149
FXUS63 KLOT 072038
AFDLOT

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

.KEY MESSAGES...

- Threat for any additional severe thunderstorms ending by 5-6
  PM CDT today

- Another round of thunderstorms may develop late Wednesday into
  Wednesday night, some of which could be strong to severe,
  mainly south of I-80 into central Illinois and Indiana

- Cooler with periodic showers Thursday and then Friday night
  into the weekend

&&

.PREVIOUS 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 338 PM CDT Tue May 7 2024

Tonight through Wednesday Evening:

A post cold frontal short-wave may touch off a few showers and
thunderstorms across portions of far northern Illinois through
the early to mid evening, with slight chance (~15-20%) PoPs for
this activity. Generally quiet weather is then expected tonight
into Wednesday morning, as the mid-level short wave lifts
northeast of the area and short wave mid-level ridging develops
overhead. Winds will become light to calm tonight as weak
surface high pressure slides across the area. Forecast lows
tonight are in the mid to upper 50s.

The most recent guidance has trended drier through much of
Wednesday afternoon, enabling another warm day (mid 70s to lower
80s F) despite increasing cloud cover. The exception to the
warmth once to the early afternoon will be lakeshore areas due
to winds turning onshore synoptically and aided by lake influence.

From late Wednesday afternoon through Wednesday night, another
round of showers and thunderstorms is expected to develop. Some
scattered showers may develop across far northern Illinois and
across the state line into Wisconsin in the late afternoon. More
notably, the approach of the next short-wave will result in a
gradual uptick in showers and thunderstorms to the south of I-80
prior to sunset, and then spreading northward in the evening and
overnight.

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.
Overall, the guidance has come into better agreement in barely
returning the front northward into our far southern CWA (south
of US-24) as a warm front by Wednesday evening. This is where
some potential may exist for surface based severe weather
threats, primarily damaging winds and hail.

Otherwise, as most of our area will be on the cool side of
the front, the main concern will be elevated hail producing
thunderstorms , some of which may become marginally severe
(hail up to 1" diameter) as mid-level lapse rates steepen
amidst supportive effective bulk shear. In addition, seasonably
high PWATs may present a localized ponding or even flooding
threat, primarily near and south of I-80.

Castro/Ratzer

Late Wednesday Night through Tuesday:

A mid-level trough extending southwest across Quebec and
southern Ontario will phase with a mid-level low over the
Missouri Valley late 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" totals by mid day Thursday) across
portions of northern Illinois into southern Wisconsin. Outside
of this band, scattered diurnally enhanced showers and perhaps a
few storms are expected. Much cooler conditions are in store,
with highs only in the upper 50s to lower 60s inland and lower
to mid 50s near Lake Michigan. Adding to the cool feel, northerly
winds will gust up to 25-35 mph, strongest near the lake.

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. It appears temperatures will
trend back to above normal by the end of the period.

Kluber/Castro

&&

.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.

&&

$$

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