011
FXUS63 KDMX 121146
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
546 AM CST Wed Feb 12 2025

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

.KEY MESSAGES...

- Significant snowfall remains on track for today, with the
  greatest amounts of 5 to 7 inches being through the heart of
  central Iowa.

- Frigid wind chills develop tonight into Thursday morning, with
  values as low as the teens to twenties below zero.

- Another winter system remains on track for Friday into
  Saturday, with greatest amounts expected over northeast Iowa.

&&

.DISCUSSION...
Issued at 407 AM CST Wed Feb 12 2025

Well its snowing. And its been doing so in much of central
Iowa since early last evening, which was quite a bit sooner than
what was anticipated by this forecaster yesterday morning.
Although not overly strong, theta e advection ahead of the
approaching wave has produced enough lift and moisture for
snowfall to overcome the low level dry air and therefore produce
a light, but persistent snowfall through last evening and into
the morning hours. We had picked up just shy of an inch of snow
by the midnight observation here in Johnston, with a steady but
very light snowfall ongoing since then. That said, flakes have
been more plate-like rather than pure dendrites, which makes
them a bit less efficient at accumulating and more susceptible
to compaction. This is expected to change as we continue through
the morning hours, and the parent wave draws nearer to the
state.

As the parent wave approaches, theta-e advection in the low to
mid levels will be increasing over the next few hours, which
will bring both a steady increase in lift, as well as slightly
warmer temperatures and moisture through the layer. This will
have a few implications on snowfall. First, we will see a steady
increase in QPF but also a slight decrease in snow ratios (15:1
to 20:1) central and south as the slightly warmer and more
moist air moves through. Then, as the wave continues eastward,
this thermodynamic forcing will be met by stronger mid and upper
level kinematic forcing, which will continue to increase the
lift through the morning hours. As these two areas of lift phase
with each other, snowfall rates will be maximized around
0.75/hr to even 1/hr in some locations for a short period of
time. This will occur around roughly 5 to 9am, and is expected
to be the period of heaviest snowfall for the day. Snow
continues to fall through mid-day, but we begin to lose the
better thermodynamic support as colder air begins to funnel in
behind, resulting in lower QPF through mid-day. However, a
secondary push of snowfall is then expected again in the
afternoon as colder air increase snow ratios and the main lobe
of mid to upper level kinematic forcing continues overhead.
While this isnt expected to be as heavy as the morning push,
the higher snow ratios will make for a more efficient snowfall,
with some areas potentially seeing snowfall ratios of 25:1 to
even 30:1. Finally, snowfall begins to diminish from west to
east through the late afternoon and early evening hours as
saturation through the column begins to wane. That said, while
accumulating snowfall will be diminishing through the evening,
very light snow/flurries may still persist until close to
midnight for most of central Iowa.

Now, translating this progression to snowfall amounts, the
highest accumulations are expected to be where the phased
thermodynamic and kinematic forcing meets with the relatively
higher snow ratios, and where snowfall occurs for the longest
period of time. This time last night, it was looking like this
band was going to occur over the southeastern into central
portions of the state. However, with a slightly stronger push of
theta-e advection, the band has drifted a bit further
northwest. Areas in southeast Iowa will still certainly see
accumulating snowfall nearing 6 inches, but the higher end
amounts are looking less likely in these areas due to lower snow
ratios and slightly lower QPF. The area that will meet the
above requirements now resides on a line going northeast from
Creston through Ames/Des Moines to south of Waterloo. This will
be where the 6 to 7"+ snowfalls are most likely. Finally, due
to this northwest trend, areas over northern Iowa have seen a
slight bump up in amounts (~1). However, some uncertainty still
remains on how much the dry air advection from the high
pressure to the north will affect amounts on the northern
fringes of the system. That all being said, have decided to
retain the same area for the winter storm warning today, but did
add one additional county to the winter weather advisory in
northern Iowa.

As the wave departs tonight into Thursday morning, cold air
will begin to advect in and skies clear out on the backside,
dropping temperatures overnight. In addition to this cooling, a
fresh snowpack on the ground will likely exacerbate cooling
overnight. This will make for a frigid Thursday morning,
dropping wind chills into the teens to twenties below zero over
much of the area. For this reason, have collaborated with
neighboring offices to issue a cold weather advisory starting
after midnight over portions of central into northern Iowa.

Finally, while todays system has taken much of the spotlight in
the last few discussions, we are still keeping an eye on another
system moving into the area Friday night into Saturday. This
system will be associated with a large push of theta-e advection
as flow becomes more southerly ahead of a digging upper wave to
the west. As this warm, moist air advects north and ascends,
snowfall is expected to develop over the area. Fridays system
will be a bit warmer, producing lower snow ratios and
potentially a wintry mix in the south. As for amounts, the
greatest QPF will be located over the northeastern portions of
Iowa, with diminishing amounts south and west. More details will
be provided on the Friday system in the coming days.&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1139 PM CST Tue Feb 11 2025

Conditions were quite variable across central and southern IA at
06z, mainly affected by areas of snowfall creating VFR to IFR
and even spotty LIFR. Farther north where there was little to no
snowfall, VFR conditions were still prevalent. Snow is expected
to intensify and expand during the early morning hours however
with at least IFR conditions expected to be widespread through
much of the day, and possibly LIFR during heavier bursts of
snow. The snow is expected to diminish by Wednesday evening,
but MVFR stratus is still expected through the end of the
period.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 539 AM CST Wed Feb 12 2025

Light snow spans much of the state today, impacting all sites.
MVFR to IFR ceilings and visibility are expected, with LIFR
visibilities likely with heavier snowfall rates. Therefore,
expect conditions to bounce around as periods of heavier
snowfall move in and out of sites. Snowfall and diminished
flight conditions are expected through most of the morning, but
should begin improving through the afternoon hours as snowfall
rates let up. Snowfall then completely departs the are through
the evening, although at least MVFR cloud cover remains until
morning.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Cold Weather Advisory from midnight tonight to 9 AM CST
Thursday for IAZ004>007-015>017-023>028-033>039-044>049-
057>059-070-071.
Winter Weather Advisory until midnight CST tonight for IAZ007-
016-017-023>028-033>035-044>046.
Winter Storm Warning until midnight CST tonight for IAZ036>039-
047>050-057>062-070>075-081>086-092>097.

&&

$$

DISCUSSION...Dodson
AVIATION...Dodson