823
FXUS63 KLOT 090555
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1255 AM CDT Thu May 9 2024

.KEY MESSAGES...

- Showers and perhaps a few thunderstorms are expected through
  mid-morning.

- Cooler weather arrives today, with periodic showers Friday
  night into the weekend.

&&

.DISCUSSION...
Issued at 246 PM CDT Wed May 8 2024

Through Thursday night:

Today has been awfully pleasant for us so far with sunny skies and
temperatures having reached the middle 70s to near 80 degrees area-
wide. This is not the story across the rest of the Midwest, however.
Early this afternoon, we find two centers of surface low pressure in
the region: one subtle little system in northwest IA and a broad,
more prominent low marching across the Ozark Plateau. The former is
producing nothing more than light showers out west while the latter
is responsible for a swath of severe storms extending from western
MO into south-central Kentucky. These two systems will phase into
one over the next several hours before moving into central IL this
evening. This will bring showers and likely a handful of storms to
the area this evening through the better part of tomorrow.

We should begin to see isolated to widely scattered showers move
into the SW CWA early-mid evening and progress across the rest of
the area. With only a little bit of elevated instability squeezing
into the CWA, thunderstorms look rather unlikely with this first
little push any farther north than say the US Rt 24 corridor. More
widespread activity will work in not long after midnight along the
northern flank of the low as it passes through central IL. It looks
like the overnight period will be our best chance at seeing embedded
thunderstorms when several hundred joules of elevated CAPE look
to build over parts of the area. The greatest thunder potential
will exist in our western and southern CWA where we`ll find the
most appreciable instability with notably steeper low-mid level
lapse rates. The environment becomes less and less favorable
approaching the lake. While CAPE values certainly won`t stand
out tonight, most of it will be confined within the hail growth
zone and we will have a respectable amount of mid level shear to
work with, roughly 25-30 kt at 3-6 km. This means it`s not out
of the question to see some small hail develop within any
relatively stronger elevated cores. Some dry air in the low
levels and a ~30 kt LLJ could also promote some gusty winds with
any thunderstorm or heavier shower tonight.

The instability will wane to the east early tomorrow pretty much
driving the thunder potential out of here through the morning.
Scattered showers are expected to continue through much of the
afternoon before high pressure builds in and clears conditions up
during the evening. A tight pressure gradient on the backside of the
system will bring breezy winds to the area during the day. Between
the cloudy, rainy conditions and cold advection, tomorrow will be
noticeably cooler with highs forecast near 60 south of I-80 and
stuck in the 50s farther north.

Doom


Friday through Wednesday:

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
thermodynamic profiles with anomalously cold mid-level
temperatures will remain to the northeast. Have therefore
continued to cap PoPs in the chance range with the expectation
that shower coverage will be no greater than scattered.

Northwest flow aloft will persist Sunday into early next week.
Another 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
precipitation as a sprawling surface ridge remains anchored in
the Deep South, but an earlier arrival of the trough could
support isolated to scattered storms in our forecast area during
the late afternoon and evening.

Beyond Sunday night, substantial differences in guidance
regarding the evolution of a building ridge across the eastern
seaboard and a central CONUS trough support leaving the NBM`s
slight chance and chance PoPs untouched from Monday through
Wednesday.

Kluber/Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1255 AM CDT Thu May 9 2024

Key Aviation Messages:
- Area of -SHRA through mid-morning, with a low (10%) chance
  for an embedded TS through around 11/12Z.
- Isolated to scattered -SHRA and MVFR ceilings through this
  evening.

An area of SHRA with some TS across eastern Iowa early this
morning is expected to drift E or ESE across northern Illinois
through mid-morning. Ongoing TS should begin to favor a general
SE trajectory along a NW to SE warm front toward central
Illinois. However, a couple widely isolated TS cannot be ruled
out into the Chicago metro roughly 09-12Z given remaining steep
lapse rates between 10-20kft. If the downward TS trend is slower
than expected, VCTS to the SW may be required for MDW/GYY.

Beyond mid-morning, isolated to scattered -SHRA with MVFR
ceilings are expected into this evening before precip coverage
diminishes and ceilings begin to SCT by late tonight.

Otherwise, NE winds will persist through today. Speed will
diminish under 10 knots tonight while direction begins to settle
between 350- 020, but should favor just east of north.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from noon today to 3 AM CDT Friday for
     the IL and IN nearshore waters.

&&

$$

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