-- Discussion containing changed information from previous version are highlighted. --
684
FXUS63 KLOT 070605
AFDLOT

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

&&

.UPDATE...
Issued at 833 PM CDT Mon May 6 2024

Generally the forecast for the next 24 hours remains on track
with very little changes made this evening.

A broad negatively tilted upper trough continues to spread
across the central CONUS which has developed a line of strong to
severe thunderstorms from South Dakota to Oklahoma. This trough
will continue to progress east overnight which will shove the
associated line of thunderstorms east as well. As far as
northern IL is concerned, this line of storms is forecast to
reach the area around daybreak Tuesday morning but in a notably
weaker and decaying state. Therefore, the threat for any true
severe weather Tuesday morning continues to look virtually zero;
however, modest mid-level lapse rates and stout winds aloft may
still promote a threat for locally gusty winds with these
storms.

The bigger question for northern IL and northwest IN continues
to be how quickly the environment will destabilize before an
upper level wave and associated 75-80 kt jet streak pivot
overhead. Latest guidance trends continue to show
destabilization occurring during the late morning and early
afternoon hours on Tuesday which should yield around 1000 J/kg
of MLCAPE and support at least isolated to widely scattered
thunderstorms. However, exact placement of the storms remains
uncertain though it does appear that the guidance consensus is
to favor areas along and south of I-80 with the greatest
coverage. Regardless of where the highest coverage occurs, the
aforementioned 500 mb jet will generate 60-65 kts of shear
overhead which is more than sufficient to promote storm
organization and a threat for severe weather. That said the
greatest threat with any severe storms Tuesday afternoon
continues to be damaging wind gusts and hail, but modestly
strong low-level shear may also support brief tornadoes to occur
where surface instability is maximized. While the severe
weather timing also remains uncertain, the general consensus is
for the greatest threat to be within the 11 AM to 4 PM CDT
timeframe.

Additionally, there is also a signal for a second round of
strong to possibly severe storms Tuesday evening, mainly across
northern IL. Obviously this secondary threat largely depends on
the afternoon round so details remain much muddier at this time.
Given the aforementioned uncertainties we recommend staying
updated with the latest forecast which will continue to reflect
the latest thinking and provide better timing and coverage
areas. In the meantime, the going forecast seems to have a
decent handle on the latest trends and therefore remains
virtually unchanged.

Yack

&&

.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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
-- Changed Discussion --
Issued at 104 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 18Z
- Scattered TSRA, possibly locally severe, mid-afternoon

A line of TS currently across central Iowa will track eastward
across northern Illinois after sunrise. TS onset timing for
ORD/MDW is favored roughly around 14Z, with an expected duration
less than one hour at any given location. While these storms
will be weakening with time, locally higher wind gusts to 30
knots are possible.

Behind the initial line of TS, 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 18Z.

Another round of TS is expected this afternoon. While
confidence in TS coverage this afternoon is lower than this
morning, it is high enough to change the PROB30 to TEMPO with
the most recent TAF issuance. TS may very well be in the
vicinity of northeast Illinois for several hours early to mid
afternoon, but the most likely window of direct impacts to the
terminals is from 19-21Z. 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
-- End Changed Discussion --

&&

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

&&

$$

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