500
FXUS63 KLOT 191546
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1046 AM CDT Wed Jun 19 2024

.KEY MESSAGES...

- Hot and muggy conditions will prevail through the end of the
  week, although cooler conditions are expected near Lake
  Michigan on Thursday and Friday.

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

&&

.UPDATE...
Issued at 1045 AM CDT Wed Jun 19 2024

Another hot day is in store today with temperatures already in
the mid-upper 80s. Expect highs to top out in the low to mid 90s
with peak heat indices in the 95-100 degree range.

Made a few minor updates to the shower/thunderstorm trends
through this evening, mainly to delay the onset of storms to
to after 12 PM CDT when most guidance indicates the lingering
weak capping inversion will begin to erode. Still not a lot of
confidence in overall thunderstorm coverage in addition to a
conditional threat of strong, gusty winds.

There are two forcing mechanisms for thunderstorm initiation we
are monitoring late this morning. The first of which being a
very weak 700 mb vort lifting north out of central Illinois,
coincident with an area of cumulus development. This feature may
provide just enough forcing to initiate a few isolated storms
this afternoon, earliest south of I-80 and then into the Chicago
metro by mid afternoon.

A second area for potential thunderstorm development is tied to
an existing cold front slowly approaching from the northwest,
currently extending just north of a line extending from Cedar
Rapids, IA to Madison, WI. The front will likely reach the
Rockford area by late afternoon/early evening. Have maintained
a 30-40% chance for showers and storms north of a Mendota to
O`Hare line to account for this. PWATs upwards of 1.75"
combined with effective shear of 20-25kts would suggest precip
loaded downbursts could produce isolated stronger wind gusts to
50 mph if storms are able to develop in this area.

Given generally weak capping in place during the afternoon and
evening hours any subtle surface convergence axes could serve
as a lifting mechanism for isolated storms today and accordingly
will maintain at least a 20% chance for the rest of the area.
Most areas will likely remain dry today, however.

Petr

&&

.DISCUSSION...
Issued at 333 AM CDT Wed Jun 19 2024

Through Thursday Night:

Convection over Iowa has pushed an outflow boundary out to the
east and it is currently entering the northwest portion of the
CWA. There are some light showers along this boundary, however,
they have been weakening as they move east. There is a slight
chance (15-20%) that these showers make into the far
northwestern CWA, however, this boundary is not expected to
initiate any new convection as it gets into the area.

The cold front that has forced this convection is draped across
Minnesota and Iowa and will continue to advance to the east
through the morning. This front will stall out in northwest IL
later this morning just northwest of the CWA. A thin band of
instability will build just ahead of the front through the early
afternoon with MLCAPE values of 1000-1500 J/kg expected. Large
scale forcing is fairly weak. The CAMs suggest that the highest
coverage of storms will be northwest of the area this afternoon
where there is more convergence along the front, although it is
possible that some showers and storms may form, generally along
and north of I-80 and in particular northwest of I-55. Any
storms that are able to form could produce localized gusty
downburst winds, however, organized severe weather is not
expected.

Hot and humid conditions will once again be in play today as high
temperatures are expected to rise into the low-to-mid 90s. Dew
points have rebounded into the upper 60s and low 70s overnight
but should slowly mix out into the mid-to-upper 60s throughout
the day today. A lake breeze will develop this afternoon,
however, southwesterly winds will keep it hung up along the
immediately lakeshore for the most part.

Model guidance continues to show a signal for a backdoor cold front
pushing down Lake Michigan early Thursday morning. It will have an
easier time racing across Wisconsin, but will likely get stuck
along the lakeshore in Illinois. It will slowly work inland
Thursday before a reinforcing push finally kicks it through the
area. This front will limit high temperatures along the
lakeshore to the upper 70s and low 80s, while areas inland will
still reach the low-to-mid 90s in the early afternoon before
falling back into the 80s into the evening. If the front is able
to surge further inland Thursday morning, this will limit
high temperatures away from the lake as well. It is also
possible that some marine fog is able to develop as the front
moves over the lake. This could push slightly inland and into
downtown Chicago and other areas along the lakeshore Thursday
morning reducing visibility.

The stalled/quasistationary cold front will once again bring a
chance of showers and storms to the area on Thursday. The
highest coverage will be along and north of I-80. It`s possible
the lake breeze acts as another focus for storm development
across parts of the Chicago metro. Organized severe weather is
not expected with these storms.

