877
FXUS63 KDMX 221112
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
612 AM CDT Sun Jun 22 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Heat headlines remain unchanged. Dangerous heat continues
  through Sunday night.

- Marginal Risk of severe weather late Sunday night, followed by
  a Marginal/Slight for continued storms throughout Monday.

- Heavy rain anticipated through at least the first half of the
  week with hydro impacts possible (see hydro discussion).

&&

.UPDATE...
Issued at 404 AM CDT Sun Jun 22 2025

Not much change in thinking regarding the forecast during the day
tomorrow, as hot humid and windy conditions are still expected
today. Most of the deliberations from the overnight forecast
pertained to handling of the heat advisory over the eastern portions
of the area. Heat indices of 105F or greater will end this
afternoon, as does the current Heat Advisory and Extreme Heat
Warning. However, while the warmest temperatures end today,
overnight temperatures still remain quite warm with lows only
falling into the mid to upper 70s tonight. Likewise, we see heat
indices climb into the upper 90s to near 100 again on Monday
afternoon, primarily in southeastern Iowa. This sparked conversation
into extending the current Heat Advisory through tonight and
potentially into Monday given the prolonged nature of the heat and
little relief overnight. That said, there remains some uncertainty
regarding the approaching front and coincident convection tonight
through Monday, which will have an impact on temperatures as it
moves into the area. A faster progression of the front and/or
outflow boundaries could bring relief to the hot temperatures
overnight, especially in the areas of the advisory farther north and
west. Similarly, the expanse of convection mid-day Monday could
squander how warm temperatures get into the afternoon as well.
Therefore, while it stands to reason that warm, humid conditions
will persist into Monday for some, they will likely be sub-advisory
values and potentially even cooler should convection be more
bullish. Given the low confidence in heat beyond today, have held
off on any changes to the current headlines for now. If confidence
increases in the progression of the front and coincident convection,
could see a need to extend at least some of the advisory into
tonight and even Monday, especially in the southeast.

As I mentioned in the previous paragraph, precipitation and
thunderstorm chances return to the forecast late tonight into Monday
as the upper high pressure to our southwest begins to form a sort of
"ring of fire" setup through the beginning of the work week. The
first of these chances will arrive with a surface boundary being
pulled behind a wave revolving around the ridging. This boundary
first lights up storms primarily over Nebraska, South Dakota, and
Minnesota late this afternoon and evening, staying primarily out of
Iowa through midnight. As we get into the early morning hours on
Monday, the surface front and any associated convection will slowly
leak southeast into Iowa. Instability will be waning as this occurs,
which should help to limit any severe convection festering over
northwest Iowa overnight. But of some concern will be the 40 to 50
kt low level jet nosed up into the state on Monday morning which
could help provide forcing for storms to take off overnight. That
said, the saving grace to overnight storms will be the elevated warm
layer still over the area Monday morning, which would need to be
overcome for any parcels to take off. This is likely the main reason
no convection is being modeled in the CAMs, despite the stout low
level jet. Still, just it`s existence demands some monitoring,
especially in such a moisture rich environment.

After day break on Monday, the boundary will continue to sag
southeastward into the area and reignite through the afternoon. This
timeframe will bring a much higher likelihood for storms. This
environment will boast 2000 to 3000 J/kg of instability which
becomes surface based by the mid-afternoon hours. Fortunately, with
primarily unidirectional flow through the layer, 0 to 6 km shear
values will be under 30 kts ahead of the boundary, but then
increasing behind where surface winds are more northwesterly.
Low level shear will also be meager, only showing around 15 to
25 kts. Some near-severe hail may be possible, but the warm
profile and lack of organization will make large hail hard to
come by. However, with a well mixed boundary layer and inverted
V soundings, wind will be a concern with any collapsing updrafts
and/or propagating cold pools. As is also mentioned in the
forecast and hydrology discussions from yesterday, efficient
rainfall will be a concern, further enhancing the threat for
downburst winds/cold pool production but also posing a hydrology
threat for both flash flooding and river flooding. This will be
especially true with a mean wind generally oriented along the
boundary, promoting training storms and prolonged periods of
rainfall. Both the storm prediction center and weather
prediction center now have slight risks for severe weather and
excessive rainfall, respectively, further solidifying the threat
for strong storms and heavy rainfall/flooding concerns on
Monday.

&&

.DISCUSSION...
Issued at 254 PM CDT Sat Jun 21 2025

Moisture was on the rise during the day and the environment
reflected this at more than just the uncomfortable, muggy conditions
at the surface. Mid-level Water Vapor imagery shows the plume of
moisture propagating around Deep South ridge, angling into
Texas and across the Central Plains. To quantify this moisture,
DDC`s 12z sounding this morning was right at the 90th percentile
for 850mb dew points (the value this morning was 14C). The
seasonally strong 50kt LLJ has been transporting this moisture
into the Upper Midwest, bearing responsibility for the multiple
rounds of convection across Minnesota and Wisconsin. Pair this
with a seasonably strong upper jet atop the ridge, contributing
to organization from shear and intensity of winds at the
surface. Enough convective precip loading is happening above the
stout 16+C EML for showers and storms to form above it in
Nebraska and into Southwest Minnesota. This being driven by a
surface low deepening over South Dakota.

