849
FXUS63 KLOT 041111
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
611 AM CDT Wed Sep 4 2024

.KEY MESSAGES...

- After a gradual warming trend midweek a cold front moves
  through late Thursday into Friday bringing scattered showers
  and low thunderstorm chances (20%)

- Lake effect rain showers could then persist Friday into
  Saturday, particularly into northwest Indiana.

- A big cooldown arrives heading into the weekend with highs in
  the 60s before warming back up early next week.

&&

.DISCUSSION...
Issued at 333 AM CDT Wed Sep 4 2024

Through Thursday:

After a multi-day stretch of highs only in the 70s(!),
temperatures are forecast to warm back into the lower 80s today,
with the exception of communities near the lake in Illinois
where 70s are still expected thanks to onshore winds. Sunny
skies and light southeast winds will prevail making for yet
another pleasant early September day.

As the expansive surface high responsible for this quiet
stretch of weather continues its gradual shift east of the
region, a plume of lower-level moisture is forecast to lift out
of southern Illinois into portions of the area late tonight into
Thursday morning. This could result in a localized increase in
cloud cover, with some hi-res guidance even attempting to
produce a few spotty showers out of it. For now have kept the
forecast dry through the afternoon, though a stray sprinkle
can`t be fully ruled out if the moisture layer ends up a bit
deeper. This increase in cloud cover could also play a greater
role in Thursday`s temperatures depending on the coverage and
how long it persists into the afternoon. For now opted to nudge
highs down just a degree with this update albeit still in the
mid to locally upper 80s.

A cold front is expected to move through southern Wisconsin
Thursday afternoon. Can`t rule out the front and associated
showers dipping into northwest Illinois as early as late
afternoon per some recent hi-res guidance, although the better
potential exists mainly into Thursday evening (after 6 PM CDT).
The shower/storm potential along the front are discussed in
more depth below.

Petr


Thursday Night through Tuesday:

The forecast in the long term begins with a cold front that will
move across the area late Thursday through Thursday night. Above
the surface, we`ll find an anomalously deep upper trough near
the northern Plains/Canadian border beginning to dig southward
into the Great Lakes region. There is some discrepancy on timing
with this front with models working it into our northwest CWA
anywhere from late afternoon to early evening. The RAP and HRRR
are the quickest introducing it closer to mid-afternoon.
Regardless, a broken line of showers and possibly thunderstorms
is expected along the front as it works across the CWA to the
southeast through late Thursday night. A dry profile and lack of
forcing should keep any pre-frontal activity at bay. The
anafrontal setup provides an opportunity for a few elevated
showers to develop behind the front with better mid level
moisture and synoptic forcing trailing the surface boundary.
However, coverage looks widely scattered at best with no post-
frontal thunder expected as instability quickly gives out.

The thunder potential, then, looks to remain closely confined to
the front. There is a good deal of uncertainty regarding
boundary layer moisture and how well dewpoints mix out during
the day ahead of this front. In any case though, it does not
appear that we`ll be looking at an environment that`s overly
favorable for thunderstorm development. With the cooler air
aloft trailing the front by several paces, mid level lapse rates
remain poor during FroPa. Closer to the surface, heating and
mixing will determine whether we`ll have to fight off more
capping or a higher LFC, but either way low level instability is
also meager. Outside of our far southeast CWA where a corridor
of higher dewpoints is well agreed upon, MLCAPE values across a
huge majority of our CWA top out in the 300-600 J/kg range. Some
of the more moist solutions pull that closer to 1,000 joules,
but keep notably more capping around as a consequence. Those
values alone may even be deceiving for what they are with
forecast soundings showing tall, skinny CAPE profiles with high
ELs. Synoptic forcing will be lacking during FroPa and forced
ascent along the front itself is conceptually sloped and
gradual. And to tack on one final damper, dry mid level air will
make it very difficult for parcels to realize the bulk of the
already modest instability, especially assuming somewhat gradual
ascent. Accordingly, decided to cap thunder chances at slight
as the front moves across.

Rain chances tied to the front should exit to the southeast
during the earlier part of Friday. The trough base will dip
south of the area during the day on Friday which could help stir
up some scattered showers in the afternoon and evening as the
cold advection destabilizes air aloft, particularly in our
north. The trough axis will meander across Lake Michigan late
Friday into early Saturday. The cold airmass spilling over the
mild lake coupled with strong northerly low level flow will
provide a good setup for the development of lake effect showers.
Such impressive thermodynamics with this setup and likely
strong convergence along the lakeshore even present a good
waterspout potential and a chance to see some lake-effect
thunderstorms. The greatest potential for this exists during the
late evening Friday into early Saturday morning. This is
primarily a concern near the Indiana lakeshore, although some
showers may spill onto the Illinois side Friday night before the
flow backs orienting convergence farther east around the
lakeshore. This backing will continue until the lake-effect
chances move to our east in what looks to be sometime Saturday
morning/afternoon. Lastly, this setup will also generate high
waves and dangerous currents on southern Lake Michigan Friday
through Saturday.

After the upper low scoots off to our east on Sunday, surface
high pressure will overspread the Plains and slowly work
across the TN and OH valleys through midweek keeping conditions
here mostly quiet. Thursday`s cold front will drag Friday`s
highs down into the upper 60s to mid 70s. 60s are expected area-
wide on Saturday with the cold upper low directly overhead. A
warmup commences on Sunday which actually looks to reintroduce
some summer-like heat for at least a few days next week.

Doom

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 610 AM CDT Wed Sep 4 2024

Light southeast winds are expected through this evening. They
may briefly shift to a more easterly direction at ORD/MDW with
the lake breeze during the afternoon though speeds would remain
light (under 10 kt). Winds then trend S to SSW early Thursday
morning along with the potential for an increase in VFR (~5000
ft) stratus coverage.

Petr

&&

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

&&

$$

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