From:  Brett Crapser <cmdrcody@nospampharowtcom.invalid>
Date:  13 Oct 2024 01:50:03 Hong Kong Time
Newsgroup:  news.alt119.net/chi.weather
Subject:  

Sat Oct 12 Meteorology for Chicago

NNTP-Posting-Host:  null

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 PM CDT Sat Oct 12 2024

KEY MESSAGES...

- Scattered showers are possible late this afternoon into the
  evening with a 20-30% chance of thunderstorms.

- A notable cool-down arrives on Sunday with the potential for
  nightly frost and localized freeze conditions through
  midweek.

- Lake effect showers are expected downwind of Lake Michigan
  late Sunday through Tuesday.

&&

UPDATE...
Issued at 1151 AM CDT Sat Oct 12 2024

No major changes made to the going forecast for later today into
this evening.

We are still expecting the best chance for scattered showers
and possible embedded storms to be across far northeastern IL
late this afternoon into the evening. While this is the case,
some isolated to widely scattered sprinkles (perhaps
accompanied by a few claps of thunder) will also be possible
early this afternoon (now through 3 pm) along the eastern
periphery of a 700 MB Theta-E axis, which is currently advecting
eastward into northern IL. There has been a persistent area of
Altocumulus associated with this feature through the morning
across eastern IA. Recently this area has even shown signs of
supporting some high based shower development (even a small
thundershower/sprinkle across southern Lee) within an
envionrment characterized by very steep mid-level lapse rates
(7.9C per KM per the 12Z DVN RAOB).

It appears this activity will likely continue to fester as it
moves eastward across our area early this afternoon. However,
the drier low-level airmass overhead, and hence the high base
level of these showers, is likely to curtail the threat for any
meaningful rainfall. Accordingly, no more than a sprinkle or
brief light shower is anticipated with this activity early this
afternoon. Also don`t be surprised to hear a clap or two of
thunder embedded within the best updrafts.

KJB

&&

DISCUSSION...
Issued at 340 AM CDT Sat Oct 12 2024

Through Sunday:

As a cold front continues to move south through the forecast
area, winds behind it have turned to the northeast with gusts
between 20 to 25 mph. Winds should slowly diminish with only
sporadic gusts leading into daybreak. As the aforementioned
surface front stalls around I-80, cold air advection and
northerly flow will help limit temperatures north of the
boundary to around more seasonable 60s and low 70s. Whereas the
southern part of the forecast area still has a chance for
warmer conditions in the upper 70s and low 80s. With dry low-
levels still in place, no rain is expected through around lunch
time. Higher res guidance still shows a weak mid level wave
transiting overhead late this morning/early afternoon. Dry
conditions are still expected, but did add some "silent PoPs"
(10 to 14 percent chance) for the low probability of maybe a
sprinkle or two ahead of the next system.

Later today, a short wave is expected to dig down the broader
long wave trough aloft centered over Quebec. As the baroclinic
zone along the previous stalled front tightens, scattered
showers are expected to develop to its north during the
afternoon and into the evening. While better lapse rates aloft
will likely be featured to the south of the front coinciding
with the warm advective wing, model soundings suggest an axis of
instability extends along and ahead of the front providing
support for elevated thunderstorms. The one lingering bit of
uncertainty will be the amount of moisture available. Increased
mid level moisture is expected to advect over the area this
afternoon and evening, but models have been struggling with the
amount. Not only does that impact the "will it/won`t it"
rain/thunder question, but also how much rain falls. There
remain some ensemble members that have isolated higher amounts,
and if there is enough moisture present perhaps some minor
training could occur. However, confidence in that scenario
playing out is low and most areas are unlikely to receive
significant rainfall. PoPs will steadily diminish into Sunday
morning. Temperatures in the city of Chicago are expected to be
in the upper 50s for a low temperature. With subsiding winds and
potentially drier conditions, it is not looking like a bad way
to start the day for early morning outdoor activities. Though
the chance for increasing winds and shower chances return by
midday.

