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Area forecast discussion 
National Weather Service Des Moines Iowa
404 am CST Monday Dec 29 2014

Synopsis...
issued at 314 am CST Monday Dec 29 2014

Weak shortwave will move across the state today as cold high pressure
continues to build south into the Continental U.S.. the high will move to the
southeastern U.S by later in the week. Upper level low tracking south
along the West Coast will push east late in the week into the
weekend.

&&

Short term /today/...
issued at 314 am CST Monday Dec 29 2014

Our snowfall potential for today has some issues that will likely
result in light snowfall amounts west and little more than flurries
or a dusting elsewhere. Yesterday the NAM hinted at two periods of
forcing and it is keeping with that solution even now. Overnight a
weak short wave pushed across northern Iowa. There was some forcing
associated with this feature which is resulting in light snow mainly
across the west. There were some isolated snow showers central and
east probably tied to some weak forcing that models are not picking
up on. Radar was interesting as well around 04z-06z it showed a
transient narrow enhanced band from Boone to northeast Story County.
There was an hour or so where up was reported at Boone. Soundings
are fairly dry so the potential for freezing drizzle appeared
realistic. Grids were updated to account for this. Additional
transient enhanced radar returns were noted across southern Iowa and
some light precipitation was added to account for this as well which will
go through about 14z.

For the rest of today...we have to look at the broader large scale
features and of note is a low over the western U.S. Sliding down the
trough out west. This will develop into a closed low later today
and tonight. This feature is noted on all models and as it moves it
appears to be pulling the shortwaves ejecting from the trough
further south. In addition...at the surface is a large Canadian
high pressure system dropping down out of Canada. This will be the
high that Ushers in colder air for Iowa later tonight but for today
it is filtering in drier air and as a result snow is having a hard
time getting organized. With the high pushing down and the flow
generally shifting south and snow potential will be relegated to the
western County Warning Area and likely not amount to much. Looking at forcing and
that is sinking south as well. For these reasons I have cut back
snowfall amounts by a fair amount.

Clouds are a bit problematic however. While saturation is weak and
spotty in the dendritic zone...low level moisture tries to re-
establish itself this afternoon and so there may be some low
ceilings coming in once again. Currently ceilings are between 2500
feet above ground level and 7000ft above ground level. There are some spotty lower ceilings tied to
the radar reflectivity mentioned earlier but this is expected to be
transient through at least early afternoon. Temperatures will be colder
today than in the past with highs in the teens north. Further south
where the high has less influence highs will be in the lower 30s but
most locations will only be in the middle 20s. Temperatures will drop off
from there.

Long term /tonight through Sunday/...
issued at 314 am CST Monday Dec 29 2014

Any snow moving through today will be south of the of the County Warning Area
tonight with cold air sinking south. Temperatures will be below zero over
the snow covered areas...though winds will keep temperatures from falling
much more than to the -5 to -10 range. Though the winds will not
be all that strong...enough wind is expected tonight that will go
with a Wind Chill Advisory for up to -25 below wind chills along
and north of a kdns-kmcw line. Expect to see similar to slightly
colder wind chills Wednesday morning and may well need another advisory
for that.

Temperatures Wednesday morning will be a little tricky. As mentioned the past
couple days...there is concern for the passage of the high
pressure center being well west of the state. Winds will be
increasing westerly by Wednesday morning. With flow not out of
the north...tendency is for temperatures to not cool as much as one would
think. Went a little above MOS for lows for that reason.

There is quite a discrepancy in the models with the evolution of
the southwest system late this week into the weekend. GFS has been
consistent with itself for the most part...though todays run is
warmer overall. The European model (ecmwf) has been the flip-flop model with its
positioning of the snow band. Today is much farther south than
yesterday and also the 12z run. Will discount that for the time
being and held fairly close to the thinking of the past few days.
The other concern is how much warm air will make it into the
system. As hinted yesterday...the amount of warm air is
significant with the warm later reaching 4 to 5 c over
southeasternia. The MOS actually GOES all freezing precipitation. Feel
that may be overdone...but did opt to word the precipitation as -sn or
fzdz for the first half of the event.

The other change with the forecast is the quicker retreat of the
cold air heading into next week. Warmed temperatures up several degrees
for the Sunday into Tuesday time frame. Even so...the change is
still 10 to 15 degrees cooler than the European model (ecmwf) forecast.

&&

Aviation...29/06z
issued at 1138 PM CST sun Dec 28 2014

Light snow is attempting to approach from the Missouri Valley
region but it is encountering powerful dry air in the lower
levels...which has also eroded all low clouds out of our forecast
area. It appears that VFR conditions will prevail through the rest
of the night...and probably through the taf period at the eastern
terminals. At fod/dsm high resolution models and forecast
soundings continue to indicate the potential for low ceilings during
the day on Monday...and have advertised MVFR in the 06z tafs but
there is some uncertainty regarding the ceiling heights. The -sn and
lower clouds should blow out late Monday as northerly breezes
strengthen and the dry air reasserts itself...allowing for a
return of VFR conditions by Monday night.



&&

Dmx watches/warnings/advisories...
Wind Chill Advisory from midnight tonight to noon CST Tuesday
for Calhoun-Cerro Gordo-Crawford-Emmet-Hancock-Humboldt-Kossuth-
Palo Alto-Pocahontas-SAC-Webster-Winnebago-Worth-Wright.

&&

$$

Synopsis...MS Dec 14
short term...fab
long term...MS Dec 14
aviation...Lee

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