Area Forecast Discussion
Issued by NWS Des Moines, IA

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530
FXUS63 KDMX 180855
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
255 AM CST Mon Dec 18 2017

.SHORT TERM.../Today/
Issued at 255 AM CST Mon Dec 18 2017

Some patchy fog and patchy dense fog has developed across portions
of the area this morning. As winds shift around to the south and
increase toward sunrise or slightly later any fog should dissipate.
For the remainder of today, the state remains sandwiched between an
area of surface high pressure to the south centered across OK/AR and
an area of low pressure to the north centered in Manitoba. The sfc
low across Manitoba is associated with an upper level trough in
nearly the same vicinity. A pseudo warm front aloft associated with
this system is moving through currently, with southerly to westerly
flow increasing ahead of this system today. This will lead to waa
across the area and clearing skies through the day. Therefore expect
a nice rebound in temps with a mild, breezy day expected. Temps
should top out into the 40s north to low 50s south.

.LONG TERM.../Tonight through Sunday/
Issued at 255 AM CST Mon Dec 18 2017

The long term forecast period can be effectively split into three
sections. Mild and dry, followed by wintry precipitation, then
much colder toward Christmas. From tonight through Wednesday
roughly zonal flow aloft and the passage and rebound of a weak
surface trough will promote a continuation of above normal
temperatures and little to no chance of precipitation. Daily highs
will range mainly in the 40s to 50s and there may be spotty fog in
the mornings, but otherwise the forecast is quiet during this
period.

By Wednesday night a large 500 mb trough will be digging into the
southwestern U.S., and will subsequently eject eastward across the
Midwest late in the work week. Initially a broad surface cyclone
will develop beneath the southwesterly flow on the forward flank
of this trough, with a surface frontal boundary extending
northeastward over Iowa by Thursday. This will result in warm air
advection above the surface and increasing atmospheric moisture on
Wednesday night and Thursday, and the development of a band of
snowfall associated with the deformation zone on the northwest
flank of the cyclone as it moves up across Missouri and toward the
Great Lakes. During the initial phase of the system for our
forecast area, Wednesday night into Thursday morning, any
accumulating snowfall will likely remain to our northwest, with
any precipitation falling across about the northwest half or so of
Iowa likely coming in the form of a light wintry mix. Forecast
soundings illustrate a pronounced warm nose above the surface,
likely leading to rain or freezing rain depending on surface
temperatures, but then transitioning through sleet to a rain/snow
mix as the warm nose cools and precipitation type becomes more
dependent on surface/near-surface temperatures. Have included
these wintry mix types through top-down process for Wednesday
night through Thursday and while accumulations are very light
during this time, travel impacts are possible Thursday morning
depending on road temperatures at that time.

By around midday Thursday the aforementioned band of snow should
be entering the north/northwest periphery of our forecast area,
with a light rain/snow mix or nothing further southeast toward
central Iowa. The questions then become, how far southeast will
the snow band progress which is dependent on the precise track of
the surface low, and how much snow will fall within the band. The
former question appears to be coming into a bit better focus
tonight, with about the northwest third or half of our area being
the most likely to see accumulating snowfall and the southeastern
half or so likely missing out for the most part. The latter
question of amounts is still nebulous, however, given the
relatively progressive nature of the system and the likelihood
that it will be weakening as it passes by, heavy amounts are not
anticipated and we are probably looking at more like advisory-
level snow amounts of a few inches or so. The snow will taper off
and move out on Friday, with most accumulation coming around
Thursday evening and Thursday night. Clearly this system has the
potential to produce travel impacts across portions of Iowa going
into the holiday weekend and we will be closely monitoring it over
the next several days.

Looking toward next weekend and Christmas, long-range models have
consistently signaled the carving out of a very large cyclonic
flow regime aloft, dominating the weather for much of the CONUS on
Christmas Eve and Christmas Day. This supports a high probability
of significantly colder temperatures, and probably fairly
persistent cloud cover with some relatively light snow chances at
times. Thus while no significant precipitation is anticipated
during this period, there is still some chance of a "barely white"
Christmas for portions of the forecast area. At any rate it will
be a frigid holiday, with highs Christmas Day probably only in the
single digits and teens across Iowa.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1143 PM CST Sun Dec 17 2017

IFR and MVFR ceilings and visibilities continue in portions of
south central and east central Iowa. Surface winds will remain
weak and from the southwest through 12z. With light winds and a
moist boundary layer anticipating IFR and MVFR ceilings and
visibilities to remain overnight. After 12z, a surface trough in
western Iowa will traverse east across the state. Look for winds
to shift to strengthen resulting in increased mixing and a steady
improvement of ceilings and visibilities to VFR.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Beerends
LONG TERM...Lee
AVIATION...Angle



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