Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 071724
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1124 AM CST Thu Dec 7 2017

.SHORT TERM.../Today through Tonight/
Issued at 345 AM CST Thu Dec 7 2017

CAA will continue throughout the day today with
mixing to around 900mb developing late this morning into the
afternoon hours. Current winds are pressure gradient driven with
shortwave exiting the state before sunrise this morning. Some
lingering low stratus across the southeast will allow for a few
flurries prior to 12z.  Otherwise, skies become mostly clear
throughout the day into the evening. Next shortwave quickly enters
the north overnight tonight, maybe as early as 06z but more likely
by around 09z.  Saturation within the dendritic layer begins past
06z across the northwest and have flurries mentioned through 12z
Friday. Similar scenario to Thursday with the low stratus deck and
decent CAA developing causing the flurries. Temperatures today,
leaned towards the cooler side of guidance.

.LONG TERM.../Friday through Wednesday/
Issued at 345 AM CST Thu Dec 7 2017

An active pattern setting up through the next week as strong
north/northwest flow dominates through much of the period as the
highly amplified flow persists. A few flurries or light snow showers
will be possible Friday morning. An intense short wave will pass
through Friday night. A short period of light snow is possible over
the far northeast including the Mason City and Waterloo areas. While
the mid levels remain mostly dry, the low level moisture will extend
up into the favorable dendritic growth region temperatures near -
15C. This system will drag a cold front through the state overnight
with good cold advection differential leading to strong mixing and
gusty northwest winds.  The momentum transfer efficiency will be
further aided by good subsidence in the wake of the departing short
wave along with an increasing pressure gradient. A period of mixed
layer winds in excess of 40 kts is likely and this may require wind
headlines. The peak winds are expected to be between 03z-12z then
gradually diminishing through the day Saturday. Saturday will be
cold with the thermal trough moving through the state. Wind chill
values will be near to slightly below zero over the northern half of
Iowa by late Friday night into Saturday morning.

Another vigorous short wave will drop southeast into Montana on
Sunday and this will push warmer temperatures into Iowa. Current
forecast highs for Sunday are in the upper 30s to upper 40s but may
very well exceed these highs with westerly winds and good mixing
through the day. The warm up will be short lived as the short wave
passes through the state Sunday night and brings another wave of
cold advection. High temperatures on Monday likely to be early as
the cold advection arrives. Light snow will be possible during the
day in addition to another round of gusty northwest winds. Yet
another strong short wave is expected in the Tuesday night/Wednesday
period with the next chance for light snow. The entire week next
week from Monday and just beyond the forecast period will feature
these fast moving/strong systems. Suspect at least one of these
systems is being under forecast by model guidance and will be a
bigger winter weather impact than currently forecast somewhere
across the Upper Midwest. Trends have continued to push the Monday
system further west and the Tuesday night/Wednesday system still has
a lot of noise within the ensemble forecasts, making each of these
candidates to be stronger than currently forecast.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1121 AM CST Thu Dec 7 2017

VFR conditions will continue across all of central Iowa through
the remainder of the day. MVFR cigs and a few flurries are
possible as a stratus deck slides south into the state late
tonight into Friday morning. Model guidance in sound agreement
with cigs further south toward KDSM and KOTM remaining in VFR
category as the cloud cover expands south.

&&

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

$$

SHORT TERM...Podrazik
LONG TERM...Donavon
AVIATION...Martin



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