Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201802
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1202 PM CST Tue Feb 20 2018

.SHORT TERM.../Today through Tonight/
Issued at 330 AM CST Tue Feb 20 2018

Main surge of warm advection and moisture transport ongoing
currently across central Iowa.  This is producing a large band of
freezing rain with embedded thunderstorms which themselves are
producing some prolific sleet showers for brief periods.  This area
will continue to move rapidly northeast through sunrise with the
bulk of precipitation out of the forecast area by 12-14Z. However,
additional freezing rain will be expected in the southeast
coincident with another wave moving northeast on the frontside of
synoptic scale trof.  Additional icing potential will now be
greatest south of a line from Waterloo through Newton to Lamoni this
morning with the exception of the far southeast corner of the
forecast area which has temperatures currently near or slightly
above freezing.  Otherwise, as northwest surface winds increase,
drier air will deepen across the state limiting precipitation
elsewhere by mid to late morning and the advisories in the northwest
may be cancelled early.  Temperatures today will see little movement
and as northwest winds increase with passage of the surface wave,
sufficient cold advection will lower readings by afternoon in most
areas.

Some clearing is expected tonight with the surface ridge approaching
the Missouri River Valley by Wednesday morning.  Temperatures should
plummet to below zero in the far northwest nearest the thermal trof
with single digits above zero and teens farther southeast.

.LONG TERM.../Wednesday through Monday/
Issued at 330 AM CST Tue Feb 20 2018

Subtropical high will remain anchored over the southwestern Atlantic
keeping the ridge over the eastern US while a longwave trough of low
pressure will persist over the western US through the end of this
week. This will keep the flow over Iowa from the southwest with
opportunities for embedded shortwave troughs to periodically move
through the state.

High pressure will pass over the state and into the Great Lakes
Wednesday into Wednesday night. With subsidence over the state much
of the day, sunshine should be in abundance. Despite this,
temperatures will still be 5 to 10 degrees below. Subtle PV anomaly
collocated with the right entrance region of a departing jet
streak will increase lift over the area Wednesday night into
Thursday. Soundings show saturation in the mid-levels into the low
levels, but is lacking saturation within the ice introduction
region until later in the day or evening. GFS profiles are colder
than the NAM during much of the day with the GFS saturating in the
ice introduction layer late Thursday into Thursday night. In
collaboration with neighboring offices, favored GFS for more ice
introduction which lessens freezing rain/drizzle. Overall,
precipitation will start mainly as light snow. It will gradually
transition from southern Iowa to around Highway 30 during the
daytime from light snow to freezing rain/drizzle to light rain,
especially over southern Iowa. Thursday night expect a wintry mix
over much of the state with light snow over far northern Iowa
before the precipitation ends. QPF amounts are under 0.2 inches
with ice amounts at this point generally under a tenth of an inch
with perhaps 1-2 inches of snow over far northern Iowa.

Another area of high pressure will pass north of the state with
associated subsidence moving over Iowa allowing for a reprieve from
the wintry precipitation on Friday. However, the ridge over the
eastern US will flatten this weekend as the trough lifts and
advances eastward across our area. This will bring another round of
wintry precipitation across the state Friday night into Saturday
night as low pressure tracks from Colorado through Missouri or into
southern Iowa. The GFS is slower compared to the ECMWF so there will
be refinement to the timing. Overall, precipitation will be
primarily snow over northern Iowa and a change from snow to rain and
back to snow over southern Iowa. Beyond this weekend, confidence
decreases as quasi-zonal flow sets up with a shortwave trough moving
through the state on Monday, but models differ on the typical
details at this time range.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1202 PM CST Tue Feb 20 2018

A wintry mix of precipitation may affect KOTM early this
afternoon, possibly down to IFR briefly, but otherwise dry
conditions are anticipated. Elsewhere stratus will linger with
widespread MVFR ceilings, but they should improve to VFR NW to SE
toward sunset with medium confidence in VFR into Wed.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Ansorge
AVIATION...Small


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