Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 222353

Area Forecast Discussion
National Weather Service Sioux Falls SD
553 PM CST Wed Feb 22 2017

.SHORT TERM...(This evening through Thursday)
Issued at 439 PM CST Wed Feb 22 2017

The tail end of a fairly strong short wave continues to move across
areas to the north of the CWA. The wave is fairly strong but it is
not sagging southward enough to impact our northern zones. Therefore
believe we will see an increase in low cloudiness from north to
south tonight behind the advancing cold front which this wave is
forcing through. Late tonight, deep saturation begins in our south
central SD zones ahead of a large upper trough digging into the
western United States. Isentropic lift is abundant in the 290-300K
layer, so therefore to the north of the aforementioned front, will
likely see some light snow develop in those zones before moving
eastward. The upper trough will then move eastward through the day
on Thursday into western Neb, southeast WY and northeast CO by the
end of the day. Surface low pressure develops in response in western
and central KS with a warm conveyor belt of moisture aloft beginning
to develop to the north of strong frontal forcing in the central
plains. Deep saturation really begins to take hold during the
afternoon hours for much of our forecast area minus our far
northeast zones from Brookings, to Marshall and Windom. In addition,
our aforementioned isentropic lift remains strong. Most notably,
strong 750-700mb frontogenesis begins to take shape close to I 90
which will aid in spreading precipitation eastward through the day.
Because of a northeasterly fetch of low level air and obvious ample
cloud cover, daytime highs on Thursday will likely not move more
then a few or several degrees off of morning lows. Given the surface
temperatures, should see primarily rainfall in northwest IA and
immediate adjacent locations, transitioning to all snow from east
central SD to south central SD, and also through a good portion of
southwest MN.

By the end of the day, 3 to 5 inches of snow is quite possible in
far southwest zones in south central SD as surface temperatures in
that area just hover around 30 degrees. They also exhibit the
coldest temperatures aloft with even a bit of dendricity to work
with. Daytime snow amounts will taper off to the east and north of
that area to eventually what is no accumulation at all in northwest
IA and immediate adjacent locations. For headlines, updated the
winter storm watch to a blizzard watch for the afternoon hours for
south central SD. We can fine tune this area in the future, but
eventually the blizzard watch area will see 45 to 50 knot winds in
the mixed layer.  This accounts for a potential model trend of
sliding southward per the GEM Global and ECMWF from their previous
runs, but noting that the GFS has slide a bit northward by late
Thursday more similar to the older runs of the GEM Global. So there
is still uncertainty in the low track which means everything where
the mesoscale dynamics and instability will eventually setup. So
therefore even for Thursday afternoon, did not want to upgrade to a
winter storm warning in south central SD in case we need a blizzard
warning in that area, but not enough confidence to go all out
blizzard warning yet due to track uncertainty.

.LONG TERM...(Thursday night through Wednesday)
Issued at 439 PM CST Wed Feb 22 2017

Models still expressing a spread in solutions with regard to upper
and surface low tracks for the Thursday night through Friday time
period. Still rejected the NAM with its far northern solution, but
the 12Z GFS, ECMWF, and GEM all shifted slightly farther south with
QPF fields/dynamics for that time period. In light of that, shifted
the heavier snow band slightly southward also. A couple model
changes of note; all models indicating instability in the form of
weak CAPE lifting northward out of eastern Nebraska on Thursday
evening, so introduced isolated thunder over portions of our lower
IA zones before 03Z. In addition, both the GFS and GEM develop strong
midlevel frontogenesis over northwestern IA in the evening, with
increased QPF amounts over that area. Temperatures aloft will be
cool enough by that point that this precipitation should be in the
form of snow, though surface temps may sit in the mid 30s, putting a
damper on snow accumulations over that area. At this point, it does
not appear that any convection over eastern Nebraska in the evening
would be enough to disrupt snow accumulations to the north, but it
will be something to keep an eye on.

By Friday morning the surface low center will be somewhere in the
vicinity of south central IA, and in a tightening gradient and mixed
layer winds increasing to 40 to 50 kts, surface winds speeds will be
ramping up later on Thursday night into Friday. With this comes an
increasing confidence of blizzard conditions developing through that
time frame, and as mentioned in the short term discussion, elected
to transition the Winter Storm Watch to a Blizzard Watch for most of
the area. The exceptions to this, which will most likely be more
refined in future forecasts, is through the western Highway 14
corridor counties where with southerly shifting model trends
snowfall may be less and winds may not be quite as strong, and
through Woodbury and Ida counties where warmer temperatures may
preclude as much blowing. These areas remain in a Winter Storm
Watch. Temperatures will move little on Friday, sitting in the mid
and upper 20s.

Light snow could be lingering Friday evening in areas east of
Interstate 29, otherwise clouds will decrease later in the night
with dying winds as low pressure moves out of the region. At this
time, snowfall totals by Friday evening will range from 6 to 12
inches, with locally higher amounts possible, and maximized amounts
running from south central SD into southeastern SD, through
southwestern MN and the IA Great Lakes area. More uncertainty exists
through the northern James River Valley with the aforementioned
southerly shifting model trends. With snow cover and decreasing
clouds/dying winds it will be colder on Friday night as lows drop
into the mid teens.

Little changes made to the remainder of the extended period with the
nearer term storm. The main theme looks to be temperatures at or
just above normal, with the next best chance of precipitation coming
in the Tuesday of next week time frame.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 550 PM CST Wed Feb 22 2017

MVFR and IFR ceilings near highway 14 will continue to advect
southward through the evening. Snow will develop late tonight in
south central and spread east through the region through midday
Thursday, but will likely begin as rain or a rain and snow mix in
far southeast SD, northwest Iowa, and portions of southwest MN.


SD...Blizzard Watch from Thursday evening through Friday afternoon
     for SDZ053>056-059>062-065>071.

     Blizzard Watch from Thursday afternoon through Friday afternoon
     for SDZ050-052-057-058-063-064.

     Winter Storm Watch from Thursday evening through Friday
     afternoon for SDZ038>040.

MN...Blizzard Watch from Thursday evening through Friday evening for

IA...Blizzard Watch from Thursday evening through Friday evening for

     Winter Storm Watch from Thursday evening through Friday evening
     for IAZ031-032.

NE...Blizzard Watch from Thursday evening through Friday afternoon
     for NEZ013.

     Blizzard Watch from Thursday evening through Friday evening for



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