-- Discussion containing changed information from previous version are highlighted. --
855
FXUS63 KLOT 220023
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
723 PM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Potential for strong to severe storms with damaging winds,
  torrential downpours and localized flooding late Saturday
  afternoon through Saturday evening.

- A brief reprieve in heat and humidity is expected Sunday and
  Monday, before it returns (with chances for thunderstorms) on
  Tuesday.

&&

.DISCUSSION...
Issued at 305 PM CDT Fri Jun 21 2024

Through Saturday night:

The remainder of the afternoon will remain very warm and humid,
especially away from the cooling influences of Lake Michigan.
Expect low to mid 90s inland, with 70s to low 80s closer to the
lake. Capping is expected to curtail thunderstorm develop across a
majority of the area through the remainder of the. The only
exception being areas across far northern IL near the WI state
line, where capping is weaker thanks to a corridor of higher
surface dew points around 70. Accordingly, a few isolated showers
and storms cannot be ruled out in this area late this afternoon
into early evening. However, expect the better focus for these
storms to largely remain north of the WI state line into this
evening.

Warm air advection will increase tonight along an strengthening
southwesterly low-level jet across much of the Corn Belt, as
low pressure shifts out across the Mid-Missouri Valley. This is
expected to foster an significant increase in the areal coverage
of elevated thunderstorms tonight, particularly across IA
eastward across southern WI. While the primarily focus for these
storms will be north of the area tonight, it does appear that a
period of showers and some scattered storms may develop into
northern IL after midnight tonight. Severe weather is not
anticipated with these storms, though a high PWAT environment
(values around 2") will support the potential for some torrential
downpours with any storms.

Another very warm (hot) and humid day is on tap for the area on
Saturday, as temperatures areawide top out in the low to mid 90s.
However, unlike today, offshore gusty southwest winds up to 30
mph on Saturday will curtail any chances for lake cooling near
the shore, so expect these very warm conditions to extend all the
way to the lake.

The threat of thunderstorms looks rather low during most of the
day on Saturday as we await the arrival of an upper trough and
surface cold front expected late Saturday into the evening. While
this is the case, severe storms are expected to develop to our
west-northwest Saturday afternoon as the moist airmass in advance
of the approaching cold front destabilizes through the day.
A favorable kinematic environment with effective shear values up
to 40 kt will support organized storm structures and clusters
capable of producing strong damaging wind gusts. Damaging wind
gusts are likely to remain the primarily threat with these storms
as they shift eastward across southern WI and northern IL Saturday
evening. Rather poor mid-level lapse rates in combination with a
very high PWAT airmass (up to 2.25") suggests the threat of hail
will be rather low outside of any well developed supercell
structures. Torrential downpours will also accompany these storms
given the near record PWATs. Accordingly, even short instances of
training storms could result in localized corridors of 2-3"+
amounts and possible flooding. This will especially be problematic
if it occurs over the urban areas in and around Chicago and
Rockford.

The arrival of these storm clusters could be as early as ~5pm
Saturday afternoon across northwestern parts of the area
(including the Rockford area), and as early as 6 or 7 pm in and
around the Chicago metro area. This threat will then come to an
end from northwest to southeast later in the evening.

KJB


Sunday through Friday:

Behind the frontal passage early Sunday morning, temperatures and
humidity finally turn more seasonal, with highs back in the 80s
Sunday and mid 80s to 90 Monday...with upper 70s to low 80s along
the lakeshore. Northwest winds arrive on the backside of the frontal
system, lingering into the start of the new work week, before sfc
high pressure drifts over the Great Lakes. While this sfc high
drifts into the region, a rapid moving upper level low will track
across the northern Great Lakes into the ne CONUS. With sfc high
pressure around during this timeframe, limited moisture at the sfc
should prevent any precip from developing. The sfc high will then be
ushered quickly to the mid-Atlantic coast Tuesday as the next
frontal system deepens across the upper Midwest.

Southwest winds return Tuesday ahead of the main frontal boundary,
bringing a brief return of hot and humid conditions to the region.
Heat indices are currently forecast in the upper 90s to low
100s, with high temps in the mid 90s. The low pressure system
driving this frontal boundary will track eastward across
southern Canada, while the trailing boundary will drape through
from the Great Lakes into the eastern Plains. Dependent on
moisture axis flowing into the region during Tuesday, could
produce early pre-frontal showers and storms across portions of
the Midwest, with focus on southern Great Lakes into the mid-
Mississippi River Valley. The main wave of showers and storms is
forecast to track through the area late Tuesday evening into
early Wednesday morning. Some model depictions are showing a
weak (almost non-existent) upper level wave developing as this
system moves out of the southern Great Lakes into the Ohio River
Valley region, with a slow southward progression Wednesday
could occur. This would linger precipitation chances through
Wednesday morning for areas south of the I-80 corridor.

After the front pushes out of the region, the remainder of the week
looks dry with seasonal conditions prevailing across the region.

Baker

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
-- Changed Discussion --
Issued at 723 PM CDT Fri Jun 21 2024

- Gusty southwest winds expected Saturday
- Thunderstorms expected across northern IL Saturday evening

Generally VFR conditions expected tonight into Saturday morning.
Southward moving outflow boundary over southeastern Wisconsin is
expected to slow down and dissipate with sunset, likely before
reaching ORD. For that reason, opted to hold off on a wind
shift to northeast, but this will be monitored closely the next
couple of hours.

Southwest winds will increase and become gusty by mid-morning
Saturday. The gusty southwest winds are expected to continue
into the evening hours Saturday as a cold front approaches from
the northwest. TSRA expected to develop along/ahead of this cold
front, likely not affecting ORD and MDW until after 00z Sunday
(Saturday evening).

- Izzi
-- End Changed Discussion --

&&

.CLIMATE...
Issued at 337 AM CDT Fri Jun 21 2024

Here are the daily high and warmest low temperature records
for Chicago and Rockford for today and tomorrow:

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

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

NWS Chicago

&&

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

     Air Quality Alert until midnight EDT tonight for INZ019.

LM...None.

&&

$$

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