Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220452
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1052 PM CST Tue Nov 21 2017

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Wednesday/
Issued at 308 PM CST Tue Nov 21 2017

Strong cold advection passed through the area this morning and the
gusty northwest winds continue to usher cold advection into the
state this afternoon. The wind gusts have diminished this afternoon
and have fallen below advisory criteria. High pressure over the
Dakotas will drop south/southeast and will be centered over Kansas
and Oklahoma and ridged through Iowa by 12z Wednesday. The wind will
remain breezy through mid-evening then will drop below 6 kts most
locations after midnight and may become light and variable,
especially west of Interstate 35. Very dry dew points have been
arriving today values in the 10 to 20 range this afternoon. Dew
points should drop into the single digits overnight as the high
pressure ridge arrives. Prime radiational cooling conditions are
expected to arrive after midnight as the winds decrease under mostly
clear skies. Have lowered forecast low temperatures tonight into the
10 to 18 degree range. Suspect a few locations may fall into the
upper single digits considering the aforementioned conditions.

The area of high pressure will move swiftly southeast on Wednesday
as low pressure approaches from the northwest. This will result in
return flow and the return of warm advection by late morning and
increasing through the afternoon.  Mid-level theta-e advection will
increase by mid to late afternoon. Increasing clouds can be expected
through the afternoon.  Sounding profiles over northern Iowa reveal
good saturation occurring within the 850-500 mb layer with modest
vertical ascent present in this layer due to the forcing. Should
have hydrometeor development within this layer. Very dry air will
remain in the lowest 2500 ft of the atmosphere. Have added sprinkle
and flurry mention over far northern Iowa for the late afternoon as
some very light precipitation should reach the surface.  Should
stronger forcing arrive and create better hydrometeor loading of the
dry layer, then low pops may need to be added for better chances for
measurable precipitation. High temperatures Wednesday are expected
to be in the mid to upper 30s north to lower 40s south.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 308 PM CST Tue Nov 21 2017

Light precip chances and temperature swings continue to be the
forecast challenges for this forecast issuance.  Wednesday evening,
we remain in weaker warm advection as a shortwave drops through
eastern Iowa.  There continue to be subtle hints that light
precip...mainly sprinkles/flurries will be possible.  Moisture is
certainly lacking however there are pockets of it and despite a
shallow dry layer, there is decent forcing around and as we saw this
morning, it doesn`t take much to squeeze out precip.  Additionally,
this time of year warm advection almost always brings a better
chance of some sort of precip.  As a result I have added flurries in
the forecast from Mason City to Waterloo for Wednesday evening, until
the shortwave passes to the east.  It is quite possible that temps
will be warm enough to actually keep precip in the form of sprinkles
but models are slow to warm evening temps a few more degrees.  From
a strictly sounding perspective, surface temps remain above freezing
through mid evening however should the shortwave pass quicker or
clouds pass sooner, then it is possible to have flurries.

We will get a quick shot of colder air Wednesday/Thanksgiving Day
but by Thursday evening winds switch to the south again and warmer
air filters in. Highs in the mid 40s to mid 50s Thanksgiving with
steady or slowly rising temps Thursday night.

Friday will be interesting.  We will be in strong warm advection but
a potent shortwave will be dropping across the Northern Plains and
northern portions of the Upper Midwest.  Clouds will increase during
the day on Friday and this will have a big impact this time of year
however there will be pretty warm air aloft and strong mixing so at
this point it looks like Friday will still see temps across the
south in the mid 60s with mid 50s north.  That stronger wave passes
east through the afternoon/evening and may bring precip across the
north.  If that does happen, temps may be too warm north.

Once that system passes to the east, we will be in deep northwest
flow with another high moving in so temps fall back into the mid 30s
to mid 40s be the end of the Thanksgiving weekend.

The cool shot may be short lived as models indicate a ridge building
across the Rockies and High Plains that will shift east early next
week as the surface high shifts east bringing a return flow back to
Iowa with temps climbing back into the mid 40s to mid 50s on Monday.
The end of the long term forecast period and beyond continues to
show an active period as a deep trough moves out of the west into
the Plains.  Tuesday will likely see a warm start with cooler air
pushing in late in the day, though keep in  mind timing this far out
is impossible to pin down.  Will also likely see precip with this
trough that models right now are passing through mainly dry.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1052 PM CST Tue Nov 21 2017

Winds continue to decrease this evening and will continue to do so
as high pressure settles into the area toward sunrise as skies
remain mainly clear. As the high moves off to the east, winds
will become from the southwest Wednesday morning. Clouds will
pass over the terminals later tomorrow morning through Wednesday
evening, but will be VFR with bases well above any ceiling
restrictions.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Ansorge



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