471
FXUS63 KLOT 020006
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
706 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
backing 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 /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 706 PM CDT Wed May 1 2024

Forecast concerns for the terminals...

- Isolated to widely scattered showers Thursday morning

- Better coverage of showers and thunderstorms Thursday
  afternoon into Thursday evening

- Winds turning easterly tonight and then south-southeasterly in
  the wake of a warm front Thursday afternoon

Mid-level VFR clouds will continue to increase this evening as
the developing surface low in the western Plains begins its trek
towards the Mississippi River Valley. While forecast soundings
continue to show copious amounts of dry air in the low-levels,
some saturation is expected to occur as the surface low gets
closer. Couple this modest saturation with weak instability
aloft and conditions continue to look favorable for isolated to
widely scattered showers to develop late tonight into Thursday
morning. However, guidance has continued to become less excited
about the already meager shower coverage Thursday morning so
confidence remains on the lower side.

Better shower coverage is expected for Thursday afternoon as a
warm front lifts into the area. Forecast soundings continue to
show instability increasing Thursday afternoon which will also
support thunderstorms, a few of which could be strong to
possibly severe with a threat for gusty winds and maybe some
hail. Though, there is growing uncertainty as to how far north
the warm front will get on Thursday which will likely dictate
how far north the better instability and storm coverage gets.
Regardless, confidence remains sufficient to warrant the
maintaining of PROB30s at ORD and MDW and the addition of
PROB30s to the other terminals for this potential. While
thunderstorm coverage is expected to wane Thursday evening as
we lose the instability, showers are expected to persist through
Thursday night as a cold front pushes through the area.

Finally, winds will become east-northeasterly late this evening
and overnight and should remain as such through Thursday
afternoon for terminals north of the warm front. However,
uncertainty is growing as to how the cooler lake may alter the
warm front as it tries to lift through the area. Therefore,
wind directions at the Chicago terminals could become squirrelly
if the front gets hung up nearby but should maintain an easterly
component regardless. As for speeds, light winds are expected
overnight before winds increase Thursday afternoon into the 10
to 12 kt range. Though, some gusts around 20 kts are possible
especially for those within the warm sector (south of the warm
front).

Yack

&&

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

&&

$$

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