Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

836
FXUS63 KDMX 050904
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
304 AM CST Fri Dec 5 2025

.KEY MESSAGES...

- Milder today with highs in the low 30s. Light dusting of snow
  possible across northern Iowa today.

- Accumulating snowfall likely Saturday afternoon into Sunday
  morning. The heavier band of 4-7+ inches of snow looks to set
  up from northwest into central Iowa.

&&

.DISCUSSION...
Issued at 304 AM CST Fri Dec 5 2025

Early this Friday morning surface low pressure sites northwest of
the area across North Dakota and northern Minnesota, allowing for
southerly flow into Iowa. This has allowed temperatures overnight to
remain fairly steady or even rise a degree or two. This surface low
will shift east across Minnesota through today with the southern
edge of snow skimming across northern Iowa. Forcing and moisture are
focus north of the area, so impacts to the area are expected to
be minimal with perhaps a dusting of snow across the northern
couple tiers of Iowa counties through this afternoon.

Our attention then turns to a more robust system set to impact Iowa
on Saturday. The 500 mb pattern doesn`t look like much, a subtle
embedded perturbation in the upper level northwest flow. But as this
wave passes across the Rockies it undergoes lee cyclogenesis with a
surface low that will track across Missouri on Saturday, placing
Iowa on the northern deformation zone side of the low. Snow is
expected to arrive in western Iowa by around noon on Saturday,
reaching the I-35 corridor by later afternoon, and western Iowa by
evening, then ending overnight through Sunday morning. Cross
sections and soundings show that lift is present through the
dendritic growth zone but not always maximized there, and the depth
of the DGZ is fairly average. This should result in snow to liquid
ratios near to just above climatology for this time of year. Cobb
output maintains SLRs of 15-20:1 through the band of heavier snow
accumulations, expected to set up roughly from northwest into
central Iowa. This placement may still shift, as the GFS remains
further south compared to the Euro. The event is now within the
range of extended CAM runs and the HRRR/RAP both seem to agree with
the Euro placement of the heaviest snow band at this time. It is
also worth noting that the 00z HREF indicates the potential for
1+"/hr rates within this heavier band. The previous discussion
mentioned an upward trend in the QPF, and indeed another ~0.05" was
added on this shift which helped to nudge up accumulations. Across
models, there is a consistent signal for 4-7+ inches of snow from
northwest to central Iowa. With onset expected in western Iowa in
the next 36 hrs, have issued a Winter Storm Watch for the area where
the axis of heaviest snow is most likely to fall. Portions of this
will later transition to either an advisory or warning with some
expansion possible.

Snow tapers off on Sunday morning with colder air filtering in
behind, resulting in another day with highs in the teens to low 20s
(single digits far north). The upcoming week brings additional
chances for snow as waves move through the active northwest flow
pattern.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1057 PM CST Thu Dec 4 2025

LLWS at all sites at times tonight. Southwest winds persist with
some 20kt gusts possible after 18z. A front will bring snow
chances to KMCW but with minimal accumulations. Its main impact
to the rest of the sites is a wind shift and cig restricting
stratus, mainly after 00z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday afternoon through Saturday
evening for IAZ004>006-015-016-023>025-033>036-045>047.
Winter Storm Watch from Saturday evening through late Saturday
night for IAZ007-017-026-027-037>039-048>050-060>062.

&&

$$

DISCUSSION...Hagenhoff
AVIATION...Jimenez