Carothers


Friday through Wednesday:

Friday looks to be another very warm and humid day as Thursday`s
cold front returns northward as a warm front. High temperatures
look to largely end up being in the upper 80s to mid 90s, and
with dew points generally favored to range from the mid 60s to
low 70s, Friday`s heat indices should once again top out at
around 95 to 100 degrees. The main exception to this will once
again be our lake-adjacent locales, where onshore flow will
keep temperatures and heat indices lower. The ridge that has
largely been responsible for the recent heat in the eastern
third of the CONUS will be in the process of retrograding
westward, and come Friday, its vertical axis will likely be near
or at our longitude, which should maximize the degree of large-
scale subsidence over our CWA. Convective activity developing
in the frontal zone atop the ridge thus looks like it should
remain displaced to our north on Friday, though the frontal zone
may still be in close enough proximity to northern Illinois for
some convection to potentially sneak in here, so have left the
NBM`s 15-30% PoPs for locales north of I-88 untouched for now.

A developing surface low to our west/northwest will yield a
tighter surface gradient across the region on Saturday. This
will afford us breezy southwesterly winds that will help advect
warmer air into the area and likely result in Saturday`s
temperatures generally being a few degrees warmer than Friday`s
in spite of increasing mid- to high-level cloud cover. These
southwesterly winds will also push the warmth all the way up to
the lakeshore for the first time in a few days. Peak heat
indices will again be influenced to some extent by how much dew
points get mixed out, but based on how the past two days have
played out in the dew point department, Saturday`s heat indices
will probably once again peak in the 95-100 degree range at most
locations. The cold front associated with the aforementioned
surface low will press southeastward as its parent low
approaches Lake Superior, and thunderstorms will likely develop
along it as it does so. These storms will likely enter our
forecast area from the north/northwest late in the day or during
the evening on Saturday and continue southeastward into the
night, likely weakening and diminishing in coverage with time as
diurnal instability is lost.

We should get a bit of a break from the recent heat and humidity
on Sunday and Monday as the post-frontal air mass settles in,
though high temperatures on both days still look like they`ll
end up being near to slightly above normal for late June.
Towards mid-week, a flattened ridge should build back into the
southern Great Lakes and allow for 90+ degree air temperatures
to make a return to the area. An active-looking baroclinic zone
setting up somewhere in the region during this time could also
end up bringing us an additional opportunity or two for showers
and storms.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 652 AM CDT Wed Jun 19 2024

Key Messages:

- Chance for isolated to widely scattered thunderstorms this
  afternoon.

- Northerly/northeasterly wind shift this evening with some
  potential for MVFR ceilings afterwards.

Isolated to widely scattered thunderstorms may develop this
afternoon, though uncertainty remains regarding where they will
develop, or even if they will develop at all as large-scale
forcing for ascent is relatively weak and any surface
boundaries in the area will likely be diffuse/subtle. If storms
develop, they will likely be "pulse"-like in nature, collapsing
and producing gusty winds soon after they pop up with new
storms developing along the outflow boundaries spit out by the
preceding storms. Generally speaking, shower/storm chances still
appear to be highest near RFD and lower the farther southeast
one goes, but due to the existing uncertainties, have maintained
the going PROB30 groups for TSRA for one last TAF package
everywhere except for GYY.

A northerly/northeasterly wind shift is expected late this
afternoon or evening at all TAF sites. The timing of this wind
shift remains somewhat uncertain as it is unclear whether this
wind shift will be caused by a true cold front later in the
evening or whether a composite outflow boundary serving as an
effective cold front will cause this wind shift to occur a few
hours earlier. Either way, after a brief uptick in winds
immediately behind the boundary, winds should become light and
remain that way overnight into Thursday morning. Post-frontal
lake-induced stratus may also ooze inland this evening and
affect the Chicago metro terminals. Thus, it is possible that
MVFR ceilings are observed for some time tonight, but
confidence in this remains too low at this time to explicitly
advertise that possibility in the TAFs.

Ogorek

&&

.CLIMATE...
Issued at 347 AM CDT Wed Jun 19 2024

With a low temperature of 78 degrees, Chicago broke its warmest
low temperature record for June 18th yesterday, while Rockford
tied its daily warmest low temperature record of 74. A few
additional temperature records (namely today`s warmest low
temperature records and Saturday`s high and warmest low
temperature records for Chicago and/or Rockford) may be
threatened over the next few days.

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

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

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

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

Visit us at weather.gov/chicago