Area Forecast Discussion
Issued by NWS Des Moines, IA

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889
FXUS63 KDMX 071121
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
521 AM CST Wed Dec 7 2016

.SHORT TERM.../Today through Tonight/
Issued at 323 AM CST Wed Dec 7 2016

Cyclonic flow aloft continues across the area today into tonight
with an upper low situated over srn Ontario/NRN MN. Area of sfc high
pressure building southeastward clipping Iowa today with the
gradient expected to tighten through the day as IA is sandwiched
between the building area of high pressure off to the southwest and
an area of sfc low pressure over Ontario. CAA in place through the
day with gusty west to northwesterly winds. Some sun expected across
portions of the CWA but will be limited late in the day with clouds
building southward as a lobe of energy rotating around the parent
low sinks southward toward the state. Expect warming today to be
somewhat limited given the push of cold air and limited sunshine.
Therefore have dropped temps a few degrees for highs with temps
staying steadier across the north where clouds move in faster this
afternoon.

Also dropped lows tonight with slightly cooler highs expected, and
some areas across the central and south to remain clear. With gusty
winds expected tonight due to the tightened sfc pres gradient,
radiational cooling will be dampened some. However airmass is quite
chilly, and should still see a decent temps drop. Therefore went
with lows around 7 to 13 degrees.

.LONG TERM.../Thursday through Tuesday/
Issued at 323 AM CST Wed Dec 7 2016

Winter has finally arrived and the seven day forecast remains,
overall, cold and somewhat active. On Thursday, one last lobe of
vorticity rounding the base of a large departing trough will move
across Wisconsin and northeastern Iowa. Forecast soundings and
forcing profiles indicate some potential for flurries during the
day across our northern and eastern counties, but there should be
no impact. This pattern will also promote biting northwest breezes
and cold temperatures, with wind chill values in the single digits
below and above normal through the day. From Thursday night
through Friday night a low-level ridge will move slowly across the
region, resulting in lighter winds and mostly dry conditions but
a continuation of cold temperatures, with overnight lows in the
single digits and teens. A dusting of snow may be possible across
far northern Iowa on Friday, but again, there should be little to
no impact with modest winds and surface temperatures well below
freezing.

By the end of the week a fast zonal flow will be in place aloft,
with an initial impulse scooting over the Pacific northwest and
northern Rockies by Saturday morning. This will be the first in a
series of shortwaves that will result in intermittent
precipitation chances and no significant warming for the remainder
of the forecast period. As the first system approaches on Saturday
a broad surface low pressure area will develop along the lee of
the Rockies, with a warm front/inverted trough extending up near
or across Iowa. In general this would favor the development of an
area of snowfall along and north of the boundary, associated with
the strong vertical forcing for ascent associated with the
shortwave aloft, and various model solutions are illustrating
this. However, there remains considerable variability regarding
the extent to which the shortwave will remain progressive or dig
and intensify over the Midwest, which in turn will affect the
degree to which cyclogenesis may occur along the inverted trough,
and the extent to which this may enhance/expand the associated
snow field. Over the last couple of runs the ECMWF has been more
open and progressive with this system, resulting in light snow
amounts mainly across northern Iowa, while the GFS has been deeper
and stronger resulting in more moderate accumulations over a
larger area. Have increased POPs to categorical in our northern
counties on Saturday, but maintained QPF significantly lighter
then GFS for now. We will be closely monitoring this system in the
coming days and headlines may eventually become necessary.

After a brief lull on Sunday, the next mid-level impulse will move
over the region on Monday or Monday night. There is some snowfall
potential associated with its passage, but overall it appears more
transient than the initial wave. It will likely also push a
strong, reinforcing cold front down across Iowa, to be followed by
a large and much colder high pressure area on Tuesday and
Wednesday. Given the agreement on this synoptic pattern, even at
six or seven days out, have deemed it prudent to lower
temperatures by several degrees during this period which brings
them closer to forecast temperatures aloft, but this still may not
be cold enough. It is quite likely that next Tuesday and Wednesday
will see the coldest temperatures of the season so far, especially
if we have snow cover in place across part of the area by then.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 521 AM CST Wed Dec 7 2016

Gusty winds expected through the period, with high clouds across
the southern half. Some low VFR to MVFR CIGS possible north late
in the period, and they should start to push into KDSM and KOTM
toward sunrise Thursday morning. Some flurries possible late
tonight at KMCW, however they are expected to be widely scattered
so have left out of the TAF for now.

&&

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

&&

$$

SHORT TERM...Beerends
LONG TERM...Lee
AVIATION...Beerends



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