Wheaton, Illinois Weather Forecast Discussion

932
FXUS63 KLOT 210532
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 AM CDT Sun Sep 21 2025
.KEY MESSAGES...
- Scattered thunderstorms expected this afternoon and early
  evening, a few could be strong with locally heavy rainfall
  also possible.
- Another round of showers of thunderstorms possible late this
  evening into the overnight.
- While there will be many dry hours, the threat of periodic
  showers and thunderstorms will continue through at least the
  first half of the upcoming work week.
&&
.DISCUSSION...
Issued at 205 PM CDT Sat Sep 20 2025
Through early evening:
Morning soundings from DVN and ILX are moist with fairly modest
lapse rates. Some clearing across our area this morning is
allowing the boundary layer to heat. Modifying the morning
soundings, it`d only take sfc temps in the upper 70s with
dewpoints in the lower 60s to get MLCAPE around 1000 J/kg with
little CINH. Given the expected destabilization and lack of a
cap, the question becomes what will force storms.
Water vapor imagery this morning depicts a negatively tilted
trough across the mid and upper Mississippi Valley. Well defined
vort max is rounding the base of this trough, moving into
western Wisconsin late this morning. Diffluence is quite
evident over northern IL on water vapor imagery in the base of
the trough and on the southern flanks of the vort max.
Given the destabilizing boundary layer and sustained ascent
associated with the diffluent and divergent flow aloft, we
should see an increase in coverage of showers and thunderstorms
the remainder of this afternoon. Greatest coverage of
thunderstorms, including a threat of a couple of stronger
storms, should be over the eastern half or so of our CWA. Weak
shear profiles will likely limit the severe threat, but
certainly can`t rule out a couple of stronger to possibly
marginally severe storms this afternoon.
Tonight:
Low confidence in evolution of things tonight. Water vapor
imagery suggests perhaps a weak MCV over southern IA which could
help maintain isolated to scattered coverage of showers and
thunderstorms through the evening and into the overnight hours.
Extrapolation of this feature would tend to suggest that the
associated ascent would largely move east of our CWA by later
tonight, though some modest (15-20kt) southerly low level flow
could maybe provide enough theta-e advection to maybe keep some
convection going in the wake of the wave. The 12z HREF was
still pretty bullish with convection overnight, so maintained
higher chances pops despite the lower confidence and less of a
signal in most other models.
Sunday:
Synoptically, there`s definitely a play for there to be less
coverage of showers and thunderstorms and possibly a mostly dry
day for some (many?) areas Sunday. Aforementioned negatively
tilted trough looks to lift north and away from our area with
neutral to even weak height rises locally Sunday. Another vort
will dig southeastward into the upper Mississippi Valley Sunday
afternoon, but forcing with this trough looks to remain well
west and northwest of our area. With a warm and moderately
unstable air mass in place, going to maintain some chance pops,
highest over the eastern CWA.
- Izzi
Sunday Night through Saturday:
Issued at 328 AM CDT Sat Sep 20 2025
Intermittent shower and thunderstorm chances will persist Sunday
night through Tuesday as additional disturbances ripple through
the west to northwesterly flow aloft. A chaotic upper pattern
into the middle and end of next week will likely continue to
result in lower-than-typical forecast confidence. Some signal
remains in the extended guidance for at least a brief return to
drier conditions (especially near/north of I-80) in the wake of
a cold front towards midweek, but depending on the influence of
another upper low towards the end of the week, this could end
up being short-lived. The latest GFS in particular would
indicate little/no break in the action with a Fujiwhara dance
taking place between several robust disturbances across the
central CONUS.
Temperatures this upcoming week will start out solidly above
normal (along with fairly humid conditions as well), with highs
well into the 70s to near 80F through Tuesday, then trending
closer to normal and a bit less humid in the mid to late week.
Carlaw/Castro
- Izzi
&&
.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1232 AM CDT Sun Sep 21 2025
Key Aviation Messages for the current TAF period:
- Multiple rounds of showers with the chance for embedded
  thunder
Loosely scattered showers remain dotted around the region. With
current coverage being somewhat low, VCSH was prevailed at all
the TAF sites through the morning. With the amount of rain on
Saturday, there is a slight chance for some lower vis to crop up
in outlying areas away from Chicago but dense fog is not
expected. Another thing that would prevent widespread fog is
another chance for showers to move over the area from Central
Illinois. PROB30s were maintained for the chance for showers
overnight with higher rain rates that could reduce vsbys down to
MVFR conditions. Technically, the chance for thunder is non-
zero, but instability looks too meager and thus did not include
it in the TAF presently though trends will be monitored through
the night.
The overnight showers are expected to dissipate after sunrise
for a break in the rain through the early afternoon. Winds will
be out of the southwest through the day, and under 10 knots.
Another wave will pass over the area late in the afternoon that
could develop another round of showers with a better chance for
thunder with it. There is some uncertainty regarding the amount
of moisture available which kept a PROB30 mention rather than a
TEMPO. However, the RFD TAF was kept dry as the better chances
are closer to Chicago and southeastward.
Showers may linger into the early evening and overnight. Models
are suggesting yet another wave of showers will move over the
region overnight into Monday morning. Similar to tonight,
instability in model soundings does not look sufficient enough
to add TS to the present TAF, but PROB30s were added for a
renewed chance for showers.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion