536
FXUS63 KDMX 171147
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
547 AM CST Mon Nov 17 2025

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

.KEY MESSAGES...

- Increasing precipitation chances this afternoon and especially
  overnight. A few non-severe thunderstorms are possible this
  evening. A brief period of light snow could occur far north
  late tonight.

- Lowered highs most areas today and the entire area for
  Tuesday.

&&

.UPDATE...
Issued at 406 AM CST Mon Nov 17 2025

This forecast update will be focused on today through Tuesday as a
system moves across the region. An upper level PV anomaly is over
Utah and Colorado early this morning and that system will move east
northeast through this time period and will be over Iowa by Tuesday
morning. Lee side surface cyclogenesis is already ongoing and that
will continue today with an area of low pressure organizing over
eastern Colorado and western Kansas. This system combined with
departing surface high pressure that is situated across the entire
CONUS just east of the Mississippi River, will lead to
increased low level moisture advection northward towards Iowa.
Stratus is already blossoming from Arkansas to southeast Kansas
as this moisture lifts north. The mid-level moisture advection
is already ongoing and is further supported by a few strong
waves of theta-e advection over the area. Mid-level cloud
development is occurring in response. The amount of cloud cover
today will be the driving factor in high temperatures today. The
mid-level clouds will be most abundant over the northeast half
of the area. The aforementioned stratus will arrive across
southwest Iowa early this afternoon and expand northeast. There
could be a gap region between the two that could have more solar
insolation today. Have shaded temperatures in the anticipated
cloud covered areas to near the NBM 25th and other areas closer
to the NBM mean. Precipitation chances will also increase this
afternoon from west to east as the moisture deepens and the
initial kinematic forcing associated with the lead PV anomaly
fragments arrives.

A few embedded thunderstorms are possible, mainly this evening as
the mid-level lapse rates steepen and parcels lifted from around 850-
800 mb realizing MUCAPEs to near 500 J/kg. While these storms
will be non-severe, this type of environment may be conducive to
graupel development. A reminder, this is not sleet. That
instability wanes after 06z as the precipitation wraps through
northeast Iowa and into southern Minnesota. Still monitoring the
potential for a transition to snow over far northern Iowa.
There are several potential failure points for the transition
to snow to lower the confidence on the probability of
occurrence. #1. Deterministic proximity soundings through the
region suggest a warm/melting layer lingering around 750 mb.
This leads to point #2. Timing of mid- level drying and the loss
of ice introduction. By the time the melting layer cools, ice
introduction is starting to be lost. If there is a cross over of
cooling prior to the loss of ice introduction, snow through
this layer is possible. Finally point #3. The near surface warm
layer. a review of thousands of surface observations and
precipitation type, in collaboration with IEM, has shown that a
surface wet bulb temperature of 33.5-34 degrees F is sufficient
to full melt snow flakes to rain. Surface wet- bulbs are going
to be close to this. The one item to monitor is the easterly
flow that is still originating from the high pressure to the
east could help lower dew point values and thus wet bulbs. There
are just too many failure points to forecast accumulating snow.
The precipitation will gradual end as that loss of ice occurs
later Monday night into Tuesday morning and may transition to
drizzle for a period prior to ending. Did increase precipitation
chances further south in the morning as a stronger vorticity
fragment is lingering on the backside of the parent PV anomaly
and that forcing should have another wave of showers moving over
central and southern Iowa. Stratus should remain in place
throughout the day and have lowered high temperatures by several
degrees for Tuesday.

&&

.DISCUSSION...
Issued at 243 PM CST Sun Nov 16 2025

<< DRY AND MILD TODAY, PRECIPITATION RETURNING THIS WEEK >>

Quiet and mild conditions are in place over the state today, as
surface high pressure dominates the pattern. Cloud cover has slowly
increased from the west, but is struggling as it moves into the very
dry airmass over Iowa. These clouds will eventually span much of the
area, which combined with increasing winds, should help keep
temperatures from plummeting tonight. The increasing winds tonight
and breezy conditions on Monday will be indicative of the tightening
pressure gradient in response to lee-side cyclogenesis in the
plains. This cyclogenesis is coincident with the PV anomaly that is
now making it`s way through the Rockies, which will eventually track
through Iowa and bring our next chances for precipitation late
Monday into early Tuesday.


