884
FXUS63 KDMX 281111
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
611 AM CDT Thu Aug 28 2025

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

.KEY MESSAGES...

- Mostly dry today and mild with highs in the upper 70s to low
  80s.

- Isolated showers and non-severe storms (<20%) possible over
  northern Iowa on Friday afternoon.

&&

.UPDATE...
Issued at 354 AM CDT Thu Aug 28 2025

A very weak boundary is located over southern Minnesota this morning
though it is very difficult to distinguish with little surface flow
near it. Its main defining feature is the pressure trough extending
through the area. Short wave energy moving over the state this
morning has resulted in a few elevated showers over east central
Iowa and there are a few sites a few hundredths inch of rain.

The southern Minnesota boundary will become a bit more
distinguishable today as it settles south into northern Iowa with
light northerly winds diurnally developing to the north and
southerly winds over the rest of Iowa. The main forecast challenge
today is the potential for any shower development in vicinity of the
boundary. Subsidence will be over much of the state in the wake of
the passing early morning short wave then the mid-levels become
neutral/static. The lack of convergence at the surface is also a
limiting factor of forcing along the boundary. The amount of
moisture available along the boundary remains in question but the
NAM continues to be much to moist through the vertical column and
thus too aggressive in instability and precipitation. Leaning the
HRRR/RAP/RRFS moisture profile blend which appears more reasonable
regarding vertical moisture availability and thus less instability.
With the lack of forcing and a moisture challenged atmosphere, have
removed shower mention with a greater chance for agitated cumulus
development with any precipitation chances <15%. There is a non-zero
chance for a few funnels to develop under the agitated cumulus
though. A nearly stalled boundary with light flow around it and
sufficient surface vorticity along with enough 0-3 km CAPE for
stretching are all enough for that non-zero funnel chance. The
potential for funnels will increase should a few showers actually
develop. A slightly greater potential for showers exists over far
north central and northeast Iowa Friday afternoon as the theta-e
differences from the boundary layer to the tropopause lower to below
10k resulting in low static stability. Thermal/moisture profiles are
also more favorable for vertical ascent.

&&

.DISCUSSION...
Issued at 202 PM CDT Wed Aug 27 2025

OAX`s 12z sounding showed just how shallow the theta-e advection is
today, with southwest flow contained between 900 and 700mb. 1-2km of
saturation is all we have to work with this afternoon, not enough
moisture to take advantage of the deep-layer shear in Iowa. A PV
anomaly seen best on GOES water vapor imagery serves as a source of
synoptic-scale lift, with a boundary remaining in southern MN. A
rumble of thunder will be possible in northwest Iowa as MUCAPE
values over 1000 J/kg in southern MN aren`t far away. Rainfall
amounts will be at most a tenth of an inch and small hail will be
possible in any robust updraft. A boundary today near the Canadian
border will sink States-side tonight, eventually weakening and
stalling out over Iowa tomorrow. Less cloud cover means we`ll mix
out better, with the chances of some light convective showers
remaining in play, especially at peak heating in the afternoon.
South of this boundary, temperatures will be in the low 80s.

For Friday, a shortwave seen this morning over parts of Washington
and Idaho will be on approach, just a slow approach thanks to the
Great Lakes high. Dry air will delay the low-level moisture
transport, decreasing intensity and coverage of rain. This is
seen best by the dry soundings with winds from the east in the
low- levels. The simulation from the NAM makes the most sense on
Friday with the bulk of showery activity occurring on the
moisture discontinuity axis between the two air masses. Trends
in 12z guidance today follows the expectation of the
stubbornness of high pressure blocking: the onset of better
precipitation is stalled. Western Iowa will have the best chance
of rain (<40%) both Saturday and Sunday. Central Iowa has about
a 20% chance each day.

The high will loosen its grip on the region early next week, giving
Iowa some low-end rain chances from the passing low pressure (<25%
chance each day). A closed low over the PNW will eject a series of
shortwaves on the northwest flow west of the high. This is why much
of next week is full of rain chances, as it remains to be seen where
in the region the waves will impact the most. What can be said is
the reinforcement of wind with a northerly component will lock highs
in the 70s for the next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 608 AM CDT Thu Aug 28 2025

VFR conditions are expected through much of the period. Cumulus
may develop over northern Iowa this afternoon around 4-5 kft
otherwise generally clear skies. Light and variable winds much
of the day though wind direction may become established for a
few periods. Generally trending more northeast to east
overnight. There may be a few pockets of fog late in the period
but confidence is low at this time.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Jimenez
AVIATION...Donavon