350
FXUS63 KDMX 091748
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1148 AM CST Tue Dec 9 2025

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

.KEY MESSAGES...

- Possible blizzard conditions developing tonight into
  Wednesday morning as strong, gusty winds of 50 to 55+ mph
  unlock existing snow pack. Gusts may surpass 60 mph overnight.
  Narrow snow bands are also possible after midnight into
  Wednesday morning.

- Active pattern continues through the rest of the week with
  additional snow chances and colder air returning.

&&

.UPDATE...
Issued at 305 AM CST Tue Dec 9 2025

The upcoming 36 hour period through midday Wednesday is setting up
to be one of high impact to Iowa and is the focus of the update this
forecast cycle. Strong winds gusting 45-55+ mph (perhaps 60-65+ mph)
will impact the state tonight through Wednesday morning and may
result in impactful blowing snow through this period.

The first phase of sensible weather impacts today will be
temperatures much warmer than much of the area has seen in a couple
weeks. Note average temperatures for this time of year range from
around 32 in northern Iowa to 37 in central Iowa and 42 in southern
Iowa. Highs this afternoon may very well mirror those averages. With
a pair of shortwaves passing north of the area, warm air advection
will surge north and push temperatures into the 30s and 40s today.
The primary complicating factor is the snow pack over central and
southern Iowa. In areas with a snow pack, warming potential
will be limited, generally closer to dewpoints as snow melts
today. In areas with snow pack have nudged temperatures down
while locations without snow pack were nudged up. While
temperatures today may be limited by the snow pack, the melting
that occurs today may have notable impacts on the next portion
of the forecast - the incoming strong winds and blowing snow
potential.

Late this afternoon the second, and more robust, shortwave will
begin to pass across northern Iowa, skimming the north and
eventually the east, with rain to start, transitioning to a
wintry mix before becoming all snow overnight. The deformation
zone with better forcing and available moisture is focused north
of the area, however this will skim northern and eastern Iowa
as the low pivots across the area. Accumulation should be light,
though northern Iowa could pick up a dusting of snow and light
ice on elevated surfaces overnight.

Strong cold air advection into the area with strong subsidence
and a tight pressure gradient will set the stage for strong
winds across Iowa tonight through Wednesday morning. Soundings
from the RAP/NAM/HRRR/GFS all indicate gusts of 45-55 kts
through the mixed layer with period of 60+ kts at the top of the
mixed layer. With the CAA/subsidence/PG parameters on the high
end, expect that area will achieve the higher end of mixing
potential and the possibility of those higher gusts reaching
nearer to the surface. For reference, 50 kts is 58 mph and 60
kts is 69 mph. Two separate surges in CAA look to occur, with
associated surges in gusts. The first occurs late tonight with
the initial cold front with the second later overnight towards
the Wednesday morning time frame with a second more robust round
of CAA. Gusts of this strength will certainly blow around snow
on the ground. There remains some uncertainty as to how
"blowable" that snow will be after melting and crusting over of
the the snow today. However, studies and past events have
indicated that with high end gusts such as those forecast
overnight, the snow age and crust can be overcome. Calculating
temperature of 32 with winds of 45-50+ kts and a blowable snow
depth over 2" yields 65-100% chances for visibility under 1/2
mile with the assumption that the model does have inherent error
and every event is different. As the previous discussion noted,
a similar event in February of 2016 resulted in notable blowing
snow with ground blizzard conditions. As such, expect that
locations with snow pack have a high chance of seeing dangerous
visibility reductions in the strong winds that will occur
overnight and into Wednesday morning.

The final factor to account for is the potential for snow
bursts/squalls as soundings contain instability and saturation
through the DGZ as we get into the morning hours on Wednesday.
Any streamers of convectively driven snow that develop would
further reduce visibility in the bands and result in a brief,
light snow accumulation.

All this to say that the next 36 hours will contain impactful and
potentially dangerous weather. A number of factors could
influence how impactful different components of this system may
be. Have issued a wind advisory for southern Iowa while
maintaining the winter storm watch for central and northern
Iowa. A transition of that watch headline will occur today.

&&

.DISCUSSION...
Issued at 253 PM CST Mon Dec 8 2025

An active weather pattern continues into this second week of
December as northwesterly flow aloft prevails and allows for a
series of shortwave troughs (aka clippers) to track through the
region. Before we get to the clippers and their associated weather,
GOES-East Day Cloud Phase Distinction RGB shows plenty of clouds
over northern and western Iowa with a few peeks at the snow pack
over southeastern Iowa. In addition, an area of fog has been
lingering over northwest Iowa with the Iowa DOT webcam at
Estherville showing a foggy scene. This fog will slowly move
eastward this afternoon into tonight largely north of Highway 30.
Will have to monitor trends into this evening if visibility declines
below a quarter mile, but so far much of today has been a 1/2 mile
at worst in Estherville, but generally above a mile.

