Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130533

Area Forecast Discussion
National Weather Service Des Moines IA
1133 PM CST Tue Dec 12 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 336 PM CST Tue Dec 12 2017

The primary concern through this period will be wind and precip
trends into tomorrow with wind conditions anticipated to reach
advisory criteria. The potent Alberta short wave/PV anomaly will
quickly race through the Dakotas tonight with leading cyclogenesis
into the Upper MS Valley. Warm/theta-e advection with limited
moisture was already in progress well ahead of this feature across
the Midwest but was producing little more than cloud cover here
with precip removed to our north and east. The 1-3km thermodynamic
forcing will strengthen further into the night but still have
insufficient moisture for precip. However the mid level mechanical
forcing more attendant to the wave will produce a brief 2-4 hour
period of lift and saturation north which may bring a light snow
to light rain mix during the morning. Any accums should stay to a
dusting at best.

By afternoon the lift departs and moisture layer shrinks with
attention turning to the strengthening MSLP gradient and mixing. A
MOS consensus projects spotty sustained winds to the 30 mph
criteria but deep mixing phased with this gradient may also
produce more widespread gusts to 39kts+ /45mph/ using a blend of
mean mixed layer wind and the top of that layer. GFS and NAM bufr
soundings show that to 00z with a fairly quick decrease after that
time. Thus have issued a Wind Advisory along and north of I-80
16-00z. There will be another fast moving Canadian short wave into
the night, but yet again with track and moisture content all
unfavorable for precip.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 336 PM CST Tue Dec 12 2017

Looking ahead into the long term, the pattern will change little
with persistent, relatively inactive NW upper level flow and
periodic short wave passages. Overall there will be little phasing
of adequate moisture and forcing for any precip with only a few
potential windows. One wave dives down the MO Valley Thu but this
will have little support below 3km and mainly track to our west.
Some token low end PoPs linger over the weekend with the passage
of a longer wave trough, but the primary low level baroclinicity
will stay well removed to our south. There is a hint a deformation
zone but the ECMWF and GFS are not in good agreement on phasing
moisture and forcing to any depth so for now any precip here looks
minor at best, and could even end up dry. Thus the overall theme
will remain with little to no precip through this period and
normal to above normal temperatures. The GFS and ECMWF do hint at
a pattern shift beyond the valid period with western CONUS trough
development and a transition to SW flow aloft, but does not look
to mature enough for much precip through the end of the next work


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1133 PM CST Tue Dec 12 2017

Little change to previous forecast with main concerns wind and
cigs. Strong cold front will push south in the morning with winds
quickly increasing aft 16z north to south during day.
Models/cross sections coming into better agreement on lowering
CIGS for much of the day following the front...with MVFR cigs
expected for most of the period through 00z. Winds to relax a bit
by 00-02z most areas. /rev


Wind Advisory from 10 AM to 6 PM CST Wednesday for IAZ004>007-



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