Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 061729
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1129 AM CST Wed Dec 6 2017

.SHORT TERM.../Today through Tonight/
Issued at 334 AM CST Wed Dec 6 2017

For today a short wave will drop through the eastern Dakotas and
Nebraska across Iowa.  There is some very weak theta-e advection
across the west central into southwest part of the forecast area.
Radar showing some returns in eastern Nebraska and South Dakota
moving south southeast.  The far west and southwest should see some
sprinkles or flurries around 12Z through about 14Z when the first
wave wanes.  For the remainder of the day we will see a brisk
northwest wind and cold advection so temps today will be similar to
a little cooler than yesterday with mid 20s to mid 30s for highs.

Tonight another decent shortwave drops across Iowa again with some
warm advection and weak forcing ahead of the wave.  Earlier model
runs of the hi-res models were putting out some light precip but new
runs seem to have taken that out.  While low moisture isn`t great
there is decent mid level moisture and the drier low levels are
pretty shallow so there is at least a slight chance for flurries or
very light snow.  For now I have PoPs across the far north where the
better forcing is but this can easily translate further south as the
wave progresses.  In any event again we are only looking at very
light snow with no impacts to travel.

.LONG TERM.../Thursday through Tuesday/
Issued at 334 AM CST Wed Dec 6 2017

Little change in the overall forecast with highly amplified
northerly flow dominating through much of the period. Thursday will
be cold and breezy in the wake of a cold front passing through the
area late tonight. High temperatures will be in the 20s Thursday and
expect early morning clouds to be clearing through the day. Another
short wave will approach the state by Friday morning. A region of
theta-e advection and isentropic lift will precede the short wave.
Cross sections and forecast soundings suggest the potential for very
light snow as this energy passes through. Another wave of cold
advection will arrive Friday night and this will be a dry boundary
passage with very dry mid levels remaining through the overnight.
Very gusty northwest winds are likely Friday night with both the NAM
and GFS suggesting mixed layer winds in excess of 40 kts. Good
momentum transfer of these winds to the surface will be in place
with the cold advection and strong subsidence behind the passing
short wave.  At this time, it appears that the winds may be strong
enough to warrant headlines. Will add mention to the HWO.

The positive trends to higher 500 mb heights through the weekend
continues as the western ridge broadens then recycles. This will
lead to warmer/near to slight above normal temperatures for Sunday
with highs reaching the low to mid 40s Sunday. Still a high amount
of noise in the spatial and temporal resolution of several short
waves dropping through the northwest flow early to mid next week. A
strong short wave will arrive late Monday into Monday night followed
by another system late Tuesday into Wednesday. Sometime during this
period expect light snow or snow showers with minor snow
accumulations possible.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1124 AM CST Wed Dec 6 2017

Primary concern with the 18z TAF package are mvfr cigs and
potential for light wintry precip. Ongoing light rain/snow in
central Iowa has continued to diminish in spatial extent as we
head toward the noon hour. Slight chance that KOTM will see some
of this activity this afternoon, but any mention was left out of
the TAF as it is unlikely that the precip would have any impact on
the terminal should it occur. MVFR cigs in central Iowa should
improve by mid-afternoon before another round of stratus builds
into north central Iowa this evening. MVFR cigs are likely in
this area through the evening but then improve overnight. Further
south the potential for MVFR is a bit more unclear. Most model
guidance favors cigs near or just above 3000 feet late tonight
into early Thursday morning. Forecast continues to hover just
above MVFR at this time. Otherwise, steady northwest winds will
persist through the forecast period and gust above 20 to 25 kts at
times.

&&

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

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Martin



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