709
FXUS63 KLOT 040549
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 AM CDT Sat May 4 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 200 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:

Next week is shaping up to be a fairly wet and active pattern.

Starting Sunday, as an upper level low drops southward into
Northern California from the Bay of Alaska (more on this later),
a surface low over Hudson Bay is forecasted to continue to move
east drawing the cold front east out of the forecast area. There
may be some lingering showers on Sunday morning, particularly
over northwest Indiana, but potentially extending as far back to
I-55. However, improving and drier conditions are expected
through Sunday. Temperatures are expected in the upper 60s and
70s away from the lake, but with cooler northerly air over Lake
Michigan, the immediate shoreline may remain in the 50s.

On Monday, the low over Northern California is expected to
weaken slightly and passes over the Rocky Mountains. Ahead of
of it, another upper level wave will pass over Northern
Illinois. Models are disagreeing on the overall strength of the
wave. The GFS is leaning deeper with an brief closed low aloft
developing, where the Euro and the Canadian depict broader
waves. Moisture availability seems limited though over most of
the area north of I-80. With some weak lobes of vorticity that
pass over, there is a chance for some light showers to pass
over, but confidence remains low as it should mostly stay south
of the area as temperatures warm back into the 70s, with low 60s
on the shoreline.

There is fairly good model agreement that the upper level low
will eject out of the Rockies over west central Montana
overnight Monday into Tuesday morning while deepening. Strong
surface cyclogenesis is expected to take place in the Northern
Plains on Tuesday; however, where and how the low develops will
largely drive what unfolds on Tuesday, and models are wildly
disagreeing on that. There is a strong 300 mb jet that will
develop just to the south of the upper level low, with its left
exit region around or just north of northeastern Illinois,
which will could assist in providing additional synoptic lift,
though the exact placement of the jet will have to be monitored
over time. But even models are disagreeing with the strength of
the jet in addition to its exact placement. Regardless, strong
mid level steering flow will draw up anomalously large
precipitable water values into Illinois with dew points
temperatures expected in the low to mid 60s.

After a weak boundary pushes through Tuesday morning providing
the first chance for rain, temperatures are projected to
increase behind the front into the upper 70s and 80s. With
better instability, showers and thunderstorms are expected
Tuesday afternoon and evening. While models are suggesting the
strongest shear and instability a little farther south in
Central Illinois and the Ohio River basin, there is the
potential for strong to even severe storms locally on Tuesday.

By Wednesday, the upper level low over the Northern Plains is
projected to eventually start to weaken; however, a positively
tilted trough axis will remain over Plains. With persistent
southerly warm air advection, there is a risk for another round
of showers and storms on Wednesday. Once again though, models
are still keeping higher moisture content and stronger
instability to the south over the Ohio River Valley as the jet
core aloft shifts slightly over Southern Wisconsin. Perhaps the
risk for stronger to severe storms will remain south of I-80 on
Wednesday, but it will be close enough to be monitored.

Beyond Thursday, there is much more uncertainty on exact timing
of showery activity. However, both Euro and GFS ensembles are
suggesting the upper level trough to continue to broaden and
trudge eastward from Thursday through Sunday. With lingering
moisture in the area and lobes of vorticity passing around the
upper level trough, there could be periods of showers from
Thursday through Saturday. It is not expected to be a complete
wash out as there should be breaks from the rain at times, but
it remains too far out for complete understanding on timing, so
the forecast maintained a 20 to 40 percent chance for rain
through Saturday morning.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1236 AM CDT Sat May 4 2024

Key Aviation Messages:

- Low probability for shallow fog through sunrise, mainly away
  from Chicago.

- Scattered showers and thunderstorms likely this afternoon and
  evening, some of which could produce gusty winds.

- Light southeast winds become south-southwest by midday, with
  possible lake breeze wind shift late afternoon for
  KORD/KMDW/KGYY.

Surface high pressure was centered northeast of the forecast
area at midnight. Farther west, an area of weak low pressure was
over the central Plains. This low will move east-northeast to
the western Great Lakes region by evening, with its cold front
moving east of the area late. Light easterly (or variable) winds
are expected early this morning, turning southeast around
sunrise and then south-southwest by midday. Several recent high-
res model runs indicate a southeasterly lake breeze will
develop, and may affect KORD/KMDW/KGYY late this afternoon. This
return to easterly winds appears to be short-lived however, as
an area of scattered showers and thunderstorms are expected to
move east across the area later this afternoon and early
evening, possibly producing westerly outflow winds of 25-30+ kts
as they move into the Chicago metro area toward evening.
Thunderstorms would likely be in a diminishing trend as they
move into the metro.

The cold front associated with this system will bring a shift
to northwest and eventually north-northeast winds later in the
evening, which would continue overnight. Guidance also indicates
a period of MVFR ceilings is possible behind the front.

Ratzer

&&

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

&&

$$

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