403
FXUS63 KLOT 221732
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 PM CDT Sat Jun 22 2024

.KEY MESSAGES...

- Potential for strong to severe storms with damaging winds,
  torrential downpours and localized flooding this afternoon
  near the Wisconsin state line and area-wide this evening.

- Breaks in heat and humidity are expected Sunday and Monday,
  and again Wednesday through the end of the workweek.

- Tuesday may feature dangerous heat and/or severe weather in
  the general region.

&&

.UPDATE...
Issued at 1111 AM CDT Sat Jun 22 2024

There are no significant changes to the going forecast thinking
at this time.

In the near term (through early afternoon), a remnant area of
warm air advection driven showers (with a few embedded
thunderstorms north of the WI state line) will continue to shift
eastward along the southern periphery of an MCV over southwestern
WI. While most of this activity will remain north of the WI state
line, current trends suggest that areas north of I-88 could
experience a period of scattered showers through early afternoon.
The threat of lightning with this activity is not zero, but poor
mid-level lapse rates is expected to keep this threat rather low
and more isolated, at least for the next few hours.

Otherwise, as we head through this afternoon, diurnal heating of a
high theta-e airmass (most prominent across IA into far
northwestern IL where surface dewpoints are in currently in the
lower 70s) will set the stage for more robust thunderstorm
development along and ahead of an approaching cold front. This
activity looks most favored to onset after 2 pm this afternoon
northwest of the area across parts of northeastern IA into far
southeastern WI. Effective deep layer shear in this region will be
supportive of supercells with an attendant threat for a couple
tornadoes.

With time, the primary threat with these storms is expected to
transition to damaging winds into this evening as congealing
outflows from developing storms begin to favor east-southeastward
organized clusters across much of northern IL. Rather poor mid-
level lapse rates in combination with a very high PWAT airmass (up
to 2.25") suggests the threat of hail will remain rather low
outside of any well developed supercell structures. Aside from the
damaging wind threat, the near record high PWATs will also
support instances of extremely heavy rainfall with the storms,
with rain rates of 1 to 2"+ per hour likely. For this reason,
even a brief period of training storms could result in some
localized 2 to 3"+ amounts before the storms sag southward
through the evening. While we still cannot rule out the need for a
short fused flash flood watch for parts of the area, we would
like to hold off at this time and monitor the mesoscale trends as
the event unfolds later today. However, we will be issuing an ESF
(hydrological outlook) for areas generally along and north of
I-80 in IL to highlight the threat of significant rises on
streams and rivers and possible localized instances of flash
flooding.

The heavy rain and severe weather threat will gradually end from
northwest to southeast later this evening into the overnight as
the clusters of storms sag southeast out of the area.

KJB

&&

.DISCUSSION...
Issued at 335 AM CDT Sat Jun 22 2024

Through Tonight:

Ongoing small clusters of convection across northern Illinois
early this morning have persisted for a few hours in response
weak low-level WAA within a 5-7kft layer of higher RH. Given
only a shallow layer of nocturnal stability, the strongest cores
have managed to generate locally strong surface winds. Expect
this activity to continue through around sunrise north of the
I-90 corridor.

In the absence of any appreciable convection into our area
today, we are well in line to have another hot and breezy day.
High temps in the mid 90s with SW gusts of 30 mph to possibly 35
mph are likely.

We will need to closely monitor the trajectory of a well-formed
MCV just north of Omaha early this morning as it tracks ENE
toward far southern Wisconsin early this afternoon. The far
northwest CWA will reside within a strong theta-e gradient east
of a mostly uncapped and increasingly unstable airmass across
eastern Iowa this morning. Broad mid-level steering flow from
the WNW should keep the MCV north of the CWA, though a
substantial swath of WSW to ENE convection over northern Iowa
may focus a formidable boundary and turn the MCV more easterly
along the strengthening theta-e gradient. Additionally, 70+
degree dew points across western Iowa will begin advecting into
the northwest CWA this morning. So while the potential for
convection across the northern CWA during the day is quite
conditional, have opted to bring low-end (15-30%) thunder
chances as far south as a Mendota to Chicago line in the
11am-3pm window. Less favorable lapse rates this far east will
limit the severe potential, but sufficient effective shear will
still support an isolated severe threat with large hail and
gusty winds. To further add to this, stout SW low-level flow
with moisture advection could result in continued generation of
convection on the tail end of the MCV and lead to localized
flooding mid to late afternoon across the far northwest CWA.

Organized deep-layer forcing with the arrival of the mid-level
wave and surface front should generate a more organized line of
strong to potentially severe convection across the area this
evening. Strong deep-layer shear along with increasing MLCAPE
values and corresponding high DCAPE values will support some
discrete supercell structures congealing into a gradual SSE
moving line with damaging winds. Expectations are that the
convection may become outflow- dominant by mid to late evening
as low-level shear vectors become oblique to the line and
consequently less favorable for balanced linear convection. With
that said, outflow winds ahead of any convection would still be
capable of producing very strong gusts.

