924
FXUS63 KLOT 131735
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 PM CDT Thu Jun 13 2024

.KEY MESSAGES...

- There is a threat for severe weather this afternoon and
  evening. Hazards include destructive hail (2"+), wind
  (75mph+), and flash flooding.

- Hot and humid conditions are expected next week.

- Periodic low chances for thunderstorms will return next week.

&&

.UPDATE...
Issued at 1031 AM CDT Thu Jun 13 2024

The forecast for this afternoon and evening appears to be on track.

A recent hand surface analysis depicts a surface pressure trough and
associated cold front extending from central Wisconsin to northeastern
Kansas with several embedded areas of relative low pressure minima.
Ahead of the cold front, a somewhat tight low-level pressure gradient
and associated breezy southwest winds are facilitating northeastward
moisture advection into northern Illinois with surface dew points
rising into the mid to upper 60s. When combined with seasonably
warm surface temperatures already nosing into the mid 80s, a
pool of instability is quickly building across the Upper
Mississippi River Valley ahead of the cold front. (With that
said, low-level thermal capping remains over our area as
confirmed by a dearth of growing cumulus fields and recent AMDAR
soundings).

Early this afternoon, an upper-level shortwave and related upper-level
gravity waves (all seen via visible and water vapor satellite imagery
along the Iowa/Minnesota borders) will move across southern Wisconsin
and provide a "glancing blow" to the destabilizing airmass over northern
Illinois. Our expectation is that the shortwave should be able to
initiate thunderstorms this afternoon across northern Illinois, though
the exact coverage, when, and where thunderstorms initially develop
remain tricky items to tack down. Initial thunderstorms may
develop out of the growing ACCAS field over southern Wisconsin
as early as 12-1pm, or later on this afternoon over the I-80 or
I-88 corridors between 3-5 pm during "peak heating." There is
also a scenario where initial thunderstorm development across
northern Illinois is relatively sparse due to the (too early?)
arrival of the shortwave and otherwise nebulous forcing as the
cold front stalls overhead. It may not be until any
convectively-generated gravity waves or an MCV in Iowa this
afternoon moves into northern Illinois toward or after sunset
that thunderstorms really take off in our local area, by that
point along and south of I-80. Generally speaking, the later it
takes for storms to develop, the further south the threat will
be.

Regardless of when and were thunderstorms develop, an extremely unstable
airmass characterized by forecast MLCAPE >3000 J/kg and convective-
layer shear >45kt will support supercell structures with a threat for
damaging to significantly damaging hail (locally >2" in diameter). In
fact, with the majority of the shear profile centered in the "mid-
levels" and >1000 J/kg of CAPE in the hail-growth layer, a few giant
hailstones >3" cannot be ruled out today somewhere in the general
region (Iowa or Illinois). And, DCAPE >1200 J/kg will support
damaging to destructive downbursts (locally >75 mph),
especially where any localized clustering or upscale growth can
occur. This is all to say that the SPC level 2/5 to 3/5 severe
weather threat (including relatively rare 10% "hatching" for
both destructive hail and winds) remains valid across most of
the area.

Finally, we will have to keep a close eye on any mesoscale corridors in
which training convection can occur as the high PWAT airmass
will be favorable for rainfall rates >2"/hr, particularly after
sunset as the cold front stalls south of I-80. Indeed, a few
individual CAM runs depict swaths of 3 to locally 6" of rain
this evening in the general region. It is not out of question
that a targeted flash flood watch may be needed as confidence
increases in where, when, and at what coverage thunderstorms
will develop.

In all, the message remains the same to remain weather aware
this afternoon and evening. Updated products are being sent.

Borchardt

&&

.DISCUSSION...
Issued at 320 AM CDT Thu Jun 13 2024

Through Tonight:

Strong warm and moist advection continues early this morning
across the area which has held temperatures in the 70s and
dewpoints in the lower 60s. After daybreak expect dewpoints to
further increase into the upper 60s and perhaps even near 70.
This will make for a warm and muggy day with highs in the upper
80s to around 90 and heat indices in the low-mid 90s.

We continue to monitor a couple areas of upstream convection at
this hour, one small cluster moving east southeast across east
central Iowa and another broader axis of convection extending
from southeast Minnesota eastward across central Wisconsin. The
former may bring a few showers (maybe a rogue lightning strike)
into areas generally southwest of a Dixon to Paxton, IL line
around daybreak. The latter may just clip areas near the WI/IL
stateline near to shortly after daybreak as well. Most areas
will remain dry but a general increase in cloud cover is likely
for the first part of the morning.

Given an overall lower shower/storm coverage, while
acknowledging a brief period of increased cloud cover may delay
destabilization initially this morning, earlier concerns of
morning showers shunting the effective front well south of the
Chicago metro are becoming a less likely outcome with each
passing hour. Accordingly, confidence is increasing in scattered
to potentially numerous thunderstorm development this afternoon
along a cold front, potentially as early as 1-2 PM CDT across
southeast Wisconsin and along the WI/IL stateline. These storms
would then likely increase in coverage with southward extent as
they move across the area into the evening. As expected, there
remain subtle timing, coverage, and placement differences among
the various suite of available model guidance but there is
enough consistency to boost shower/storm chances into the
"likely to definite" range (60-80%), highest south of I-80.

