Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 040602

National Weather Service Des Moines IA
1202 AM CST Sun Dec 4 2016

Issued at 913 PM CST Sat Dec 3 2016

There will be no appreciable changes to the forecast this evening.
Confidence in amounts is medium at best due to melting concerns.
Have solicited for reports on social media and dispatch offices
with all melting near Fort Dodge, and just token grass accums and
some slushy roads around Denison and Carroll where road temps have
dipped to freezing or below.


.SHORT TERM.../Tonight through Sunday/
Issued at 300 PM CST Sat Dec 3 2016

Strong short wave lifting northeast through Nebraska and South
Dakota this afternoon with good QG forcing and moisture advection
moving into western Iowa. An area of snow has developed over west
central Iowa and eastern Nebraska. Several observing sites across
this region have visibilities dropping below 1SM at times in snow.
South of this area another short wave is lifting rapidly north
through Texas with a strong moisture transport ahead of the system
streaming north through Oklahoma and eastern Kansas.
Precipitation should fill in south of the current precipitation
region in response to the moisture lifting north. The entire area
of precipitation will then move northeast through central Iowa.

Precipitation type between rain and snow is the biggest question
overnight. Temperatures above 1000 ft AGL are cold and below 0C
through the period for all areas except for far south central Iowa
where a weak warm nose arrives. A ribbon of higher theta-e/sfc wet
bulb air will lift north into central Iowa. Have used a RAP/NAM
compromise on the 34.5 F surface wet bulb line as the are where the
precipitation switches to mainly rain with little to no snow
accumulations south of this area.  This line will be close to
Interstate 80 with the exception of the region from Grinnell to
Ottumwa where snow may become more predominant. Generally 1-3 inches
forecast north of the 34.5 F line with locally up to 4 inches up
near Waterloo. Will be issuing a winter weather advisory for areas
where 2 inches or more falls overnight when the snow has a better
opportunity stick to roadways and cause a traffic hazard.  The
advisory is also being issued considering this will be the first
winter driving hazard of this winter season.

.LONG TERM.../Sunday night through Saturday/
Issued at 300 PM CST Sat Dec 3 2016

Sunday Night through Tuesday Night...Confidence Medium: Though
confidence will be high for the early portion of the
late Tuesday night the development of another potent southern
stream shortwave will result in some challenges. Namely...the
models currently are handling the system different with the
European model farther north into southeast Iowa while the 06z run
of the GFS continues to push the system east...just south of the
border. There is also some uncertainty with regard to the timing
of the arrival of the Arctic airmass. The GFS is slightly faster
than the Euro with the transition day being Wednesday. With the
airmass being cold enough for all snow...the system Tuesday night
will bring either a period of light snow or perhaps some light
accumulations in the southeast third of the area. Though the
trends have been for lighter precipitation and colder air by
midweek...will need to monitor for potential drift in model
solutions though at this point would expect that at least for the
storm side of the equation...we would remain on the light side.

Tuesday Night through Saturday...Confidence Medium to High:
Models continue to come into better agreement with passage of
southern stream system just grazing southern Iowa with light snow
and perhaps some accumulation early Wednesday. With cloud cover
present most of the day...diurnal range will be limited to about 5
to 10 degrees at best. Strong cold air advection will continue
through the day with H850 temps of -8 to -9C in the early portion
of the day falling to -10 to -12C by days end. A second push of
Arctic air will drop south late Thursday with H850 temperatures
nearing -18C by 00z Friday. The thickness will bottom out to about
505 to 510 dm at 00z then warm air advection with some moderation
in temperature will occur overnight. Some clouds may help to keep
temperatures a few degrees warmer by Friday morning...though
overall the 2 day cold spell will be the coldest temperatures
regionwide so far this season. Ensemble guidance suggests that
mins have a chance of being under 10 degrees both Thursday and
Friday over the northern half of the forecast area. With the
center of the cold high pressure settling into the southeastern US
by 00z Saturday the Gulf of Mexico return moisture will temporarily
be limited through Friday morning but with return flow beginning
by late Friday there will be another chance of precipitation toward
the end of the period. At this time...even with warming...the
precipitation looks to be all snow as highs reach the lower 30s.
Given the lack of consensus in the out periods the past 3 to 5
days...will not discuss amounts or location at this point.


.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1201 AM CST Sun Dec 4 2016

Confidence has increased somewhat since 00z. Widespread IFR/LIFR
conditions continue along and west of an KAIO-KAMW-KMIW-KALO line
at 06z...and these conditions are expected to spread eastward into
KDSM and KOTM early this morning. All TAF sites will likely see a
period of IFR visibility restrictions due to snow or a rain/snow
mix. Conditions should then increase sharply toward midday as the
precip departs with VFR conditions returning through the rest of
the period.


Winter Weather Advisory until 11 AM CST Sunday for IAZ017-



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