-- Discussion containing changed information from previous version are highlighted. --
628
FXUS63 KLOT 031122
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
622 AM CDT Fri May 3 2024

.KEY MESSAGES...

- Increasing chances of showers and thunderstorms capable of
  producing gusty winds Saturday afternoon and evening.

- Periodic shower and thunderstorm chances next week, with some
  potential for strong to severe storms Tuesday.

&&

.DISCUSSION...
Issued at 335 AM CDT Fri May 3 2024

Through Saturday:

Ongoing convective activity across portions of northern
Illinois and northwest Indiana continues to wane early this
morning as remaining instability and associated mid-level lapse
rates decrease. In fact, as of 3am CT, very little lightning has
been observed in the forecast area over the past hour. While a
few lightning strikes are still possible through sunrise, a
departing mid-level wave will limit remaining deeper-layer
support. Meanwhile, a cold front roughly parallel to the
Mississippi River will continue to shift a band of shallow
showers across the area into mid-morning.

High pressure will quickly settle across the western Great
Lakes through tonight. The combination of a rather dry airmass
evident by upstream dew points generally in the 30s along with
fairly clear skies today should allow seasonably warm conditions
in the 60s along the shore and lower 70s inland today.

Today`s cold front will stall across the south half of
Illinois, allowing for a return of low-level moisture behind the
surface high on Saturday. A small mid-level wave lifting
northeast across the far Upper Mississippi River Valley Saturday
morning may brush the northwest CWA. Return of sufficient
moisture by this point remains in question, though an isolated
storm cannot be ruled out for areas west of I-39 late morning
into early afternoon. A disjointed trough passage into the
western Great Lakes later Saturday and into Saturday night
should better align with increasing shallow moisture to produce
scattered showers and storms as far east as around I-57 by
sunset. However, fairly shallow low-level moisture return and a
deep reservoir of dry mid-level air may limit storm coverage but
also support gusty winds with any convection.

Kluber

Saturday Night through Thursday:

Showers and thunderstorms should be ongoing along and ahead of
a cold front Saturday evening. Some of these storms could be
strong to severe early in the evening, with mainly localized
gusty winds, though this threat should diminish with eastward
extent due to diurnal loss of low-level instability and
generally weak deep-layer shear. Sunday through Sunday night
looks to be largely dry and cooler, as Saturday night`s cold
front settles toward the Ohio Valley and surface high pressure
builds east across the upper Midwest and western Great Lakes
region. Temperatures will be limited to the lower 70s well
inland Sunday afternoon, while northeast winds off of Lake
Michigan will keep highs only in the upper 50s near the lake.
Lows will dip into the mid-upper 40s in most spots Sunday night.


Ensemble guidance depicts deep upper level trough developing
across the western and central CONUS heading into next week,
with a closed upper low developing across the Northern Plains
Monday. A series of mid-level short waves are progged to rotate
around the south and southeast periphery of the circulation as
it slowly drifts east, eventually lifting the front across the
lower Ohio Valley northward as a warm front later Monday night
into Tuesday. Some spotty elevated showers are possible as early
as Monday afternoon and evening as warm/moist advection
develops atop the warm frontal slope, though really becomes more
focused into the local area Tuesday into Tuesday night. This
appears to be the period with the greatest potential for
thunderstorms, some potentially strong to severe given strong
mid-level wind fields and warm/moist/conditionally unstable low
levels. Temperatures looks to surge back into the upper
70s/around 80 Tuesday afternoon, with surface dew points rising
into the 60s as the warm front lifts across the area.

Periodic shower and thunderstorm chances linger through the
mid-week period, as ensembles continue to indicate the upper
trough drifting east across the upper Midwest/Great Lakes region
through the end of the forecast period. Blended NBM guidance
maintains chance pops both Wednesday and Thursday, especially
during the more diurnally-favored afternoon/evening hours.
Higher low-level Theta-E air mass gradually gets shunted east of
the area during this time, with temperatures gradually settling
into the lower 70s by Thursday.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
-- Changed Discussion --
Issued at 612 AM CDT Fri May 3 2024

Key Aviation Messages...

- Period of low-MVFR and IFR ceilings early this morning,
  scattering to VFR by mid-morning.

- North-Northwest winds 10-15 kts early, becoming northeast
  around 10 kt late morning/midday.

- Low-confidence potential for MVFR ceilings at KMDW/KORD early
  Saturday morning.

Surface cold front was pushing through northeast IL early this
morning. A fairly narrow band of low-MVFR and patchy IFR
ceilingswas along/behind the front, and will affect the
Chicago terminals for a few hours this morning before moving
east and allowing VFR conditions to prevail. A few spotty
sprinkles may linger for an hour or so, though no impactful
showers are expected. With the front, winds shift north-
northwest 10-15 kts, and these will persist through late morning
before winds veer northeast at around 10 kts. High-res model
guidance has shown quite a bit of spread as to just when the
shift to northeast will occur, but should occur in the late
morning to midday time frame. Winds will then remain northeast
through this evening, eventually becoming light east late
tonight and southeast Saturday morning.

Guidance continues to suggest some low-MVFR stratus may form
over Lake Michigan and northern Indiana early Saturday morning,
and could possibly spread west into KMDW/KORD around sunrise.
Confidence is low at this distance, but will continue to monitor
with later forecasts.

Ratzer
-- End Changed Discussion --

&&

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

&&

$$

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