737
FXUS63 KDMX 251806
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
106 PM CDT Wed Jun 25 2025

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

.KEY MESSAGES...

- Heavy rainfall and thunder continues over the area early this
  morning. Localized flash flooding and ponding of water is
  possible. Severe weather threat is low.

- Additional thunderstorm development this afternoon and
  evening, mainly over northern Iowa. Heavy rainfall is likely.
  Severe weather is possible with strong winds being the main
  concern. Hail and tornado risk will be conditional.

- One more round of widespread storm chances Thursday afternoon
  and evening. Strong wind gusts and heavy rainfall will be
  possible with these storms.

&&

.UPDATE...
Issued at 331 AM CDT Wed Jun 25 2025

We begin yet another morning with heavy rain falling over portions
of Iowa, with rain and thunderstorms slowly drifting north along the
nose of a 30 to 40 kt low level jet. These storms are moving
relatively slowly and produced anywhere from 1 to 2 inches of rain
in portions of southwestern and south central Iowa early this
morning. Fortunately, precipitation will continue to slowly drift
northeastward out of the areas hit with the heaviest rainfall
yesterday and into central and eventually northern Iowa by later
this morning. Individual storm motions this morning are slow,
but the mass of rain should be progressive through the morning.
This will help to mitigate major flash flooding issues, but we
will be monitoring observed rainfall rates and amounts through
the morning in the event that flash flooding does occur. The
severe weather threat this morning is low given the saturated
profiles reducing lapse rates and limiting instability.

This heavy rainfall and thunderstorms are attached to a
shortwave currently lifting northeastward over Nebraska and into
South Dakota and Minnesota through the morning. This low will
eventually lift into Minnesota round mid- day but leave an
elongated trough of low pressure over northern into northwestern
Iowa this afternoon. Convergence along this trough will be the
genesis region for additional storms this afternoon and evening
across northern Iowa. At the same time, clearing skies and theta
e advection occurring over much of central Iowa today will
generate a moist, unstable environment southeast of this
genesis region, providing fuel for storms into the evening. This
environment will boast 2000 to 3000 J/kg of SBCAPE, dewpoints
in the 70s, and LCLs of 500m or lower. However, despite the
favorable thermodynamics, wind fields will again be the
determining factor of how strong storms will become this
afternoon. Deep layer and low level shear values this afternoon
look a bit better than what we`ve seen with recent severe
weather events, but the severity of storms is somewhat
conditional and dependent on how convection this morning and
into the afternoon plays out.

Model guidance is depicting two different solutions in terms of the
wind fields. The NAM suite shows more south southeasterly surface
winds this afternoon, while models like the HRRR, RAP, and GFS have
winds swing to more southwesterly. The differences between
these two camps is how they handle convection this morning prior
to initiation in the afternoon. The HRRR, RAP, and GFS have a
stronger meso-low with an MCS-like feature moving through
northern Iowa and into Minnesota, which keeps convection over
the north for longer and doesn`t allow for any significant
surface boundary development. Conversely, this feature doesn`t
exist in the NAM and therefore allows for a surface boundary to
develop in the afternoon, keeping winds more south
southeasterly. The net result is better turning in the low
levels (higher SRH) and higher 0-3 km and 0-6 km shear values
from the NAM, but more modest shear values with less turning in
the hodographs from the HRRR/RAP/GFS. Therefore, the evolution
of mesoscale features this morning and through the day today
will have a significant impact on the severity of storms this
afternoon. Should the NAM solution pan out, rotating
storms/supercells would be possible, increasing the chances for
all severe hazards, including tornadoes. If the morning
convection mucks up the wind fields, as the HRRR, RAP and GFS
suggest, severe weather will still be possible (modest 0-3 km
shear should still help to keep cold pools from racing out ahead
of storms), but it would be primarily a wind threat with any
storms that organize along the boundary and less of a tornado or
hail threat. Therefore, will be monitoring convective and
mesoscale trends closely today. The storm prediction center has
now placed a slight risk (level 2 of 5) for severe weather over
northern Iowa for today. In addition to the severe weather
potential, the threat for heavy rainfall will continue. For more
information on heavy rainfall and potential flooding/flash
flooding concerns, please see the updated hydro discussion
below.