The synoptic pattern won`t deviate much for the next several days,
at least as long as the ridge in the Deep South is stalled. The
short-term impact from this is the muggy heat. There could be some
spots in the west today that reach the century mark for highs, this
being less common due to the moisture content in the air that
is difficult to warm. Regardless, temperatures statewide will be
in the mid to upper 90s the next two days, highest in the north
and west. The moist air also holds heat well, which creates a
scenario where there is little to no reprieve overnight. Lows
will be in the upper 70s and Apparent Temperatures around 80
degrees. Not much deviation in the temperature forecast for this
weekend and consequently headlines. The bottom line is that
dangerous, prolonged heat is here to stay today and tomorrow.
The other aspect to the short term is the winds, which may
provide marginal relief to the heat. Sustained winds up to 30mph
will be common this afternoon. The jet keeps going through the
day so winds not far aloft in the PBL are valued at and above
40kts, some of this will be able to mix down the rest of this
afternoon and on Sunday as well, especially in the north. This
will be sporadic enough to not issue a Wind Advisory, at this
time as the WAA regime and already warm temperatures will
inhibit momentum transfer.

Now for the more loaded  part of the forecast, that being the
precipitation, beginning with today. Elevated thunderstorms will be
possible across northwest Iowa this afternoon, some of which could
have small hail with them that will have mostly melted in the warm
air mass. Chances of storms is below 10%. Moving on to Sunday night,
a deepening trough from the Intermountain West will take the surface
low to the northeast, setting up its cold front over northern and
northwest Iowa. Wind shear will be parallel to the front and will be
weaker due to upper-level ridging amplifying in response to the
trough. It will also be poorly oriented for storms that make their
way southeast towards central Iowa. Over central Iowa, NAM soundings
indicate the potential of elevated hailers ahead of the front after
3am. The sustainability of damaging winds and hail with these is
going to be short-lived given the shear profile mentioned before.
The storm mode on the front will likely be a line given the shear
orientation, which will drop an outflow boundary southeast towards
central Iowa. Inverted-V soundings and CAPE values around 2000 J/kg
offers a window of severe weather along the outflow, but with the
better shear remaining along and behind synoptic front; storms that
fire along this boundary will have an early window for more intense
severe weather, but quickly become disorganized as they grow
upscale. This covers the marginal risk in place for Monday, both
with damaging winds and hail in play. Storm modes will be messy
clusters. Theta-e differences exceeding
-20K will generate stronger, but disorganized cold pools
which will only add to the mesoscale forcing mechanisms for CI.
Heavy rain, perhaps with localized flash flooding, will be possible
due to slow storm motions and moist profiles.

That brings our main focus: the heavy rain setup for the week. PWAT
values are over 2 inches (175% above climo), warm cloud depths are
pushing 12kft, and the mean wind parallels that of the LLJ. On the
synoptic front left behind by its parent low: it will be draped SW
to NE beneath weak synoptic flow. Since its orientation mimics that
of the ridge, the flow that does exist runs parallel to it. This is
a synoptic/frontal setup primed for multiple days of heavy rain. The
combination of mesohighs and nocturnal jets will wobble this front
north and south. A wave that arrives late Wednesday will finally
make the boundary more progressive in nature. However, zonal flow
sets in following this front`s departure, leaving the window open
for more rainfall into next weekend. So far, there are slight risks
for excessive rain through the first half of the week. It is likely
(80-100%) that most of northern and northwest Iowa will see at least
2 inches of rain total this week. High end amounts right along the
front itself could be 5+ inches, the location of which will be
better understood as mesoscale features are captured in the CAM
window. The flash flooding confidence is increasing for this time
frame and a flood watch will likely be needed in the coming days.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 608 AM CDT Sun Jun 22 2025

VFR conditions prevail through the period, with gusty south
southwest winds being the primary impact to TAFs this period.
Sustained winds of 20 to 25 kts expected with gusts up to 35 kts
possible. &&

.HYDROLOGY...
Issued at 254 PM CDT Sat Jun 21 2025

Current precipitation trends have the heaviest rain starting late
Monday into Monday night on a line extending from Creston up to
Waterloo. As mentioned in the discussion above, high confidence in 2
inches of rain for the week across much of the state with some of
the highest amounts found somewhere in this corridor. Rainfall
amounts exceeding 1 inch per hour possible Monday night.

HEFS guidance reflects the higher sensitivity of the Cedar and
Raccoon River basin which has received the most rain recently. The
25th-75th percentile curve comes into minor flood stage for sites
such as PROI4, MCWI4, and ALOI4. More runoff is possible in the
Cedar compared to the Raccoon as flash flood guidance in northeast
Iowa is some of the lowest in the state right now (the 6 hour
guidance is 2 inches). Less certainty of flood potential on the Des
Moines and Skunk basins as these have been less impacted in the past
week, making their flood potential more dependent on where the heavy
rain axis ends up. The high-end scenarios bring both of these into
flood.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Extreme Heat Warning until 9 PM CDT this evening for IAZ004-
005-015-023-024-033>035-044>046-057-058-070-071-081-092.
Heat Advisory until 9 PM CDT this evening for IAZ006-007-016-
017-025>028-036>039-047>050-059>062-072>075-082>086-093>097.

&&

$$

UPDATE...Dodson
DISCUSSION...Jimenez
AVIATION...Dodson
HYDROLOGY...Jimenez