Instability upwards of 2000-3000+ J/kg combined with deep layer
shear of 35-45+ kts will support supercell structures with
initial storm development this afternoon. Main hazards include
significant hail (2"+) and wind gusts (75mph+). With the arrival
of a subtle mid-level wave, further weakening of any remaining
convective inhibition should occur toward late afternoon/early
evening, suggesting storm mode transitions to congealing multi-
cell clusters and bowing segments as storm coverage increases,
particularly south of I-80.

The SPC Day 1 Outlook for severe weather remains mostly
unchanged with a level 2 of 5 risk for much of northern and
northeast Illinois and northwest Indiana and a level 3 of 5
risk confined to areas southwest of the Chicago metro where the
greatest instability is expected. A northeastward expansion of
this area into the metro certainly cannot be ruled out with
later updates. Any storms that develop across the entire area
will be capable of destructive hail (2"+) and wind gusts
(75+ mph).

A tornado cannot be fully ruled out today given an initial
supercellular mode in the afternoon, and then later into the
evening after storms grow upscale, but weak low-level shear and
high LCLs support no more than a 2% tornado risk.

Lastly, high PWATs and the potential for a period of training
storms this evening, supports a continued mention of a localized
flash flooding risk.

Petr


Friday through Wednesday:

Primary forecast concerns for the extended remain heat/humidity
and thunderstorm chances.

High pressure will be moving across the upper Great Lakes
region Friday and this will send a weak cold front down Lake
Michigan, arriving in the mid/late afternoon. While winds will
already be northeast near the lake, along with cooler temps,
this front will usher in cooler air across the entire cwa
through Friday evening. After highs in the lower/mid 80s for
most locations Friday, temps will be falling into the 60s by
sunset along with lower humidity levels. Low temps by Saturday
morning will be in the 50s for most areas.

High temps will be back in the 80s for most areas on Saturday
with dewpoints still only in the 50s. East/southeast winds will
keep the IL shore of Lake Michigan cooler.

As has been advertised for several days, the upper ridge will
begin to build north across the region on Sunday. To some
degree, the models and their ensembles have had a chance of
thunderstorms from early Sunday morning through early Monday
morning, as the ridge has not yet fully spread north of the cwa.
And while confidence remains low, this time period, Sunday-ish,
seems to be a time period to monitor for the potential for some
storms, with uncertainty for timing/location. Highs on Sunday
could easily be in the mid 90s for most of the area absent of
any significant cloud cover/precipitation.

After Sunday, confidence is very low for thunderstorm chances.
The ECMWF and its ensembles are hot and dry for several days
next week with the upper ridge firmly in place while the GFS
continues to show frequent precip chances, which would also
impact high temps, pushing them lower then currently
advertised. Blended pops have some form of low chance pops from
Saturday night onward and while any one of these periods may
have some precip, much of the extended is likely to be dry with
little skill trying to time any impulse.

Uncertainty increases further by Wednesday/Thursday when a cold
front may approach from the northwest bringing what will likely
be the next best chance of precipitation (after Sunday) and if
the front does move through the area, a break from the hot and
humid conditions.

As for heat index values, current highs in the lower/mid 90s and
dewpoints generally in the upper 60s to around 70 yield heat
index values in the upper 90s to around 100. Again, with no
precip influences/effects. Plan to starting mentioning this heat
potential in the HWO this morning. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1235 PM CDT Thu Jun 13 2024

Forecast concerns for the terminals...

- Scattered to widespread thunderstorms this afternoon and
  evening, some storms could be severe

- Winds become northerly behind a cold front this evening

- Lake breeze to move through the Chicago terminals Friday
  afternoon

Breezy west-southwest winds have developed at the terminals,
especially the Chicago ones, this morning as the atmosphere has
mixed into a low-level jet overhead. While this jet is gradually
diminishing, gusts in the lower 20 kt range will likely persist
through early afternoon.

At the same time, an upper disturbance and associated cold front
will continue to move towards northern IL from WI which will aid
in developing scattered to widespread thunderstorms this
afternoon and evening. While confidence is high that storms will
develop, the confidence on exact timing and coverage over the
terminals, especially at RFD, is lower. Therefore, I have
maintained the TEMPOs and their respective timings which seem
reasonable based on latest guidance trends. However, I did
extend the VCTS mentions at MDW and GYY an hour later to account
for the potential for the front and associated storms to stall
near I-80 this evening. Additionally, there is also the threat
that any storms this afternoon and evening could become severe
with the threats of large hail (possibly in excess of 2 inches),
damaging winds (possibly in excess of 60 kts), and locally
heavy rain which will result in reduced visibilities.

Storms will gradually come to an end from north to south late
this evening as the front moves through. As a result winds will
become northerly (possibly varying between 350 and 010) with
speeds in the 6 to 8 kt range overnight and persist into Friday
morning. Though a lake breeze is expected to develop early
Friday afternoon and push through the Chicago terminals between
18z and 20z. Otherwise, expect mainly VFR conditions outside of
storms with gradually scattering clouds overnight into Friday.

Yack

&&

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

&&

$$

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