-- Discussion containing changed information from previous version are highlighted. --
553
FXUS63 KLOT 190833
AFDLOT

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

.KEY MESSAGES...

-- Changed Discussion --
- 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.
-- End Changed Discussion --

&&

.DISCUSSION...
-- Changed 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 Tuesday:

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 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
-- End Changed Discussion --

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 119 AM CDT Wed Jun 19 2024

Key Messages:

- Westerly wind shift possible at RFD around 08-09Z. Low chance
  of a shower accompanying this wind shift as well.

- Chance for thunderstorms this afternoon.

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


A decaying line of convection is entering northwest Illinois at
press time. The associated precipitation probably won`t survive
to RFD (only about a 15% chance that it does), but even if it
does, notable visibility reductions are even less likely. The
outflow from this convection is more likely to reach RFD and
cause a westerly wind shift some time around the 08-09Z time
frame, though a return to prevailing southerly winds is favored
soon thereafter as the boundary lifts northward.

This afternoon, thunderstorms are favored to develop along this
remnant outflow boundary, and could also freely develop in the
muggy and uncapped air mass south of it. These storms will
likely be "pulse"-like in nature, collapsing soon after they pop
up with new storms developing along the outflow boundaries spit
out by the preceding storms. Thus, confidence is low in a storm
affecting any one TAF site. Convective coverage will likely be
greatest closer to the remnant outflow boundary, so think that
RFD stands a better chance at seeing storms this afternoon
compared to the Chicago metro TAF sites, but for the time being,
will let the inherited PROB30 groups for TSRA ride a while
longer in all five of our TAFs given the remaining uncertainties.

Lastly, a northerly/northeasterly wind shift is expected late
this afternoon or evening at all TAF sites. The timing of this
wind shifts 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 in the
immediate wake of the boundary passage, winds should become
light and remain that way overnight. Behind the boundary, lake-
induced stratus may ooze inland and affect the Chicago metro
terminals, potentially resulting in MVFR ceilings being
observed for some time.

Ogorek

&&

.CLIMATE...
Issued at 305 AM CDT Tue Jun 18 2024

With a high temperature of 97 degrees recorded at O`Hare Airport
yesterday, Chicago broke its daily high temperature record of 96
degrees for June 17th, which was previously set in 1887 and
1957. A few additional temperature records (primarily some of
the record warmest low temperatures for both Chicago and
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:           6/18 6/19 6/20 6/21 6/22
---------------------------------------
Record High:     98  102  104  101   97
Record Warm Low: 77   78   78   74   76

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

Ogorek

&&

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

LM...None.
-- End Changed Discussion --

&&

$$

Visit us at weather.gov/chicago