Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 282124

Area Forecast Discussion
National Weather Service Des Moines IA
324 PM CST Tue Feb 28 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 323 PM CST Tue Feb 28 2017

Confidence:  Medium to High.

Several impacts to consider over the next 24 hours. Though front
continues to mover into southeast Iowa now...still a brief
opportunity for thunderstorms this afternoon/early evening over the
east/southeast in area of best instability. The window will be short
lived with only a 2-3 hour at most opportunity for any stronger
storms. With unidirectional shear...wind or hail most likely though
0-3km SRH and sfc to 2km sfc relative flow would promote some
turning in the lowest layers.  With southeast areas currently
relegated to marginal/ dynamics now expected to
translate southeast and out of the area. By mid to late evening the
instability will wane with rain developing with the approach of the
main H500 shortwave. Eventually colder air will drive the
precipitation to mix and change to snow overnight with the best
precipitation production over the north from northern Iowa to
southern MN/WI. Models continue to differ on axis of maximum qpf and
overall amounts...but conservatively 1 to 2 inches of snow still
looks likely over the far northeast...north of highway 20. Recent
warm ground temperatures and highs in the 40s and 50s over some
of the region will initially help to melt some of the snow. Once
the system passes southeast on Wednesday northwest winds will
increase with sustained 20 to 25 gusting to 30 to 35 mph during
the daytime hours. Colder air will return with highs back in the
30s to 40s over the region. The light rain/snow will be ending in
the afternoon hours.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 323 PM CST Tue Feb 28 2017

Main concern initially in the longer term will be system passing
through the state on Thursday. Upper system is forecast to race
southeast through the state during the day with attendant surface
low exiting the area by early afternoon. Will see a tight pressure
gradient on the backside with deep mixing suggested by soundings
on all models. Have accordingly raised winds on Thursday although
it may not be enough given the strength of the winds aloft. This
will likely be a wind advisory type of event with strong gusts by
afternoon up to 50 mph. Will also see the potential for convective
snow showers in the northern third or so of the forecast area with
low level saturation and some weak instability. While overall snow
amounts may not be much, the combination of stronger winds and
snow showers may produce brief periods of poor visibilities during
the afternoon.

Surface ridging will slide through the state on Thursday night
with warm advection increasing on Friday. This will continue into
the weekend as broad upper ridging moves into the central United
States. Temperatures will climb daily with readings into the 60s
for most locations by Sunday once again. Models diverge somewhat
after this point with GFS keeping bulk of system early next week
north of the state and the Euro a bit farther south. In either
case, the threat of precipitation is limited as most lift and deep
saturation remain north and east of Iowa. Temperatures will
continue their run of above normal readings into next week as


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1153 AM CST Tue Feb 28 2017

Plenty of considerations this period. Southeast expected to see
convection this afternoon between 20-01z mainly for KOTM. With low
moving out tonight...northwest flow increases overnight with rain
increasing and CIGS lowering to LIFR/IFR most areas through 09z.
Light rain likely to change over to -SN with some lower visby aft
09z northeast at KALO and KMCW. Confidence remains limited on how
low to drop visby northeast...though period of light snow and
wind should result in MVFR possible IFR vsby for a period early
Tuesday 12-18z. Northwest winds remain 16G26kt or higher aft 12z
as well. /rev





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