Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 120552

Area Forecast Discussion
National Weather Service Des Moines IA
1152 PM CST Mon Dec 11 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 331 PM CST Mon Dec 11 2017

Confidence: High

Main concerns overnight will be wind and temperatures...then onto
Tuesday highs along with cloud cover returning.  Vigorous but
somewhat moisture starved shortwave quickly diving south through
Iowa this afternoon. The H500 parent low quite visible on the water
vapor imagery continues to spin up light rain/snow mix over the east
this afternoon.  The main impacts today have been wind along with
brief periods of light accumulating snow (north/northeast) along
with light rain/drizzle/graupel as the system tracks southeast. With
the upper level system expected to be southeast of the forecast area
by 23z...the more widespread precipitation should come to an end
with anything left being light in nature - either flurries or light
drizzle. Winds have lessened across the east behind the boundary but
remain strong in the advisory area and will keep advisory going
until expiration. Though synoptic models are quick to clear out
clouds this evening...upstream conditions/hires models suggest
otherwise and will be keeping clouds through 03-06z most areas.
Stronger subsidence will keep the boundary layer at least partially
mixed through 06z with gusts remaining up to 35 mph at times this
evening. Late evening will see break in clouds between systems
though with warm air advection and mid level clouds on the horizon
to the northwest...clouds will increase over the west during the
afternoon hours tomorrow. With the sustained cold air advection lows
tonight will drop to the lower teens north to upper teens south.
Early morning wind chills north will hover near the 0 degree mark
with northwest winds at 10 mph. By mid to late day tomorrow...H850
temperatures will be on the increase west during the day allowing
highs to warm into the 30s to lower 40s over the far west. Winds
Tuesday will be much less active than today with westerly winds
about 10 to 15 mph during the bulk of the afternoon hours.

.LONG TERM.../Tuesday night through Monday/
Issued at 331 PM CST Mon Dec 11 2017

No big changes to the forecast and it remains tricky with the region
being mired in persistent northwest flow.  There will be several
shortwaves dropping down the flow and for the most part, systems
appear to either slip to the northeast of Iowa or the southwest of
the state.  Also difficult to establish is a where if any, pockets
of moisture will be when these systems pass.  There is no tap of
real moisture.

The next wave (after today) to impact us will be Tuesday night/
Wednesday.  All models suggest the wave deepens over southwest Iowa
and there is a big slug of forcing coming across the state moisture
is either non existent or or in narrow, shallow corridors, none of
which suggest any major precip event.  If precip did occur further
northeast as the GFS suggests, it would be flurries or freezing
drizzle.  Southwest, the precip would be light rain. But even at this
point confidence is too low to delineate one area over another and
moisture is too sparse to put a PoP in.  It does appear windy again
Wednesday/Wednesday night as colder air spills in behind the wave
for Thursday/Friday.

Another shortwave drops across the region Thursday with the same
concerns for this period, though the area of best forcing does seem
to be in agreement with the models...which is over the west and
southwest.  Slight chance PoPs were left in this area for what
should start as rain but as we get towards 00Z Friday, precip should
mix or change to snow.

Models now vary on the weekend with respect to location/strength of
any shortwave/low but still bring a more zonal flow to the region
thus temps will be a touch cooler but still in the mid 30 to mid 40
range over the weekend.  The Euro is the outlier at this point but
it is suggesting a low dropping across Missouri with a quarter inch
of precip...which hasn`t happened here in a long time...but should
this model verify, this would be a snow event for much of the state
with several inches possible north and light amounts south.  While
this bears watching, it is, for now, the outlier.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1152 PM CST Mon Dec 11 2017

Skies have cleared many of the sites and should clear the others
in the next few hours. Some mid clouds and low VFR to move back in
Tuesday afternoon, then across the north low level saturation to
increase and could see CIGS drop to below MVFR. Confidence is not
high, so have only highlighted with a sct deck currently. Winds
diminishing as the clouds clear, and will continue to back around
to the west and lighten up through Tuesday. Winds then shift
around to the south/southeast Tuesday evening.





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