811
FXUS63 KDMX 182008
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
308 PM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Thunderstorm and heavy rain potential tonight into Saturday
  morning. A few strong storms are possible across western into
  northern Iowa, with wind and hail the primary risks.

- Additional showers and storms possible Saturday afternoon and
  evening. Greatest severe risk over southern Iowa with wind and
  hail main risks.

- Active weather continues Sunday into early next week.
  Potential for reprieve through mid-week.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 305 PM CDT Fri Sep 18 2026

The weather across Iowa today varies quite a bit depending where you
are at. For much of the area (central into northern and eastern
Iowa), cloudy skies, mild temperatures and drizzle/rain have
characterized much of the morning hours, with conditions now drying
out this afternoon. However, other areas in southwest Iowa that
cleared out from the stratus this morning are seeing quite
different conditions. Clear skies and warm temperatures aloft
have allowed temperatures to climb into the upper 80s and even
low to mid 90s, a far cry from the upper 60s and low 70s farther
north and east. Likewise, dewpoints in the low to mid 70s are
resulting in muggy conditions along with these warmer
temperatures. Fortunately for those under the stratus, clearing
has been far less robust than what was previously modeled this
morning, keeping those warmer temperatures at bay.

The extent of the stratus becomes important as we look ahead to
storm chances this afternoon and evening. Destabilization is
occurring in areas where skies are clear, primarily over Nebraska
and into western and southwestern Iowa. Warm mid-level temperatures
will generally inhibit vertical parcel acceleration, however, a
surface boundary over Nebraska may provide enough mechanical lift to
overcome the convective inhibition. While the storms that fire along
this boundary will have access to a healthy amount of instability
(2000 to 3000 J/kg of MLCAPE), weak flow aloft will result in poor
vertical wind shear, limiting storm organization. Likewise,
these storms would be arriving in our area later in the evening
as mixed- layer instability will be decreasing. Therefore,
primary concern with any of these storms would be downburst
winds and small hail as activity pulses up and down.

A second area of storm development is expected across northwestern
into northern Iowa later this evening, firing along the triple point
as it arrives in our northwest.  A healthy amount of elevated
instability will be in place across northwest Iowa early this
evening and moderately better wind shear will be more favorable for
a few organized storms. That said, lack of solar heating from
stratus in this area should keep most of the activity elevated,
posing mostly an isolated damaging wind and hail threat. However, if
a storm can get rooted at the surface, low level wind and 0-3 km
CAPE fields would be favorable for a brief tornado, although this
seems like an unlikely scenario at this point. The storm prediction
center has since backed off on the tornado potential with their day
1 marginal risk (level 1 of 5), removing the 2% tornado risk area
and keeping the hazards to wind and hail.

While any severe risk today will generally be in the afternoon and
evening, storms are expected to persist in some capacity through the
overnight hours. A 30 to 40 kt 850 mb low level jet will nose up
into the east west oriented boundary across northern Iowa, producing
steady lift through the region. Storm motions to the east will favor
training storms as convection fires at the nose of the jet through
the night. Likewise, very moist profiles with PWATs of 2"+ will
promote efficient rainfall with any convective activity through the
night. This will pose a heavy rain and flash flooding threat over
northeastern Iowa tonight. More on this in the hydrological section.

As we get into Saturday, the overall system will slide east
southeastward, posing some potential for showers or storms during
the day. The greatest chances will be focused at the nose of the jet
over northeastern Iowa through the morning and mid-day, with
additional chances along the trailing boundary as it passes into
central and southern Iowa Saturday evening. A few stronger storms
will be possible again along this boundary, although wind seems to
be the primary threat. Slightly better shear oriented along the
boundary will promote multicell storms. Likewise, healthy
instability (2000 to 3000 J/kg) will promote hail development with
any more organized and discrete updrafts. Finally, plentiful
moisture will once again promote heavy rainfall, this time over
southern Iowa. The primary risk will be Saturday evening, with the
boundary eventually pushing through by Sunday morning. Behind the
boundary, there is a signal for light drizzle Sunday morning.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 305 PM CDT Fri Sep 18 2026

Our active pattern continues Sunday with additional
rain/thunderstorm development possible in the afternoon and evening,
as another trough digs in on the lee side of the Rockies. This
overall pattern persists into early next week, with rain and
thunderstorms possible each day. That said, the 12z global model
guidance that`s come in today does show some reprieve may be on the
horizon toward the middle of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1236 PM CDT Fri Sep 18 2026

Widespread stratus and fog/mist is resulting in poor flight
conditions as low as LIFR this afternoon. While these have been
slow to improve, steadily improving visibilities and ceiling
heights are still expected through this afternoon. Some breaks
in the clouds have developed near KALO and KMCW but low clouds
and patchy fog/mist generally fill back in and/or stick around
through the evening and overnight, especially at northern
sites.

In addition to the visibilities and ceilings, we also expect
scattered showers and thunderstorms to move through the area
tonight. Have timed TAFs out to the most likely periods, but
confidence is low in exact timing and evolution, so amendments
are likely.

&&

.HYDROLOGY...
Issued at 306 PM CDT Fri Sep 18 2026

Heavy rain and training storm potential over northern Iowa tonight
may pose a risk for flash flooding, especially in urban areas. The
main axis of heaviest rainfall has wavered quite a bit just in
today`s guidance, but the general consensus is putting our forecast
area on the southwestern fringe of the highest QPF. Should the nose
of the LLJ and coincident storms set up to our north, our area may
be spared from the worst of it. However, there are solutions that
show the heavier QPF band a bit farther south into north central
Iowa. It`s also not uncommon for these areas to leak slightly
farther south. Therefore, while the greatest risk does set up to our
north and east, have decided to put out a targeted Flash Flood Watch
across northwestern Iowa that includes Mason City and Waterloo.

In addition to the flash flood risk, similar areas received 1 to 2"
of rain this morning, and could see another 1 to 3" tonight,
depending on the main axis. Therefore, river responses will be
possible, primarily along the Cedar and Iowa Rivers. The HEFS output
suggests gages along these rivers could reach into action or minor
flood stage, with higher end scenarios pushing into higher stages.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 7 PM CDT this evening through Saturday
afternoon for IAZ007-017-027-028-039.

&&

$$

SHORT TERM...Dodson
LONG TERM...Dodson
AVIATION...Dodson
HYDROLOGY...Dodson