Beyond today, we will see one more round of widespread storm chances
passing through the area on Thursday as troughing to our west
eventually lifts up over the upper ridge and pulls a north-south
oriented boundary from west to east through the state. Storms will
fire along this boundary through the afternoon and evening, taking
advantage of the unstable airmass overhead. Wind shear values will
again be borderline for severe weather, with 25 to 30 kts of deep
layer shear along the boundary as it moves through Iowa. The main
risk with these storms looks to be strong winds, but will see how
storms this morning and again this afternoon play out before getting
too ingrained in the specific details of Thursdays severe
threat. The storm prediction center has left the severe weather
risk as a marginal threat (level 1 of 5) for Thursday.

&&

.DISCUSSION...
Issued at 258 PM CDT Tue Jun 24 2025

Heavy, efficient rain of early this morning has gradually decreased
in intensity with time through the later morning to early afternoon
hours today but rainfall amounts into late morning vary from 4-6"+
in portions of southern to southwest Iowa, approaching 1-2" in
portions of central Iowa with 2-4" in locations in between and
lighter amounts in much of the north except for isolated locations.
Limited reports have been received in the way of impacts from this
excessive rain thus far in southern Iowa though certainly conditions
are more primed for hydro issues going forward this week with
subsequent rounds given some of these higher end rain amounts in the
last 24+ hours with some noted rises already seen on some area
rivers. See the hydro discussion below for more details on the hydro
threat through the next several days.

For the rest of today, generally expecting isolated to scattered
showers and storms to persist through the rest of the afternoon and
evening hours. Although the overall severe threat has lowered for
this afternoon and evening with the lingering rain and cloud cover
through much of the day, a few stronger storms remain possible in
portions of the area like where clearing has occurred more in
portions of far southern Iowa, or near to south of the lingering
surface boundary that is roughly in between the Highway 30-20
corridors. Overall parameter space is most favorable in portions of
southern Iowa where instability/shear are highest (outside of areas
receiving rain early this afternoon), per SPC mesoanalysis, and as
reflected in the SPC Day 1 Level 1 (of 5) risk which has been pulled
back to southern Iowa. For any stronger storms that can develop,
gusty winds remains the primary threat, with some small hail
potentially possible if a more organized updraft can develop. Storm
motions will remain slow, around 10-20 knots, so heavy rain will
continue to be a threat with any activity that occurs. In addition,
cannot rule out some funnel cloud development in portions of north
central/northeast/east central Iowa where there is an overlap
between the 0-3km CAPE and surface vorticity with a few funnel
reports having been noted in adjacent CWAs already this afternoon.
Have addressed this threat with a SPS.

As we get into tonight, forcing begins to increase as a shortwave
nudges into the region into Wednesday as well as the LLJ redevelops
late pointing into eastern NE/western IA. With mean flow overall
still largely parallel to the surface boundary, expect more training
storms that could be very efficient rain producers, especially on
the nose of the LLJ. Thus, we once again transition late tonight
into the overnight into another heavy rain threat, though as noted
in the previous update, CAMs continue to suggest disorganized
convection developing along the instability gradient in the
south/southwest with storms pushing northeast with time. Should
these become more organized, some wind threat could develop, but
this is lower probability scenario with shear remaining weak.

The severe threat increases some diurnally Wednesday, especially
north, as instability balloons into the afternoon and shear is
actually a bit stronger than in previous days. This is also where
the surface boundary looks to lift (towards the MN/IA border) by
Wednesday afternoon providing a further focus for storm development.
However, as in the case of today, overall threat and location will
be dependent on the morning activity, cloud cover, location of the
boundary and necessary destabilization. As with the previous days,
wind will remain the primary severe threat, though some small hail
may be possible if more organized storms can develop. Will also need
to watch the flow near the boundary as if any backing can occur,
than a tornado cannot be completely ruled out either, though this
remains less likely. Although this activity Wednesday
afternoon/evening looks to be in northern Iowa, displaced from the
heavy rain here to start the week, we`ll still need to continue to
monitor the heavy rain threat with still efficient rain likely given
pwats remain near 2" with favorable warm cloud depths even though
storm motions themselves should become slightly more progressive.

