646
FXUS63 KLOT 131159
AFDLOT

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

.KEY MESSAGES...

- Strong storms this morning along and north of a Rockford to
  Chicago line. Hazards include lightning, small hail, and gusty
  winds.

- 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.

&&

.UPDATE...
Issued at 655 AM CDT Thu Jun 13 2024

The forecast has been updated to account for a cluster of
strong storms that have persisted into northern Illinois early
this morning, mainly areas along and north of a Rockford to
Chicago line. Small hail to nickel size and gusty winds to 40
mph can`t be ruled out. These storms could persist as far
southeast as downtown Chicago over the next 1-3 hours.

Petr

&&

.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 /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 647 AM CDT Thu Jun 13 2024

Forecast concerns include...

Scattered thunderstorms this morning with possible wind shift.
Scattered strong/severe thunderstorms this afternoon.
East/northeast winds late this afternoon into this evening.

Scattered thunderstorms have continued to develop across
southwest WI early this morning and are now moving into far
northern IL. This activity is expected to continue southeast
and will move across the Chicago terminals over the next few
hours. A northerly wind shift is possible as well with this
activity. Further development is possible on the southern flank
of this activity, as it moves to the east/southeast.

How this current activity may affect this afternoon`s convection
is unclear. There is another upper wave that will be approaching
and thunderstorm development still looks on track. The question
now will be if it develops south of where this current activity
moves. Trends will need to be monitored this morning into early
this afternoon, but for now, opted to make no changes to the
current forecast. Thunderstorms this afternoon into this evening
will likely be strong to severe with gusty/erratic winds.

If winds turn northerly this morning and assuming they turn back
southwesterly this morning, they`ll likely gust into the 20kt
range through early afternoon and then turn more westerly,
possibly northwesterly. Depending on storm coverage/location,
winds are expected to turn back north/northeasterly late this
afternoon into this evening and then should turn back to a light
north or north/northwest direction this evening. cms

&&

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

&&

$$

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