486
FXUS63 KLOT 130820
AFDLOT

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

.KEY MESSAGES...

- Level 2 to 3 out 5 Risk for severe weather this afternoon and
  evening. Hazards include destructive hail (2"+) and wind (75mph+)
  and localized flash flooding.

- Hot and humid conditions expected next week.

- Periodic low chances for thunderstorms next week.

&&

.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 /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1232 AM CDT Thu Jun 13 2024

Forecast concerns include...

Scattered strong/severe thunderstorms this afternoon.
Gusty southwest winds later this morning/early afternoon.
Possible east winds after storms.

Various areas of thunderstorms nearby this morning. Storms over
southeast Lake Michigan will move away from the terminals while
storms over eastern IA will move southeast into west central IL
and remain southwest of the terminals. But the area of storms
across eastern MN into central WI will continue moving southeast
and is expected to slowly weaken and dissipate. However, if
this area of convection does not dissipate fully, showers may
reach into far northern IL and RFD by daybreak this morning.

The focus will then shift to new thunderstorm development which
is expected to develop just north or potentially right over the
terminals this afternoon. There is still some uncertainty for
timing and location and there is a small chance thunderstorms
will develop south of the terminals. However, the consensus
among the latest guidance is that thunderstorms will develop
over northern IL ahead of an approaching cold front and then
push to the southeast this evening with some isolated thunder
potential remaining through sunset. Opted to trim tempo mention
to just 2 hours, which may still need some refinement as trends
emerge and then include vicinity mention for a longer time
period. Any of the thunderstorms that form today may become
severe with locally strong/erratic winds/gusts. Ifr/lifr cigs
and vis will be possible along with very heavy rain.

Southwest winds will slowly increase early this morning with
some gusts possible by daybreak. Then gusts into the 20kt range
are expected through early afternoon. Winds will turn westerly
ahead of the front and then possibly northerly though will be
impacted by thunderstorms. Models are now showing an easterly
wind for a few hours behind the storms, that settles into a
light northerly wind for the rest of tonight. cms

&&

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

&&

$$

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