Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 242358

Area Forecast Discussion
National Weather Service Des Moines IA
658 PM CDT Sat Mar 24 2018

Issued at 658 PM CDT Sat Mar 24 2018

Radar is indicating a quick decrease in coverage of returns as the
forcing subsides. Will let the headlines expire at 7 pm with
little additional accumulation/impact expected. Drier air pushing
in from the northeast is allowing some erosion of clouds from the
northeast and will be tracking this progress for any additional


.SHORT TERM.../Tonight through Tonight/
Issued at 245 PM CDT Sat Mar 24 2018

Updated through the day adjusting snow amounts with very heavy
amounts reported close to 1.5 feet of snow in the far northeast.
Snow is beginning to taper off from northwest to southeast across
the area. However some accumulating snowfall is expected to linger
across the far east central portion of the forecast area into
early evening. Also with these areas still seeing visibilities in
the snow area around 1/2 to 1 mile, still expect some additional
accumulations in the extended portions of the headlines of 1 to 3
inches. As ice introduction is lost aloft...may see the snow mix
with some freezing drizzle at times into the early evening
especially in the far south. Otherwise precipitation should taper
off completely toward mid evening with cloudy conditions

.LONG TERM.../Sunday through Saturday/
Issued at 349 PM CDT Sat Mar 24 2018

There is a potential for widespread freezing rain/drizzle Sunday
night into Monday morning. From Sunday through about Tuesday Iowa
will remain beneath relatively warm, moist southwesterly flow
aloft ahead of a longwave trough over the western U.S. A series of
subtle shortwave impulses ejecting out of this trough will produce
several rounds of precipitation across the region during this time
frame. At the surface, a quasi-stationary frontal boundary will
develop from around Grand Forks down to Dodge City by late Sunday
night, then eventually sweep across Iowa on Monday night, pushing
precipitation off to the southeast on Tuesday. From Sunday night
into Monday night this will result in a scenario where our
forecast area sees relatively cool easterly flow undercutting warm
southwesterly flow aloft, with rounds of precipitation moving
through. Forecast soundings indicate a lack of ice introduction
with the cloud layer being below the -10C level, and an
increasingly pronounced warm nose multiple degrees above freezing
around 850 mb. This is a classic setup for freezing rain/drizzle,
dependent on surface temperatures. At this time it appears the
first, lighter round of precipitation will affect primarily
northwestern sections of the CWA Sunday evening, possibly
expanding toward central Iowa overnight, then a surge of more
substantial precipitation will cross much of the area during the
day on Monday. Sunday night lows will be in the upper 20s across
the north and near freezing further south, so freezing
rain/drizzle could be extensive in area. Fortunately the forcing
for ascent in the cloud layer with the first wave is weaker and
QPF lighter, but we could still see several hundredths to perhaps
around a tenth of an inch of ice northwest by Monday morning. As
the stronger, second wave produces more QPF during the day Monday
temperatures will be rising above freezing, so most of the
additional QPF will likely not freeze. However, travel conditions
could be dangerous Monday morning and we will be monitoring the
situation closely the next couple of days. A winter weather
advisory may be issued in the near future.

There are several possible effects of rain/freezing rain falling
on the cold and snowy surfaces of the north. First, what will road
temperatures look like across northern Iowa? Most indications are
that the snow that fell today will melt off the roads nearly
completely by Sunday afternoon, and in some cases already has
melted off completely. If the road surface is then able to dry
out, it will likely cool to below freezing on Sunday night before
precipitation begins, leading to icing on roads by Monday morning.
In addition, we are forecasting roughly half to two thirds of an
inch of QPF in many areas from late Sunday night through Monday
night. What effect will this have, hydrologically, when combined
with the melting of 12-20 inches of snow across our north and
northeast? How much of the snow will melt on Sunday, Monday, and
Tuesday? Certainly there is a possibility of some rises on local
streams and rivers, as well as ice jams, and we will also be
closely monitoring this potential.

The western longwave trough will finally swing through around
Tuesday night or Wednesday and clear things out for a bit.
Thereafter we will remain beneath cyclonic flow through Friday and
roughly zonal flow next weekend. This will keep things fairly
active, with periodic impulses providing additional precipitation
chances, but the timing and sensible weather effects of these
impulses are difficult to predict at this range, and no notable
changes were made to the forecast as we focus on the event ending
today and the potential for freezing rain on Sunday night/Monday.


.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 658 PM CDT Sat Mar 24 2018

Dry low level flow from the east has allowed ceilings to climb to
VFR at northern locations and this flow will eventually also raise
ceilings in the south this evening. Anticipate mostly VFR
conditions for the rest of the forecast despite some mid and high
cloudiness. Surface winds will remain from the east to southeast
for the duration of the forecast as well.





SHORT TERM...Beerends
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