Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 072124
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
324 PM CST Wed Dec 7 2016

.SHORT TERM.../Tonight through Thursday/
Issued at 316 PM CST Wed Dec 7 2016

The upper level low pressure over Lake Superior will begin to move
east tonight and will move Iowa out of the lows base and more zonal
flow and back to cyclonic flow. This will drag another wave of cold
advection south into the state along with stratus and the potential
for flurries and/over very light snow to northern Iowa tonight into
Thursday. Widespread flurries and light snow have been going on
upstream through the eastern Dakotas and Minnesota today. This
region will shift south and should clip northern Iowa by 09z
tonight. Low level moisture will increase through a deep dendritic
growth zone region that will exist from near sfc to 700 mb. Despite
limited vertical ascent and turbulence within this layer, it will be
enough for some dendrites to develop. This is the potential for
minor accumulations of a few tenths of an inch near Mason City and
northeast. Temperatures overnight will be in the 5 to 15 range with
highs on Thursday in the teens to low 20s. The wind will remain
breezy with gusts over 30 mph possible at times. The combination of
the wind and temperatures will push wind chill readings to 5 to 10
below 09-15z tonight into Thursday morning.

.LONG TERM.../Thursday night through Wednesday/
Issued at 316 PM CST Wed Dec 7 2016

We will be entering a more active pattern but the big story remain
the "introductory" cold temps this week followed by really cold
temps next week with a potentially headline type snow event North on
Saturday/Saturday night.

Lows Thursday night...coming out of the coldest day this week...will
be in the single digits for across the forecast area for the most
part.  There is some concern with winds staying up a bit and cloud
cover North that temps would stay up a bit more than what we have
forecast.

Despite a weaker Northwest flow on Friday...cloud cover will
increase ahead of the next system and given the cold start to the
day we`re not seeing much of a recovery of temps on Friday.  Friday
night we start a brief period of warm advection as a surface low
develops over the Plains and an associated warm front lifts from SW
IA Northeastward.  This is setting up to be an interesting system
with a potential for a headline event snow.  Currently models are
not in decent agreement with the placement and evolution of the low
but there does appear to be a rapid deepening of moisture with deep
forcing through the dendritic layer.  Snow ratios during the best
period of forcing will be changing from around 18 to 1 down to
around 12 to 1 or so.  Even with that there is a decent shot at
several inches across the far North by Saturday evening. We will be
monitoring forecast trends for better model consensus.  At this
point the Euro remains with the surface low weaker and further south
and a weak shortwave moving across with the GFS and having Canadian
models having a deeper shortwave and the surface low further North.

Model differences continue into Sunday but for the most
part...Sunday will be dry except for maybe the far East with only
slight chances for snow...depending on which model is right with
respect to how the surface low moves off.

Then Monday we start our change.  After highs in the 20s and 30s
over the weekend...a large low over Canada will act to sharpen a
ridge over the West Coast and with time this low will deepen a
trough over the upper midwest and drop some very cold air into the
region.  The Euro has the low further West sending a couple of
shortwaves across Iowa along with precip/snow chances.  The GFS and
Canadian are both further East with the low but the GFS skews the
trough axis further East as well sending the shortwaves further
South.  Either way we will be much colder by Tuesday/Wednesday with
highs in the single digits and teens and lows in the single digits
above and below zero.  There will also be periods of wind which will
bring wind chills into the conversation. Depending on which model
verifies...we could also be adding some snow accumulation to the
forecast which will definitely impact temps as well.  All of this
will be better defined in the coming days as model agreement occurs
but one thing for sure is that the mild weather we have seen is done
for awhile.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1137 AM CST Wed Dec 7 2016

MVFR cigs will be in the vicinity of KMCW/KALO this afternoon with
other sites expected to remain VFR. MVFR cigs are expected to
increase from north to south late tonight and persist through much
of Thursday morning. A few flurries or very light snow is possible
over northern Iowa late in the period. The wind will remain gusty
from the west/northwest through the forecast period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Donavon



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