Heavy Rain in Illinois, June 20, 2024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Isolated to scattered storms into early evening. Locally gusty
  winds and torrential downpours possible with these storms, but
  organized severe weather is not expected.

- Period of dense fog possible near the Lake Michigan Shore
  tonight.

- Hot and muggy conditions will prevail through Saturday,
  although cooler conditions are expected near Lake Michigan
  today again on Friday.

- Highest chance of thunderstorms over the next 7 days late day
  Saturday and Saturday evening, with the next chance of
  thunderstorms probably on Tuesday.

&&

.DISCUSSION...
Issued at 258 PM CDT Thu Jun 20 2024

Through Friday Night:

The threat of widely scattered thunderstorms will continue across
parts of northern IL through late afternoon before waning into
this evening. Most of this activity is expected to remain
focused in northern IL west of a lake breeze boundary in close
proximately to a west/southwest-to-east/northeast oriented
surface frontal boundary currently sagging southward near the
I-88 corridor. Severe weather is not anticipated with these
storms, owing to the overall weak deep layer shear. However, the
potential exists for some isolated wet microbursts, which could
thus result in some localized instances of strong gusty winds
with any of the stronger storms. Otherwise, the very moist
airmass in place (featuring near 2" PWATs) will also support
torrential downpours with these storms trough early evening.

Cooler onshore flow off of Lake Michigan will continue into
tonight as high pressure remains in place across the northern
Great Lakes. This could allow low clouds and dense marine fog to
bleed inland across some of the collar counties of southern
Lake Michigan after sunset this evening. If this does occur, a
short fused dense fog advisory could be needed, especially for
areas within a few miles of the lakeshore. Confidence on the
inland extent of the fog remains low at this time, though I did
opt to add areas of fog to the forecast database near the lake
for tonight.

Low clouds and fog may persist along the lakeshore areas into
early Friday morning before conditions diurnally improve through
the morning. Otherwise, a frontal boundary stalling out south of
I-80 tonight will begin shifting northward again on Friday as a
surface wave of low pressure shifts east-northwestward into the
Mid-Missouri Valley. Because the front may get hung up along the
southern shores of Lake Michigan for several hours, the surface
flow may remain onshore for a majority of the day across parts of
far northeastern IL, thus resulting in another day of cooler
weather along the IL shores. Farther inland, however, another hot
and humid day is expected. Inland high temperatures are expected
to top out in the low to mid 90s amidst dewpoints well into the
60s. This will result in heat indices up to near 100. The primary
threat for thunderstorms on Friday is expected to largely remain
north of the area into southern WI, though some chance for a few
scattered afternoon storms will exist across far northern IL
(north of I-88).

KJB

Saturday through Thursday:

A developing surface low to our west/northwest on Saturday will
yield a tighter surface pressure gradient across the region.
This will afford us breezy southwesterly winds (gusts up to
30-35 mph) that will help facilitate warm air advection and
push the warmth all the way up to the lakeshore for the first
time in a few days. Saturday's highs are pegged to top out in
the lower to mid 90s (locally upper 90s) across the area, and
with dew points generally expected to span from the mid 60s to
upper 60s F, this will translate to another day of heat indices
peaking in the 95-100 degree range at most locations.

The cold front associated with the aforementioned surface low
will press southeastward on Saturday as the low center
tracks toward the northern Great Lakes, and thunderstorms will
likely develop along it as it does so. These storms will enter
our forecast area from the northwest as early as the mid-late
afternoon, but more likely during the evening, and continue
southeastward into the night, likely weakening and diminishing
in coverage with time as diurnal instability is lost.

Deep-layer shear will be better than it has been the past
several days given the presence of enhanced flow through the
tropospheric column on the southeastern periphery of the
deepening low pressure system, and with pre-frontal MLCAPE
potentially pushing 2000-2500 J/kg, the overall kinematic and
thermodynamic parameter space will likely allow for some of the
frontal convection to become severe. Whether any severe
thunderstorms occur in our forecast area will largely hinge on
the timing of the cold frontal passage.

An earlier frontal passage while diurnal instability is still
plentiful would be more favorable for severe weather prospects
here, while a later frontal passage would see convection arrive
as the boundary layer stabilizes and our supply of instability
has been depleted to some degree, thus keeping the potential for
severe weather minimal. The level 1/5 (Marginal) risk introduced
into the northwest half or so of our CWA in SPC's Day 3
Convective Outlook early this morning still seems appropriate
for now. However, there has been a recent trend in the guidance
toward a bit faster cold frontal approach. Considering that we
will have the latest sunsets of the year through July 2nd (and
associated diurnally higher instability until past sunset),
this would likely put a level 2/5 severe threat in play for
portions of the area and extend low severe probs farther southeast.

