390
FXUS63 KLOT 071800
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
100 PM 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.

&&

.MESOSCALE UPDATE...
Issued at 1259 PM CDT Tue May 7 2024

We`re closely monitoring for an increased severe weather threat
within the CWA through the mid to late afternoon. The area of
greatest concern is across northwest Indiana where there is a
more distinct tornado threat. The threat for damaging hail has
also increased further northwest in the CWA, encompassing much
of if not the entire Chicago metro area. Our window of severe
weather potential is approximately from 1-5PM CDT.

17-18z analysis indicated a deep occluding surface low across
the ND/SD border, its warm front lifting into southern
Wisconsin, and its cold front back across eastern Iowa. A
seasonably warm and moist air mass has developed across the warm
sector, with temperatures in the 70s and dew points in the mid
60s, except across our far northwest (upper 60s) and the
northeast IL shore (50s and 60s due to onshore winds).

Elevated convection is ongoing across northwest Illinois, where
a hail and gusty wind threat is present. This area will continue
to be monitored for severe hail up to 1" or so in diameter. A
warning was recently issued for Ogle County.

The main focus for the more appreciable severe threat will be
spurred by strong large scale ascent (mid-level PVA and height
falls and upper jet support) and cold frontal convergence. Given
already weak and eroding MLCINH, isolated cells across west
central IL at the top half of the noon hour may be the
beginnings of the anticipated convection ahead of the cold
frontal zone.

While surface winds had been backed, we`re seeing an anticipated
veering to southwesterly taking place across northern and
central Illinois. Effective bulk shear of 60+ kt, steepening
mid-level lapse rates, sizable CAPE in the hail growth zone,
and straightening hodographs with decreased low level shear
present a favorable setup for splitting supercells with damaging
hail as their main threat, some of which could be significant.
As such, in the 1630z SPC outlook update, severe hail probs have
been spread farther northwest, and include the potential for
2"+ stones.

Near and east of I-57 in eastern Illinois and particularly
across northwest Indiana is the area most likely to have a ~2-3
hour overlap of more backed southerly breezy southerly surface
winds, strong low level shear with decent low-level hodograph
curvature, 100-150 J/kg of 0-3 km MLCAPE, and aforementioned
very strong deep layer bulk shear and steepening mid-level lapse
rates. Supercells in this corridor certainly could be
supportive of all hazards, including tornadoes. Given the
shorter residence time of the most favorable parameter space,
confidence in significant (EF-2+) tornado occurrence within the
CWA is on the lower side and appears more favored just east and
southeast of the CWA, though given the conditional sig-tor
potential, SPC`s 10% hatched tornado probs appear appropriate.

Farther north and west, certainly can`t rule out a brief tornado
threat, though the shortening and straightening hodographs seem
to be a key limiting factor. The cold front passage this
afternoon will quickly end the severe threat by 5 PM CDT or so.

Castro

&&

.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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1259 PM CDT Tue May 7 2024

Key Aviation Messages:

- Scattered TSRA, possibly locally severe this afternoon
- SE winds shifting SW and gusting over 20 knots around 19Z

Satellite analysis depicts MVFR clouds growing in depth to become
more concerning for thunderstorm expansion across NE Illinois in the
18z-19z hour and then expanding into NW Indiana after 19z or so. The
storms in around Chicago and points westward do pose a threat for
hail along with winds in excess of 35 kt. The main window for storms
will continue until about 23-0z, longest in NW Indiana, with the
main window for the Chicago terminals through 2230z.

VFR conditions will return behind the storms this evening, with
initially breezy SW winds easing overnight. Winds will shift
southeasterly and then easterly through the late morning into the
afternoon Wednesday. Another batch of precipitation will shift
toward NE Illinois later in the day. Confidence in thunder toward
the tail end of the 30 hr TAF is still below 30 percent.

KMD

&&

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

&&

$$

Visit us at weather.gov/chicago