002
FXUS63 KDMX 291151
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
651 AM CDT Sun Jun 29 2025

 ...Updated for the 12z 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.

&&

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

The one-day reprieve of higher humidity yesterday is a distant
memory as the high plains surface low advects a low-mid 70F
surface dew point air mass within the warm sector across the
state today. Aloft, the H3 jet core remains confined to
northern Minnesota, with mid-level shortwave energy riding thru
it`s southern periphery. At the surface, the warm front resides
in a similar area, extending back to the the high plains sfc
low. As such, the convective focus overnight will remain north
and northwest closer to better theta-e advection and forcing.
Similar to last night, all of the CAMs develop a MCS across the
Dakotas, sliding southeast in some form on Sunday morning. As
typical in relatively light mid/upper-flow situations, models
underachieve consensus regarding MCS sustainability and timing.
This time is no different. The HRRR is the most aggressive in
maintaining it the longest - well into Sunday morning across
northern & central portions of the CWA, which if it would occur,
would have impact on where and when/where storms initiate on Sunday
afternoon and evening. Alternatively, the majority of the other
model output weakens/dissipates the activity before
redeveloping storms across southern Iowa along a south moving
"cool" front in afternoon/evening. Although best deep-layer
shear lags the boundary near the jet, values of 25-30 knots
should be sufficient for the potential for a few strong/severe
storms once they do develop. Although initial updrafts may
present a brief hail threat, downdraft CAPE values in excess
1000 j/kg (due to mid- level dry air) points to damaging winds
as the primary threat, especially once convection congeals.
Also, 0-3 CAPE values remain above 150 j/kg which suggest at
least the potential for a few funnel clouds with LCLs near 1 km.
However tornadoes are not expected given the environment. Heavy
rain will be a good possibility as well, with precipitable
water values progd to be 1.5-2" statewide. More details can be
found in the Hydrology section below.

Storms should depart into Missouri Sunday night giving way to
slightly cooler and dry conditions for Monday & Tuesday.
The progressive pattern returns by mid-week which will bring
storm chances (20-40%) and above normal temperatures along with
it for the latter half of the week and into the holiday weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 650 AM CDT Sun Jun 29 2025

Showers and weakening storms are expected to continue moving
east/southeast into Iowa this morning, with low probabilities
of this activity making it to KFOD, KDSM, and KMCW by mid-late
morning. Following this morning activity, uncertainty remains
higher on the chances for showers and storms later this
afternoon to evening as a cold front passes across the state.
Following recent guidance would suggest storms to fire after
20-21z over northern Iowa with the front dropping south into the
evening tonight. PROB30 groups have been included to account
for this potential later in the day, but adjustments will be
necessary if expectations change through the day. Otherwise,
outside of brief MVFR ceilings with any showers and storms, VFR
conditions are expected generally across the terminals. Variable
but light winds will gradually shift more northwesterly into
Monday following the departing front.

&&

.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.

&&

$$

UPDATE...Ansorge
DISCUSSION...Hahn
AVIATION...Bury
HYDROLOGY...Hahn