Weavers Weather

206
FXUS63 KLOT 132342
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 PM CDT Sat Sep 13 2025

.KEY MESSAGES...

- Areas of fog may develop late tonight outside of urban areas.

- Generally warm and dry conditions are expected early-to-mid
  next week.

- Temperatures will turn more seasonable and periodic shower and
  storm chances will return toward the end of next week.

&&

.DISCUSSION...
Issued at 212 PM CDT Sat Sep 13 2025

Through Sunday:

Thunderstorms continue to regenerate this afternoon in
northwestern Indiana along the nose of a 40-50kt 700mb jet and
along the eastern edge of an EML plume extending eastward from
the Upper Mississippi River Valley. This region is along the
downwind side of an upper-level ridge centered over the Upper
Mississippi River Valley. The expectation is for the
regenerating thunderstorms to continue while gradually shifting
eastward this afternoon and evening. Convective-layer shear of
around 35 kt will continue support episodic bouts of dime to
locally quarter size hail as the activity scoots eastward. (In
fact, have noted periodic mid-level mesocyclones, confirming
that convection is taking advantage of the shear). Elsewhere,
it`s turned out to be a pretty nice day with filtered sunshine
and a southeasterly breeze. Without additional forcing (away
from the nose of the 700mb jet), our thinking is that additional
storms are unlikely to develop northwest of where they are now.
With that said, will hold onto low (20%) chances for storms
through this evening across much of the area just to be safe (as
anything that does develop would be prime to be strong given a
better overlap of shear and instability with westward extent).

Tonight, the center of the upper-level ridge will shift eastward
toward the Great Lakes. At the same time, a weakness in surface
pressure will develop, altogether leading to mostly clear skies
and relatively light winds. As a result, do think areas of fog
will develop especially where rain fell earlier today. The
inherited mention of fog in the forecast is in good shape.

Tomorrow, a surface high pressure system centered near the
borders of Ontario and Quebec will strengthen, causing winds in
the southern Great Lakes to be decidedly onshore/easterly. As a
result, anticipate there will be a gradient in temperatures
ranging from the mid 70s along the lakeshore to the upper 80s
far inland. Otherwise, mostly sunny skies will prevail.

Borchardt

Sunday Night through Friday:

The upper level pattern next week will feature high amplitude
ridging across the Great Lakes paired with a closed upper low
in the Southeast setting up a pseudo-Rex Block pattern across
the eastern CONUS. This should keep the various western CONUS
shortwave troughs and associated showers and storms mainly west
of the region through the first half of the upcoming week. Long-
range guidance continues to diverge slightly on the position and
strength of the upper low which has some implications for the
degree of warming each day and timing of the onset of more
unsettled weather later in the week. The GEFS has the upper low
closer to the coast allowing much warmer air to lift into the
region paired with an earlier breakdown of the ridge by midweek
and return to shower chances. Meanwhile, the EPS maintains a
further west position of the upper low (closer to the more
classic Rex Block pattern) which slows the breakdown of the
ridge and delays the arrival of precipitation to later in the
week. This still favors warmer (albeit cooler than the GEFS) and
dry conditions prevailing in the local area for the majority of
the workweek.

With all of that said, there are no major changes in
expectations through the upcoming work week with the going
forecast favoring the EPS solutions with dry and generally warm
conditions expected well inland of Lake Michigan through
midweek. Expect highs in the 80s to potentially near 90 toward
western and central Illinois. Light surface flow will allow
daily lake breezes to surge well inland each afternoon which
should hold highs in the 70s along the lakeshore with falling
temperatures in the afternoon expected for inland areas in its
wake.

The upper ridge/blocking pattern begins to breaks down toward
the end of the week with the western trough axis gradually
shifting toward and over the area into next weekend. This will
lead to increasing shower and thunderstorm chances Thursday
night onward; however, there is not a strong signal for more
organized convection or heavy rain at this time. In tandem,
temperatures will trend cooler and more autumn-esque with highs
in the 70s and lows in the 50s.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 642 PM CDT Sat Sep 13 2025

The only aviation weather concern is the potential for BR/FG and
spotty IFR/LIFR cigs.

Winds will generally acquire an east/southeasterly direction
this evening, before becoming light and variable overnight.
Increased near-surface moisture and weakening surface winds will
support the potential for some fog development late tonight
into early Sunday morning. Have introduced TEMPO groups for
high-MVFR vsbys at ORD, MDW, and GYY, and expanded the time
window for low-MVFR vsbys at DPA and RFD. If winds just off the
surface weaken a bit more than currently anticipated, the
potential for IFR cigs/vsbys could increase a bit, but
confidence in this occurring at any of the TAF sites remains too
low to introduce IFR at this time.

VFR conditions are expected during the day on Sunday. Easterly
winds will increase a bit through the day, with some
intermittent gustiness possible at ORD/MDW during the afternoon.

Carlaw

&&

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

&&

$$

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