Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 240445
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1145 PM CDT Sat Mar 23 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Light snowfall this afternoon and evening with accumulations
  generally under an inch.

- Stronger system moving into the area tomorrow, initially
  bringing moderate to heavy snowfall rates in the northern half
  of the state then switching to rain area-wide by the evening.

- Gusty conditions Sunday night into Monday with a few stronger
  thunderstorms possible during the day Monday.

&&

.DISCUSSION...
Issued at 316 PM CDT Sat Mar 23 2024

Increasing clouds across the area are the first signs of this
weekend’s active pattern creeping into the area. Light snow has
begun to fall over the western portion of the state and will
continue to slowly make it’s way east through the evening. This
precipitation has struggled initially to overcome the low level dry
air, lagging slightly behind when it was expected yesterday. Better
forcing and saturation is being realized further south which is
allowing for precipitation in this area as well. Some accumulation
is possible this afternoon and evening, but should be generally less
than an inch. This snowfall is resulting from weak energy and
Pacific moisture passing through the upper level flow, as an H500
trough begins to dig in over the west coast.

This upper level trough and it’s associated surface low will be our
main weather bringers starting early tomorrow morning through much
of the day Monday. Large scale and mesoscale forcing will be
plentiful as the surface low begins to churn to our southwest, and
strong theta-e advection streams into the area. The primary
challenge with this forecast lies with the progression and evolution
of the precipitation, as snow falls initially before switching over
to rain through the day Sunday. The million dollar question is how
much snow will fall before temperatures increase and rain takes over
as the dominant precipitation type. As of right now, this looks to
be of no concern for much of central and southern Iowa as the
transition will occur quickly and bring only a brief period of
snowfall with minimal accumulations. However, the tricky locations
will be further north where the forcing increases and frontogenesis
is maximized.

In northern Iowa, there looks like there may be multiple hours of
moderate to heavy snowfall occurring before the switch to rain.
Recent trends in CAMs and global models have shown a slightly
southeastward trend in higher totals through the last 24 hrs, with
probabilistic output from the HREF and LREF pinging a 50 to 70%
chance of seeing snowfall greater than 3 inches near the
Iowa/Minnesota border and increasing exponentially further north.
This is reflected by forecast soundings near Mason City, which show
a deep moist layer with strong lift through the DGZ and a near
isothermal layer near 0C, suggesting intense snowfall rates and
large aggregates. This would lead to heavy, wet snow, with
temperatures near freezing in the lower levels and at the surface,
making accumulations difficult to quantify. Right now, running with
an expected snowfall of around 3 inches in our far northeastern
counties. However, this will be a high “boom or bust” scenario, as a
faster progression of the warm air would all but eliminate the
snowfall amounts in NE IA and keep us well below the 3 inch forecast
but a slower progression would shift the heavier snow totals further
south and above the 3 inch forecast. Regardless of the bust
potential, have decided to go with a small Winter Weather Advisory
in this area, just to raise some awareness of the potential for some
heavier snowfall on Sunday near the I-35 corridor and will allow
future shifts to adjust this headline should the trend continue
towards higher snow totals.

The impacts from precipitation wane once the snowfall switches
completely over to rain by Sunday evening, however, the system is
far but done at this point. Winds will increase as the surface low
draws nearer and pressure gradients tighten Sunday afternoon through
Monday. The wind gust magnitudes have diminished some since this
time yesterday, with Sunday looking to have widespread gusts of 30
to 35 kts and Monday having slightly higher values but only a few
areas pushing over 40 kts. Therefore, have held off on any headlines
due to the anticipated winter weather as well as the greater wind
risk being on Monday. That being said, will want to continue to
monitor wind trends and prepare for gusty conditions to start next
week.

Finally, models continue to hint at thunderstorm potential on Monday
as the surface low passes through western Iowa and puts central Iowa
right in the warm sector. Not much has changed in our thinking for
severe threats since this time yesterday, as any stronger storms
will have plenty of shear to work with, but will be limited by the
amount of instability that can be realized. SPC issued a day 3
marginal risk for our area with the arrival of the warm sector and
coincident cold front, which is reasonable given the conditional
threat for severe weather. With it still being outside of the high-
resolution guidance window, not going to speculate on too much at
this time, but given the setup and parameter space, all severe
hazards would be possible provided any strong storms develop.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1145 PM CDT Sat Mar 23 2024

Light snow is ongoing across the northern terminals early this
morning and is expected to continue over the next few hours or
so, which will allow for continued MVFR/IFR conditions due to
ceilings and visibility restrictions. There will be a brief dry
period for a few hours across the state before sunrise, when the
main wave associated with the large low pressure system
circulating further west pushes into the region. This will
bring returning widespread snow mainly over KMCW and KALO, while
the remaining terminals see a wintery mix around 12z through at
least mid to late morning. Following this, warmer air surging
into the state with increasing gusty southerly winds will allow
for a transition to rain around 18-21z from south to north.
Overall, expecting continued periods of MVFR/IFR conditions with
this precipitation. Rain will linger through the rest of the
day, though may be on and off in nature.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 AM to 7 PM CDT Sunday for
IAZ006-007-016-017.

&&

$$

DISCUSSION...Dodson
AVIATION...Bury


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