Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

482
FXUS63 KDMX 291754
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1254 PM CDT Sun Jun 29 2025

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Storms moving into northern and western Iowa this morning may
  play a role in how storm chances (currently 40-60%) and
  hazards evolve later today into tonight. For now, at least a
  few storms may be capable of damaging wind gusts and efficient
  rainfall.

- 20% chance of storms Monday afternoon along with a cooler
  airmass.

- More storm chances (20-40%) and seasonally warm temperatures
  (similar to today) return for Wednesday into the holiday week
  (not washouts).

&&

.UPDATE...
Issued at 222 AM CDT Sun Jun 29 2025

Regional radar is quite active to the north and northwest of the
state early this morning showing a mesoscale convective systems
(MCS) over Minnesota with several convective clusters over South
Dakota into northern Nebraska. The MCS has an east-west line of
storms that has reached northwest Iowa. The storms in the
Yankton vicinity are along the cold front. This as well as the
MCS are being fueled by a 30 knot, 850mb low level jet that
comes up over Nebraska and then veers into southwest Minnesota.
Convective allowing models (CAMs) have a decent handle on the
Minnesota MCS and that east-west line. Meanwhile, CAMs are
struggling with the southeastern South Dakota convection, but
timing this out it will reach our northwestern service area
likely around 5am. The expectation is that these storms will
not be severe given they are already out of balance with the
outflow surging out ahead and they are moving into an
environment with less favorable shear.

All of this storm activity and their evolution into Iowa has sent
confidence in reverse with more uncertainty. One cluster of storms
we have not discussed is farther west over south central South
Dakota. Many of the recent CAMs runs this evening have these storms
decaying as they reach or move into western Iowa later this morning
and this may leave behind some sort of residual boundary. In this
scenario, there would be less cloud cover and thus a higher degree
of instability. If and whatever residual boundary is around may play
a role in where convection develops, but the cold front would
be the main focus for convection. The environment in this case
would feature strong instability both overall and low level and
steep lapse rates supportive for storm growth. This would point
to some initial updrafts that could support 1" severe hail.
However, storm organization remains lack luster until near and
behind the cold front. Of course, this could be locally
increased by the leftover boundary. Mid-level dry air would
contribute to large downdraft CAPE values around or a bit over
1000 J/kg and thus strong downdraft winds would be the concern
as storm cores weaken. Depending on where the storms form,
funnel clouds cannot be ruled out given the strong low level
instability and weak shear, but tornadoes remain unlikely in
this environment.

Now, the alternative solution to the convection over south central
South Dakota are that a few CAMS such as the 12z and 0z HRRR
and the 0z RRFS have this cluster surviving into Iowa with some
potential gusty winds, though to what degree remains to be seen
if at all. If this latter solution plays out with convection
and cloud debris moving through the state, there would be lower
instability and the coverage of storms and their intensity may
be less than currently forecast.

As was the case yesterday, heavy rainfall parameter space is
favorable given deep warm cloud depths, high precipitable water
values nearing 2 inches, and slow mean flow around 10 to 15 knots.
Deterministic and HREF forecasts show generally 1 to 2 inches with
isolated amounts over 3 inches still on track. While soil moisture
capacity did increase yesterday, northern Iowa is still above
the 70th percentile. So, storms with these rainfall amounts may
breach flash flood guidance, but given we are in peak
agriculture growth, flash flooding concerns may be more
relegated to if one of these bullseye amounts land over an urban
center. Even then and in agreement with WPC, the marginal risk
of excessive rainfall seems reasonable with just isolated flash
flooding potential.

Storms will diminish and move out overnight as the cold front pushes
through. It looks like Monday afternoon may have isolated to
scattered showers and storms developing in the steepening low
level lapse rates as a shortwave trough drops in from the
northwest.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1246 PM CDT Sun Jun 29 2025

Morning complex of showers and thunderstorms has shrouded the
state in thick layer of clouds today; which has limited warming
and thus rain chances. So, although isolated shower and storms
will still be a possibility yet later this afternoon, coverage
and strength will be much more limited compared to earlier
expectations. What does that mean for terminals? Well, we left
in Prob30 groups highlighting this possibility although with
uncertainty. Best time will be generally 22-03z. Should storms
materialize, they won`t take long to dissipate and/or move east
this evening. MVFR CIGS at KDSM will likely persist for the next
1-2 hours as well, but with better daytime mixing as clouds
move east, expect them to lift by around 1930z. Otherwise, VFR
conditions can be expected through the period. There may be
isolated shower/storms on Monday, however, coverage will be very
limited - similar to today.

&&

.HYDROLOGY...
Issued at 243 PM CDT Sat Jun 28 2025

Soil moisture remains high though across much of the CWA owing
to last week`s heavy rainfall. NASA SPoRT soil moisture
percentiles for the top 40 cm of the soil (usually considered
for river forecasting purposes) remain above normal north of US
30, while generally more normal south.

Additional rainfall is forecast Sunday into Sunday night, however.
At this point, the best chances for heaviest rainfall appears
to be with most likely location of more organized storms in the
central and south - generally away from the aforementioned area
of above normal soil moistures (in the north). Latest tools
suggest this additional rainfall will have a minimal impact on
existing river trends especially at those locations where levels
are or will be running high. The rainfall may add minor amounts
to the crests or slow falls. Although not a great chance given
that southern Iowa is less flash flood prone, localized flash
flooding would be a possibility over urban areas or if storms
are able to move parallel to the boundary.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Ansorge
AVIATION...Hahn
HYDROLOGY...Hahn