036
FXUS63 KLOT 062033
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 PM CDT Mon May 6 2024

.KEY MESSAGES...

- Thunderstorms are likely tomorrow morning and may produce
  gusty winds and/or small hail.

- A second round of thunderstorms appears probable during the
  afternoon, with the highest confidence in storm redevelopment
  being east of a Pontiac, IL to Chicago, IL line. 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 332 PM CDT Mon May 6 2024

Through Tuesday night:

All eyes are upstream on the central/southern Plains, where a
significant severe weather outbreak is underway and will
continue well into the night. Strong forcing associated with an
ejecting upper-level trough and an attendant cold front at the
surface will promote rapid upscale growth across Kansas and
Nebraska, with the resultant elongated QLCS then expected to
track into Iowa and Missouri late this evening and eventually
into Illinois late overnight. By the time this strongly-forced
line of storms reaches our forecast area, it will likely be
elevated and on a general downward trend in intensity, so
prospects for severe weather in our forecast area with this
early morning QLCS appear to be minimal. Near-surface stability
won`t be overly strong, however, and MUCAPE could still be as
high as about 1000 J/kg (highest in our southern counties) as
this line of storms rolls through, so there could still be some
gusty winds and/or small hail with any of the stronger
individual updraft cores within this line.

The remnants of the weakening QLCS should skitter out of our
forecast area from west to east by about 11 AM CDT or so, and
rapid destabilization efforts will begin as soon as it departs.
Cold mid-level temperatures associated with the aforementioned
trough pivoting overhead coupled with strong upper-level forcing
from DPVA and divergence within the left exit region of an
upper-level jet should not have much of a problem with
steepening mid-level lapse rates in the wake of the morning
convection, which is not likely to cause substantial convective
overturning of the troposphere in the first place. Warm/moist
advection will also modestly increase temperatures and dew
points at the surface going into the afternoon, with pockets of
sunshine likely to provide a boost to air temperatures as well.
Taken together, it appears that at least part of our forecast
area will end up realizing 1000+ J/kg of uninhibited MLCAPE
around come the early-mid afternoon, with the greatest degree of
instability (potentially up to 2000 J/kg of MLCAPE) focused
across our southeastern CWA. The aforementioned large-scale
forcing, plus the presence of a low-level jet and isentropic
ascent, thus appears likely to reignite an additional round of
scattered to potentially numerous thunderstorms in at least part
of our forecast area during the afternoon tomorrow, and the
majority of the latest 12Z CAM suite seems to be on board with
this idea.

Wherever instability recovers appreciably by the time the core
of an inbound mid-level vorticity maximum arrives, convection
appears likely to develop, and deep layer shear will be plenty
strong enough to be able to sculpt any mature convection into
supercells, with the largely unidirectional shear profiles
promoting both large hail and damaging downburst winds and the
most likely severe weather hazards. While not particularly
likely, some threat for tornadoes does also appear to exist,
mainly in our far southeastern CWA, where low-level flow looks
like it will be a little more backed than elsewhere. All that
said though, if adequately strong instability is unable to be
realized to balance out the very strong deep layer shear, then
updrafts are likely to be sheared apart by the strong mid- and
upper-level flow and not really be able to get well-organized,
which would greatly limit the overall scope of the severe
weather threat that may be realized. At any rate, the overall
prospects of convective redevelopment and severe weather in our
forecast area tomorrow afternoon appear to be greatest across
our southeastern counties, where the Storm Prediction Center has
delineated a Slight (level 2/5) Risk in their Day 2 Convective
Outlook. Farther northwest in our CWA, confidence in adequate
destabilization occurring in time for convection to develop and
mature is lower, and hence, a Marginal (level 1/5) Risk appears
to more appropriately characterize the overall severe weather
threat there at this time.

After the afternoon convection has departed, there may be one
additional opportunity for isolated to widely scattered
thunderstorms to affect our forecast area in the evening as a
secondary, more subdued vorticity maximum swings into the area
and kick-starts additional convective development to our
northwest. It`s possible that this activity could trickle into
our northern CWA near or after sunset and pose some threat for
hail and/or damaging winds, but with gradually waning
instability and sounding profiles looking noticeably drier by
then, confidence in this occurring is relatively low at this
time.

Ogorek


Wednesday through Monday:

In the wake of the Tuesday system the parent mid-upper low will
remain situated over the Central Plains with the core of the
northern stream upper jet directly overhead. Low-level
trajectories from the Gulf of Mexico will allow for a quick
moisture recovery across the Mid-Mississippi Valley on
Wednesday. A semi-coupled upper jet structure will provide
additional synoptic support for widespread thunderstorm
development across this region then spreading east across the
Ohio Valley. The question locally is how far north can this
moist warm sector reach which will impact our severe
thunderstorm potential. Latest trends in 12Z guidance keep the
northern extent to the better instability just south of the
area, confined mainly to central/southern IL/IN and areas along
the Ohio River Valley. However, given the strong dynamics in
play, if better instability can lift into the area, especially
into our southern CWA (along/south of the Kankakee River Valley)
the environment would be supportive of all severe hazards. Will
continue to monitor trends closely.

While the severe threat is more in question, showers and
thunderstorms are expected across the area by Wednesday evening
and overnight. As the surface low occludes and becomes more
elongated, an east-to-west axis of cool wind-whipped rain likely
continues into the day on Thursday, especially along/north of
I-80. The cooler north to northeast winds off the lake in rain
will limit our daytime warming on Thursday to the 50s to lower
60s.

The upper low begins to split, with the western lobe cutting
off over the western CONUS and the eastern lob phasing with a
wave extending across Ontario/Quebec with a northwest flow
pattern setting up, with the next wave diving out of Canada
bringing our next chance for showers an perhaps a few
thunderstorms late Friday into Saturday morning. There are
differences with the handling of the western CONUS cut-off upper
low and resultant upper level pattern though it appears the
pattern remains active with off and on precipitation chances
continuing next week.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1240 PM CDT Mon May 6 2024

Key Aviation Messages:
 - Line of TSRA with gusty winds early Tuesday AM
 - Scattered TSRA possible Tuesday afternoon (30% chance)

VFR conditions are forecast through this evening and early
overnight hours with generally east to southeast winds around
10kt.

A line of thunderstorms is expected to cross the area early
Tuesday morning, potentially as early as 10Z toward RFD and 12Z
for ORD/MDW with associated vsby/cig reductions down to as low
as IFR at times. This line of storms may be accompanied by a
gusty westerly wind shift before settling back to a southeast
direction.

Once the initial line of storms moves east of the area,
conditions become favorable for renewed isolated to scattered
storm development by Tuesday afternoon as winds turn south and
then southwesterly. Confidence in storm coverage is lower for
areas north of I-80 and accordingly opted to introduce this
potential with a PROB30 from 18-22Z. If storms do develop they
would have the potential to become strong to severe.

Petr

&&

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

&&

$$

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