956
FXUS63 KLOT 180138
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
838 PM CDT Mon Jun 17 2024

.KEY MESSAGES...

- Hot and muggy conditions will prevail through the end of the
  week.

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

&&

.DISCUSSION...
Issued at 255 PM CDT Mon Jun 17 2024

Through Tuesday Night:

Isolated thunderstorms will be possible through sunset. The
focus appears to be a weak boundary lifting north, currently
between I-55 and I-57. As this boundary lifts north, isolated
thunderstorms will continue to be possible. Otherwise, it
remains hot with temps in the lower/mid 90s. Southwest winds
gusting as high as 30 mph have allowed dewpoints to mix out, in
some areas into the lower 60s. This is keeping the heat index
values generally in the mid/upper 90s with a few locations
tagging 100.

Winds will diminish with sunset this evening but may remain
gusty at times, especially in the metro area as low level winds
increase this evening. Combined with dewpoints likely rising
back into the upper 60s/lower 70s, low temps tonight will likely
be in the mid 70s for most locations, upper 70s/near 80 for
Chicago.

The next chance for thunderstorms will be overnight through
mid/late morning Tuesday as a weak wave lifts north across the
area. Confidence remains low for coverage, especially at that
time of day and its possible precip may be in form of showers,
vs thunderstorms. Kept pops in the slight chance (20%) range for
now but as trends emerge tonight, these may need to be
increased. Isolated thunderstorms are possible again Tuesday
afternoon, but could be dependent on morning coverage and
then most of Tuesday night looks dry for now.

Did not make any changes to high temps on Tuesday, which is
mainly lower 90s areawide. However, these are generally above
most guidance highs. Possible cloud cover and precip in the
morning, if it occurs, could keep high temps lower. Though if
clouds clear out during the afternoon, highs may still reach the
lower 90s. Southwest winds gusting into the 25-30 mph range may
once again keep dewpoints in the 60s with heat index values only
in the upper 90s. cms

Wednesday through Monday:

The focus mid to late week remains the continued hot and humid
conditions and periodic shower and thunderstorm chances. The
amplified upper level ridge across the eastern CONUS will begin
to flatten out some across the Great Lakes region with the heat
dome still locked in place. Forecast high temperatures Wednesday
through Saturday are in the mid-upper 90s which is 10-15
degrees above the 30-year normals for this time of year. Dew
points likely mix out enough each day to keep our max heat
indices in check in the 95-100 degree range which remains below
our local heat headline criteria. However, given this is the
first prolonged stretch of heat this year we will continue
messaging heat safety related information. A backdoor front
looks to keep wind directions onshore Thursday and Friday which
will likely hold high temperatures near the lake in the 80s.

As for shower and thunderstorm chances, a stalled surface
boundary will be near to just northwest of the area as early as
Wednesday morning that could serve as a focus for thunderstorm
development through Wednesday, particularly northwest of Chicago
toward the Rockford area. Given lingering uncertainties with
the timing and placement of this boundary precip chances remain
capped at 30-50% for areas northwest of I-55. For areas
southeast of I-55 isolated showers and thunderstorms can`t be
ruled out each afternoon.

A shortwave moving across the Northern Plains/Canadian border
and associated surface low will eventually swing a cold front
across the area during the Saturday night into Sunday period.
This will bring shower and thunderstorm chances and at least a
temporary reprieve from this current heat wave with highs back
in the 80s early next week.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 640 PM CDT Mon Jun 17 2024

Ongoing showers and thunderstorms near GYY should continue to
move offshore and away from the terminals over the next hour,
though trends (including any signs of redevelopment of
thunderstorms out of cumulus clouds extending along Interstate
80 upstream of DPA/ORD/MDW) will be watched with an eagle eye
over the next few hours. Otherwise, breezy southwest winds at
press time should ease in magnitude after sunset, though
occasional gusts up to 20kt may prevail through the overnight
hours. Scattered high-based cumulus should become replaced by
thickening upper-level clouds overnight.

Toward or just after daybreak, an MCV currently located near
the bootheel of Missouri will arrive over northeastern Illinois
or northwestern Indiana. At least scattered showers but
possibly also scattered thunderstorms may accompany the MCV,
though confidence in the exact track of the circulation, let
alone coverage of any shower or thunderstorm, is not high enough
to warrant any mention of such in the outgoing TAF package. In
addition, local pockets of MVFR cloud cover may accompany the
MCV, but confidence is likewise not high enough to advertise
more than a period of SCT025. Adjustments to the TAF at 03Z or
06Z may be warranted as the circulation lifts through central
Illinois and forecast confidence increases in where it will go.

Southwest winds will increase in magnitude after sunrise and
gust 25-30kt through the morning. Depending on the thickness of
cloud cover tomorrow, rising mixing heights may allow for a
period of wind gusts over 30kt tomorrow afternoon. Finally, like
today, isolated thunderstorms cannot be ruled out in the
afternoon with a probability of occurrence at any given point
somewhere around 20% or lower.

Borchardt

&&

.CLIMATE...
Issued at 441 AM CDT Mon Jun 17 2024

Several temperature records will be threatened over the next few
days. The records that appear to be in greatest jeopardy are
today`s (June 17) record high for Chicago and several record
warmest low temperatures for both Chicago and Rockford through
Saturday, June 22nd.

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

                  Chicago
--------------------------------------------
Day:           6/17 6/18 6/19 6/20 6/21 6/22
--------------------------------------------
Record High:     96   98  102  104  101   97
Record Warm Low: 78   77   78   78   74   76

                  Rockford
--------------------------------------------
Day:           6/17 6/18 6/19 6/20 6/21 6/22
--------------------------------------------
Record High:     99   99   99  101  100   97
Record Warm Low: 75   74   76   73   71   73

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
     ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...None.
LM...None.

&&

$$

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