A slower frontal passage could also see showers linger around
into Sunday morning in our southeastern counties, but otherwise,
no additional precipitation is expected on both Sunday and
Monday as surface high pressure and a drier post-frontal air
mass settle into the region. Averaged across our forecast area,
both days should generally be cooler and less humid than the
past few days, but high temperatures on both days still look
like they'll end up being slightly above normal for late June,
coolest near the lake. Towards mid-week, a flattened ridge
should build back into the southern Great Lakes and allow for
90+ degree air temperatures to make a return to the area on
Tuesday. The presence of a baroclinic zone in the region could
also present us an additional opportunity to see showers and
storms at times Tuesday-Tuesday night.

The cold front passage Tuesday night will probably shunt the
baroclinic zone and any meaningful convective chances safely
south of the area on Wednesday and Thursday. High temperatures
on Wednesday will be cooler than Tuesday, but likely still above
normal away from the lake (mid-upper 80s in many areas), with
seasonable lower to mid 80s on Thursday. Lakeshore highs are
forecast to be in the upper 70s to around 80F on Wednesday and
lower to mid 70s on Thursday.

Ogorek/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1242 PM CDT Thu Jun 20 2024

The primary aviation weather concerns through the 18Z TAF period
include:

* Isolated to widely scattered thunderstorms this afternoon and
  evening

Light NE winds will continue through the remainder of today.
Isolated to widely scattered showers and thunderstorms are expected
to begin developing during the mid-late afternoon and continue into
the evening. While storms aren't expected to be overly aggressive,
brief heavy downpours and localized gusty winds will be possible.
Thunderstorm chances drop considerably following 00Z, although
pockets of showers may persist late into the evening. Barring
any downpours overhead, conditions look to remain VFR through
the event.

Winds overnight will be light and predominantly NE, variable at
times. There is a modest, but growing, signal for MVFR cigs and
possibly even some fog to move inland off the lake and over the
Chicago sites overnight. With the majority of model guidance holding
onto VFR, kept the TAFs as such. A much stronger and consistent
signal exists over GYY for MVFR, or even IFR, cigs tonight into
Friday morning. Friday should see easterly winds over Chicagoland,
southerly to southwesterly over RFD, near and under 10 kt.

Doom

&&

.CLIMATE...
Issued at 337 AM CDT Wed Jun 19 2024

After breaking a couple of temperature records this week, the
high and warmest low temperature records for both today and
Friday are unlikely to be broken, though Saturday's temperature
records for both Chicago and Rockford could potentially be
threatened.

Here are the daily high and warmest low temperature records
for Chicago and Rockford for each day through Saturday, June
22nd:

           Chicago
-----------------------------
Day:           6/20 6/21 6/22
-----------------------------
Record High:    104  101   97
Record Warm Low: 78   74   76

           Rockford
-----------------------------
Day:           6/20 6/21 6/22
-----------------------------
Record High:    101  100   97
Record Warm Low: 73   71   73

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT Friday night for INZ001-
     INZ002-INZ010-INZ011.

     Air Quality Alert until midnight EDT Friday night for INZ019.

LM...Dense Fog Advisory until 10 PM CDT this evening for the IL and
     IN nearshore waters.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook...CORRECTED
National Weather Service Chicago/Romeoville IL
332 PM CDT Thu Jun 20 2024

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-211645-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
332 PM CDT Thu Jun 20 2024 /432 PM EDT Thu Jun 20 2024/

This Hazardous Weather Outlook is for portions of north central
Illinois...northeast Illinois and northwest Indiana.

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk.
  Limited Flooding Risk.

DISCUSSION...

Isolated to widely scattered thunderstorms are expected this
afternoon. Severe weather is not expected, though the strongest
storms could produce gusty winds and torrential downpours.
Another hot and humid day is expected with peak heat indices
generally in the 95 to 100 degree range. However, cooler
temperatures are expected closer to the Lake Michigan shore.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Limited Thunderstorm Risk north of I-88.
  Limited Excessive Heat Risk away from the lakeshore.
Saturday...
  Limited Thunderstorm Risk...with an associated:
     Level 1 of 5 Severe Thunderstorm Risk.
  Limited Excessive Heat Risk.
Sunday Morning...
  Limited Thunderstorm Risk well southeast of I-55.
Tuesday...
  Limited Thunderstorm Risk.

DISCUSSION...

Peak afternoon heat indices in the 95 to 100 degree range along
with intermittent thunderstorm chances will continue through
Saturday, although cooler conditions are expected near Lake
Michigan on Friday. A few storms could be strong to severe
northwest of Interstate 55 late Saturday afternoon and evening.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east-northeast at 20-25 mph.

LMZ740>745-211645-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
332 PM CDT Thu Jun 20 2024

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Limited Thunderstorm Risk.
Saturday...
  Limited Thunderstorm Risk...with an associated:
     Level 1 of 5 Severe Thunderstorm Risk.
Tuesday...
  Limited Thunderstorm Risk.

KJB

Visit us at www.weather.gov/chicago

Follow us on Facebook, Twitter, and Youtube at:
www.facebook.com/nwschicago
www.twitter.com/nwschicago
www.youtube.com/nwschicago