666
FXUS63 KLOT 090812
AFDLOT

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

.KEY MESSAGES...

- Showers and thunderstorms this morning, with locally heavy
  downpours and perhaps some isolated wind gusts. Showers become
  more scattered this afternoon/tonight.

- Breezy and much cooler today with temperatures in the 50s and
  60s.

- Periods of showers and possibly some thunderstorms early next
  week.

&&

.DISCUSSION...
Issued at 312 AM CDT Thu May 9 2024

Through Friday:

Showers and some thunderstorms were blossoming across the area
early this morning, in association with increasing forcing for
ascent and warm/moist advection ahead of a mid-level short wave.
This wave, the old mid/upper low circulation which has been
hanging out over the northern Plains over the past few days, is
shearing east and will eventually be absorbed into another mid-
level trough propagating southward from the upper
Midwest/northern Great Lakes. Showers and thunderstorms will
continue to overspread much of the area early this morning
(thunderstorms more likely west and south of the Chicago area,
given the northwest-southeast orientation of the MUCAPE field
across the southwest half of the cwa) with some locally heavy
downpours for the morning commuter rush, before becoming more
scattered/showery toward midday and into the afternoon hours as
the best forcing shifts off to the east. Scattered showers will
linger tonight as the combined upper trough axis continues to
shift slowly southeast across the area, though should become
less numerous and eventually north of I-80 overnight. Weak mid-
level ridging briefly develops overhead on Friday, between the
departing upper trough to our southeast and another digging
southeast into the upper Mississippi Valley. This will allow for
mainly dry weather during the day along with diminishing winds,
though a fairly extensive fair weather cumulus field should
develop with cool temps aloft.

With the primary surface low tracking across downstate IL/IN,
low level winds will shift from east-northeast to north during
the day, becoming somewhat blustery with gusts around 25 mph at
times. This will draw cooler air into the region, limiting high
temperatures to the 50s across northeast IL and areas along Lake
Michigan, and the low-mid 60s farther west and south. Temps
will dip into the 40s tonight, with low-40s especially near the
IL/WI border where cloud cover should decrease after midnight.
Highs on Friday will rebound slightly to the mid-60s in most
areas, though a weak lake breeze along the IL shore and into
northwest IN will keep readings in the low 60s in those
locations.

Ratzer


Friday Night through Wednesday:

A compact upper-level low will dig southeast over the western
Great Lakes Friday night and produce a period of showers
primarily across the southeast half of the forecast area mid-
evening into the overnight hours. Did not include a mention of
thunderstorms in the current forecast, though steepening mid-
level lapse rates on the back edge of the showers could support
an embedded lightning strike or two.

A broad upper-level low across far northern Ontario on Sunday
will pivot around southern Hudson Bay and into northern Quebec
through the middle of next week. A cold front associated with
the low will eventually stall nearby or potentially over the
forecast area on Monday. Meanwhile, a mid-level low over the
Four Corners region this weekend will drift eastward across the
central Great Plains and Ohio Valley then to the mid-Atlantic.
This low should interact with the stalled boundary while drawing
low-level moisture back northward into the area by Monday.
While confidence in the exact evolution remains medium at best,
particularly whether the low slows too much and becomes detached
from the broader flow pattern, consensus forecasts support
periods of showers and possibly some storms Sunday night into
Tuesday night (highest chances on Tuesday).

Kluber

&&

.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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