447
FXUS63 KLOT 072107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
407 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

&&

.MESOSCALE...
Issued at 407 PM CDT Tue May 7 2024

Surface trough/confluence zone continues to move eastward
across extreme eastern IL. Storms developing along this boundary
continue to struggle, likely a result of weak low level
convergence and a very dry mid-upper level air mass. Air mass
ahead of this boundary remains unstable and uncapped based on
RAP mesoanalysis and confirmed by blossoming Cu field on visible
satellite imagery. Winds are slightly more backed across
northwest Indiana, so convergence may be a bit stronger as the
boundary gets into northwest Indiana, so it is still possible we
could see a ramp upward in the intensity of some of these
storms over the next couple of hours.

IND and IWX WSR-88D VWPs both show strong deep and low level
shear which will continue to be supportive supercells, even
where surface winds are slightly west of due south. Until the
boundary clears our CWA, we will need to continue to closely
monitor storms for any signs of sustained intensification and
growth. Hail and damaging winds would be the main threats if any
storms can grow sufficiently and become sustained. However,
hodograph derived from latest IND VWP shows sufficient low level
shear for a tornado threat given the low LCLs and strong low
level CAPE.

- Izzi

&&

.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, locally lower 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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