Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161135

Area Forecast Discussion
National Weather Service Des Moines IA
635 AM CDT Fri Mar 16 2018

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

.SHORT TERM.../Today through Tonight/
Issued at 406 AM CDT Fri Mar 16 2018

Nighttime Microphysics imagery from GOES-East shows low clouds over
roughly the southwestern half of Iowa with higher level clouds over
much of the rest of the state. KDMX WSR-88D indicated rain showers
oriented in an arc from west central into south central Iowa were
slowly moving northeastward. These showers were being forced
primarily by 700mb frontogenetical forcing, but were encountering dry
air in the low levels. 00z KOAX RAOB showed some mid-level
saturation between 700 and 600mb, but quite a bit of dry air from
700mb to the surface. Dewpoints over central Iowa were in the upper
teens to low 20s indicated the dry airmass being advecting into the
region via the easterly winds. Despite this dry low level air, have
received an mPING report of rain reaching the ground near Murray
shortly before 3am. So, isolated saturation of the low levels is
taking place, but will be slow to do so throughout the day as the
easterly winds continue to move dry air into the state.

Of concern initially this morning is what will the near surface
environment be for any precipitation that is able to reach the
ground. The warm air aloft from roughly 0.5 to 2.5km remains helping
to melt any hydrometeors as they fall. Soundings show a sub-freezing
layer below this while the air temperature will be around freezing
with dewpoints below freezing. So, expect freezing rain or possibly
ice pellets depending on depth of the near surface cold air. Road
temperatures from Iowa DOT and forecasts for where we expect
precipitation this morning look to be slightly above freezing. There
may a glaze of icing on elevated surface out toward Crawford County
before temperatures rise enough to end any icing concerns this

Strong breezes are expected much of today as high pressure squeezes
down into the western Great Lakes between low pressure over Nova
Scotia and our approaching low pressure over western Kansas. The
pressure gradient will be tightest this morning and winds will be
close to advisory criteria for a few hours this morning. Winds will
back off a bit this afternoon, but will remain breezy into this

As the aforementioned low pressure over western Kansas moves east
into the Ohio Valley by Saturday morning, moderate to strong Q-
vector convergence will spread from southwest to northeast over the
state. As this better lift arrives, saturation will occur and allow
rain showers to also spread northeastward. Expect rain will be the
predominate precipitation type from mid-morning into this afternoon,
though snow may mix in over our far northwest counties for a bit. It
will take until late this afternoon before saturation reaches our
far northeastern counties. As temperatures slowly fall tonight, will
see a transition to a wintry mix. Road temperatures will be a
delaying factor for impacts as they are forecast to be well into the
40s during the afternoon and do not look to drop below freezing
until mid to late evening over northern Iowa. This is where the
concern would be for any icing on roadways, especially over
northwestern Iowa where road temperatures will fall to near 30
degrees versus closer to or just above freezing over much of north
central Iowa. Went ahead and issued a Winter Weather Advisory for
Emmet and Palo Alto counties tonight for the concern of icing. There
will be some snow as well, but amounts look to be generally an inch
or less at this time over the far north. Light icing of a glaze to
perhaps a few hundredths and a dusting of snow will be possible
toward I-80 as well overnight.

.LONG TERM.../Saturday through Thursday/
Issued at 406 AM CDT Fri Mar 16 2018

Subsidence will spread quickly east across the forecast area on
Saturday morning with precipitation quickly ending across the
east. However, a secondary weak shortwave will arrive by afternoon
helping clouds to linger in western and southern areas for much of
the afternoon. Surface ridge is expected to slide into the state
on Saturday into Sunday morning with clearing skies and light

By Sunday, a strong shortwave currently off the Oregon/California
coast will be rapidly progressing east across the southern
Rockies. Both GFS and Euro have continued to show a farther
southern track with this system into early next week as it passes
east through the central United States. While some mid level
forcing passes across the southern half of the state, dry low
level flow from the east and northeast will limit any
precipitation to near the far southern border of the state.
Otherwise, conditions will be dry for the rest of Iowa on Monday
despite a cold brisk wind and below normal temperatures. A
secondary trof axis will approach the state on Tuesday although
forcing will be waning along with limited moisture. Therefore, the
threat of precipitation will be low and diminishing with time as
the system approaches. Low pops will remain across the northwest
and west central areas into Tuesday with any precipitation
remaining very light. Tranquil weather is then expected into the
middle of next week before another shortwave approaches toward the
end of the period. Both GFS and Euro are indicating a broad swath
of warm advection precipitation into the state toward the end of
the work week. At this point, temperatures appear warm enough for
primarily a rain event but will continue to monitor.


.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 635 AM CDT Fri Mar 16 2018

Moderate to high confidence in this TAF period. Precipitation
will spread into the state from southwest to northeast today
primarily as rain with a wintry mix expected tonight. VFR
conditions will give way to MVFR and IFR restrictions late this
afternoon or this evening as the precipitation arrives. Have also
included high end IFR ceilings tonight at FOD, DSM, and OTM.
Winds from the east will be sustained at 15 to 25 knots before
slowly decreasing later this afternoon into tonight where winds
will drop below 10 knots towards 12z.


Winter Weather Advisory from 7 PM this evening to 7 AM CDT
Saturday for IAZ004-015.



SHORT TERM...Ansorge
AVIATION...Ansorge is the U.S. government's official web portal to all federal, state and local government web resources and services.