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311
FXUS63 KLOT 011751
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 PM CDT Wed May 1 2024

.KEY MESSAGES...

- Widely scattered showers with a few embedded non-severe storms
  are expected late Wednesday night into Thursday morning.

- Another period of showers and storms is expected late
  Thursday into Friday. A few storms late Thursday afternoon
  into Thursday night could be strong to severe and produce
  locally heavy rainfall -- most likely near and west of the
  I-39 corridor.

&&

.DISCUSSION...
Issued at 330 AM CDT Wed May 1 2024

Through Thursday:

A cluster of showers continues south of I-80 and east of I-55. A
few cells have become more consolidated across Kankakee county
and remain tall enough to produce occasional lightning strikes
and gusty winds for the next hour or so as it moves east into
Indiana along the Kankakee River Valley. The earlier concern
about potential wake low winds developing thankfully haven`t
come to fruition though there have been sporadic gustiness in
the 30-40 mph range.

The rest of the day looks dry across the area as surface high
pressure settles over the region amidst slight mid-level height
rises. Winds will be breezy out of the west to start the day
with temperatures warming into the 70s this afternoon (warmest
south of I-80).

Heading into tonight, modest warm advection and isentropic
ascent sets up across the region well ahead of a weakening
surface low cindered over eastern KS/NE. This should result in
the development of widely scattered showers during the overnight
hours and into Thursday morning (a few embedded non- severe
elevated storms can`t be ruled out). It is possible the late
morning and early afternoon period is dry for much of the area
though confidence is not especially high. Now that the
convective allowing models are beginning to capture this period
there are signs that isolated thunderstorms may attempt to
develop within the warm sector near the warm front in the
afternoon as it lifts north across the area. If this were to
occur, a strong to severe storm couldn`t be ruled out though the
better potential exists west of I-39 toward early evening with
the front and matches up with the latest SPC Day 2 Outlook
highlighting a level 1 of 5 severe weather threat in those
areas.

Temperatures on Thursday will be dependent on the placement of
the warm front and how quickly it lifts to the north. Areas
generally along/south of I-88/80 will be within the warm sector
for the longest duration and could very well warm into the low
to mid 80s! With a potentially slower arrival of the front and
hence slower warming, areas along/north of I-88 may only warm
into the 70s. Meanwhile, areas along the lakeshore in far
northeast Illinois may struggle to warm out of the lower 60s!

Petr


Thursday Night through Tuesday:

Shower and storm coverage is likely to become increasingly numerous
going into Thursday night as a cold front approaches from the west.
In addition to the threat for a few potentially strong to severe
storms in our western CWA during the evening, a potential for
locally heavy rainfall exists Thursday night into early Friday
morning given the presence of climatologically high precipitable
water values and that the mean storm motion relative to the
orientation of the frontal zone will be conducive for training
convective elements. EPS/GEFS ensemble QPF probabilities suggest
that this potential appears to be maximized to the west of our
forecast area, and rain rates may very well taper off as this
corridor of showers and storms slides eastward into our forecast
area Thursday night, but given the recent rainfall, it may not take
a whole lot of rain to cause minor flooding here, so forecast trends
in guidance still bear watching.

The rain is likely to linger into the daytime on Friday across at
least eastern portions of our forecast area before ending as the
cold front finally pushes east of here. When the rain and associated
clouds will clear on Friday is still somewhat in question with
guidance still offering mixed opinions on the overall
progressiveness of the front. How quickly the rain and clouds clear
the area will have an effect on high temperatures for Friday, with
temperatures likely to climb into the low to mid 70s wherever some
sunshine can be realized and remain in the 60s wherever the rain and
stratus clouds remain pervasive. The exception to this will be areas
closer to Lake Michigan, where the presence of onshore flow all day
will keep temperatures in the 60s (and perhaps even the 50s along
the immediate lakeshore) regardless of when the rain and clouds
clear out.

A signal for another round of precipitation in the area remains for
the late Saturday through early Sunday time period as a shortwave
impulse tracks into the region, though it`s not an overly coherent
one with lots of variance still present in medium range guidance
regarding the overall strength of this disturbance and what sort of
surface response it might induce. Temperatures over the weekend are
favored to be near to slightly above normal and again will be cooler
closer to Lake Michigan, before most signs point towards another
warm-up early to mid-week next week. The overall weather pattern
across the central part of the CONUS also looks to be quite stormy
during the first half of next week, and thus, there will likely be
additional opportunities for us to see showers and storms towards
the end of the current forecast period.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
-- Changed Discussion --
Issued at 1251 PM CDT Wed May 1 2024

Key Messages:

- Period of scattered showers (and maybe a storm) early to mid-
  Thursday morning. Confidence is moderate.

- Possibility for more isolated to widely scattered
  thunderstorms mid to late Thursday afternoon. Confidence low
  at this time.

- Easterly winds expected later this evening through the day
  Thursday.

All will remain quiet this afternoon, though winds will remain
gusty from the west through sunset. Winds will turn
northeasterly later this evening, then easterly by early
Thursday morning as a surface warm front shifts northward across
central parts of IL. A few scattered showers (and maybe an
embedded thunderstorm or two) may develop around daybreak
Thursday morning as this warm front approaches. Opted to convert
the PROB30 mention of TS Thursday morning to a tempo for SHRA
early to mid-morning. While a brief storm or two cannot be
totally ruled out during this time, it appears a slightly better
chance of thunder will come in the afternoon.

The surface warm front is likely to remain south of the
terminals through most (if not all) of the daylight hours of
Thursday, though it should be approaching the I-80 corridor
during the afternoon. For this reason, east to southeast winds
are expected to prevail across the terminals. Given the close
proximity of the warm front Thursday afternoon, there is now
some concern that some afternoon thunderstorms could develop
near the terminals as the airmass near the front destabilizes
through the day. While confidence is not high with the extend of
thunderstorms near the terminals in the afternoon, we have
opted to add a PROB30 mention for them from mid to late
afternoon at the 30 hour TAF sites.

KJB
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&&

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

$$

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