Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150426

Area Forecast Discussion
National Weather Service Des Moines IA
1026 PM CST Thu Dec 14 2017

Issued at 847 PM CST Thu Dec 14 2017

Updated forecast to include patchy freezing drizzle mention
northwest/western areas mainly before midnight. Few sfc reports
and radar returns over the west/northwest...IDOT also out with
salters over northwest areas closer to Estherville/Spirit Lake
south to Storm Lake region. /rev


.SHORT TERM.../Tonight through Friday/
Issued at 400 PM CST Thu Dec 14 2017

The primary concern through the period will be subtle precip
trends mainly in the form of flurries. The current scenario has a
SW-NE oriented long wave trough moving through the Midwest with
the primary short waves/PV anomalies to our west. Most models show
little in the way of forcing over Iowa or upstream, but satellite
imagery shows a somewhat deeper SW-NE axis of moisture
approaching northern IA at 21z, which is also reflected in the
models. It`s admittedly difficult to see where this and its
associated flurries are coming from exactly, but considering its
orientation must be more mechanically forced than
thermodynamically. Nevertheless a few CAMs reflect this and it
needs to be accounted for in the forecast so have scattered
flurries moving N to S through Iowa tonight, mainly this evening.

Considerable cloudiness will remain in place into Friday. As the
long wave trough exits, warm/theta-e advection increases across
much of the area through the day, but insufficient moisture here
will keep any precip relegated to MN and WI even though decent QG
forcing will be in place, especially 850/700.

.LONG TERM.../Friday night through Thursday/
Issued at 400 PM CST Thu Dec 14 2017

Attention in this period will eventually turn to a pattern change
with some form of a long wave trough developing across the
western CONUS. Overall models have a good handle on this
progression, as well as a compact short wave ejecting into the mid
MS Valley Sunday. There is little forcing below 3km, but mid
level deformation and mechanical forcing look to bring a glancing
shot of light snow to light rain over southeast sections during
the morning. Little to no accumulation is anticipated. What`s left
of the longer wave trough will pass to our south into early next
week with our attention turning to the next cycle of a western
CONUS trough. Our weather looks to remain dry in the mean time
with another stretch of above normal temperatures.

By Thursday the long wave trough will mature through the Rockies,
but as one would expect at Day 7 there are varied solutions among
the ECMWF and GFS deterministic runs and their associated
ensembles with uncertainty regarding short wave strength and
timing. Thus confidence is fairly high in some form of
precipitation around this time across the Midwest, likely snow of
some degrees over portions of IA, especially north, but
confidence in details is low. Confidence is also high in an
associated nearby frontal system and baroclinic zone, but the
aforementioned variability will result in low confidence down to
the scale of our forecast area with the surface low track anywhere
from IA to MO. This will result in a fairly wide range in
potential temperatures. Although it will be beyond the forecast
period, there is relatively high confidence in colder weather
behind the passage of this system to end the work week with what
would likely be the coldest temperatures of the season.


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1026 PM CST Thu Dec 14 2017

MVFR conditions mixed with VFR periods most areas through 12z with
periods of MVFR conditions holding on north sites KMCW/KALO
through around 18-19z. Tonight additional concern of patchy -fzdz
mixing with flurries with weak trough passing through area.
Coverage rather limited with an area along highway 20 near KFOD at
04z and another area near KADU/KAIO. Possibility of brief period
at KDSM between 06-08z but confidence remains low. Otherwise aft
15z west of I35 clouds will thin first as warmer air push arrives
remainder of period...and later north. /rev




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