Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 050547
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1147 PM CST Sun Dec 4 2016

.SHORT TERM.../Tonight through Monday/
Issued at 329 PM CST Sun Dec 4 2016

Light snow ended across the far eastern portions of the forecast
area by early afternoon while clearing was moving into western
Iowa. Much of central Iowa remained cloudy though this afternoon
and this held temperatures in the low to mid 30s along and east of
Interstate 35. The greatest forecast challenge overnight is fog
potential. Dew points in the 30s over the melting new light snow
cover will aid development. The main negative overnight is the
pressure gradient will not completely relax leaving south to
southwest wind of 4 to 7 mph. Should the fog form it could become
locally dense over the southeast half of the forecast area then
north towards Mason City.

High temperatures on Monday will be dependent on cloud cover.
Current forecast highs in the low to mid 40s may be too
optimistic if the fog holds longer in the morning and mid level
clouds in advance of an approaching short wave arrive early.
Strong forcing and deeper saturation begins to arrive over the
northwest mainly along and northwest of a Mason City to Sac City
line by mid to late afternoon and will bring a chance for light
rain to that area.

.LONG TERM.../Monday night through Sunday/
Issued at 329 PM CST Sun Dec 4 2016

The long term forecast period appears relatively quiet but also
much colder than we have experienced since last winter. In fact at
Des Moines the high temperature forecast for Thursday is colder
than the lowest temperature recorded so far this fall/winter. Put
another way, the temperature has only dipped to freezing or lower
on four days so far this season at Des Moines, but the current
forecast holds the temperature below freezing continuously for
four days, from Tuesday through Friday of this week.

The shortwave discussed in the short term section above will curve
from the Dakotas up across Minnesota on Monday night and be the
first in a series of impulses that will carve out a large cyclonic
flow regime across much of the CONUS through at least Thursday
night. As this leading shortwave moves by it will push a cold
front across Iowa on Monday night. A narrow band of forcing along
the front will move quickly overhead, mainly across northern Iowa
into Minnesota/Wisconsin, and may generate a period of light
snowfall. Moisture is limited and any amounts should be light. Low
confidence in both occurrence and timing of any snowfall
associated with this feature dictate only slight chance POPs on
Monday night.

On Wednesday another fairly robust shortwave will cross the
central U.S. but will dig further south. This system will generate
another swath of snowfall, however, the NAM and GFS continue to
depict this precipitation across Missouri while the ECMWF insists
that southern Iowa will be affected. Given the dry low-level air
being pumped in on northwest winds during that time and the
persistence of these solutions, have maintained a fairly steady
state forecast with low POPs south of I-80 and no snow wording
around the Des Moines metro. Even if snow does fall that far
north, it would likely be very light and of little to no impact.

On Friday or Saturday the deep cyclonic flow aloft will finally
shift off to the east, allowing for a brief period of weak ridging
and a modest warming trend. Even then, high temperatures on
Saturday will only get up to around 30 in southern Iowa. This
will be followed by an active roughly zonal flow regime in which
temperatures will be more moderate but intermittent precipitation
chances will occur. At this range the timing of the impulses
responsible for any such precipitation is nearly impossible to pin
down.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1145 PM CST Sun Dec 4 2016

The primary concern remains fog and stratus trends through the
early morning hours. Currently, 05z obs and satellite imagery
shows the back edge of IFR/LIFR stratus as far west as a KTVK-
KOTM-KVTI-KCCY line with patchy MVFR fog just to the west of those
areas. With the stratus still right on the doorstep of KMCW and
KALO, and variable fog trends, confidence is only low to medium
through 14z with essentially persistence expected with winds
hopefully too strong for visibilities to drop too much.
Confidence is higher that VFR conditions will rebound by late
morning however, and continue through the day, with gusty peak
heating winds mainly west of TAF sites.

&&

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

&&

$$

SHORT TERM...Donavon
LONG TERM...Lee
AVIATION...Small



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