Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201122

Area Forecast Discussion
National Weather Service Sioux Falls SD
522 AM CST Sat Jan 20 2018

.SHORT TERM...(Today and Tonight)
Issued at 351 AM CST Sat Jan 20 2018

Temperatures today will again run from 10 to 20 degrees above their
climatological normals, although not as warm as yesterday, with most
locations in the 40s. Meanwhile, the upper level flow buckles out of
the southwest today with signs of northward moisture return ahead of
our next system already noted on the GOES 16 nighttime microphysics
product in the way of an expanding low level stratus deck across

First issues from the incoming winter storm begin by as early as
this evening across our far western counties. A band of snow is
expected to develop along a mid level front for points mainly north
and west of a line from Chamberlain to Huron to perhaps Brookings,
with some model guidance perhaps pulling the best forcing even
further north/west of this area. Pretty good dry layer indicated in
the forecast soundings for this area and thus left chances on the
lower side until later overnight with increasing chances thereafter.
The other potential concern will be the possibility of some freezing
drizzle across our southeast counties in NW Iowa with saturation in
the low levels but drier air in the ice growth zone.

.LONG TERM...(Sunday through Friday)
Issued at 351 AM CST Sat Jan 20 2018

Will focus on the winter storm impacting this forecast area in the
Sunday through Monday night time frame. The outer periods of the
forecast from mid week on look rather benign at this time, with
temperatures near or above normal. Of course, the temperatures could
be affected by significant snowpack where it ends up occurring.

The deterministic models are in good agreement with both the low and
upper level synoptic features. Albeit the ECMWF is just a tad slower
with its upper QG forcing late Monday and Monday evening over the
eastern plains. The large scale features show the upper low just
east of the four corners area Sunday morning, lifting east to
northeast toward Des Moines IA by Monday afternoon, and finally
toward the lower Great Lakes Monday night. At the surface, low
pressure begins to form in southeast CO early Sunday with an
inverted trough extending northeastward into southern IA.
Cyclogenesis then consolidates the low, with movement into central
IA by Monday afternoon.

What is significantly different in the forecast from the 00Z Friday
model runs is the jet structure. Our forecast area is still placed
in a broad right entrance region on Sunday with the jet streak over
the Great Lakes region. However on Sunday night into Monday morning,
the jet streak ahead of the upper low now angles more northward into
the eastern plains placing our forecast area in a left exit region
along with strong curvature which will greatly aid in ascent.

The mid level frontogenesis is still maximized near the maximum QPF
area, from along an axis near Yankton/Tyndall SD, northeastward into
the northern sections of southwest MN. The NAM continues to paint
its heaviest QPF in our western zones, made worse by the 06Z run
when compared to the 00Z run. Our west is largely stable but does
have deep saturation. But otherwise prefer model consensus in
placing the heaviest QPF and snowfall along either side of an axis
from Yankton SD, to Sioux Falls, to Windom MN. The ECMWF places its
heavier QPF just south of that axis more along the low level
instability, and our southeast zones are pretty unstable Sunday
night above 850mb. Further north, instability is pretty strong from
700mb on up in southeast SD and southwest MN Sunday night. Above the
frontogenesis, less stable to unstable EPV* is noted in the 650-
600mb layer. At any rate in terms of QPF, the ECMWF was just enough
south that we were very hesitant to place the heaviest QPF and
snowfall any further northwest than it already is, preferring a
consensus. In fact, the QPF plumes on the various ensembles and SREF
are all over the place, with little in the way of clustering. The
deterministic models are actually in better agreement with the QPF
placement as they should be due to their agreement with the synoptic
features. Even the ECMWF is only about 30 or 40 miles south of the
GFS and CMC, while the NAM only about 30 or 40 miles west and north
of the GFS and CMC.

Noting that the storm system is just now entering California, and
the plumes show little clustering with QPF at this point, decided to
leave our area as a watch with some tweaking of the timing. In
addition except for our far western zones, decided to add blizzard
possible wording to the watch due to the winds. Blended consensus
mos and mav guidance to make the winds stronger. There is strong
model consensus that strong winds will move eastward Sunday night
and lasting into Monday, even Monday evening in our far east. But
our far western zones could miss the worst of the wind, or at the
least, not lasting as long. But generally along and east of the
James River, models are showing 35 to 40 knots at 925mb where the
gradient is the strongest which is very concerning. Therefore
expecting 25 to 40 mph winds in this event when coupled with the
potential for moderate to heavy snow at times, could make for
greatly reduced visibilities especially outside city limits.

The far southeast zones around Spencer and Storm Lake Iowa continue
to be rather of a challenge. Warm air aloft will likely create some
freezing drizzle/freezing rain in those areas for a good portion of
Sunday night. In addition, there is some dry mid level air remaining
in those areas all the way through 12Z Monday hindering ice
production. Therefore have a glaze in those locations and even into
the southeast part of southwest MN, where ice accumulations could be
around five hundredths of an inch. To our far northwest, do not have
Beadle and Kingsbury county in any headlines yet as the
aforementioned heaviest QPF/snowfall consensus is just to their
south. If these trends continue without changing, it is likely that
our southeast and northwest corners will need an advisory. Our
southeast for a wintry mix with strong wind, and our northwest for
advisory type snow and blowing snow.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 506 AM CST Sat Jan 20 2018

Some high level clouds have kept any fog at bay thus far but there
is still an outside chance at some patchy fog developing through
the early morning hours. Chances are too low to include any
restrictions in the TAF however. A band of snow will develop by
late afternoon/evening near KHON although confidence in timing and
placement, and thus potential impacts, is low at this time.
An incoming winter storm will lead to increasing probabilities of
MVFR and lower ceilings/visibilities at the very end of this TAF
period and beyond, with KFSD likely to be the last terminal


SD...Winter Storm Watch from Sunday afternoon through Monday
     afternoon for SDZ040-054>056-059>062-065>071.

     Winter Storm Watch from Sunday morning through Monday morning
     for SDZ050-052-053-057-058-063-064.

MN...Winter Storm Watch from Sunday evening through Monday evening
     for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Sunday afternoon through Monday
     afternoon for IAZ020-031.

     Winter Storm Watch from Sunday evening through Monday evening
     for IAZ001>003-012>014-021.

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



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