-- Discussion containing changed information from previous version are highlighted. --
821
FXUS63 KLOT 181113
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
613 AM CDT Tue Jun 18 2024

.KEY MESSAGES...

- Hot and muggy conditions will prevail through the end of the
  week, although a potential for cooler conditions near the lake
  exist Thursday and Friday.

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

&&

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

Through Wednesday Night:

A slug of deeper moisture from the Gulf of Mexico is on our
doorstep early this morning, and upstream RAOBs sampled PWATs
just around two inches in the core of this plume. Mid-level
temperatures are warmer within this moist axis, and lapse rates
will correspondingly diminish, limiting the amount of
instability available to elevated parcels this morning which
should limit the lightning threat. Have boosted precipitation
chances a bit through mid morning across the eastern half of the
forecast area as a series of embedded vort maxes meander
northward across the region, although any showers generally look
to remain pretty light.

The core of this moist axis will begin to shift east of the
region through midday and will take the greatest precipitation
chances and the bulk of mid and high-level broken to overcast
cloud cover with it. Suspect that we`ll quickly build lower
cumulus/stratocu through the morning as deepening mixing taps
into the increased moisture off the surface, but don`t see much
of a reason why this won`t eventually lift and scatter out
through the afternoon. Latest thinking is we`ll end up with
generally partly cloudy conditions this afternoon (maybe a bit
more in the way of high cloud cover in our south) which should
still allow temperatures to climb into the low to mid 90s--
especially considering the very warm start we`ll have this
morning. Thicknesses are actually a bit higher than they were on
Monday, with the increased cloud cover and increased moisture
content the main limiting factors behind warmer conditions. More
blunted mixing today should mean that dewpoints don`t mix
out/fall as readily as they did on Monday. All of this results
in a bit lower air temperatures and high dewpoints, yielding
peak afternoon heat indices in the mid to upper 90s. Winds will
gust at times up to 35 mph this afternoon with a bit better flow
available off the deck.

Expect that we`ll be in a bit of a subsident regime this
afternoon as the aforementioned vort/shortwave peels off to
our east. We`ll be uncapped again though during the afternoon,
so any subtle boundaries could serve as a focus for isolated
storms, but coverage is expected to be quite low.

The synoptic cold front across Minnesota and Iowa is expected to
become very active this afternoon with renewed thunderstorm
development. This boundary will then inch eastward overnight,
and it`s probable that convective outflow augments the
east/southeast movement of the front, with a composite boundary
sagging very close to (or in) our NW and stateline locales
Wednesday morning with at least a low threat for a few showers
and storms entering the CWA. It`s pretty difficult to pinpoint
how this will play out at this range, but it seems like the
front/outflow boundary will end up close enough to the CWA to
support some higher midday and afternoon shower and storm
coverage during the afternoon, with the greatest chances north
and west of about I-55. It doesn`t look like there`s going to be
any well-defined large scale forcing mechanism aloft though,
with general height rises and the best upper jet
forcing/divergence displaced across Iowa, so have capped things
to scattered/chance PoPs for now. Pulse convection with a
downburst wind threat would be favored with paltry mid and upper
level flow/deep layer shear.

With such weak cloud-bearing flow, individual cell motions (if
storms develop in the area) would be quite slow and generally
northeastward, paralleling the cold front/remnant outflow
boundary yielding a localized heavy rain threat. Not sure if
coverage will be high enough to make this more of an issue
though, but something we`ll keep an eye on.

Lastly, if we aren`t gunked up by convective debris cloud cover
on Wednesday, temperatures should be near or perhaps a degree
or two warmer than today but with generally lower dewpoints with
deeper mixing again. The main exception would be in the
vicinity of the front/outflow if storms fire by late-
morning/midday which would result in lower high temperatures
than currently advertised. Overall, heat indices once again look
to top out in the mid to upper 90s with some late-day relief
possible across lake-adjacent locales in NE Illinois from a lake
breeze.

Carlaw


Thursday through Monday:

The primary story through the end of the week continues to be
the hot and humid conditions brought by the ridge over the
eastern CONUS and the periodic shower and thunderstorm chances
brought by the stalled front to the northwest. Although high
temperatures will be in the low-to-mid 90s through Saturday, dew
points should be able to mix out each day enough to limit heat
indices to the upper 90s. This keeps us below Heat Advisory
criteria, however, we will continue to message heat safety
through the end of this first extended period of heat this year.

On Thursday, a backdoor cold front will move down Lake Michigan
and sweep across the area. Model guidance is showing a
consistent signal for the front being at the doorstep of Chicago
by 12Z Thursday morning (if not earlier) before advancing
southwest. This timing will limit high temperatures near the
lakeshore to around 80 degrees. Although highs will likely be in
the 90s across much of the area, temperatures may actually fall
through the afternoon hours as the front moves overhead. If the
timing of this front continues to move up, then some areas
further inland could also see their high temperatures limited on
Thursday. The stalled cold front over Wisconsin may also sag
slightly southeastward Thursday as the ridge begins to flatten
out. This will bring a chance (30-50%) for showers and
thunderstorms into the northwestern CWA Thursday afternoon.

Friday looks to be a pretty quiet day, although onshore flow
associated with the remnants of the backdoor cold front and a
lake breeze will likely limit high temperatures along the
lakeshore while the majority of the area sees highs in the low-
to-mid 90s once again. Isolated showers and thunderstorms also
can`t be ruled out Friday afternoon, mainly in the northern
portion of the CWA.

The next widespread chance for showers and thunderstorms comes
Saturday afternoon/evening into Sunday as a shortwave moves
across the Northern Plains and an associated surface cold front
pushes through the area. This cold front will bring an end to
this extended period of heat and humidity. Following this
frontal passage, zonal flow looks poised to return with dry
conditions early next week.

Carothers

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
-- Changed Discussion --
Issued at 609 AM CDT Tue Jun 18 2024

Some scattered showers and lowered ceilings associated with a
remnant MCV are currently moving into the area from the south.
These lowered ceilings are creating scattered MVFR conditions.
Showers and MVFR conditions should not be long-lived at KORD,
KMDW, KDPA, and KGYY, so opted to go with a TEMPO group as
opposed to prevailing. Lower ceilings are more widespread
further west, leading to MVFR conditions in the prevailing line
at KRFD. However, showers will remain east of KRFD with this
system. Ceilings will rise as the MCV continues lifting
northeast and low-level moisture begins to mix out with a good
amount of clearing taking place this afternoon.

Winds will be gusty out of the southwest with gusts up to 30 kts
expected due to a tight pressure gradient. Winds will remain
gusty into the overnight hours before backing down early
tomorrow morning. Precipitation chances are low through the TAF
period, although the possibility of an isolated shower or storm
can`t be ruled out this afternoon.

Carothers
-- End Changed Discussion --

&&

.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...
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM this morning to 10 PM CDT this
     evening for the IL and IN nearshore waters.

&&

$$

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