Williamstown, Iowa Weather Forecast Discussion

183
FXUS63 KDVN 200834
AFDDVN
Area Forecast Discussion
National Weather Service Quad Cities IA IL
334 AM CDT Sat Sep 20 2025
.KEY MESSAGES...
- On and off periods of showers and thunderstorms are expected
  through the weekend and the first half of next week. The
  threat for severe thunderstorms remains low.
&&
.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 327 AM CDT Sat Sep 20 2025
Not much change to the going forecast throughout the weekend,
with daily chances of showers and a few storms. Early this
morning saw a line of isolated showers and storms extending from
southwestern Wisconsin south towards west-central Illinois,
going through our region. This activity had developed ahead of
an area of low pressure, which can be seen very well in the
GOES-East mid-level water vapor imagery over central Iowa. Due
to increased convective inhibition, this activity has mostly
dissipated. The upper-level low will continue to translate
eastward with time this morning, placing our area under the
associated mid-level trough. As of this AFD issuance, we can see
another area of showers along the IA/MO border region
approaching our southern CWA. Due to this area of rainfall and
associated cloud cover from the approaching low, we have lowered
high temperatures today by at least a few degrees, resulting in
highs in the lower 70s west to the upper 70s east.
There could be additional showers and a few storms possible this
afternoon into tonight (20-50% chance) as the trough continues to
lift northeast through the area. Both deep-layer shear and
instability appears quite meager, with SBCAPE values per the 20.00z
HREF ensemble of less than 1000 J/kg, and indeed, the HREF 1-hr
probabilities of thunderstorms are around 10 to 20%, so showers
will be more common than thunderstorms.
In the wake of today`s trough, yet another shortwave is expected to
move through the area for Sunday and Sunday night. Similar to
today/tonight, precipitation chances are around 20-50%. However,
different from today will be slightly stronger deep-layer
shear, with values progged around 25 to 35 knots per the GEFS
ensemble plumes, thanks to stronger 700 mb level flow. At this
time, SPC currently only has us under a general thunderstorm
risk, with a Marginal Risk well east of us over the Ohio Valley
region, but if the shortwave slows down any, given the
environment, there could be some stronger convection possible
in our region. There are some mixed signals in the machine-
learning severe output, but the CSU and NSSL AI outputs do have
a low-end potential for strong to severe storms for our region
Sunday. For what it`s worth, ensemble cluster analysis is in
good agreement with the timing of the trough, so confidence is
higher that we should see only garden variety thunderstorms, if
any do develop.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 327 AM CDT Sat Sep 20 2025
We will continue to see daily chances (20-50%) of showers and
occasional thunderstorms Monday through Wednesday as the large-
scale weather pattern continues to feature a series of
shortwave troughs moving through the central CONUS region. Any
storms that develop are expected to be your garden variety
storms (not likely to become strong to severe) due to a lack of
robust deep-layer shear, but modest instability will be enough
to generate some storms at times. NBM exceedance probabilities
of a quarter inch or more of rainfall in a 24 hour period is
generally between 20 to 40%, so it`s possible that some
locations could see some wetting rainfall. Again, these
showers/storms should be hit-and-miss in our region - not a
complete washout. In fact, by the time we get to Friday, we
could see some relief from the daily chances of precipitation as
some of the guidance suggests stronger mid-level ridging to
develop, which would help kick out any troughs that would
otherwise support rain chances.
In terms of temperatures, there doesn`t appear to be too much of a
change over the next several days, with highs maintained in the
upper 70s to lower 80s for most locations. A slight cooldown is
possible for Wednesday through Friday as winds shift more
northeasterly as opposed to southerly, with the NBM indicating highs
in the lower to middle 70s each day.
&&
.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1244 AM CDT Sat Sep 20 2025
We continue to track a line of isolated showers/storms from
southwestern Wisconsin south towards far west-central Illinois.
This activity has developed ahead of an approaching area of low
pressure, currently over central Iowa as of TAF issuance. There
is one lone storm just northeast of MLI, which brought brief IFR
visibility reductions there around 05z. We expect impacts from
this line to be minimal, if any, as it should continue to
slowly track eastward away from our local TAF sites. Outside of
another round of isolated showers/storms this afternoon and
evening that could support MVFR visbys, conditions should remain
mostly VFR. Light and variable winds early this morning should
turn more southerly this afternoon, but generally stay around 5
to 10 knots.
&&
.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.
&&
$$
SHORT TERM...Schultz
LONG TERM...Schultz
AVIATION...Schultz

NWS DVN Office Area Forecast Discussion