Joliet, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

333
FXUS63 KLOT 210532
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 AM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Isolated to widely scattered showers and thunderstorms
  possible through early evening, mainly along and east of I-57

- Unseasonable warmth will continue through Saturday, with one
  last 90 degree day of 2024 possible

- Periods of showers and some thunderstorms late Saturday night
  possibly continuing into Monday

&&

.DISCUSSION...
Issued at 325 PM CDT Fri Sep 20 2024

Through Saturday:

Isolated to widely scattered showers and thunderstorms will
continually try to develop through early evening ahead of a cold
front for areas along and east of I-57. While forecast
soundings continue to show around 30 kts of effective shear, the
modest instability looks like it will limit the intensity of
any storms that develop so the threat for severe weather appears
to be decreasing. Therefore, expect any storms to remain sub-
severe with the main threat being lightning and perhaps a
localized wind gust of 30-40 mph.

Showers and storms will taper by 7 PM this evening as the
aforementioned cold front exits into north-central IN. As a
result, rain-free conditions are expected overnight as much
drier air moves in. Though, there is the potential for some
patchy fog to develop late tonight into Saturday morning due to
the combination of strong radiational cooling and light winds.
While dew points are forecast to diminish overnight, it looks as
if some residual moisture will linger near the aforementioned
front in northwest IN and adjacent areas of eastern IL. Since
forecast soundings show the moisture being rather shallow
confidence is lower on coverage and intensity of fog especially
with westward extent. Nevertheless, felt the potential was
sufficient to warrant the introduction of a formal patchy fog
mention to the forecast mainly in northwest IN and far eastern
IL.

Any fog that does develop tonight will erode by mid-morning on
Saturday. So expect another partly sunny and unseasonably warm
afternoon with highs once again forecast to top out in the upper
80s to around 90 degrees. The exception, however; will be for
areas along the northern IL lakeshore which will see highs in
the upper 70s to lower 80s due to onshore winds and a lake
breeze.

Heading into Saturday night, another weather system is expected
to develop across the central Plains as a broad upper trough
ejects out of the southwest CONUS. This weather system will then
begin to move into northern IL (and eventually northwest IN)
late Saturday night into Sunday resulting in another period of
showers and thunderstorms with beneficial rainfall for our
ongoing drought.

Yack


Saturday Night through Friday:

Late Saturday into Sunday a closed mid-upper low will shift east-
northeastward from the Desert Southwest out across the central
Plains. As this occurs, a lead impulse emanating from this low
is expected to foster shower and thunderstorm development across
much of the Corn Belt late Saturday into Saturday night as it
ejects out across the Mid-Missouri Valley in tandem with a
southeastward shifting cold front. We are likely to see this
activity shifting/developing into our local area Saturday night
into Sunday. Favorable deep moisture pooling along this frontal
boundary should favor some beneficial rainfall for the area,
though possibly enough to be an overall drought breaker. The
threat for showers and thunderstorms will continue Sunday night
into Monday, though the highest chances by Monday look to be
sagging south of I-80 with the surface frontal boundary.

Following the departure of this early week system, mean long wave
upper troughing and northerly lower-level winds across the Great
Lakes will foster a much cooler, more seasonable type pattern
across our area. Accordingly, expect daily high temperatures to
primarily be in the middle 70s following early morning lows in the
50s.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1233 AM CDT Sat Sep 21 2024

- Dense fog over northwest IN, including GYY, until just after
  sunrise
- Lake breeze expected to result in wind shift to easterly
  (ORD/MDW) and northeasterly (GYY) this afternoon
- SCTD SHRA chances will increase this evening for RFD and late
  evening into the overnight at immediate Chicago area
  terminals

Dense fog bank over northwest Indiana is expected to persist and
likely expand westward into far eastern Illinois until quickly
burning off just after sunrise. Suspect urban heat island warmth
will keep MDW warm/dry enough to limit threat of fog, but will
need to monitor for a brief period of fog around sunrise.

Otherwise, VFR conditions today, before some SCTD SHRA begin
affecting northern Illinois tonight. Initially, highest chances
are west in the RFD area this evening, but spreading east into
the immediate Chicago area terminals as well by late this
evening into the overnight.

Light/variable winds should become light southerly this morning,
but lake breeze is likely to develop and move inland resulting
in a wind shift to easterly at MDW and ORD later this afternoon.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

Visit us at weather.gov/chicago