559
FXUS63 KLOT 180546
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 AM CDT Tue Jun 18 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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1243 AM CDT Tue Jun 18 2024

Satellite imagery shows a MCV moving north through Illinois
toward the area. This system has MVFR ceilings and light
showers associated with it. It will be arriving in northern
Illinois/Indiana around daybreak this morning. Widespread MVFR
ceilings look more likely in the vicinity of KRFD with periodic
MVFR ceilings likely at KORD, KMDW, KDPA, and KGYY. Light
showers should stay in northeastern Illinois and northwestern
Indiana as the system tracks to the NE. Questions still remain
about the exact timing and longevity of these showers as the
reach KORD, KMDW, KDPA, and KGYY around 12Z, so have opted for a
TEMPO group rather than a prevailing at this point. Ceilings
will rise this afternoon as the MCV lifts out of the area and
low-level moisture mixes out.

Winds will remain gusty out of the SSW overnight tonight. This
will continue today as winds increase to around 20 kts with
gusts up to 30 kts due to a tight pressure gradient. Similar to
yesterday, there is a slight chance (15-20%) for isolated storms
this afternoon. The pressure gradient will finally weaken
tomorrow night around 06Z allowing winds to back down and gusts
to fall off.

Carothers

&&

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

&&

$$

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