Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 041744
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1144 AM CST Mon Dec 4 2017

.SHORT TERM.../Today through Tonight/
Issued at 333 AM CST Mon Dec 4 2017

The weather will be quite changeable through the period with temp,
wind and precip trends all concerns. Currently the parent upper low
which will drive our weather changes is moving through the Rockies
with broad diffluence helping aid mid and upper level lift. This
layer is struggling to saturate however so nearby precip over
eastern Iowa is driven more by lower 1-3km forcing in warm/theta-e
advection focused by 0-2km moisture convergence. MUCAPEs are 500
j/kg+ resulting in some lightning to our NE, which could possibly
ooze back just into our forecast area. This forcing looks steady
state into the morning so PoPs are relegated to eastern sections.
Although the weaker and limited moisture mid level kinematic forcing
will persist into the afternoon, the 1-3km thermodynamic forcing
will end with the passage of the cold front aloft and associated
frontogenesis zone.  Thus will leave all but eastern sections dry
with attention turning to the west.

From 18-00z the deformation zone and strong mid level QG forcing
will mature and intensify and begin affecting northwestern sections,
although the max response will remain north of Iowa. This forcing
will be inverted from earlier today with strong subsidence below 3km
and any precip driven by mid level lift and eventual saturation. The
window will be brief, but soundings suggest a few hours of snow
likely NW during the early evening hours. Although accums will only
be a half inch or less, it will be wind blown when it falls as both
the MSLP gradient and mixing increase as the surface low moves into
NW Wisconsin. The precip should end during the late evening hours
with attention turning entirely to strong winds. MOS suggests
sustained speeds will reach at least 30-35 mph with Fairmont and
Mason City Bufkit soundings suggesting gusts may reach 45-50 mph
with gusts 35-45 pm in the southern portions of the advisory area
through at least 06z and possibly 09z so have extended the valid
time to account for that. Although it will be brief, the phasing of
snowfall and strong winds may make for hazardous travel conditions
far NW, more due to visibility restrictions than slick roads.

Temp wise, have nudged up highs which will boost to 70F in spots
with 09z readings already in the lower 60s central and south. Low
level moisture will keep mixing from being too much, but it should
be sufficient to reach those temps. Readings should then begin
falling during the afternoon west with the frontal passage and then
farther east into the evening. The strong winds into the night will
likely keep readings up somewhat versus what is shown by MOS so have
leaned toward the warmer raw models temps which often perform better
with steady mixing into the night.

.LONG TERM.../Tuesday through Sunday/
Issued at 333 AM CST Mon Dec 4 2017

On Tuesday an upper trough swings through the state and cold air
continues to spill into the Upper Midwest.  This will be the first
day where very cold temps are felt as highs only make it to the mid
20s to mid 30s.  The gradient over the state remains tight so winds
will still be an issue for Tuesday.  Bufkit soundings show 40+kts
atop the mixed layer at Mason City through Tuesday evening and winds
at ALO will be that strong through at least Tuesday afternoon.
Bottom line is that wind headlines may need to be extended across
the north/northeast through Tuesday.  Will let the day shift have
one more model run to assess before extending the headlines already
in place.  The other problem with these strong northwest flow cases
is that there is usually vort maxes/shortwaves that drop through the
flow and these are very hard to time and place spatially.  Tuesday
afternoon most models indicate some sort of brief snow shower
activity with such a feature in the afternoon though confidence is
low in the placement/timing of this so just have some flurries going
in the forecast for now. A second shot at flurries comes Tuesday
night with yet another disturbance in the deep northwest flow.

On Wednesday the upper low and trough shift east which will relax
the gradient over the region giving us less wind but we will still
be cold with highs similar to Tuesday.  At this point I am keeping
Tuesday/Wednesday dry but again, low confidence in where another
round of weak, fast moving disturbances pass through the flow.

Late Thursday into Saturday a big ridge over the Western U.S.
sharpens and this deepens the trough over the Upper Midwest/Great
Lakes bringing more cold air into the Upper Midwest.  Highs
Friday/Saturday will only be in the 20s across the forecast area
with lows both nights in the single digits and teens.  It also looks
windy again Thursday through Friday so wind chills will be very
cold.  In addition all models suggest a stronger shortwave dropping
into the state and have some qpf into central Iowa.  We could see
some snow Friday into Saturday but this is near the end of the
forecast period so there is much uncertainty with this but it will
be something to watch going forward.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1144 AM CST Mon Dec 4 2017

Quickly evolving conditions through at least 03z. Strong south
winds continue ahead of the primary cold front...switching to sw/w
following the passage. Winds briefly increasing just behind the
boundary in pocket of clear air/dry slot. A band of MVFR/IFR low
stratus has developed as well over far western Iowa and will need
to monitor progression east. Additional concerns will be brief
period of possible convection at KOTM from 21-01z...strong
northwest winds of 30 to 45 kts aft 00z through 09z...and
continued strong gusty winds for the remainder of the period.
Brief period of light snow possible from 00z through 04z mainly
north areas and may affect KMCW between 02 to 04z with strong
winds and visibility reductions due to falling/blowing snow. /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 3 PM this afternoon to 3 AM CST Tuesday for
IAZ004>007-015>017-023>028-033>039-044>049-057>060-070.

&&

$$

SHORT TERM...Small
LONG TERM...FAB
AVIATION...REV



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