Grimes, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

393
FXUS63 KDMX 282347
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
547 PM CST Fri Nov 28 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Prolonged snowfall through Saturday has begun in the west
  early this afternoon. Significant snowfall amounts of 8 to 12"
  are expected through Saturday evening, with some locally
  higher amounts possible.

- Gusty winds behind the snowfall may lead to blowing and
  drifting of fresh snowpack on Saturday night.

- Much colder temperatures move in on Sunday, with wind chills
  in the single digits above and below zero.

&&

.DISCUSSION...
Issued at 250 PM CST Fri Nov 28 2025

The well-advertised winter storm for this weekend is now knocking on
our doorstep early this afternoon. The responsible wave is now
making it`s way over the Rockies, with surface low pressure building
over the plains. Pressure gradients are tightening in response to
this, increasing southerly flow and beginning to stream gulf
moisture northward. Weak frontogenesis on the periphery of the
building low pressure is currently producing radar returns
(snow) over eastern Nebraska, western Iowa and up into the
Dakotas where a lobe of better upper level moisture is also
present. This precipitation is combating some drier air below
and struggling to reach the ground, although surface
observations and MPING reports are already beginning to report
snow over the western portions of Iowa.

As we progress through this afternoon, this precipitation aloft will
slowly drift east and continue to saturate down through the dry
layer beneath. At the same time, increasing low level moisture
transport and theta-e advection will begin to lift northward ahead
of the building surface low, giving this area of precipitation the
extra push it needs to reach the surface. This will mark the
beginning of our prolonged period of snowfall over the state.
Precipitation coverage will initially be focused under this
initial lobe of weak frontogenesis and theta-e advection.
However, as the 500 mb wave continues toward the central CONUS,
and deepens into a pronounced trough, the building surface low
pressure will close off and lift toward the state. This will
bring strong isentropic lift up into Iowa, causing the
precipitation to expand over the entirety of the state
throughout the day Saturday.

Snowfall amounts have continued the upward trend this shift, now
increasing southward and slightly eastward. The expectation is still
that most of the area will see amounts in the 8 to 12" range, with a
band of higher amounts up to 15". This heavier band likely won`t be
widespread, but instead in the "Venn diagram" where the heavier
snow from the theta-e wing tonight meets with the heaviest
rates from the actual low pressure passage tomorrow. This area
of higher (13"+) amounts varies somewhat from run-to-run and
model-to-model, causing NBM to produce a fairly large area of
13"+ values in the forecast. Generally, the expectation is still
for these higher amounts to be along interstate 80 and north,
favoring the eastern portions of the forecast area. Of course,
once you reach a certain point with snow amounts, it becomes
more nitpicky than impact-driven. This prolonged snowfall is
going to create travel impacts for most of the area on Saturday,
whether your location ends up on the higher or lower end of
that spread. Likewise, areas that only briefly cross through the
heavier bands will still see periods of 1"/hr rates or higher,
which will lead to visibility reductions.

Farther south in the state, there is still some question on snowfall
amounts as warmer surface temperatures attempt mix rain with the
snow. However, with the slight southwestward trend in higher
amounts, have seen an increase in forecasted snow totals over
southwestern Iowa. For this reason, have upgraded a few counties in
southwestern Iowa from the advisory to a winter storm warning. It`s
still possible the warm air wins out in these areas, but model
soundings indicate the surface warm layer will be extremely shallow
and wetbulb temperatures only around 33 to 34 F at their warmest. It
stands to reason that heavier rates will be able to overcome this
shallow warm layer, and snow through it. Dynamic cooling of the
layer may occur as well, as snow falls an melts (removing heat from
the air) but southerly flow/warm air advection will work to negate
this cooling. Given this delicate balance, this area definitely has
the most uncertainty in snow amounts. In addition to this rain/snow
mix, there may also be a brief period early Saturday morning where
ice introduction is lost and freezing rain/drizzle develops. This
seems to be a fleeting threat and significant accumulations aren`t
expected. However, be on the lookout for isolated patches of ice in
southwestern Iowa.

The heaviest snowfall rates should be wrapping up around the early
evening in our forecast area with lingering light snow lasting till
about 10 pm to midnight. While the falling snow should be improving
through this timeframe, there is a growing signal for wind gusts
increasing to around 25 to 35 mph behind the cold front late
Saturday afternoon and evening. These winds should primarily be
displaced from the heavier snowfall rate that are associated with
the warmer portion of the system, but a brief phasing of falling
snow and increasing winds could lead to reduced visibilities into
Saturday evening. Likewise, fresh snowpack/loose snow on the ground
will be susceptible to blowing/drifting around. This blowing snow,
in conjunction with snow still on the ground, will prolong travel
impacts into Saturday night and early Sunday morning. Winds/gusts
will then diminish through Sunday morning.

High pressure and colder air fills in behind the system on Sunday,
dropping temperatures over the area. Wind chills on Sunday morning
will be near and below zero, making for quite chilly conditions for
anyone getting up to move snow on Sunday morning. These cold
temperatures and wind chills continue through the first part of the
week, with highs on Sunday and Monday only reaching the upper teens
to low 20s and overnight lows falling into the single digits Monday
and Tuesday mornings. Wind chills through this same time will be
quite cool in the single digits above and below zero. Of note,
another system looks to clip the state on Monday into Tuesday,
bringing additional snow chances to the area. Fortunately, amounts
look marginal compared to this weekend`s system, with ensemble
probabilities showing a roughly 50 to 60% chance for an inch or
more of snow in southern Iowa and only a 30 to 40% chance for
two inches or more. Additional details on the early week system
will be provided in the coming days.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 536 PM CST Fri Nov 28 2025

Worsening conditions through the TAF period due to snow. Periods
of moderate to heavy snow possible through the day Saturday,
which is reflected by the SN groups in the TAFs. BLSN starts to
replace the falling snow as northerly winds begin to gust over
20kts. BLSN risk with vsby reductions will continue after 00z.
IFR cigs will continue to move in this evening. IFR to LIFR
cigs commonplace during the day Saturday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM CST Sunday for IAZ004-005-015-
023>025-033>036-044>047-057>059-070>072.
Winter Storm Warning until 6 AM CST Sunday for IAZ006-007-016-
017-026>028-037>039-048>050-060>062-073>075-081>085-093>095.
Winter Storm Warning from 9 PM this evening to 6 AM CST Sunday
for IAZ086-096-097.
Winter Weather Advisory until 6 AM CST Sunday for IAZ092.

&&

$$

DISCUSSION...Dodson
AVIATION...Jimenez