Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182116

Area Forecast Discussion
National Weather Service Des Moines IA
316 PM CST Mon Dec 18 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 316 PM CST Mon Dec 18 2017

Benign weather through the short term as a dry airmass pushes across
the state. We will be in continued warm advection this evening
but overnight a cold front will sink south across Iowa. There is
almost no moisture to work with so other than some high clouds and
a wind shift, there will be little sensible weather with the
frontal passage.

Temperatures on Tuesday will be a little cooler north but southern
Iowa should still see highs in the 50s.  In any event, still well
above normal for mid to late December.

.LONG TERM.../Tuesday night through Monday/
Issued at 316 PM CST Mon Dec 18 2017

There will be a significant transition in the upper pattern later
this week bringing and end to the relatively mild conditions of
the past several weeks. However, at the beginning of this
timeframe, Iowa will still be in a mild regime with increasing
warm advection as the trof in the west drops south into the
intermountain west. Above normal readings with little threat of
precipitation are expected on Tuesday night into Wednesday evening
before the changes arrive. A cold front will approach the state on
Wednesday night and begin to moves southeast into Iowa by Thursday
morning as a surface low deepens in western Kansas.

The real action begins in earnest on Thursday with both the Euro
and GFS now a bit farther south in their respective solutions.
This will usher in colder air a bit quicker on Thursday and also
set up a better threat of snow across northern Iowa. Isentropic
lift will intensify through the day with precipitation expanding
on the cool side of the front. Initially, light rain is forecast
for much of the forecast area although a smaller area of mix and
snow is expected in northern Iowa in the morning. This transition
area will drop south and east during the day and impinge on the
Des Moines metro area toward sunset. This could be quite
problematic around rush hour Thursday evening as temperatures drop
below freezing near sunset with falling precipitation leading to
quick development of ice on area roadways. Deformation snowfall
will continue across the northwest half to third of the state on
Thursday night with relatively dry conditions in the south where
the dry slot will move overhead. In addition, the winds will be
quite brisk and may lead to some reduce visibilities in the snow
areas of the north overnight. Snow amounts appear light to
moderate across northern Iowa, likely in the snow advisory range,
especially when combined with the winds.

The system will depart by Friday morning with a deepening trof
across the central United States. This will allow cold Canadian
air to spill in successive waves into the region including Iowa
through Christmas Day. Given the new snow cover in northern Iowa,
readings below zero for lows seem quite likely by the weekend in
those locations. There may be some light snow during the weekend
as well, although the models have been all over the place in
regards to timing and location of this event.


.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1146 AM CST Mon Dec 18 2017

VFR conditions are expected through the forecast period.  Brisk
southwest wind today will diminish with sunset but soundings
indicate winds just off the surface rapidly increase to 35-40kt so
wind shear was added to the TAF`s for a period around 00Z.





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