Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 092123
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
323 PM CST Mon Jan 9 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 323 PM CST Mon Jan 9 2017

Warm advection underway across central Iowa this afternoon with
initial surge of lift and precipitation now north and east of the
forecast area. The warm advection will persist into tonight as
surface low approaches from the west and will see limited moisture
return with a couple of different issues will developing. Lift
will reinvigorate over northern Iowa with approach of the system
with both good isentropic lift and QG forcing. Deep saturation
continues to be the main concern with deeper saturation remaining
in MN but clipping northern IA at times. It appears that at least
light precipitation will occur at times overnight with a mix of
freezing drizzle or some light snow/ice pellets should saturation
become deeper. Recent short term models have also indicated some
heavier precipitation late tonight in the north and will continue
to monitor closely for potential impacts/headlines should this
scenario come to fruition. Farther south, temperatures will likely
cool briefly this evening, but begin to climb overnight with the
warm advection and increase in moisture. The lower levels will
likely saturate later tonight with light fog and some drizzle
becoming possible across the southeast half of the forecast area
overnight.

Surface low will progress across the state on Tuesday morning with
boundary sweeping across the state.  Models continue to indicate a
strip of heavier rain across eastern portions of the forecast area
but solutions continue to jump spatially.  Have continued to chances
pops during the morning but will drying by midday after passage of
the boundary.  Will see good cold advection by afternoon although
the best surge is across northern Iowa as approach of next system
will shunt to bulk of the colder air to the northeast.

.LONG TERM.../Tuesday night through Monday/
Issued at 323 PM CST Mon Jan 9 2017

The long term forecast period can be roughly split into four
segments. During the first, from tomorrow night into Wednesday, a
brief period of weak ridging will be quickly succeeded by a subtle
and fast-moving mid-level shortwave moving over the region. In
advance of the wave a surface low will develop around northeastern
Colorado on Tuesday night, with an inverted trough extending up
across Nebraska and northern Iowa. This will keep overnight temps
steady or slowly rising late as southerly flow picks up south of
the boundary. On Wednesday as the shortwave passes this boundary
will push south across the area as an effective cold front, with a
large area of high pressure across the Dakotas spilling down over
the rest of the Midwest. The second segment of the forecast
pertains to the passage of the front and initial building in of
the colder airmass behind it.

On Wednesday afternoon and evening as the surface front pushes
through and the mid-level shortwave moves overhead, it will
provide an opportunity for light precipitation to develop.
However, forecast soundings indicate that saturation will be
shallow with only modest lift in the cloud layer, so any amounts
will be very light and measurable precipitation chances are fairly
low. In addition, across about the southern two thirds of the area
the saturated layer exists at temperatures warmer than -10C, so
any precipitation that does fall will likely come in the form of
drizzle or freezing drizzle, depending on surface temperatures.
Given the lack of QPF the only impact of this would be if any
light glazing manages to accrue on roads and other surfaces. The
window of opportunity for this to occur appears fairly narrow at
this time given the dry air in place, lack of organized low-level
lift, fairly warm surface temperatures, etc. However, we will
continue to monitor the potential over the next couple of days.
On Wednesday night, in the colder post-frontal airmass behind the
front, it appears that there will be an opportunity for flurries
or light snow across northern Iowa but with little to no
accumulation or impact.

On Thursday and Friday the cold high pressure area will dominate
most of the region, especially northern Iowa, as it slides by
across Minnesota and Wisconsin. This will result in hazardous wind
chills, especially on Friday morning when the coldest temperatures
will be observed but winds will hold up through the morning hours
as the center of the high remains to our north. Around sunrise
Friday morning wind chill values are currently forecast to range
from 20 to 30 degrees below zero roughly north of Highway 20, and
it is likely that a headline will be warranted at some point.

The fourth and most complex segment of the long term forecast is
next weekend, with multiple rounds of winter/mixed precipitation
possible. On Saturday morning a cutoff low will reside near the
California/Arizona/Mexico border, with a northern stream impulse
crossing the Rockies and another right on its heels coming ashore
over the Pacific Northwest coast and British Columbia. There will
be a complicated interaction between the two northern stream waves
and the southwestern U.S. low and long-range prognostic models
have been having trouble resolving this over the last several
days. The EC continues to scoop most of the southwestern vorticity
out with the leading northern wave, while the GFS persists that it
will come out more in broken pieces between the two waves. In any
event the response will likely be prolonged precipitation to our
south across Kansas, Missouri and Illinois, probably surging up
into our area on one or two occasions over the weekend. With a
fairly cold airmass still in place the most likely precipitation
type will be snow, but at this time timing and any accumulations
are beyond any reasonable confidence level of prediction. In
addition, depending on the track of any reflected surface low
centers and the degree of low-level warming mainly over our south
and southeast, there may be opportunities for rain or icing at
times, mainly south of Interstate 80. At this range the high
variability in the forecast and difficulty in forecasting any
narrow corridor of icing precludes a mention in the outgoing
forecast, which contains only snow for now. We will be watching
model trends in the coming days and solutions will likely coalesce
as the southwestern U.S. low develops and the initial northern
shortwave moves ashore later in the week.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 545 AM CST Mon Jan 9 2017

Stratus will push down across mainly Northern TAF sites producing
MVFR cigs. We could also see some light snow across the Norther
but confidence isn`t high enough to include in TAF`s at this time.
Lower stratus will return tonight with better precip chances aft
09Z for a wintery mix North and rain South. Icing will be possible
at KFOD, KMCW and KALO aft 06Z.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Lee
AVIATION...FAB



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