Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 170932

Area Forecast Discussion
National Weather Service Sioux Falls SD
332 AM CST Wed Jan 17 2018

.SHORT TERM...(Today and Tonight)
Issued at 328 AM CST Wed Jan 17 2018

A ridge of high pressure will allow clear skies to continue
throughout the day. The combination of warm air advection under
this ridge and sunshine will lead to high temperatures near
seasonal averages, with upper 20 and low 30s for most. Those in
south central south Dakota will be a few degrees warmer yet, with
highs in the upper 30s.

The only other notable feature in the near term is the continued
light southwest surface winds, which will be enhanced by downsloping
off the Buffalo Ridge. Winds have already been gusting in the 20 to
30 mph range for communities in the lee of the ridge, such as Canby,
Marshall, and Worthington. These winds continue through the evening,
and weaken as wind direction turns more westerly overnight.

.LONG TERM...(Thursday through Tuesday)
Issued at 328 AM CST Wed Jan 17 2018

Thursday morning, a short wave moves southeastward through the
western Great Lakes area. This wave does not have any affect on the
FSD forecast area and does not even provide a wind shift. The
surface flow of air is from the southwest but appears fairly light
at this time, thus providing not the highest degree of mixing
potential. Therefore given the 925mb temperatures, superblended
highs did not look too bad giving well above normal readings for
this time of year. On Friday, the upper flow becomes more zonal. A
short wave passage moving through western Ontario ushers in a wind
shift. But ahead of the wind shift across northwest IA and extreme
northeast NE, surface winds remain out of the southwest with a
ribbon of warming noted at 925mb placed squarely in this southwest
flow. Therefore highs will be warmer on Friday when compared to
Thursday, but especially over northwest IA where adding about three
degrees is warranted by blending in some ECMWF and weighted model.

Saturday will offer one more above normal day. A strong upper trough
of low pressure moves into Utah and Arizona by 00Z Sunday, with the
ECMWF a bit slower then the Canadian and GFS. Surface cyclogenesis
develops near the CO/KS border with an inverted trough extending
northeastward toward the southern portion of northwest IA. It
appears stratus begins to move northward first into northwest IA by
late in the day, then across the forecast area Saturday night.
Between the stratus possible in northwest IA, coupled with a
northerly flow dominating our other zones, likely means a cooler day
when compared with Friday and enough uncertainty with the stratus to
just keep the superblend highs going.

There is a lot of discrepancy with the details in how the various
deterministic models eject the upper and surface lows into the
plains on Sunday and Sunday night. The GFS and Canadian are both
quicker at 500mb then the ECMWF. Furthermore, the Canadian brings
the upper and surface lows on a decided northeasterly track with the
northern jet stream well north in southern Canada. The GFS and ECMWF
are much closer with their placement of the northern stream jet,
which is across ND and northern MN. If it were not for the timing
differences, the ECMWF and GFS would be very close. The GEM Global
would initially bring in warmer and dry mid levels into our eastern
zones on Sunday, with a strong winter storm then taking shape Sunday
night and Monday morning with plenty of wind and jet stream dynamics
in play. With a more southerly track, the ECMWF and GFS saturate the
mid levels sooner, or at least by late in the day on Sunday, and
produce and overall colder solution with most snowfall once again
Sunday night and Monday morning with a bit less wind, but still
windy. At this time, using ensemble means are prudent with this much
discrepancy in place. Highs on Sunday at this time appear to be in
the lower to mid 30s, providing perhaps a light rain or snow mix in
northwest IA and northeast NE. Precipitation Sunday night and Monday
would be all snow.

The errors in this forecast is once again watching the northern
branch of the jet stream. The split flow so far has been strong this
winter and cannot be ignored. If the Canadian model pans out closer,
the northern stream would be a non player. But the northern stream
on the GFS and ECMWF certainly needs watched for anymore southern
movement which would cut off the deep moisture supply. Therefore
ascertaining snowfall amounts this far out is very difficult with
the low level cyclogenesis placement very uncertain.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1016 PM CST Tue Jan 16 2018

VFR conditions are expected through the TAF period.




SHORT TERM...VandenBoogart
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