783
FXUS63 KLOT 171955
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 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.
en 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 we will continue to message the
importance of taking frequent breaks if spending prolonged time
outdoors and to stay hydrated. Don`t forget to check on
neighbors and family that may be especially vulnerable to the
heat. 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
which 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.
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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1230 PM CDT Mon Jun 17 2024

Forecast concerns include...

Isolated thunderstorms.
Gusty southwest winds.
Possible low level wind shear tonight.

Isolated thunderstorms developed south of the terminals this
morning and are expected to continue this afternoon and remain
south of the terminals, including GYY. However, there is still a
chance of isolated thunderstorms across the entire area through
sunset but no mention in this forecast due to low coverage and
uncertainty for location.

Another weak wave may bring some showers and isolated thunder to
the area from the predawn hours through mid morning Tuesday.
Confidence is also too low for any mention with this forecast.
There is also some potential for mvfr cigs Tuesday morning and
these would be dependent on precip coverage. For now, maintained
scattered mention.

Southwest winds have been gusting into the mid 20kt range so far
and gusts into the mid/upper 20kt range will likely continue
through sunset. Low level winds are expected to increase through
the night and this should allow some gusts to continue at the
surface but if not, then mention of low level wind shear may be
needed. Southwest winds will gust into the mid 20kt range again
Tuesday. cms

&&

.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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