn
the page into October.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 619 PM CDT Mon Sep 23 2024

Key Messages:

- Rain move in late tonight, with gradually deteriorating
  cig/vsby into Tuesday morning.

- Uncertainty remains with the track of the low pressure center,
  making for some uncertainty in where steadiest rain and
  resulting cig/vis impacts set up.

Regional radar display depicts rain across MO and downstate IL
associated with the next weather disturbance which will spread
into the forecast area after midnight tonight. As rain develops,
ceilings will settle into low-MVFR, and potentially IFR through
Tuesday morning. Some uncertainty remains with the foot print
of the steadier rain area however, with some recent guidance
suggesting a slightly more south-southeastward shift with the
area of heaviest precipitation. With this lingering lower
confidence, have maintained the prob30 mention for the lowest
conditions through late Tuesday morning. If guidance continues
to the trend to the southeast, the potential for TSRA would
likely decrease as well. It continues to look as if KRFD will
remain off to the northwest of the poorest conditions. While
rain is expected to diminish and become more showery later in
the day, scattered showers are expected to linger across the
Chicago metro terminals into the evening hours on the
northwestern periphery of the departing system.

Wind direction is expected to remain NNE through most of the
period, though confidence does decrease during the day Tuesday
with winds likely more northerly for a time. Winds should
eventually shift to the NNW Tuesday evening.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 10 PM CDT this evening for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 10 PM CDT this evening for
     INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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