627
FXUS63 KLOT 031842
AFDLOT

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

.KEY MESSAGES...

- A dry and cool night is in store with overnight lows in the
  mid to upper 40s.

- Tomorrow (Saturday) will be warm with highs in the upper 70s
  to lower 80s, and increasing chances (60 to 70%) for showers
  and thunderstorms in the evening especially across northern
  Illinois.

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

&&

.DISCUSSION...
Issued at 130 PM CDT Fri May 3 2024

Through Saturday night:

A surface high pressure system continues to meander through the
Lower Great Lakes region and is leading to mostly sunny skies,
light north to northeasterly winds, and generally pleasant
temperatures (at least away from Lake Michigan).

With our area on the western flank of the surface high tonight,
quiet conditions will prevail. Light winds, clear skies, and a
dry airmass will facilitate efficient radiational cooling and
allow for overnight lows to fall into the mid to upper 40s.

Tomorrow, low-level flow will turn southeasterly and eventually
southwesterly as the surface high pulls further away to the east and
an upper-level short-wavelength trough approaches from the west. Low-
level moisture advection will allow for humid air across central
Illinois and Indiana to quickly return northward and into our area
by mid-afternoon. Even with filtered sunshine, the advection of a
plume of seasonably warm 850mb temperatures near +13C (into which
the boundary layer will build) will help boost afternoon highs into
the upper 70s to lower 80s.

Chances for showers and thunderstorms will increase during the
afternoon hours in two apparent waves. The first will be with an
axis of remnant convection (originating from the Plains
tonight), due to arrive sometime in our area early tomorrow
afternoon. An ensemble of CAM guidance suggests the leading axis
of showers and storms may outrun the best forcing provided by
the upper-level shortwave and an associated cold front lagging
to the west, and fight a pocket of dry mid-level air in place
across the Lower Great Lakes. As a result, chances for showers
and storms appear higher later in the afternoon and toward
evening along the cold front as it moves across the area (50 to
70% chance from west to east ahead of the cold front). With the
upper-level shortwave and associated jet streak expected to
lift northeastward toward the Upper Peninsula of Michigan (e.g.
remain somewhat displaced from the instability axis further to
the south across our area), any convective cell tomorrow
afternoon and evening should behave in a "pulse" like fashion
with overall limited storm-scale organization. As a result, the
main threats with any storm tomorrow will be locally gusty
winds, small hail, downpours, and lightning strikes, similar to yesterday
(and consistent with a level 1/5 threat for severe weather). In
all, tomorrow looks like another quintessential spring-like
day.

Coverage of showers and storms will decrease after sunset as the
cold front approaches northwestern Indiana. Temperatures behind the
front will fall into the upper 40s (northwest) to mid 50s
(southeast) early Sunday morning as northwesterly winds become
predominant.

Borchardt

Sunday through Friday:

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 /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1231 PM CDT Fri May 3 2024

- There is no impactful weather expected through the current
  forecast period

Lingering MVFR cigs over Northwest Indiana are slowly eroding
away as a cold front exits the area through the afternoon.
Northeast winds around 10 knots are expected through the day,
gradually rotating to the southeast by Saturday morning and
diminishing slightly. Guidance is suggesting a fairly low level
inversion developing Saturday morning. With enough flow above
1000 feet, it should result in a FEW low to mid level clouds
developing; however, some areas in Northwest Indiana might have
moments of lower vis around 4 SM, but dense fog is not expected.
By Saturday afternoon, winds will become southwesterly ahead of
the next passing cold front. Rain showers are expected to
accompany this front, but not arriving until after the current
forecast period for any of the TAF sites.

DK

&&

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

&&

$$

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