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Area forecast discussion 
National Weather Service Des Moines Iowa
548 am CST Sat Jan 31 2015

Short term /today/...
issued at 313 am CST Sat Jan 31 2015

Precipitation trends are the obvious concern in this period. Confidence is
high in progression of precipitation as a whole...which is just trying to
cross the MO river...but maximum temperatures in lowest 1km are quite variable
in time and space through the rain/snow/mix 1-3c threshold. Using a
GFS/NAM/rap model consensus for these temperatures results in a rain/snow
mix advancing into central Iowa this morning and into the Highway 20
or 30 corridors by early afternoon with mainly rain south. Temperatures
begin to cool late however with less variability changing to mainly
snow outside of a rain/snow mix southeast quarter toward 00z. Any
accumulations should mainly be under an inch through 00z due to
either late arrival north or melting and mix farther south. Accums
should really pick up just after this valid period however.

Models are in good agreement with deep weak/moderate kinematic and
thermodynamic forcing accompanying slab of moisture to similar depth
in right entrance region of Great Lakes to East Coast jet segment.
Hi res reflectivity simulations show this nicely as well so have
categorical wording south half by afternoon. Generally followed a
blend of NAM/GFS MOS for highs with a somewhat higher blend toward
the cooler NAM numbers due to precipitation.

Long term /tonight through Thursday/...
issued at 313 am CST Sat Jan 31 2015

A prolonged winter storm is set to affect the area from tonight
through Sunday. As approaching northern and southern stream
systems merge and phase over the Midwest we will see an inverted
trough extend from Kansas and Nebraska across central Iowa by this
afternoon...with a large precipitation shield moving northeast across
Iowa late today in association with the broad region of ascent
ahead of the approaching 500 mb trough. Temperatures will be warm
enough to support rain initially...especially along and south of
the surface trough...but after dark tonight it will transition
over to snow pretty quickly. Surface temperatures will likely
remain above freezing across about our southern half or so through
the late evening and perhaps until close to midnight...however
forecast soundings indicate that temperatures will be below
freezing just above the surface and dynamic cooling within the
column should produce the quick change to snow. Meanwhile...with
frontogenetical forcing overhead and precipitable waters above the 95th
percentile for the season given the system plenty of moisture to
work with...qpfs are unseasonably high and approaching an inch in
some areas which is near the climatological average for the
entire month of January. Combined with a fairly deep dendritic
growth zone...this should lead to fairly heavy snow with rapid
accumulations overnight especially after about 8 or 10 PM as the
snow really starts to stick.

On Sunday the surface low/trough will slide off to our southeast
but the 500 mb trough will just be passing overhead...providing
an additional slug of forcing and cold air supporting more snow
with an even deeper dendritic growth zone indicated by all model
soundings. In addition...the tightening pressure gradient between
the departing surface low and a large ridge building in from the
northwest will lead to strong north winds of around 20 miles per hour with
higher gusts during the day Sunday. With multiple inches of wetter
snow already on the ground from Saturday night and winds of this
caliber blowing around the drier falling snow...blowing snow
issues could be significant on Sunday. The wind speeds are too low
for blizzard conditions to be reached...but travel could be badly
affected especially while the snow is falling. Given the
combination of all of these conditions and impacts...have issued a
Winter Storm Warning for much of the forecast area from late this
evening through late Sunday evening.

The snow will gradually end on Sunday evening as the
aforementioned high pressure ridge builds into the area bringing
much cooler weather especially with the fresh snow pack in place.
Iowa will then remain beneath generally northwesterly 500 mb flow
for the bulk of next week...preventing any significant warming and
preventing the snow from melting much. A series of mostly weak
impulses will move through this northwest flow producing
additional light snow at times. The strongest such wave may
produce accumulating snowfall somewhere around Tuesday night...and
while amounts appear light it may still have some travel impacts
with ditches already full. This system will also push through a
reinforcing shot of cool air which could make Wednesday night into
Thursday morning the coldest period of the week. Toward Thursday
and Friday there are some indications of weak ridging aloft and
low level warm air advection...but these effects will likely be
tempered by the deep snow pack expected to be in place.


issued at 548 am CST Sat Jan 31 2015

Conditions will continue to deteriorate today transitioning from
VFR this morning to certain low end MVFR and possible IFR at times
tonight and into Sun morning. Widespread rain/snow mix or Kansas and
western MO will advance into Iowa south to north today and transition to
all snow by this evening and continuing into sun. Winds will
increase somewhat north and west /kfod/kmcw/Kalo/ 09-12z leading
to blowing snow as well. The snow and blowing snow with IFR/MVFR
conditions will expand state wide and continue beyond the valid
period /12z/ as well.


Dmx watches/warnings/advisories...
Winter Storm Warning from 7 PM this evening to 9 PM CST Sunday
for Adair-Adams-Appanoose-Audubon-Black Hawk-Boone-Bremer-Butler-

Winter Weather Advisory from 7 PM this evening to 9 PM CST
Sunday for Cerro Gordo-Emmet-Hancock-Kossuth-Palo Alto-Winnebago-



Short term...small
long term...Lee

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