Additional activity is expected overnight into Thursday and then
again diurnally Thursday afternoon and evening continuing our active
week with additional chances for severe weather and heavy rain that
will continue to heighten the flooding threat into the end of
the week both from a flash flooding and a river flooding
standpoint (again, see hydro discussion). Refinements to the
going forecast will continue to be made in subsequent days with
again the extended NBM forecast overdone in space and time
though with boundary uncertainty, little confidence to make too
many adjustments outside of the short term forecast.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1252 PM CDT Wed Jun 25 2025

Lingering showers over northern Iowa early this afternoon with
new showers and storms starting to develop as well. Challenging
aviation forecast going forward with thunderstorm timing and
impacts given lingering uncertainty exactly when/where
showers/storms occur. Most favorable locations remain north -
KFOD or KMCW, though new development is occurring over southern
Iowa as well, near KOTM. Confidence in showers/storms reaching
ALO remains lower, but kept with previous Prob30 group.
Adjustments in timing/impacts likely as storms develop this
afternoon but localized visibility drops could occur as well as
gusty winds with any stronger showers/storms. Although VFR
conditions will largely prevail, MVFR CIGs expected at times
with some IFR possible north, especially through the overnight
as additional rounds of showers/storms move through.

&&

.HYDROLOGY...
Issued at 331 AM CDT Wed Jun 25 2025

Both flash flood and river flooding remain a concern today through
later this week, with river flooding concerns persisting well into
next week.

There has been a persistent signal of heavy rainfall mainly through
Thursday night into Friday morning, however uncertainty in the exact
placement of highest rainfall amounts remain. The latest 25/00Z HREF
and NBMv5.0 continue to show the axis with the highest QPF amounts
transitioning to portions of northwest and north-central Iowa
starting this afternoon, specifically along and north of US Highway
30. Confidence is increasing on seeing widespread amounts of 2-4" in
this area, with higher end amounts (from the HREF PMM or NBM 95th
percentile data) showing pockets of 5-8", with max amounts of 10+"
still in play. As is typically the case in these scenarios,
mesoscale details /e.g. exact storm initiation locations and
placement of outflow boundaries/ cannot be accurately predicted at
this time range, so the placement of the heaviest precip may need to
be refined.

The hydrologic response will depend heavily on the meteorological
activity and resultant QPF. Flash flooding will be a possibility
anywhere in the CWA where persistent heavy rainfall occurs, with the
flash flooding possibly being triggered by either the rainfall
intensity or the sheer volume of rainfall.

In terms of river flooding, we have extended the QPF used in our
official river forecasts from the normal duration of 24 hrs during
the warm season, to 72 hrs for this event -- recognizing the longer
term duration of the heavy rainfall. This change to QPF duration
pertains only to those streams in the Mississippi River drainage
portion of our CWA, or roughly across the northeast 2/3rds of our
CWA.

Given the change in QPF duration for our official river forecasts,
those forecasts along with our QPF ensemble hydrographs and HEFS are
all suggesting significant rises on many streams with minor to
moderate flooding indicated later this week into at least early next
week. The larger streams of most concern presently include the Iowa,
Cedar, Winnebago, Shell Rock and Raccoon Rivers. As always, though,
the actual geographic placement and amount of the rainfall will be
important in determining the actual hydrologic response. Even a
difference in heavy rainfall placement of 10-20 miles, for example,
can significantly impact the actual vs. anticipated hydrologic
response in neighboring stream basins.

Given the timing in those forecasts (i.e., forecast to reach flood
stage generally 3, 4 or more days out from the present time), as
well as lower confidence since it so far out in time, we opted to
mention these threats in our Hazardous Weather Outlook (HWO) instead
of issuing river flood watches or warnings at this time. As the time
draws closer and confidence increases then we will migrate to river
flood watches or warnings as needed.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday afternoon for IAZ004>007-015>017-
023-024-033-034-044.

&&

$$

UPDATE...Dodson
DISCUSSION...05
AVIATION...05
HYDROLOGY...Fowle/Zogg