Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 241129
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
629 AM CDT Sun Mar 24 2024

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

.KEY MESSAGES...

- Snow arrives for parts of central and northern Iowa with a
  band of moderate or greater of snow in some areas. High
  snowfall rates of 1 to 2 inches per hour may occur.

- Temperatures warming from south to north today into tonight.
  Significant snow melt will occur into Monday.

- A few strong storms may occur Monday with hail the primary
  threat.

&&

.DISCUSSION...
Issued at 428 AM CDT Sun Mar 24 2024

Well here we go again, March wants to hang out with the winter crowd
again and not with the popular April and May spring months. A strong
upper level system is moving across the intermountain west with lee
side cyclogenesis ongoing. The developing low pressure combined with
the high pressure ridge moving east from Michigan to Louisiana, has
setup good southerly flow and a surge of moisture transport from
Texas and into Kansas this morning then angling northwest into
southwestern Nebraska. Precipitation continues to blossom in that
vicinity early this morning with some lightning noted along this
band. The low level jet will continue to increase this morning and
may top 60 kts sometime this morning as it points into central Iowa
and bringing the strong surge of moisture with it. The arrival of a
short wave this morning along with the speed convergence of the low
level jet and the theta-e advection, will provide provide ample
forcing for convective snow potential and high snowfall rates of 1
to 2 inches per hour at times. Thermal profiles are also favorable
for aggregate snow growth with near isothermal layer in the 0C to -
3C range. This often results in the large aggregate snowflakes which
is a wet and heavy(by weight) snow with snow to liquid ratios often
in the 7:1 to 8:1 range.

One forecast challenge is setting the southern edge of the impactful
snow potential. Using only a surface wetbulb of 34 degrees as a low
level melting layer (which assumes no melting layer aloft), the snow
region may linger near the Interstate 80 to Highway 30 corridor
through 10-11 am or so before a strong surge of warmer and drier low
level air arrives. Also, will continue to monitor the convective
nature of this precipitation lifting towards Iowa. The convection
while will produce higher snowfall rates, it will also come in waves
as opposed to continuous over an X period time which could lower the
ceiling for snow. The expectation is the precipitation shield will
fill in as it arrives into central Iowa. These are the challenges
for today. The snowfall rates will overwhelm any above freezing road
temperatures and lead to quickly deteriorating travel conditions.
Conditions will then improve from south to north as the warmer air
arrives. Have opted for a Winter Weather Advisory to cover this
event for now though there is a non zero chance that an upgrade to a
warning would be required if the heavy(by depth)snow band occurs
with slantwise convection. Technically patches of freezing rain
could occur but given this event will be occurring diurnally, any
impacts to surfaces will be minimal with the March solar radiation
helping keep road temperatures at 32 or greater where not
overwhelmed by the snow.

Any lingering snow by this evening should be confined to far
northern Iowa and will be completely out of the area by mid to late
evening as the warmer air continues to surge north then the snow
melt will be on as snow eater dewpoints in the 40s arrive. Note
those dewpoints will be in central Iowa by this afternoon. A few
elevated showers and storms are possible overnight as the next surge
of moisture and forcing arrives. It is possible some small hail
occurs. Still monitoring the severe weather potential on Monday with
a marginal risk out mainly west of Interstate 35. The shear and wind
profiles are not near as impressive as they appeared 24-48 hours ago
and for the lack of a better term, are meh below 600 mb. There is
still enough instability for hail potential but the potential for
organized storms and supercells has decreased. That also goes for
the wind potential on Monday. The area of low pressure lifts through
with more shower and storm potential Monday night and there still
may be a transition late into Tuesday of light snow.

Beyond Tuesday, warmer weather will make its way to Iowa as March
feels peer pressure from April to finally get its act together.
Highs in the 50s and 60s are expected to return Thursday and into
the weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 625 AM CDT Sun Mar 24 2024

A large area of precipitation moving into southwest Iowa
currently will move across most sites by mid morning with the
possible exception of KOTM. The precipitation will begin as snow
at KMCW/KFOD/KALO while KDSM will be closer to the rain and snow
transition line. Cigs and vsbys will become MVFR to IFR in the
areas with snow. Cigs will improve from south to north this
afternoon and evening and any lingering snow will transition to
rain. A few thunderstorms are expected tonight along with some
low level wind shear. Southeast winds today will be gusty at
times.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 7 PM CDT this evening for
IAZ004>007-016-017-025>028-037>039.
Winter Weather Advisory until 1 PM CDT this afternoon for
IAZ024-035-036-047>050-060-061.

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon


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