Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 112049

Area Forecast Discussion
National Weather Service Des Moines IA
249 PM CST Sun Feb 11 2018

.SHORT TERM.../Tonight through Monday/
Issued at 249 PM CST Sun Feb 11 2018

Few overall concerns through tomorrow. Well advertised break from
active pattern taking place for a few days as H500 parent low over
northern Canada begins to lift northeast. Airmass farther south
being replaced with milder air as upper level flow becomes more
zonal after Monday. In the short term one more surface cold front
will approach northern Iowa this afternoon as it begins to
experience frontolysis with ridge of high pressure expanding south
tonight. The ridge will push one last area of colder air into the
state overnight providing the opportunity for another night of
sub zero cold over the area. Aloft the frontal boundary will
remain intact through early Monday. This will provide enough
convergence and lift for mid to high clouds to drop southeast into
northern Iowa overnight. It looks like the thicker cloud cover
will reside north of US20 and mainly high thin clouds over the
central areas. Normally overnight lows might better reflect cloud
cover but with the albedo quite high over the region today due to
snow cover and expected low level cold air advection
tonight...mins over the north will likely drop below zero...along
with many of the usual river valley areas. Winds across the region
tonight will swing from the northwest to the northeast at about 5
to 10 mph by morning. Though we are just shy of wind chill
advisory criteria over the north half of the forecast area, it is
likely that most locations will wake to wind chill values of
between 15 and 20 below with a north northeast wind around 7 to 10
mph. Certainly something to be aware of with morning commuters
and schools in session.

Otherwise Monday promises to be another mostly sunny day south with
some clouds across the north. Highs will be similar to
today...though a few degrees cooler due to the cold start...little
mixing...and continued neutral advection processes through midday.
By mid to late afternoon weak warm air advection will once again
return aloft as a new system approaches for Tuesday/Wednesday.  The
warming will be reflected in that time period despite the onset
Monday. Highs tomorrow should reach the lower teens northwest to the
lower 20s in the south.

.LONG TERM.../Monday night through Sunday/
Issued at 249 PM CST Sun Feb 11 2018

High pressure will slide from Minnesota into the Great Lakes Monday
night into Tuesday. As winds rotate clockwise to southeast by
Tuesday morning, warm air advection will begin and may have some
light snow or flurries skirt the northwest forecast area before
daybreak. There is still plenty of dry air between 950 and 700mb to
mention above slight chance PoPs at this point. Otherwise, 850mb
temperature will rise 6-8C from Monday to Tuesday with another rise
of 8-10C from Tuesday to Wednesday. Soundings show that there is a
steep low level inversion that will likely limit surface
temperatures from realizing the full warming of the 850mb
temperatures. In addition, cross sections across the state from the
NAM and GFS show saturation will increase below 925mb starting as
early as Tuesday afternoon with the NAM/GFS/ECMWF all agreeing in
relative humidity above 80% by early Wednesday. This could mean more
cloud cover than currently mentioned as well as fog as warm air
moves over a widespread snow pack of at least 6 inches deep across
central Iowa. Overall, trended temperatures down from initial
guidance, but unless things drastically change will likely need to
lower highs further, raise lows, increase cloud cover, and mention

The next cold front will arrive across Iowa late Wednesday into
Thursday from the north with the main forcing moving over the Great
Lakes. A secondary, weak surface low forms along the front and may
provide enough lift for some light rain or snow showers over the
area. However, models have been fairly inconsistent with their
timing and placement and this still persists on the 12z runs. For
example, the GFS soundings show mid-level dry air at 18z Thursday
while the ECMWF is much moister in the same layer. As the front
clears the state later on Thursday, will see winds increase from the
northwest through early Friday. These winds will draw in colder air
bringing temperatures back below normal again by Friday. This shot
of cold air may be short-lived. The GFS/ECMWF show surface high
pressure moving away from the region allowing for warmer air to
return while the CMC maintains the cold with precipitation chances.
For now, have leaned toward the GFS/ECMWF lowering precipitation
chances on Saturday.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1134 AM CST Sun Feb 11 2018

Few impacts next 24 hours. Expecting VFR conditions to remain.
Increase in mid/high level clouds northwest/north aft 00z with
cigs below OVC110 northwest of KFOD by 11-12z. Otherwise SCT to BKN250
remainder of north half. Will see winds become more northwest aft
00z with north/northeast winds aft 12z as high slides southeast.
Winds remain generally below 10kts aft 00z. /rev




LONG TERM...Ansorge
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