Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 140442

Area Forecast Discussion
National Weather Service Sioux Falls SD
1042 PM CST Fri Jan 13 2017

.SHORT TERM...(This evening through Saturday)
Issued at 351 PM CST Fri Jan 13 2017

A surface high pressure will be building across the CWA giving way
to a quiet and more pleasant weekend.

Another chilly night is on tap but not as cold as last night. Zonal
upper level flow will be advecting warm air, bringing milder
temperatures into the weekend. Tonight`s low will range from 5 below
to 10 above. Lowered temperatures across snow covered areas,
especially from a line from Huron to Marshall. Decreasing clouds
from west to east will continue throughout the overnight hours;
followed by a partly to mostly clear skies on Saturday. Near normal
temperatures are expected with highs climbing into the low 20s north
of I-90 to near 30 south. Winds will remain light and variable, as
the surface high shifts east by Saturday night.

.LONG TERM...(Saturday night through Friday)
Issued at 351 PM CST Fri Jan 13 2017

Focus on the medium range in on the southern stream system that is
expected to move into the plains on the Sunday into Monday time
frame. Models are still disagreeing on the amount of warming aloft as
this system lifts north and east, making precip type difficult to
discern. As system is lifting northeast, a short wave working south
in the northern stream potentially interacts with southern stream
system. The 12Z GFS has the greatest amount of interaction between
the two systems, introducing cooler air a bit quicker from the
northwest and keeping a well defined back edge to the precipitation
shield.  WPC was not in favor of the 00Z Euro, nor the GFS given the
northern and southern stream interaction, but instead favored the
NAM solution.  For this forecast iteration, followed mainly the GFS,
with both a NAM and Canadian influence. The 12Z European has shifted
further east with the track of the southern stream system, cooling
the forecast 850 temps across the forecast area.

Icing potential in northwest Iowa, particularly down in the Sioux
City to Storm Lake area looks to be significant with 0.1-0.25" of
icing potential.  The icing potential decreases as one moves north
and west into the forecast area, but how quickly it decreases is
still in question depending on the precise location of the warm air
in the 850-800 mb layer. Precipitation amounts will also likely
decrease to the north and west, with little precipitation expected
in north central South Dakota.  With the icing potential in our
south, reasonable to continue the winter storm watch to the south.
Further to north, precipitation type is still a mystery, but with
potential icing and 2-4" of snow, went ahead and expanded the watch
slightly on the north and west side.

Snow ratios of 2-3:1 during the precipitation event were
significantly too low, especially where primarily snow is expected.
Have raised snow ratios into 7-12:1 based on location, but further
refinement will likely be needed once details of the warm air aloft
become more clear.

Behind the system, still looks to be a chinook pattern.  Have not
altered superblend temperatures in the extended, but if there is a
swath of snow across the area, it would likely impact the warming
potential in the middle and late portions of next week.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1040 PM CST Fri Jan 13 2017

VFR conditions and light winds will dominate across the area
through the TAF period. Some models hinting at development of
sub-VFR clouds/visibility east of I-29 after 15/00Z, however
confidence in this development is low and will not include any
sub-VFR conditions in our forecast area at this time.


SD...Winter Storm Watch from Sunday afternoon through Monday evening
     for SDZ062-066-067-069>071.

MN...Winter Storm Watch from Sunday afternoon through Monday evening
     for MNZ089-090-098.

IA...Winter Storm Watch from Sunday afternoon through Monday evening
     for IAZ001>003-012>014-020>022-031-032.

NE...Winter Storm Watch from Sunday afternoon through Monday evening
     for NEZ013-014.



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