821 
FXUS63 KDMX 261745
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Sun Mar 26 2023

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

.DISCUSSION.../Today through Saturday/
Issued at 315 AM CDT Sun Mar 26 2023

Key Messages:

- Narrow band of accumulating snowfall anticipated this morning,
  with several inches of snowfall possible in some areas and
  little to none elsewhere.
- Fairly quiet from later today through Wednesday, with some
  fleeting precipitation chances and steady temperatures.
- Large, dynamic spring storm system set to impact the region 
  later in the week, around Thursday and Friday.

All eyes are on the precipitation approaching the area early this
morning. So far things have been setting up more or less as
expected. An initial band of scattered precipitation pockets has
move from southwestern toward central Iowa, producing light
rain/snow/mix but with little accumulating and nearing the Des
Moines metro at this time. Meanwhile, snow is spreading across
eastern Nebraska and regional radar imagery shows the
frontogenetical forcing axis beginning to set up our anticipated
WSW-ENE band within the last hour or so. Thus far it has not
matured and snowfall accumulations in that area appear modest, but
high-resolution near-term models continue to indicate it will get
its act together quickly over the next hours in the east/southeast
Nebraska to west/southwest Iowa region, with rapid snowfall rates
of an inch-plus per hour possible before 12Z or so. The expected
location of this threat falls within the ongoing advisory, and no
changes are being made at this time. As the band matures this
morning, we will monitor closely in case any partial cancellation
or expansion of the advisory becomes warranted. Overall forecast
snowfall amounts have not changed appreciably from previous,
however with such a narrow feature likely to have sharp cutoffs
and pockets of enhanced snowfall, near-term adjustments will be
likely throughout the morning. It should also be noted that most
of the high-res runs since Saturday evening have consistently
showed a fading of snowfall intensity as the band works eastward
later this morning, a combination of waning forcing and post-
sunrise warming/melting, though there are some exceptions
including recent HRRR runs. For now, it is likely the highest
snowfall accumulations will fall in a swath from roughly Omaha to
Audubon to Boone, which again lines up with the current advisory.

The band of precipitation will move quickly eastward out of our
area during the late morning and early afternoon hours, after
which a surface high pressure ridge will build down across the
region through Monday. This will keep things cool, dry and quiet
to start the coming week. Depending on how much snow falls this
morning and how much of it melts this afternoon, any lingering
swath of snow cover tonight could lead to potential for some fog
and temperature variations, but these would be localized and if
most or all of the snow melts this afternoon not a factor. Another
mid-level shortwave will move over the region late Monday night 
into early Tuesday, but has considerably less focused forcing or 
available moisture than this mornings system and the forecast 
remains dry other than some low POPs in our south/southwest with 
little accumulation or impact. Another fast-moving but more robust
impulse will then mover across the Upper Midwest around Tuesday 
night into Wednesday, likely generating a swath of precipitation
across southern Minnesota and Wisconsin during that time frame. By
the time this encroaches on northern Iowa much of the lift
associated with the shortwave will already be moving eastward into
the Great Lakes, and another surface high building in from the
north behind it will bring dry air intrusion, the combination of
these effects being that the precipitation should fade out as it
enters Iowa. However, the timing/location of this will be close,
and some model runs do paint light to moderate accumulations in
our northern counties late Tuesday night/Wednesday morning. Have
held only 20 POPs for now, but this bears watching as an increase
might be needed in coming forecast updates.

The most impactful weather period/system of the coming week, after
this mornings focused snow band, will come from around Wednesday
night through Friday night. On Wednesday a deep 500 mb trough will
round ashore over California, turning downstream flow more
southwesterly. 500 mb ridging over Iowa will thus come to an
abrupt end as leading ripples ejecting out of the western trough,
coincident with surging warm air/moisture advection, will bring an
initial round of transient precipitation likely in the form of
rain to our service area roughly around Thursday morning.
Subsequently, as the large western trough progresses eastward it
will result in Lee cyclogenesis with a deepening surface low
developing over eastern Colorado and extending a sharpening
inverted trough/front northeastward over Iowa in a rather classic
synoptic setup. This will promote strengthening flow/surface 
winds and a rapid northward surge of moisture and surface 
dewpoints south of the boundary, leading to burgeoning instability
and thunderstorm potential by late Thursday and Friday. As the 
surface low finally kicks out into the Midwest on Friday it will 
then likely generate additional precipitation, ranging from 
thunderstorms on the southeastern flank of the cyclone to snow on 
the northwestern flank. However, the details of this evolution are
where the models really show disagreement, namely with the GFS 
being considerably stronger with both the 500 mb and reflective 
surface low, and correspondingly a bit further north with its 
track, while the EC and GEM show more of an open wave at 500 mb 
and a relatively weaker surface low moving a bit further south. 
These variations will have significant impacts on sensible weather
and hazards for Iowa, so we will be monitoring the evolution of 
the forecast this week as theoretically severe weather and winter 
storm conditions could both be possible in parts of Iowa around 
Friday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1245 PM CDT Sun Mar 26 2023

Area of snow is pushing through central into eastern Iowa early
this afternoon and initial LIFR/IFR conditions will improve into 
VFR through the afternoon or early evening from west to east. 
While not in the official forecast at this time, there is the 
possibility of fog development tonight due to the recently fallen 
and melting snow from a DNS/CIN to ALO line. As the fog dissipates
in the morning, will need to monitor for the potential of 
borderline MVFR ceilings, though have left out since this is 
nearing the end of the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 1 PM CDT this afternoon for 
IAZ034>039-044>050-057>059-070.

&&

$$

DISCUSSION...Lee
AVIATION...Ansorge