Area Forecast Discussion
Issued by NWS Des Moines, IA

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568
FXUS63 KDMX 200930
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
330 AM 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
colocated 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 06Z TAFS through 06Z Tuesday night/
Issued at 1127 PM CST Mon Feb 19 2018

Freezing precipitation obviously biggest concern. Will have to
keep an eye on OTM. At this time have -RA, but cannot completely
rule out -fzra/-fzdz potential. KALO touchy with fog... vsbys
likely 1/4 mile or less in eastern Black Hawk county, but around 2
miles in western Black Hawk county. KALO sited in the middle, so
will likely oscillate between 1/4 mile and 2 miles this morning.
Once precip moves east of area generally by noon except for KOTM,
winds will begin to increase and will persist in the 10kt to 20kt
range overnight.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon CST today for IAZ004>007-
015>017-023>028-033>039-044>050-057>062-070>075-081>085-092>096.

&&

$$

SHORT TERM...Cogil
LONG TERM...Ansorge
AVIATION...Kotenberg



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