222
FXUS63 KLOT 190530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Thu Jun 19 2025

.KEY MESSAGES...

- Severe weather threat has ended, though the threat for
  isolated thunderstorms and localized flooding continues in
  portions of the area into early this evening.

- Stretch of hot and humid conditions with peak afternoon heat
  indices possibly over 100 degrees this weekend into early next
  week.

&&

.DISCUSSION...
Issued at 435 PM CDT Wed Jun 18 2025

Through Thursday Night:

The seasonally strong upper level trough will continue through
the lower Great Lakes region tonight. Showers and isolated
thunderstorms this evening will continue and may start to
taper, however an approaching upper level jet on the back side
of the trough axis should aid to reinvigorate several vorticity
maximum rotating about in the trough. Therefore expect
additional showers to continue overnight, though likely with a
bit lower coverage. Mid level lapse rates remain rather weak at
least during the overnight hours, and thus the thunder chances
will be minimal.

Northeast Illinois will be on the backside of the upper trough
axis through the morning Friday. In spite of some increased
lapse rates and some uptick in instability through the day,
height rises will ensue and the atmospheric column will also dry
out significantly such that any shower activity through the day
would be minimal late morning through the afternoon. Skies will
turn partly cloudy and highs will be fairly seasonal (the
coolest we will see for the next week), generally in the lower
80s.

On Thursday night, a northern stream upper jet in northwest flow
will and a corresponding low level jet response will drive some
scattered showers and storms, but this will be favored north of
the Wisconsin border. See the long term discussion for more
regarding the heat which still looks on track to peak over the
weekend into Monday, with heat indices getting in the low to mid
100s for several days which will lead to an elevated heat risk.
This building heat with northwest flow close by on the edge of
the building ridge may lead to some showers or storms on Friday
for area north of I-80.

KMD

Friday through Wednesday:

Main focus through the extended period continues to be building
heat and humidity this weekend into at least early next week.

Broad upper level ridging will be building across the Rockies
and high Plains Thursday as upper troughing shifts east across
the Great Lakes. This will place the local area under a
northwesterly flow aloft ahead of the building ridge to the
west. This set up will support the passage of subtle shortwaves
through the northwest flow which may result in some isolated
shower/thunder chances, especially across far northern Illinois
later Thursday or Friday. Better chances look to be to the north
so most if not all areas may remain dry.

A closed upper low upstream over the west coast is progged to
further amplify the downstream ridge and push it eastward into
the weekend. This will bring in much warmer air by Saturday with
a breezy southwesterly low level flow setting up across the
area. With such an amplified pattern, the ridge axis will extend
well north of the area by Sunday, which combined with the
fairly northerly position of the base of the western trough
would keep the favored track for showers and thunderstorms north
of the area through the weekend. Given that we`re still a few
days away from this pattern shift, we`ll have to monitor how
the amplitude of the upper features evolves over the next few
days to see if the signal for convection to be favored to the
north persists. The question going into early next week is how
quickly the amplitude decreases and when a favored track for
thunderstorms returns to the area as well as how quickly the
heat abates.

As far as temperatures, 850 mb temps are expected to warm into
the 21-23C range for the weekend. Breezy south to southwest
surface winds will support strong warming with highs expected to
reach the mid 90s for most areas both Saturday and Sunday, and
likely into Monday and perhaps Tuesday as well. Dewpoints are
forecast to reach the lower 70s but this will be another element
to monitor as drought currently persists across the area. Rain
from today`s thunderstorms may impact this, but Thursday and
Friday will likely allow for drying out of the top layers of the
ground. Given this, and the early stages of crop growth (as
opposed to mature stages in July and August which allows for
crops to add a notable amount of water vapor to the air) there
is some concern that current dewpoint forecasts could be a bit
too high. A drier airmass would allow for higher temperatures in
the upper 90s. Ultimately, a hotter/drier scenario could net a
similar heat index as a slightly less hot/more humid scenario
with the resultant messaging of taking heat precautions being
the same. At this point, the bigger concern is the duration of
the heat as opposed to heat reaching an extreme or unusual
magnitude.

MDB/Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1230 AM CDT Thu Jun 19 2025

Key Aviation Messages for the current TAF period:

- Lingering showers around terminals until daybreak

- IFR/MVFR conditions improve by 12Z as showers exit eastward

The main impacts to area terminals through 12Z will be
scattered showers and IFR/MVFR cigs. Looking at current radar,
there is a break in the shower activity over RFD that could make
its way to Chicago terminals by around 08Z. However, there are
additional showers (and an isolated storm) currently in
southwest Wisconsin that can move through northern Illinois
between 9Z to 12Z. Any current thunder upstream is expected to
dissipate over the next couple hours allowing thunder to remain
out of the TAF. Isolated downpours may provide brief moments of
reduced vis, but generally P6SM showers are expected. While
pockets of higher cigs exist, most of the area will find itself
with MVFR cigs, locally IFR, through around 9Z. Improving cig
trends are expected between 9Z and 12Z as the showers exit to
the east. While there is lower confidence in the exact timing
for VFR cigs, there is higher confidence in VFR after 12Z.

Drier conditions are expected today. Models are suggesting a
weak lake that will remain east of airfields this the
afternoon, but confidence is low so the TAF was kept dry at
present. Winds will remain light and out of the northwest and
slowly become west by midday. Lighter south- southwest winds are
expected this evening and through the overnight.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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