A more robust shortwave with a 120+ knot jet core will slowly
move towards the region on Sunday. A cooler air mass will
continue to move south over the forecast area with high
temperatures on Sunday in the 60s (warmer to the south, cooler
to the north). As the pressure gradient strengthens through the
day, northwest wind gusts will increase out of the northwest in
the afternoon to 25 to 30 mph. With lingering moisture present,
there is a (20 to 30 percent) chance for some pop-up showers in
the afternoon. While much of the forecast area will probably
remain dry, the better chances will be closer to Lake Michigan.
Those shower chances become more focused over northwest Indiana
Sunday as night, as mentioned in the discussion below.

DK


Sunday Night through Friday:

A vigorous upper-level wave and closed upper low will continue
to dig across the broader Great Lakes Region late Sunday. This
will usher in a multi-day stretch of below normal temperatures.
Strong northwesterly winds are forecast to continue well after
sunset in the wake of an associated surface trough
axis/reinforcing cold front. While the official wind gust
forecast is closer to 25-30 mph, gusts in excess of 30-35 mph
can`t be ruled out if the low-level pressure gradient ends up
being stronger. In fact, wind chills are forecast to dip into
the 30s Sunday night into early Monday morning.

We continue to head toward needing Frost/Freeze related
headlines sometime during the Monday night-Wednesday night
period. At this time, Tuesday night has the best potential
(~30-50% chance) to see sub-freezing temperatures outside of
the urban core of the Chicago metro and away from the
lakeshore/lake effect clouds. It`s still too far out for a
potential Freeze Watch with some lingering questions on
overnight wind speeds and potential cloud cover influences.
Nevertheless, those with sensitive plants should plan to cover
or bring them indoors during this period.

In addition to the cooler temperatures, a favorable pattern for
a long duration lake effect shower event will exist periodically
Sunday night through Tuesday. Modeled equilibrium levels exceed
15-25 kft at times as continued shots of cold low-mid level air
steep lapse rates over a warm lake. Deep saturation (and a
narrow unstable layer) extends well into the -10 to -20C layer,
suggesting there is some potential for enough charge separation
for lightning as well (particularly Monday night into early
Tuesday morning). The initial band of lake effect showers will
extend mainly into portions of Northwest Indiana and southwest
Michigan. However, as another mid-level wave rounds the closed
upper low late Monday into Tuesday morning, this band may drift
back to the west into northeast Illinois. In fact, the wave
itself could support scattered showers further inland across the
area.

Ensemble guidance continues to suggest a return to near to even
above normal temperatures late week as upper ridging builds
back across the region in the wake of the departing eastern
CONUS upper level trough/closed low.

Petr

&&

AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1242 PM CDT Sat Oct 12 2024

Forecast concerns include...

Chance of thunderstorms this evening.
Possible mvfr cigs overnight/Sunday morning.
Wind shift to northwest Sunday morning.
Strong/gusty northwest winds Sunday afternoon.

Current isolated showers southwest of the terminals are expected
to remain south of the terminals this afternoon. There is a
chance of an isolated lightning strike with this activity.
Additional showers and isolated thunderstorms are expected to
develop late this afternoon and continue through mid evening
across far northern IL and over southern Lake Michigan. There
remains uncertainty for exactly where this activity may develop
and the current prob mention for DPA/ORD/MDW seems reasonable
for both the potential and the best timing, 00z-04z. If this
activity develops further north, it is possible that all of the
terminals will be dry.

After the chance of precipitation ends late this evening, there
will be the potential for mvfr cigs. Confidence is rather low as
recent guidance trends suggest mvfr cigs will be patchy an
perhaps short in duration. Opted to maintain mvfr cigs for now,
but trends will need to be monitored. If they do form, they
should scatter Sunday morning with perhaps a low vfr deck,
3-4kft possible at times Sunday.

Northeast winds are expected to remain around 10kts through mid
afternoon then increase into the 12-15kt range by sunset with
higher gusts possible. These may be short lived with speeds/
gusts diminishing again in the mid/late evening. Winds will
slowly turn more northerly overnight and likely be north/
northwest by daybreak Sunday morning, except at GYY, which will
turn northwest by mid/late Sunday morning. Speeds/gusts will
steadily increase from late morning into the afternoon, with
gusts to 30kt expected for mid/late Sunday afternoon. cms

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL
     nearshore waters.

     Small Craft Advisory until 3 PM CDT this afternoon for the IN
     nearshore waters.

&&

$$