Crystal Lake, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

426
FXUS63 KDVN 122317
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
617 PM CDT Sat Sep 12 2026

.KEY MESSAGES...

- An active pattern is in store next week with multiple rounds
  of showers and storms likely through Friday. Currently, the
  greatest severe potential is Monday PM when there is a
  Marginal Risk, Level 1 out of 5, for eastern IA and northwest
  IL.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 230 PM CDT Sat Sep 12 2026

As of early this afternoon, a weak cold front was positioned
near the Mississippi River. Humidity is much lower to the
northwest of the front with dewpoints in the 50s, compared to
mid/upper 60s to the east into northwest/central Illinois. Weak
convergence along the boundary and the absence of forcing aloft
should preclude showers/storms through this afternoon and
evening across eastern Iowa and northwest Illinois. Heading into
tonight, subtle WAA aloft along an axis of 850-700mb westerly
flow could kick off scattered showers and isolated storms mainly
after 3 AM, likely becoming more widespread later into Sunday
morning (20-50%).

As an upper wave works across the region during the day on
Sunday, scattered showers and isolated thunderstorms will
continue to be possible through the afternoon, with relatively
light rainfall amounts anticipated. Northerly surface winds,
combined with periods of cloud cover, will act to hold down
temperatures into the low/mid 70s for most locations for Sunday
PM. With a drier air mass in place for Sunday night, cool
conditions are expected with lows in the upper 40s NE to upper
50s SW.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 230 PM CDT Sat Sep 12 2026

Monday-Tuesday...The longwave ridge over the Northern Rockies shifts
eastward over the Mid-to-Upper Mississippi Valley as a closed
heights center replaces it over the Northern Rockies. Diffluent
flow aloft and strong low-level WAA should support storms
(50-80% coverage) along the warm front in eastern Iowa, but
limited moisture, especially near the surface, will put a
hamper on the coverage (20-40%) across west-central and
northwest Illinois. Regardless, the southerly return flow behind
the warm front is forecasted to allow dew points to climb back
into the upper 60s and low 70s along the cold front diving
southeastward as the trough over the Northern Rockies lifts
into south-central Canada. After another dry period Monday
afternoon, storms will once again redevelop Monday evening
along the approaching cold front where a 40-knot LLJ is
expected to enhance the low-level shear. Current LREF guidance
has the sfc-to-500 mb wind shear vector oriented at a 45-degree
angle and close to 35 knots which would suggest a mixed
convective mode. However, by the time any convection reaches our
CWA, it will likely be in the form of linear clusters,
promoting a more damaging wind threat. This is why the SPC
has a Marginal Risk, Level 1 out of 5, for severe storms Monday
night in our northwestern counties.

By Tuesday, the storms are expected to decrease in intensity
along the front as upper-level dynamical support is lost. A
weak high pressure is forecasted to slide into Minnesota
Tuesday night, reinforcing the northeasterly flow over the CWA
and pinning the stationary front in our south. This stationary
front will act as the focal point for showers and storms through
the middle of next week.

Wednesday-Friday...The entire area will be sandwiched between a
ridge over the Southeast and a positively tilted trough over Hudson
Bay that extends back towards the Pacific Northwest. This
pattern should support diffluent flow aloft over the residual
stationary front as rounds of stratiform rain and storms develop
along it on Wednesday and Thursday. As the Southeast ridge
retrogrades westward, the flow is forecasted to return to a more
stable zonal flow as the boundary begins to wash out Friday
without any major air mass changes. By the end of the next week
though, there is a good chance (50-80%) that areas across
eastern Iowa and northwest Illinois will see over 2 inches of
rainfall with swaths of 4+ inches possible (20-30%).

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 614 PM CDT Sat Sep 12 2026

Clear skies and excellent visibility will continue through this
evening, now that the weak cold front has shifted southeast of
all terminals. Overnight, mid level clouds will increase, with
some scattered showers around Sunday morning. Rainfall should be
scattered for several hours, and is introduced as a Prob30, with
visibilities to 5SM, and cigs down to 3500 ft. Most models
suggest predominantly VFR weather through Sunday, but with some
downpours, I feel it is worthy of the MVFR forecast. Winds be
northwest around 10 kts this evening, decreasing overnight and
turning northeast for Sunday morning.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...Uttech
LONG TERM...Delaune
AVIATION...Ervin