A corridor of impressive PWATs surpassing 2" and potentially as
high as 2.3" will feed into developing convection across
northern Illinois early this evening. While deep-layer and
propagation vectors suggest that convection should be
progressive this evening into the early overnight hours, intense
rainfall rates over any period of training convection will be
capable of producing local instances of flash flooding. A Flash
Flood Watch may ultimately be needed for areas around and north
of I-80 this afternoon and evening if convection north of the WI
state line begins to drift southward (i.e. regeneration behind
MCV) or the main line of convection is trending slower than
currently forecast.

Kluber


Sunday through Friday:

In the wake of a cold front, Sunday will be noticeably cooler and
less humid with highs in the low to mid 80Fs. A sharp upper-level
shortwave will dive southeast across the Great Lakes but should
remain north of our area. Hence, Sunday looks dry. Upper-level
ridging will start to build into the region Monday causing
temperatures to rebound into the upper 80s to around 90F. A weak
surface pressure gradient (thanks to the passage of a surface
high) will allow for a lake breeze to surge inland, so lakeshore
locations should be cooler with highs topping off in the low to
mid 70s. All in all, Sunday and Monday continue to look
pleasant, relatively speaking.

Tuesday into Tuesday night continues to look like a period of
interest with respect to both the chances for severe weather as well
as a brief stint of dangerous heat. With the core of upper-level
ridging and an associated pool of 850mb temperatures >20C moving
overhead, temperatures are poised to skyrocket into the low to mid
90Fs. Ensemble model guidance also advertises a rapid increase in
column moisture partly thanks to increasingly breezy southwest
winds, with mean PWATs climbing above 1.75" and surface dew points
rising into the low to mid 70Fs. Taken together, Tuesday appears to
be the first day this summer where widespread heat indices of 100-
105F+ are possible, including up to the Lake Michigan shoreline.

Now, anytime we deal with heat indices above 100F, we often have to
consider the threat for severe convection. Big-picture wise, our
area will be pretty far removed from the upper-level jet located
along the US/Canadian border. However, upper-level shortwaves
embedded in the jet to our north should sideswipe the deep reservoir
of instability across the Upper Mississippi River Valley, which may
provide the means for several outflow-dominant MCSs to parade into
the Great Lakes (perhaps starting Monday night, lasting through
Tuesday night). A southward-moving cold front may also act as a
trigger for severe thunderstorms Tuesday evening, though our area
may be fairly capped in the absence of large-scale forcing. (In
other words, we may need an MCS with a deep cold pool to activate
the deep reservoir of instability). Even with the questions of how
and when thunderstorms may happen, it`s worth noting that nearly all
the individual GEFS/EPS members show QPF sometime between Monday
night and Tuesday night, which is a pretty strong signal for
convection in this type of pattern. Moreover, GEFS-based ML severe
guidance is fairly bullish with Level 3/5-caliber severe weather
probabilities already advertised across our area. For now, our
official forecast will feature broad-brushed 40 to 60% PoPs in the
Tuesday to Tuesday night timeframe in favor of refinements going
forward. Needless to say, any thunderstorms (or associated southward-
moving outflows) on Tuesday would complicate the forecast for
temperatures.

Wednesday through the end of next week, ensemble model guidance
favors a pool of relatively cool low-level air spilling into the
Great Lakes region leading to lowering humidity levels and near
seasonable temperatures. There is a weak but growing signal in
ensemble guidance for another period of hot, humid, and stormy
conditions next weekend.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1232 PM CDT Sat Jun 22 2024

Key Aviation Impacts for the current TAF period:

- Southwest winds gusting up to 30 knots through this evening

- A line of thunderstorms is expected to move over the airspace
  tonight through early Sunday morning. Downpours and strong
  wind gusts are the main impacts

- MVFR cigs are expected to move in with the squall line and
  continue to lower behind the line, with the slight chance for
  IFR cigs.

Other than some VFR showers around KRFD at the time this
discussion was published, main impacts through 00Z will be from
strong southwesterly wind gusts. Observations have been
consistently between 25 to 30 knots, which is expected to
continue through the afternoon; however the occasional gust
just greater than 30 knots cannot be completely ruled out though
not expected to be prevailing.

A squall line is still expected to drop southeastward over
Northern Illinois this evening. Timing has wavered slightly but
it still looks like it should arrive around KRFD just before
01Z, then by 02Z to the Chicago terminals. Warm temperatures and
sufficient moisture should make highly efficient rain rates
resulting in downpours and reduced vis and MVFR cigs as the line
passes over. Strong northwesterly winds are expected along the
front with gusts 25-35 knots.

Winds become westerly behind the line and diminish briefly
before gusts up to 20 knots return. Higher confidence in MVFR
cigs remaining through mid Sunday morning, but there remains a
weak signal on the potential for IFR cigs. There was not enough
confidence to add to the TAF currently, but left the SCT
sub-1000 ft group in TAF. Sunday should be VFR with
northwesterly winds around 15 knots, potentially with the
occasional gust up to 20 knots.

DK

&&

.CLIMATE...
Issued at 335 AM CDT Sat Jun 22 2024

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

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

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

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 AM CDT Sunday for the IL and IN
     nearshore waters.

&&

$$

Visit us at weather.gov/chicago