970
FXUS63 KLOT 201744
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1244 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

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

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

- Periodic chances for thunderstorms will continue, though most
  hours will remain dry.

&&

.UPDATE...
Issued at 1105 AM CDT Thu Jun 20 2024

The threat of widely scattered thunderstorms continues to be
the primary focus this afternoon into early this evening. This
will particularly be the case to the south/southeast of the area
of cloud cover currently draped from eastern IA northeastward
across far northwestern IL into southern WI. Diurnal heating of
a very moist low-level airmass (dew points near 70) in this
region is expected to weaken surface convective inhibition this
afternoon as a short wave impulse moves eastward into northern
IL. This is thus expected to foster at least some widely
scattered thunderstorm development after about 2 pm this
afternoon. While isolated storms will be possible across most of
northern IL into northwestern IN later today, most of the
activity may tend to become more focused in close proximately to
a sharpening west-to-east oriented surface frontal boundary
near the I-80 to I-88 corridors, and west of a lake breeze
boundary.

Widespread severe weather is not anticipated with these storms
this afternoon, owing to the overall weak deep layer shear.
However, the potential exists for some wet microbursts, which
could thus result in some isolated instances of strong gusty
winds with any of the stronger storms. A very moist airmass
(featuring near 2" PWATs) will also support torrential
downpours with these storms this afternoon. The threat of
storms will then diminish through the evening.

KJB

&&

.DISCUSSION...
Issued at 337 AM CDT Thu Jun 20 2024

Through Friday:

A cold front has moved into the CWA and brought light northerly
winds to the northwest half of the area. Through the morning
hours this front will retreat back to the north, setting up
along the IL/WI state line. A backdoor cold front is also
expected to advance down Lake Michigan and approach the area
this morning. The backdoor front/lake breeze will likely be
held up just inland, but will surge southward across northwest
IL and northern IN early this afternoon into this evening.
Temperatures behind the backdoor front/lake breeze are much
cooler than out ahead of it, so the timing of the front could
impact high temperatures today. Areas that are unaffected by the
backdoor front/lake breeze through roughly 18Z will see
temperatures climb into the low 90s, bringing maximum heat
indices into the mid 90s.

Behind the backdoor front/lake breeze, conditions appear
favorable for marine fog to form this morning. This will
decrease visibility over the lake and may seep slightly inland
through the morning, but is not expected to persist inland into
the afternoon.

The presence of both of these fronts brings a chance of showers
and storms across the area this afternoon. Highest coverage
will likely be along and north of I-80 where a band of 1000-1500
J/kg of MLCAPE is expected to build. Where all of these
boundaries converge may also serve as a prime location for
convective development. Any storms that are able to form here
will likely move into the Metro area. Organized severe weather
is not expected, although some storms may be able to produce
gusty downburst winds. Coverage will be highest through the
afternoon with some storms lingering into the evening hours,
however, the overnight hours are expected to be dry.

Friday looks to be a fairly quiet day. A shortwave will move
across Wisconsin bringing a chance of showers and storms into
the far northern portion of the CWA, however, most areas should
remain dry. Temperatures will return to the low-to-mid 90s
except along the lakeshore where a lake breeze will keep things
cooler. This will create heat indices in the mid-to-upper 90s
away from the lake.

Carothers


Friday Night through Wednesday:

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 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 currently pegged to top out in the low to mid 90s
across the area, and with dew points generally expected to span
from the mid 60s to near 70F, 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
approaches Lake Superior, 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 Marginal (level 1/5) Risk
introduced into the northwest half or so of our CWA in SPC`s Day
3 Convective Outlook seems appropriate for now given the
current distribution of frontal passage timing in the latest
available deterministic and ensemble guidance.

A slow 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.
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 or two to see showers and storms
mid-week next week.

Ogorek

&&

.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.

&&

$$

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