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772
FXUS63 KLOT 212005
AFDLOT

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

.KEY MESSAGES...

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- 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.
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&&

.DISCUSSION...
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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
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&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1246 PM CDT Fri Jun 21 2024

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

* Uncertainty regarding a lake breeze and associated wind shifts
  this afternoon

Early this afternoon, we find a lake breeze sitting on the eastern
doorstep of MDW. It`s struggled to make much headway in recent hours
since the opposing SW flow has ramped up a bit. Confidence remains
rather low as to whether the boundary will make it across MDW,
although most recent model guidance thinks not. Models have backed
off on the idea that the lake breeze will make it to ORD and have
accordingly removed any mention of an easterly shift from the TAF,
although it`s still not out of the question that the lake breeze
surges across ORD later this afternoon. On the warm side of the lake
breeze, SW winds near 10 kt will continue gusting into the teens
while lighter easterlies can be found behind the front.

A complex of showers and thunderstorms will move across southern
Wisconsin this evening and tonight. A few showers may find
themselves over RFD at times, although low chances kept a precip
mention out of the TAF. It`s even possible that a little bit of
light rain makes it over the Chicago sites overnight, though odds
are better at RFD.

SW winds will build through the morning on Saturday achieving gusts
to 25 to 30 kt by mid-late morning and continuing through the
afternoon. There is a signal to watch for a period of even higher
gusts in the late afternoon into the evening as a system of showers
and storms moves into the area, slated for just beyond the current
30-hr period.

Doom

&&

.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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