462
FXUS63 KLOT 221937
AFDLOT

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

.KEY MESSAGES...

- Showers will gradually come to an end this evening leaving
  mostly dry conditions for tonight and Monday

- Another period of showers and possibly a few thunderstorms is
  expected Monday night and Tuesday especially along and east of
  I-55

&&

.DISCUSSION...
Issued at 237 PM CDT Sun Sep 22 2024

Through Tuesday:

Widespread showers continue to pivot across northern IL and
northwest IN in association with a mesolow that has developed
ahead of the cold front that is now over northwest IL. These
showers are expected to persist into the early evening hours
before gradually tapering from northwest to southeast as the
cold front advances through the area. So far the lightning
coverage across IL and IN has been virtually non-existent likely
due to the lack of instability. Therefore, the thinking is that
the thunder potential is near zero at this point. However, have
decided to maintain a slight (15-20%) chance for thunder in the
forecast mainly for areas south of I-80 in case the
aforementioned mesolow is able to overcome the weak instability
and generate a rogue lightning strike or two.

As showers come to an end this evening, winds will quickly
increase behind the front with gusts in the 15 to 25 mph range
expected especially over the open waters of Lake Michigan. These
wind speeds in combination with the northerly direction will
also build waves on the lake into the 6 to 10 ft range this
evening and overnight making for dangerous swimming and
hazardous boating conditions. While the strongest winds speeds
will taper Monday morning, the persist onshore flow should allow
waves to remain elevated through Monday afternoon and possibly
into the evening. Therefore, a Beach Hazard Statement remains in
effect for all northern IL and northwest IN beaches through
Monday evening for this threat.

Outside of the winds and waves, Monday looks to be a decent day
weather wise with mostly dry weather expected as the front is
expected to be stalled across central IL and IN. However, a weak
disturbance and lingering mid-level moisture overhead does look
to keep a fair bit of cloud cover around. Regardless, more
typical temperatures for late September will be in place with
highs Monday afternoon in the 70s with overnight lows in the 50s
both tonight and Monday night.

Unfortunately, the break in the weather will come to a close
Monday night into Tuesday as the upper-trough over the central
Plains pivots into the Great Lakes and lifts the stalled front
back north. While this will once again bring some meaningful
rainfall to northern IL and northwest IN, the greatest coverage
of showers looks to be along and east of I-55 where better
moisture and instability are forecast. Given that forecast
soundings show a similar CAPE profile to what was seen today, am
not overly impressed with the coverage of thunderstorms Monday
night and Tuesday. Though, did maintain a 20% chance of thunder
in the forecast, mainly for areas south of the Kankakee River
Valley, in the off chance the dynamics can overcome the modest
instability. The showers and any storms that do develop are
expected to conclude Tuesday evening as the trough moves east
and drier air filters back into the area.

Yack


Tuesday Night through Sunday:

Medium range forecast guidance continues to build a high
amplitude mid/upper-level ridge across the Midwest into the
western Great Lakes during the mid to late week period. This help
foster a blocking pattern across the central CONUS later in the
week as a cut off upper low meanders south of this ridge across
the central/southern Plains eastward towards the Mid-Mississippi
Valley. Unfortunately, the predictability associated with these
of these types of patterns tends to be lower than normal,
particularly with smaller scale details. Accordingly, this lends
to lower forecast confidence in specific forecast details beyond
Thursday.

The weather for Wednesday and Thursday looks to largely be dry and
seasonable across our area as surface high pressure dominates
across the western Great Lakes. This pattern should support high
temperatures in the mid to upper 70s both days (warmest on
Thursday) under mainly sunny skies. Onshore flow may keep areas
along the Lake Michigan lakeshore a couple of degrees cooler,
however.

The forecast beyond Thursday will largely be dependent upon the
interaction that occurs between the cut off low and a northward
shifting tropical system expected to lift northward from the Gulf
late in the week. If they interact favorably, heavy rainfall
associated with the remnant tropical wave could potentially
develop northward into our area late in the week as the tropical
wave slingshots north-northwestward. However, confidence with this
occurring into our area remains rather low at this point. Given
the low predictability and overall low confidence with the
forecast beyond Thursday, I made no changes to the NBM
initialization, which does include some very low pops Friday into
the Weekend.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1242 PM CDT Sun Sep 22 2024

Key Aviation Impacts for the current TAF period:

- Continued showers through the afternoon, tapering off this
  evening. Lower vis possible within periods of heavier showers

- Low cigs creating MVFR/IFR conditions through 03Z. Potential
  cig improvement tonight, but MVFR conditions expected through
  Monday morning

- Winds will be out of the NW shifting to the north. Stronger
  gusts around 20 knots possible late this afternoon

A subtle surface boundary is moving over the city of Chicago
providing light east-northeast winds at the time this discussion
was published. However, as the next wave of showers moves in
from the west, the prevailing wind pattern is expected to return
to the northwest.

As this system continues to move east over the airspace,
showers are expected to continue through the afternoon. It has
already been observed that during the more intense showers,
ceilings and visibilities have reduced down into IFR
conditions. Given that the next two bands of showers is headed
for the Chicago area, a TEMPO was added for the risk of SHRA
bringing lower cigs and vis once again. The heaviest
precipitation is moving east of the RFD airspace and though
showers will continue this afternoon, the TEMPO for lower
conditions reflects the earlier departure. There is no thunder
expected with this afternoon`s showers at any TAF sites.

Given the lingering rain in eastern to central Iowa, VCSH was
added to the TAFs extending into early this evening for
lingering light rain longer than originally anticipated. Cigs
will continue to lower for an expected window of prevailing IFR
just before the 00Z push. The northwest winds will increase
later this afternoon with gusts at or just over 20 knots
possible. As they increase and the front moves through, these
winds are expected to drift to the north. Winds will gradually
diminish and cigs should begin to recover back to MVFR levels
through the overnight. Low confidence on the return of VFR
conditions Monday morning, but there was enough of a model
signal to leave MVFR through 18Z.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Monday evening for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement from 7 PM CDT this evening through
     late Monday night for INZ001-INZ002.

LM...Small Craft Advisory until 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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