644
FXUS63 KLOT 012103 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
403 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

- Showers and storms are expected late Thursday into Friday. A
  few storms late Thursday afternoon into Thursday night could
  be strong to severe

- Wet antecedent conditions and strong storms may also occur
  Thursday into Friday, most likely near and west of the I-39
  corridor

- Periodic chances for showers and thunderstorms next week, with
  the best chance possibly Tuesday afternoon/evening when there
  may be a severe potential.

&&

.DISCUSSION...
Issued at 235 PM CDT Wed May 1 2024

Through Friday:

Despite the clearer skies over Illinois, stratus clouds over
central Iowa are slowly moving east. While there are some weak
returns showing up on radar, drier air in place will keep
conditions dry. As additional moisture moves north with
isentropic upglide providing enough lift that may result in some
showers or even an elevated thunderstorm overnight, though the
highest probability (around 30 percent) will be in northwest
Illinois. Winds are expected to turn east-northeasterly
overnight in response to the next area of low pressure shifting
towards the Mid-Missouri Valley. The continued influx of a drier
low-level airmass on these easterly winds is leading to models b
acking off on rain potential tonight.

With a surface low out in the Central Plains today, it will
move north and east through the end of the week, drawing a warm
front northward Thursday. As the warm front moves, temperatures
and dew points to its south will steadily increase through the
afternoon, with highs in the upper 70s and low 80s forecast.
However, exact temperature values across far northern IL will be
strongly dependent on how far north the warm front actual
reaches. However, as winds will be out of the east to northeast
on Thursday north of the warm front, cooler conditions (in the
60s) can be expected along the immediate Illinois lakeshore.

Model soundings show a strong capping inversion for much of the
day with all instability aloft. While scattered showers and
even isolated elevated thunderstorms are possible through much
of the day, there is increasing chances for storms to develop in
the late afternoon if the cap does erode. While deep layer
shear does not look particularly strong tomorrow, there would be
enough instability with steeper lapse rates loft to allow for
some strong to even severe convection to develop. While it
appears to be a conditional threat, it prompted the expansion
for the convective threat to a level 1 out of 5 for tomorrow.

By Thursday night, the cold front associated with a surface low
will arrive and finally move east through northern Illinois
through the overnight. This would allow for another chance for
showers and some storms to pass over the area. Additionally,
with the anomalously high moisture content for this time of the
year (above 1.3") and the antecedent wet soils, there is a
potential for localized flooding to occur in the area where
heaviest rain rates develop. The front is expected to move east
of the area Friday morning as winds switch back to the west to
northwest. Lingering showers may provide pockets of light rain
through the early afternoon, though probability for
precipitation diminishes through the day. Cooler air behind the
front will keep high temperatures in the upper 60s to 70s on
Friday.

DK

Friday Night through Wednesday:

The extended period looks active with various systems expected
to transverse the region, the first on Saturday/Saturday evening
and a second Sunday night into Monday. Still uncertainty for
timing and neither of these systems look all that strong and
instability/ shear also remain limited. Nevertheless, low chance
pops for both time periods are warranted.

The pattern begins to shift on Monday, with warmer and more
humid air expected to spread north and across the region. Precip
chances certainly warranted during this time period but trends
are unclear for timing/coverage Monday and Monday night. Its
possible that convection is focused further west of the area on
Monday afternoon/evening and if it moves into the local area, it
would be in a decaying phase later Monday night. However, by
Tuesday, there has been some consensus that thunderstorms will
develop across/near the area in both previous runs and the
ensembles. With highs possibly tagging 80 and dewpoints in the
60s, there should be plenty of instability. While not yet shown
in SPC outlooks, this time period will need to be monitored for
a severe potential. Precipitable water values also look to
increase into the 1.5 inch range, which if these materialize
would lead to the potential for heavy rain. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
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

&&

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

&&

$$

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