436
FXUS63 KLOT 240625
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
125 AM CDT Tue Sep 24 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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 125 AM CDT Tue Sep 24 2024

Key Messages:

- Rain showers and drizzle at times through the TAF period.

- Ceilings as low as LIFR and visibilities as low as IFR
  expected at times through this afternoon.

A baggy low pressure system centered near KSTL at press time
will continue to lift northeastward throughout the day today and
bring periodic showers through the area today. During the
overnight hours, drizzle may be observed at times and produce
IFR/MVFR visibilities and LIFR/IFR ceilings. Towards and
especially after daybreak, precipitation should become more of a
true rain, with the potential for a few-hour long period of
steady rainfall to produce visibility and ceiling reductions
that will be comparable to what the overnight drizzle has been
producing. RFD may be far enough removed to the northwest to
avoid seeing the more impactful ceiling and visibility
reductions that are expected in the Chicago metro, but could
still nevertheless experience MVFR conditions at times.

A few lightning strikes could not be ruled out at/near the
Chicago metro terminals during the daytime today, but it
appears now that the better chances for thunderstorms will
remain southeast of the terminals. Most of the rain should come
to an end by this evening with visibilities and ceilings likely
to improve by this time as well, but at least isolated showers
and MVFR ceilings will likely linger around in the vicinity of
Lake Michigan Tuesday night into Wednesday morning.

The wind direction forecast for today is a bit tricky with the
low pressure center passing by and the expected rain, but
guidance has generally trended towards winds turning northerly
and northwesterly a little earlier compared to previous model
runs, so have adjusted the wind directions in the TAFs a bit to
account for these trends.

Ogorek

&&

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

&&

$$

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