896
FXUS63 KLOT 071107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
607 AM CDT Tue May 7 2024

.KEY MESSAGES...

- Thunderstorms expected this morning, which may produce gusty
  winds and/or small hail.

- A second round of scattered thunderstorms is likely this
  afternoon, with the highest confidence east and southeast of
  the I-55 corridor. A few of  these afternoon storms could
  become severe with large hail and damaging winds as the
  primary hazards.

- Another round of storms may develop late Wednesday, some of
  which could be strong to severe, mainly south of I-80 into
  central IL/IN.

&&

.DISCUSSION...
Issued at 344 AM CDT Tue May 7 2024

Through Wednesday Evening:

Early morning Midwest radar mosaic depicts an extensive north-
south linear MCS propagating east-northeast across eastern
IA/MO. This MCS, associated with a potent negative-tilt mid-
level short wave and surface cold front spreading east from the
mid/upper Missouri Valley, will continue to push into northern
IL and the WFO LOT forecast area after 5 am. The surface cold
front was in the process catching up to a warm front stretching
from IA into central IL, which should allow storms to become
slightly elevated farther east into IL, which along with the
gradual diurnal decrease in low-level instability should result
in a slow weakening trend as it moves into our area. Despite
these trends, localized sub-severe gusty surface winds up to 50
mph will likely still be possible across our western cwa as this
line arrives. While a few isolated to widely scattered cells
may develop ahead of the line within increasing warm advection
flow, the main line of storms should reach the Chicago metro
area around 7-8 am.

The convective outflow/cold pool footprint from these morning
storms will stabilize low levels through midday, with some
question remaining regarding location/extent of diurnal
destabilization across the forecast area this afternoon.
Kinematics certainly favor the potential for strong to severe
storms, with the mid-level short wave trough steepening mid-
level lapse rates and a 75+ kt mid-level jet spreading across
the area. Guidance continues to indicate the bulk of morning
convection clearing the area around noon, with around 1000 J/kg
of MLCAPE developing by early afternoon. CAMs depict at least
widely scattered to scattered thunderstorm redevelopment early-
mid afternoon, though location and evolution is of somewhat low
confidence. Severe storms are possible area-wide this afternoon
with all hazards possible, though areas east/southeast of the
I-55 corridor look to have a higher overall and higher tornado
threat better low-level instability and stronger shear as the
mid-level jet spreads east. The window for greatest severe
threat is roughly from 1-2 pm through about 5 pm, with storms
becoming more isolated thereafter. Coverage may be a bit better
across northwest IL and along the IL/WI border late in the
afternoon.

Generally quiet weather is expected tonight into Wednesday
morning, as the mid-level short wave lifts northeast of the area
and short wave mid-level ridging develops overhead. Breezy
southwest winds later today will diminish overnight, as weak
surface high pressure slides across the area.

Another round of showers and thunderstorms is expected to
develop Wednesday afternoon into Wednesday night, as the next
short wave approaches the area. A surface low pressure wave is
expected to develop along the cold/stationary front which will
trail across the southern Plains in association with this wave,
which will then propagate northeast into the mid-Mississippi
Valley by Wednesday evening. Renewed warm advection return flow
will help push the front north across central IL/IN as a warm
front during the period, with showers and thunderstorms
developing northeast into the forecast area during the
afternoon. Details remain somewhat unclear with the track of the
surface wave and frontal position by Wednesday evening, with
various guidance showing quite a bit of spread. A more northerly
low/warm front track into our southern cwa would likely present
a greater severe weather threat.

Ratzer


Late Wednesday Night through Monday:

A mid-level trough extending southwest across Quebec and
southern Ontario will phase with a mid-level low over the
Missouri Valley Wednesday night. As the phasing wave crosses the
area on Thursday, a residual low to mid-level TROWAL combined
with PWATs around 1" and a deep layer favoring warm rain
processes supports the potential for a low-end localized band of
heavy rain (1-2") across portions of northern Illinois into
southern Wisconsin late Wednesday night into Thursday. Outside
of this band, scattered diurnally enhanced showers and perhaps a
few storms are expected.

After a quick-moving ridge yields dry and seasonable temps on
Friday, another upper-level low is progged to cross the western
Great Lakes on Saturday. Anomalously cold mid-level temps
shifting across the area through the day will yield scattered
showers and storms, with some stronger cores likely producing
locally strong wind gusts. Diverging solutions by early next
week with regards to handling of the departure of Saturday`s
upper low and a building ridge to the west greatly diminishes
the forecast by early next week.

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 605 AM CDT Tue May 7 2024

Key Aviation Messages:
- Line of TSRA with potential gusts to 30 knots 13-15Z
- SE winds shifting SW and gusting over 20 knots around 19Z
- Scattered TSRA, possibly locally severe, mid-afternoon

A line of TS moving across northern Illinois will reach ORD/MDW
roughly around 1330Z, with an expected duration one hour or
less. While these storms are gradually weakening, locally higher
wind gusts to 30 knots are possible.

Behind the initial line of TS, BKN MVFR ceilings are expected
to develop as isolated -SHRA persist through the remainder of
the morning. SE winds will then veer SW and gust to 25 knots
beginning around 19Z.

Confidence continues to increase with another round of TS mid
to late afternoon. TS may be in the vicinity of northeast
Illinois for several hours, but the most likely window of direct
impacts to ORD/MDW is from 19-22Z. These storms may be strong
to severe and capable of wind gusts over 30 knots and hail.

Beyond the second round of TS, VFR conditions and SW winds are
expected through tonight.

Kluber

&&

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

&&

$$

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