Bannockburn, Illinois Weather Forecast Discussion

150
FXUS63 KLOT 171957
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
257 PM CDT Wed Sep 17 2025
.KEY MESSAGES...
- Warm and primarily dry conditions through the day on Friday.
- Periodic chances for showers and some thunderstorms will
  return Friday night into next week, though there will continue
  to be plenty of dry hours.
&&
.DISCUSSION...
Issued at 257 PM CDT Wed Sep 17 2025
Through Friday:
Expect very similar weather conditions to the past 24 hours over
the next 24-30 hours, followed by early signs of the gradual
breakdown of the entrenched blocking regime. Lows will be in the
mid to upper 50s outside of Chicago (a bit milder than this
morning) and in the lower to mid 60s in the city. While we
don`t have any patchy fog explicitly mentioned in the gridded
forecast late tonight into early Thursday, wouldn`t be surprised
at very patchy ground fog in typical favored spots.
On Thursday, skies should trend to partly cloudy into the
northwest 1/3 or so of the CWA (vs. nearly full sun everywhere
today). Otherwise, 925 mb temps will nudge up 1-2C, supporting
highs a bit warmer than today, primarily in the mid-upper 80s
away from the lake breeze cooling footprint inland of Lake
Michigan. A few 90F readings are possible near and south of a
Sterling to Watseka line. Thursday night will be tranquil with
lows in the upper 50s to mid 60s, mildest in and near Chicago.
A slow moving upper low will drift southeast across the Dakotas
on Friday. This will sharpen up the mid-upper ridge axis over
the northwestern Great Lakes, helping maintain the dry daytime
conditions. As the moist axis finally begins to slosh east from
near the MS River on Friday afternoon, can`t completely rule out
a few showers near/west of I-39 (20-25% chance). Friday`s high
temps will be similar to those of Thursday. The minor exception
will be near Lake Michigan, where the lake breeze will surge
farther inland earlier than on Thursday as it becomes enhanced
by a backdoor cold front.
Castro
Friday Night through Wednesday:
The aforementioned upper low will move across Minnesota Friday
night and then become an open wave embedded in a larger trough
over the north central US and Canadian Prairies on Saturday.
Associated weak surface low pressure will lift north and drag a
warm front slow across the region. A fairly sharp instability
west-east instability gradient will nudge eastward and open up
the potential for scattered showers and isolated to widely
scattered storms Friday night, aided by the closest pass of the
trough to our northwest. The evening should be dry over the
eastern half of the area, including the Chicago metro, with low
rain chances shifting east overnight.
Large scale forcing will shift farther north on Saturday-
Saturday night, with any isolated to widely scattered showers
and isolated storms confined to north of I-80 (20-30% PoPs) but
particularly up into Wisconsin (closer to the better forcing).
Expect another warm day on Saturday, also paired with a bit
higher humidity (dew points in the 60s), followed by a mild
night Saturday night by late September standards.
A more amplified trough then looks to eject out of the northern
Plains Sunday-Sunday night and move across the Great Lakes
Sunday night through Monday night. Associated with this feature
should be a more formidable cold front, potentially accompanied
by a bit higher coverage of showers and thunderstorms to the
area during this timeframe. Wind shear values and PWATs Sunday
through Monday look to be rather modest, so the threat for
severe weather and/or torrential rainfall (and flooding) still
looks low at this time. Additionally, southerly flow will keep
high temperatures in the 80s through the early part of next
week. To note, the Autumn Equinox is on Monday September 22nd at
1:19 PM CDT this year, so temps will be well above normal to
close out astronomical summer.
Looking towards Tuesday-Wednesday next week, the forecast
becomes much more uncertain as guidance is struggling with the
evolution of the Monday/Tuesday trough. Some guidance suggests
the trough will morph into an upper low and remain parked over
the Great Lakes through a good portion of next week while others
suggest a return of upper ridging as the upper low becomes
"buried" in the southern Plains. If the upper low solutions
materialize then cooler and wetter conditions would be favored.
However, if renewed upper ridging comes to fruition then a
return to dry conditions with generally above normal temps would
be favored, albeit with persistent northerly flow keeping it
cooler near the lake. After our official PoPs peak in the 40-50%
range Sunday evening through Monday evening, they do trend down
to only 20-30% on Tuesday and 

NWS LOT Office Area Forecast Discussion