Area Forecast Discussion
Issued by NWS Des Moines, IA

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814
FXUS63 KDMX 050002
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
602 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 00Z TAFS through 00Z Monday evening/
Issued at 600 PM CST Sun Dec 4 2016

The primary concern will be fog and stratus trends into the
night. The current scenario has MVFR/IFR stratus along and west of
a KCNC-KTNU-KEBS-KCAV-KFXY line at 00z and continues to slowly
move west. Just behind, to the west of the clearing, visibilities
are dropping to MVFR. Confidence is admittedly low in how this
will progress with a wide range of solutions in the models. The
TAFs have played close to persistence which would suggest the
stratus not changing much, possibly back building, and some
IFR/MVFR fog developing where skies have cleared. Confidence is
higher than VFR conditions will rebound by late Mon morning
however and continue through the day.

&&

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

&&

$$

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



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