Sycamore Swimming Pool, Illinois Weather Forecast Discussion

922
FXUS63 KLOT 201905
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
205 PM CDT Sat Sep 20 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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 106 PM CDT Sat Sep 20 2025
Key Messages:
- Periodic showers and thunderstorm chances through the TAF
  period.
- Low confidence in the prevailing wind direction over the
  course of the TAF period.
Periodic showers and chances for thunderstorms are forecasted
through much of this TAF period as a couple of upper-level
disturbances work their way through the region. The initial wave
of shower activity will occur this afternoon into this evening,
and some thunderstorms are likely to be seen during this time
as well, particularly at/near the Chicago metro terminals. The
most robust thunderstorms could produce gusty winds and knock
visibilities down to MVFR, if not IFR, levels.
Another wave of shower activity is then possible late tonight
through tomorrow morning (perhaps with not much of a break from
the afternoon/evening showers/storms), though just how widespread
this second wave of predominantly warm air advection-driven
showers will be remains uncertain given that low-level flow
won`t ramp up all that much tonight. Some embedded thunderstorms
would be possible within this activity, though thunderstorm
coverage will likely be less widespread than it is expected to
be this afternoon. MVFR conditions could also be realized for a
time if shower coverage does end up being fairly widespread.
While there is a scenario where some shower activity could
remain present near the terminals just about all day tomorrow, a
break in the precipitation is favored to occur at some point
tomorrow morning through mid-afternoon. Then, another round of
showers and possibly thunderstorms could occur late tomorrow
afternoon and evening as the next upper-level disturbance
approaches the area.
Confidence in the prevailing wind direction through the TAF
period also remains low on the whole, especially with showers
and storms complicating the picture. Fortunately, outside of any
thunderstorms, wind magnitudes are favored to remain under 10
kts through the TAF period.
Ogorek
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion