826
FXUS63 KDMX 140809
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
309 AM CDT Sat Jun 14 2025

.KEY MESSAGES...

- Storm chances increasing later this afternoon into the
  overnight and Sunday. A few strong to severe storms may be
  possible.

- High confidence in stormy pattern starting late Sunday into
  next week. While lacking specifics such as timing, location,
  and hazards, one and likely more than one round of strong to
  severe storms will be possible over the state in this period.

- Increasing confidence in heavy rainfall next week that could
  lead to flash flooding and/or river flooding later in the
  week. Similar to above, the location is not clear, but chances
  will increase if rain can repeat over the same basins and as
  time goes on in the week.

- Staying warm and humid most days through late next week.

&&

.UPDATE...
Issued at 309 AM CDT Sat Jun 14 2025

Patchy dense fog has developed in far southern Iowa into Missouri
early this morning with plenty of available low level moisture in
place. Areas that have seen fog development so far have been where
winds have gone calm in a bit of a saddle region between the low to
the southeast and the shortwave off to the northwest. By 3 AM fog
was not widespread in the area, but this will be closely monitors
through the morning for expansion in case an advisory is warranted.

Convection this afternoon is expected to develop along the warm
front that has been draped across northern Iowa for several days.
Available instability is higher than it has been the past couple
days, but shear continues to remain fairly weak. A notable
difference in the most recent model runs (00z to 06z so far this
morning) is that while earlier runs developed convection along the
boundary that would remain anchored there and percolate into the
late Avignon before fading out. That has been the case the past
several nights. The latest data, however, has escalated the
situation as it were. The boundary is further south into Iowa
than previous forecasts and there is a weak perturbation in the
flow that, while subtle, helps to weakly fuel convection across
the area resulting in less isolated and more scattered to
perhaps widespread convection in northern to central Iowa this
evening. Severe potential remains low, but not non-existent.
HRRR soundings across northern Iowa show 2000+ J/kg of ML and SB
CAPE, however entrainment CAPE reduces these values by 600-800.
With dry air being entrained and weak shear in place the sever
threat remains low, however some hail of gusty winds would
certainly be possible with these storms.

Convection will percolate along the boundary in northern Iowa
thorugh the overnight and on Sunday morning. By afternoon the
western shortwave drops across the area, reinvigorating convection
across the area. This sweeps across much of the state through the
afternoon and evening. The better instability axis at this time
remains off to the west and shear is, again, pretty weak so the
severe potenitial remains fairly low with this activity. As we
look ahead to the upcoming week the forecast is active with
multiple chances for severe storms and heavy rain. The setup
remains similar to that outlined below in the Friday afternoon
discussion.

&&

.DISCUSSION...
Issued at 257 PM CDT Fri Jun 13 2025

Upper level water vapor imagery shows an active northern stream
with multiple shortwave impulses traversing north of the state
this afternoon. To the south of Iowa, the upper closed low
continues to slowly spin east-northeastward. This latter feature
has helped to provide a focus for agitated cumulus and light
showers over our extreme southeastern forecast area. In the low
levels, the effective front is over southern Minnesota into far
northeastern Iowa and this may be the focus for a few storms yet
this afternoon to early this evening. SPC mesoanalysis shows
decent surface based instability of 1000 to 2000 J/kg, but as
has been the case the last few days effective shear is rather
marginal at or under 30 knots. Thus, not expecting anything
severe with the storms that do form this afternoon. In addition,
storms will be efficient rainers and slow movers at 10 to 15
knots at most and thus could produce a very isolated 1 inch or
so of rainfall.

After this activity diminishes early this evening, winds will be
light and with the more humid airmass may see patchy fog develop afd
midnight. For now, guidance points towards southern Iowa and have
focused the fog mention in those locations. Into the day Saturday,
the boundary to the north should begin to sag into Iowa and as such,
so should the storm chances return late in the afternoon into
Saturday night. There should be more surface based instability
Saturday afternoon than Friday and low level lapse rates are
favorable. However, the wind profile through the column has around
or less than 20 knots and thus there is not enough shear for
organized storms and the severe risk remains quite low. With the
weak flow, pinpoint rainfall amounts around an inch once again
will be possible.

The upper level pattern from Sunday into next week is looking quite
supportive for several rounds of thunderstorms. The first shortwave
trough will ride overtop the ridge on Sunday and while there are
higher amounts of instability, shear looks to remain more marginal
and highest out over western Iowa. This latter location is closer to
the day 3 SPC outlook. Additional shortwaves will ride over an
increasing favorable airmass characterized by several thousand J/kg
of instability and deep layer shear supportive of storm
organization. Thus, strong to severe storms will be possible over
some portion of the state and some of the time next week. Details on
hazard types, location, and more specific timing will be better
defined in the coming days. Further, the airmass will also be
supportive of efficient rainfall with precipitable water values at
least 1.5 to at times around or a bit above 2 inches, deep warm
cloud depths, and 850-300mb flow that will be slow early in the week
and see increasing speed later in the week. ECMWF extreme forecast
index is honing in on Wednesday into Wednesday night with some level
of ensemble support and NAEFS/ECMWF low level specific humidity is
above the 97.5th percentile. Thus, heavy rainfall could lead to
flash flooding with WPC excessive rainfall outlooks marginal most of
the week over some portion of central Iowa to slight risk over
northern Iowa by Tuesday. Obviously, these locations can change as
the rainfall forecasts change, but if rainfall can repeat, or train,
over the same area this will increase the chances for flash
flooding. Further with the rainfall, the 12z GFS forced National
Water Model (NWM) annual exceedance probabilities (AEP) at 5 days
and 10 days show low values in parts of the Cedar and upper Des
Moines River basins. Similar to the WPC outlooks, the NWM AEP does
highlight the potential for increased flows and possible river
flooding. While there will likely be a day in next week that may be
less stormy, which currently looks like Thursday, it will certainly
be an active weather week across the region with most days being
warm and humid.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1121 PM CDT Fri Jun 13 2025

VFR conditions currently prevail over the area, although stratus
and patchy fog is expected to develop through the morning
leading to MVFR to IFR conditions at most sites. Confidence in
these impacts is highest at KMCW, KALO, and KOTM, but KDSM will
also be right on the fringe of MVFR ceilings/visibility.
Conditions improve through mid-day. Showers and thunderstorms
then develop in the afternoon and evening, mainly at KMCW and
KFOD, but may drift further south later in the evening.

&&

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

&&

$$

UPDATE...Hagenhoff
DISCUSSION...Ansorge
AVIATION...Dodson