618
FXUS63 KDMX 072356
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
556 PM CST Fri Feb 7 2025

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

.KEY MESSAGES...

- Winter weather expected late tonight and Saturday. Light snow
  with accumulations north and freezing drizzle possible central
  and south. Minor icing may occur (50%)

- Colder Sunday and into next week. A few chances for light
  snow next week, mainly central and south.

&&

.DISCUSSION...
Issued at 258 PM CST Fri Feb 7 2025

High pressure is centered over the upper Ohio valley and northwest
through central Minnesota and North Dakota. Mid-level moisture is
beginning to lift north into southern Iowa with cloud bases around 7
kft. Over northern Iowa, had returns on radar from mid-level forcing
but no precipitation has occurred with very dry air below the 10 kft
cloud bases. The forecast challenge remains with two systems
potentially impacting Iowa later tonight and into Saturday.

The main focus of this forecast discussion will be regarding the
freezing drizzle potential over central and southern Iowa and will
have a more brief discussion regarding the light snow for northern
Iowa. Monitoring a couple areas for moisture analysis this afternoon
for the freezing drizzle potential as there is a wide variety in the
initialized 0-1.5km moisture by the deterministic models. The first
is over the Missouri/eastern Kansas/eastern Oklahoma area where the
most robust models like the HRRR/RAP are over initializing this
moisture and with each RAP run, it backs off only to try to force
more moisture into the solution 3 hrs out. It is also over
initialized in moisture in the second area of focus over northwest
South Dakota. The 12 NAM moisture initialization was better and it
had a good solution regarding the moisture progression into Iowa,
though note the 18z solution is back to the over moist low levels
NAM bias. Then models such as the GFS tend to have too dry of
solutions of low level moisture.

This system is very different from the freezing drizzle event on
Wednesday though both have little to no ice introduction into the
saturated layer. The easterly flow from the high pressure to the
east is very pronounced and will impact the low level saturation and
in fact, several model solutions are keeping cloud bases AOA 1kft,
which is not favorable for drizzle formation. Also, reviewing areas
with condensation pressure deficits within that 0-1.5km layer, these
are sporadic and short duration when they do occur. In addition, the
low level isentropics levels (290K-300K) show near neutral flow and
then down glide (subsidence) after 12z. Due to these potential
negative factors for freezing drizzle production, do not have high
enough confidence in widespread and impactful freezing drizzle. Will
need to continue to monitor through tonight on how much moisture
does return and how that low level saturation process proceeds.
Obviously should the widespread moderate freezing drizzle indeed
develop, then some icing impacts are quite possible. Perhaps this
occurs within the moisture axis along the trough that will move
through in the morning. Again, that should be brief though if it did
occur due to the quick eastward progression.

Light snow is still expected over northern Iowa where deeper
saturation occurs and ice introduction should occur. That said,
there is event a wedge of dry air around 700 mb that could impact
the precipitation over northern Iowa that may limit the amount of
precipitation and perhaps there could be brief periods with a loss
of ice introduction and freezing drizzle potential. Still have 1-2
inches of snow in the forecast. Breezy conditions during the
afternoon around 25 mph could cause some minor blowing snow but with
no current snow cover, it should be limited.

Colder air will follow the system for Sunday and into much of next
week. Next week still looks more active which could result in a few
rounds of light snow especially over central and southern Iowa.
Arctic high pressure will be situated north of Iowa during much of
this period and the dry air originating from this high could have a
big impact on precipitation chances during the period.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 545 PM CST Fri Feb 7 2025

VFR conditions continue through the evening hours but ceilings
will start to drop overnight and continue to lower to at least
MVFR into Saturday morning. Some IFR possible at times during
precipitation which will be in the form of mainly snow in
northern Iowa near KMCW, and a mix of freezing drizzle and snow
towards KFOD and KALO prior to the start of snow or after with
loss of ice introduction. Further south, freezing drizzle
remains the main precipitation type for KDSM and KOTM, though
some uncertainty remains with freezing drizzle timing and
impacts especially so left with Prob30 groups at this time.
Edits may certainly be needed in impacts/timing at all sites as
precipitation develops, especially to visibilities which also
remain more uncertain. Precipitation ends around midday into
early afternoon with overall aviation conditions improving
through the later afternoon into evening hours Saturday.


&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...05