<< MONDAY/TUESDAY SYSTEM: MOSTLY RAIN, BUT SOME THUNDER AND SNOW? >>

As the wave passes across the plains tomorrow, a strong push of
theta-e advection will lift into the area. The associated plume of
low and mid-level moisture will work to saturate through the dry
antecedent air mass, eventually saturating down to the surface
through late Monday afternoon and evening. Precipitation will
increase in likelihood farther north and east in Iowa, although
occasional rain/showers are possible throughout the area Monday
afternoon and evening. A stout warm nose in the mid-levels will keep
precipitation as mostly rain over central and southern Iowa. With
this warm nose in place, lapse rates even steepen enough to
produce some instability aloft, introducing the potential for a
few rumbles of thunder as rain moves through. This will be most
likely in the southern half of the forecast area where
instability values could approach 200 to 300 J/kg, depending on
the model you pick. Any convectively driven parcels would be
rooted quite a ways aloft, but with the cold profiles, can`t
rule out some small hail, which this time of year may perceived
as sleet, although the processes for creating the two types of
ice pellets does differ.

The precipitation forecast becomes a bit trickier over northern Iowa
where colder air and stronger forcing will bring potential for
wintry precipitation Monday night into Tuesday. Along and just north
of the Iowa/Minnesota border, a band of stronger frontogenetical
forcing will set up in the 800 mb to 700 mb layer generating strong
lift up into the dendritic growth zone. The warm nose aloft is
coming in a bit weaker with today`s guidance as well, resulting in a
temperature profile nearly isothermal along the 0 C isotherm
once saturation occurs. Surface temperatures and wet bulbs are
still forecast to be above freezing, but if that stronger
forcing leaks into far northern Iowa, it`s possible the shallow
surface warm layer won`t be able to melt the volume of snow
falling into it. If these heavier rates come to fruition, areas
in northern Iowa could pick up a quick inch or two of snow
Monday night/Tuesday morning. Of course, if this better forcing
sets up north of the area, or the warm nose is a few degrees too
warm, precipitation would fall as a cold rain instead. Right
now, the expectation is still that the bulk of the snow is going
to stay to our north. Mostly rain will fall in our forecast
area, with occasional snowflakes mixed in along the northern
border. If a change over to snow does occur and we see
accumulations south of the Iowa/Minnesota border, deterministic
and ensemble guidance both suggest it would be most likely in
areas north of highway 18. Even then, recent warm conditions
should have ground temperatures warm enough to melt the snow as
it falls, further reducing any accumulations.

Through early Tuesday morning, deeper saturation will fade in and
out as drier air works into the mid and upper levels, reducing ice
introduction. Low level saturation will stick around through the
morning hours, keeping low clouds. Brief drizzle may still be
possible through the morning on Tuesday, but a relative lack of lift
in the low levels is going to be working these chances. Likewise,
the saturated layer isn`t looking nearly as deep as it did in
soundings yesterday, further diminishing the potential. Like with
the snow, if drizzle does occur, surface and ground temperatures
should be warm enough to negate significant freezing in northern
Iowa.


<< REST OF WEEK, THURSDAY/FRIDAY SYSTEMS >>

Although they may not be as supportive for drizzle, saturation in
the low levels will keep low cloud cover overhead through much of
Tuesday, keeping temperatures cool through the day. For this reason,
have knocked high temperatures down a few degrees on Tuesday as
guidance is likely too aggressive with mixing out the boundary layer
given the cloud cover and CAA. By Wednesday, return flow develops on
the back side of the Great Lakes high, bringing warmer temperatures
through the middle of the week. By Thursday, two 500 mb waves will
pass through the central US and Canada, influencing the weather
through the second half of the week. How these two waves interact
will be worth watching, as the GFS brings the northern wave far
enough south to keep the southern wave suppressed and the
precipitation south of Iowa, while the ECMWF shows the southern wave
lifting up into southern Iowa, bringing widespread rain chances to
Iowa on Friday. More on these systems in the coming days.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 541 AM CST Mon Nov 17 2025

Deteriorating conditions from west to east this afternoon and
the overnight as cigs lower and shower and isolated
thunderstorms develop. East/southeast wind will increase this
morning and will be gusty at times through the period. VFR
conditions through mid afternoon is expected before the
precipitation arrives and cigs gradually lower to MVFR at times.
At this time, only included thunderstorm mention this evening at
KDSM. IFR cigs may develop over northern Iowa after 06z.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Dodson
AVIATION...Donavon