373
FXUS63 KLOT 222337
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 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 /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 637 PM CDT Sun Sep 22 2024

Key Messages:

- N winds become gusty around/above 20 kts this evening behind
  cold front. Eventually winds ease and turn NE late
  tonight/Monday.

- MVFR/patchy IFR ceilings this evening gradually rise to VFR
  by or prior to midday Monday.

Surface low pressure was over south-central lower MI early this
evening, with a slow-moving cold front trailing through
northwest IN and central IL. Tightening pressure gradient lags
the front a bit, though we`re starting to see increasing NNW
winds spread from WI into northern IL, with gusts above 20 kts
at times. These breezy N winds will likely persist across the
terminals this evening before speeds/gusts ease, and eventually
turn more NE late tonight and Monday.

Spotty -SHRA/-RA lingers behind the front, as well as MVFR and
patchy IFR ceilings, though the breezy north winds will
gradually bring drier air into the area and allow showers to
dissipate and cloud bases to rise later this evening. MVFR
ceilings may linger through Monday morning, especially near Lake
Michigan as winds turn onshore, though confidence is somewhat
low in how quickly conditions improve to VFR. It`s possible
lower clouds scatter out or rise to VFR later tonight and we see
only patchy MVFR ceilings across the Chicago terminals Monday
morning before all sites go VFR by midday.

Ratzer

&&

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

IN...Beach Hazards Statement through late Monday night for INZ001-
     INZ002.

LM...Small Craft Advisory until 7 PM CDT Monday for the IL
     nearshore waters.

     Small Craft Advisory until 10 PM CDT Monday for the IN
     nearshore waters.

&&

$$

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