Area Forecast Discussion
Issued by NWS Des Moines, IA

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209
FXUS63 KDMX 011009
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
409 AM CST Thu Dec 1 2016

.SHORT TERM.../Today through Tonight/
Issued at 400 AM CST Thu Dec 1 2016

Broad cyclonic flow will continue across the upper Midwest
including Iowa today into tonight although it will be weakening
with time as main upper low pulls into southeastern Canada. Low
level moisture will remain in place with extensive cloudiness into
tonight although clearing will approach toward the end of the
period. Will see some increased mid-level forcing by later today
into the evening in northern Iowa which may help linger some
patchy precipitation, however moisture is not nearly as deep as on
Wednesday with precipitation production marginal at best. May see
a few sprinkle/flurries with the lift but not much else. Winds
will remain brisk but a downward trend will continue in overall
speeds as pressure gradient weakens with the approach of surface
ridging. However, wind velocities tonight likely too light into
model guidance with the extensive cloudiness helping low level
mixing to persist well after dark.

.LONG TERM.../Friday through Wednesday/
Issued at 400 AM CST Thu Dec 1 2016

Forecast concerns involved light snow accumulations Saturday night
into Sunday and with active weather pattern and temperatures next
week. Models continue to show large discrepancies after Monday of
next week and much uncertainty remains wrt to any significant
storm and even temperatures next week. Leaned closer the GFS
through Sunday and a blend thereafter.

Friday into Sunday...Upper low over California becomes cutoff from
the main 500mb flow Friday into Friday night. This allows a weak
shortwave embedded in the zonal flow aloft to impact the state
Saturday night into Sunday. The precipitation with this system
appears to fall primarily as snow, with some rain possible early
in the event and towards the end Sunday morning. Light snow
accumulations will occur overnight Saturday night into Sunday.
There is little, if any, forcing within the dendritic layer
Saturday night even though there is a quick burst of decent QG
forcing b/t 06-12z Sunday over the forecast area. The better
moisture seems to be across the eastern portions of the CWA during
the same time and thus have around a half inch or less going. Any
forcing remains below the dendritic layer Saturday night. Some
weak WAA at the surface helps warm the boundary layer and likely
switch precipitation over to rain mid to late morning Sunday.
However, much of the forcing looks to be quickly moving east b/t
12- 15z Sunday and have low confidence with any rain after 15z.
Depending on how quickly the cloud cover exits, temperatures may
be too cold for Sunday, particularly over the western portions of
the CWA.

Monday into Wednesday...Weak surface high pressure begins to move
east of the state during the day Monday and allows for some return
flow to develop. Monday will be the warmest day of the extended
and nudged up the maximum temperatures to coincide with the
decent WAA and mixing. Next, the aforementioned cutoff low finally
gets back into the main upper level flow Monday and quickly moves
east-northeast through Tuesday as it tracks across Missouri.
Further north, a surface low over the Dakotas into southern
Canada bring a cold front into the region and thus the combination
of the two systems will be the focus for some snow and rain
potential Tuesday into Tuesday night. Although it appears more
likely become split between these two system and thus likely drier
during the day Tuesday. The main discrepancies really show up by
Wednesday into Thursday. The ECMWF is much slower than the GFS
with deepening the trough and moving it eastward into the region.
Regardless of the solution, much colder temperatures look to
impact the region mid to late next week. 850mb temperatures as
cold as -12C Wednesday per 01.00z GFS and the ECMWF shows a
persistent few days of -6C 850mb Wednesday into Friday. Current
forecast is too warm if this solution holds true and will need to
trend much colder if GFS/ECMWF continue this solution. Wednesday
into Thursday remains the best time frame for any significant snow
with the CAA and both ECMWF/GFS hinting on a decent shortwave
cutting across the region. Plenty of timing and location
questions to be determined and remains something to monitor
closely.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1136 PM CST Wed Nov 30 2016

Confidence is fairly high that little change is anticipated
through much of the period. Although the vast majority of the
precipitation has ended, MVFR ceilings will persist into Thu,
possibly with periods of IFR ceilings north and east. Winds will
essentially remain unchanged.


&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Podrazik
AVIATION...Small



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