291
FXUS63 KLOT 081740
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM CDT Wed May 8 2024

.KEY MESSAGES...

- Another round of showers and thunderstorms develops late today
  and tonight. A few strong to severe storms are possible mainly
  this evening, though greater severe chances will be south of
  the area. Locally heavy rainfall in excess of a half inch is
  possible tonight into Thursday morning as well.

- Cooler weather arrives Thursday, with periodic showers
  Thursday and again Friday night into the weekend.

&&

.DISCUSSION...
Issued at 356 AM CDT Wed May 8 2024

Through Thursday:

A lull in our active weather pattern is expected for most of
today, as subsidence develops within mid-level short wave
ridging in the wake of a departing disturbance. Weak surface
high pressure was reflected at the surface, and will drift east
across the area this morning. Light westerly winds will back to
the southeast and eventually to the east this afternoon as the
ridge moves off to the east, and surface low pressure develops
into Missouri to our southwest. Mostly sunny skies are thus
expected into early afternoon, with temperatures warming into
the mid-70s to lower 80s (warmest south), though developing
onshore lake breeze winds will cool the immediate Lake Michigan
shore to the mid-upper 60s.

Clouds will increase from the southwest later this afternoon
however, especially across southern parts of the forecast area,
in response to renewed warm advection/isentropic ascent ahead of
a short wave trough (old northern Plains upper low circulation)
wrapping east-northeast across the mid-lower Missouri Valley.
Shower and thunderstorm chances will also increase across the
southern and western counties this evening, and eventually
across the remainder of the area overnight, as the surface and
mid-level waves advance into the region. The effective low-level
frontal zone from yesterday`s cold frontal passage and
convective outflow is expected to remain south of the cwa with
the main threat of severe weather remaining to our south, though
some near-surface based storms and locally gusty surface winds
can`t be ruled out along the ILX/LOT WFO border. Otherwise,
elevated storms farther north should pose mainly a marginal hail
threat, and SPC`s Day 1 marginal risk appears reasonably with
higher risk categories across central IL/IN. Locally heavy
rainfall amounts are likely a greater risk across the WFO LOT
cwa later tonight into early Thursday, as the sheared positive-
tilt upper trough propagates across the region. EPS and GEFS
ensembles probabilities (40-70%) of rainfall amounts greater
than a half inch are indicated from northeast IA east-southeast
across the southern half of the our forecast area, with WPC
including our area in a marginal Excessive Rainfall Outlook
during the Days 1-2 period.

A turn to much cooler weather is also anticipated for Thursday,
as east winds increase and turn northerly during the day with
the passage of the surface low pressure wave to our south. After
summer- like temperatures today, highs on Thursday will only
reach the upper 50s to lower 60s, with locations near Lake
Michigan struggling to make it out of the lower 50s. Showers and
blustery north-northeast winds will make for a chilly feel to
the day.

Ratzer

Thursday Night through Tuesday:

A southeastward tracking trough responsible for more active
weather on Thursday will gradually cross the forecast area
Thursday night. Isolated to widely scattered showers may persist
well through the evening.

The trough will shift southeast of the CWA by around daybreak
Friday. Shortwave ridging will produce stout mid-level warming
through the day Friday, though low-level moisture will remain
sufficient for a decent coverage of shallow cumulus during the
afternoon. On the heels of the ridge, a compact upper-level low
will dig southeast over the western Great Lakes Friday night
into Saturday. Earlier guidance had advertised a diurnally
favorable passage of the low directly over our area midday
Saturday, but has since supported a faster solution with the low
passing to the northeast around daybreak. Additionally, more
favorable thermo profiles with anomalously cold mid-level temps
will remain to the northeast. Have therefore limited forecast
coverage to scattered showers Friday night into Saturday, with a
slight chance of thunder during the afternoon.

NW flow aloft will persist Sunday into early next week. A more
formidable shortwave trough is progged to cross the area
sometime late Sunday afternoon through Sunday night. Low-level
moisture availability looks marginal at best for any widespread
precip as a sprawling surface ridge remains anchored in the Deep
South. But any earlier arrival of the trough (e.g. late
afternoon/early eve), could support isolated to scattered
storms.

Beyond Sunday night, substantial differences in guidance
regarding the evolution of a building ridge across the eastern
seaboard and a central CONUS trough support maintaining
consensus forecast guidance with a chance of showers and some
storms centered around Tuesday.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1218 PM CDT Wed May 8 2024

Aviation Forecast Concerns:

- Secondary low pressure system develops tonight into Thursday
  morning across central Illinois.

- Multiple rounds of precipitation expected late tonight through
  Thursday, bringing rainfall to northern IL terminals and
  reducing ceilings.

- Breezy conditions arrive this afternoon, with gusts increasing
  late Thursday morning as the low exits.

Low pressure system will begin to move into the Ohio River
Valley this evening, with convective storms remaining south of
the local area. A secondary low pressure system is modeled to
develop overnight in the mid-Mississippi River Valley shifting
slowly into central Illinois Thursday morning. This is expected
to bring a wave of precipitation to northern Illinois terminals
late tonight, lingering throughout much of Thursday. Model
indication is for -SHRA chances throughout the event, with more
steady RA coming in two waves early Thursday morning (07-14z)
and then again midday (16-22z) as the backside of the low
pressure system exits the region. As this system develops across
the region, ceilings are expected to decrease to MVFR with the
early morning activity, with TEMPO LMVFR possible. Then LMVFR
ceilings are forecast with the second wave midday into the
afternoon.

Breezy conditions are anticipated to begin moving back across
the area this afternoon into the evening with the leading low
pressure system. As the secondary system begins to pull away
from the northeast IL and northwest IN terminals Thursday,
gusty winds are anticipated to pick up from the nne.

Baker

&&

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

&&

$$

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