Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 081146
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
546 AM CST Fri Dec 8 2017

...Updated Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 359 AM CST Fri Dec 8 2017

Weak shortwave currently moving through the state early this
morning is mainly bringing some mid-level cloud cover. A few
flurries maybe found over western to northern portions of the
forecast area, but with the cloud bases AOA 7000 feet, any flakes
that are produced are likely to not reach the surface. Only have
flurries mentioned through 12z this morning. The next shortwave
looks to push across Wisconsin this evening and quickly push east
of the area past 06z Saturday. Some light snow is possible b/t
00-06z tonight across the northeast sections of the forecast area.
MCW forecast soundings suggests ice introduction by around 02z
but loses it by 06-07z time. The depth of the moisture is only to
about 5000 feet but there is saturation with the dendritic layer.
Winds atop the mixed layer increase to 35-40 knots and certainly
cannot rule out some visibility issues with the falling snow and
blustery winds developing. However, with the snow likely to be
very short-lived, no headlines are anticipated attm and could just
be handled with a Special Weather Statement. Windy conditions
remain throughout the overnight hours with some wind gusts over 40
are likely across northern Iowa prior to sunrise Saturday.

.LONG TERM.../Saturday through Thursday/
Issued at 359 AM CST Fri Dec 8 2017

The gusty winds will persist into Saturday though will be gradually
subside through the day as the pressure gradient relaxes and the
mixed layer winds diminish. Temperatures will be cold once again as
cold advection continues on Saturday with high temperatures in the
low 20s to low 30s. Ridge of high pressure will reach far western
Iowa by late afternoon and will move quickly east across the state
through the early evening. This will bring a switch to southwest
winds and the beginning of warm advection. Saturday night lows
should occur during the evening followed by steady to slowly rising
temperatures after midnight. A stronger push of warm advection will
arrive Sunday morning fueled by westerly winds and downslope flow.
The very warm air arriving around 850 mb will create an inversion
and will limit mixing potential to near 900 mb. Expect to have
passing mid-level clouds through the day. How persistent and opaque
these clouds are and how much sun is filtered will have an impact on
high temperatures. Downslope flow can offset some of the cloud
impacts. Considering all the above, have remained close to guidance
for now with highs in the upper 30s northeast to near 50 southwest.

The active northwest flow for next week still leading to a lot of
spatial and temporal variability for potential outcomes. Trending
to higher 500 mb height across Iowa next week which would shift
the more active weather further to the northeast and impact
Minnesota and Wisconsin and leaving Iowa in the drier side which
includes more subsidence and breezy to windy conditions for much
of the period. A strong cold front will arrive on Monday. The
boundary will be accompanied with stratus but very dry air above
700 mb. The moisture will be in a thermal layer warm than -10C
mitigating any flurry threat and profiles would not be conducive
for any other precipitation. The primary threat with the boundary
passage will be strong and gusty northwest winds. The wind
potential looks stronger than the potential for tonight and will
continue to monitor for headline potential. Hints that the large
amplified ridge over the western CONUS will break down late next
week as a strong system moving of the Pacific squashes it and
eventually leads to a warmer westerly flow for the Midwest beyond
the extended forecast period and towards next weekend.

&&

.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 538 AM CST Fri Dec 8 2017

VFR conditions expected throughout the day today before the next
shortwave tonight looks to provide some MVFR stratus across
northern Iowa. Left out mention at DSM and OTM, as low confidence
in the duration, but certainly the potential is there for some
brief stints of MVFR ceilings tonight.

&&

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

&&

$$

SHORT TERM...Podrazik
LONG TERM...Donavon
AVIATION...Podrazik



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