As we move into Tuesday, one clipper will be passing into the
western Great Lakes with the next robust clipper moving into North
Dakota Tuesday morning. Between these, strong low level warm air
advection will set up and bring milder air into Iowa. However, cloud
cover will be stout, especially over northern into central Iowa, and
with the snow cover over the state, the initial National Blend of
Models (NBM) is overdone with its degree of mixing and thus warming.
So, have shrunk the diurnal temperature range and am now closer to
the dewpoint temperature, which cuts highs by 3 to 4 degrees. As the
robust clipper approaches from the northwest, its strong forcing
will move over northern and eastern Iowa in the afternoon and
evening. While low level temperatures will be warm enough for rain,
near surface temperatures could be close to or just below freezing.
This may result in a short period of freezing rain over the far
northern and northeastern counties, which transitions to rain as the
warmer air and diabatic processes win out. Any icing looks to be
minimal and relegated to elevated surfaces. With the clipper racing
off to the east, the rain may switch briefly to and end as a bit of
light snow with accumulations similarly minimal.

The big story will be the strong winds that crank up as a cold front
slams through the state Tuesday night with the potential for ground
blizzard conditions developing. There look to be two waves of cold
air advection with the first arriving in the evening. BUFKIT
soundings from the NAM and RAP show impressive winds at 850mb
topping 55 knots for 6 or more hours dropping over much of the
state. As the profile cools, deep, mechanical mixing will begin to
tap into these stronger winds so wind speeds were adjusted upward
with a blend of WRF-ARW and NBM 90th percentile. This pushes gusts
up to around 50 knots/58 mph and high wind warning criteria over
northern Iowa to as far south as Highway 30. However, it is December
and there is a snow pack on the ground, which raises the question of
blowing snow and ground blizzard. The Waterloo snow observer and
here at our office in Johnston both have a blowable snow pack on the
ground at this hour. Temperatures into Tuesday are likely to rise
above freezing, creating a crust on the snow. However, the crust is
likely to be thin and breakable given the prolonged strong wind
speeds forecast Tuesday night. We`ve seen past events where this has
occurred, including the February 7, 2016 blizzard that had higher
temperatures and a deeper crust than this event will have. So, as
temperatures cool and the winds blow, this snow pack will be
released resulting in significant blowing and drifting snow and
visibility reductions. This has resulted in the issuance of a Winter
Storm Watch for (ground) blizzard conditions. As another round of
cold air advection coupled with a trailing strong pressure rise
drops in towards sunrise Wednesday with an elevated front, this
should keep the blowing snow going into the daytime hours waning
towards Wednesday afternoon. As if this all was not enough, the
second push of cold air also steepens low level lapse rates
resulting in low level instability with saturation reaching into the
dendritic growth zone (DGZ). Snow squall parameter, especially based
off of the 1km layer version, are pinging with this boundary from
around a little after midnight over northern Iowa to early to mid-
morning over southern Iowa. Any falling snow will add to the blowing
snow and visibility reductions. Bottom line is that blowing
snow is likely to have an large impact late Tuesday night into
Wednesday morning, especially over northern Iowa, and impacts to
travel in the form of slick roads and reduced visibility are
increasing.

The active pattern continues through the end of the week with
additional clippers and snow chances with temperatures colder
Wednesday and Thursday lower compared to Tuesday. Even colder air
may arrive by late this week into this weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1148 AM CST Tue Dec 9 2025

VFR conditions are forecast to deteriorate later this afternoon
into this evening across the terminals. First, there is light
precipitation that may pass over northern Iowa with highest
chances at MCW, including a low chance of a short period of
freezing rain with temperatures rising causing a transition to
rain. At FOD and ALO, just light rain, if anything, is forecast.
As the precipitation slides out this evening, very strong and
gusty winds from the northwest will spread over the state along
with MVFR ceilings tonight. Wind gusts over 40 knots are likely
at all terminals and a few gusts to or a bit over 50 knots are
possible, especially over western and northern Iowa. Low level
wind shear may result and have left previous forecast LLWS;
however, the vector difference may not be great enough for true
LLWS and this may end up being more turbulence. With the strong
winds, visibility restrictions may occur if the snowpack on the
ground is able to be unlocked and blown and have this
advertised at MCW, FOD, and ALO to varying degrees. Narrow,
intense snow bands, or snow squalls, may develop after 6z until
around 12z, with this shown at PROB30 groups over northern
Iowa. Ceiling restrictions will persist through the end of the
period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from 9 PM CST this evening through
Wednesday morning for IAZ004>007-015>017-023>028-033>039-
044>050.
Wind Advisory from midnight tonight to noon CST Wednesday for
IAZ057>062-070>075-081>086-092>097.

&&

$$

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