Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 152344

Area Forecast Discussion
National Weather Service Sioux Falls SD
644 PM CDT Thu Mar 15 2018

.SHORT TERM...(This evening through Friday)
Issued at 336 PM CDT Thu Mar 15 2018

A dangerous spring storm system will impact the area early Friday
through Friday night. An upper level low will slide across the base
of a ridge centered over the Great Plains. The center of the surface
low pressure looks to be well south along the Nebraska-Kansas
border. This will bring stout east winds 15-25 mph to most.
Meanwhile, mid-level warm air advection from the southeast ahead of
the surface low will provide moisture and instability aloft, fueling
showers with some moderate rainfall rates Friday. It`s this
superposition of cold surface air and warm air aloft that will bring
a threat for freezing rain.

Accumulations of ice could be significant where conditions perfectly
align. There is still low confidence in exactly where this band
will set-up, as a mere one or two degrees will make a drastic
difference in precipitation type. Currently models highlight the
highest icing (in the quarter inch range) along the Missouri River
valley, from south central South Dakota along the river border
with Nebraska (roughly a Gregory to Vermillion, SD line). Further
south and east, including the vicinity of Sioux City, temperatures
Friday afternoon could sneak above freezing, reducing the amount
of ice accumulation. Once again, a degree or two of temperature
difference could shift the region of greatest impact, and errors
of 1 or 2 degrees in a temperature forecast is common even a few
hours ahead of time. Stay tuned to the latest warnings and social
media posts for where the greatest impacts are expected.

Impact-wise, power lines and tree branches could break where
icing reaches near a quarter inch, which is most likely in that
Missouri River corridor described earlier. Areas north of I-90 and
Sioux Falls northeastward will still have potential for light
icing, but the higher rainfall rates look to stay southwest of
these areas. The timing also looks to be delayed with dry air
keeping Sioux Falls dry into the afternoon Friday before soundings
saturate. Nevertheless, even light icing can cause travel
headaches with patchy slick spots on area roads.

Sleet and snow will also be impacts with this system, with several
inches of snow and some sleet mixing in with freezing rain, but
the freezing rain looks to be the biggest impact at this time, and
is the primary reason for current Advisories and Warnings. Snow
amounts in the 2 to 4 inch range are expected along a Huron, SD
to Sioux Falls to Spencer, IA line.

.LONG TERM...(Friday night through Thursday)
Issued at 336 PM CDT Thu Mar 15 2018

Precipitation quickly decreases after midnight early Saturday
morning, and by sunrise, a building surface high and ridge aloft
will clear skies. Temperatures will remain below average, but above
freezing, which should help in melting any ice accumulation from

Sunday will be seasonal temperature-wise, bringing mostly low
40s, but then another winter weather system moves south of the
region Sunday night into Monday. This system looks further south
than previous runs, with lighter precipitation amounts for the
tri-state area. Given the incoming system Friday, the details of
this next system have not been overly analyzed, but light rain and
snow again look possible for many areas as an elevated front
forces light accent over the Northern Plains.

A couple shortwave troughs could move southeast along the active jet
stream Tuesday into Wednesday, keeping active weather around through
Mid-week. Next Thursday looks to bring quieter weather as a ridge of
high pressure develops.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 626 PM CDT Thu Mar 15 2018

Breezy easterly winds are expected through the TAF period.
Conditions are expected to deteriorate through the morning hours
on Friday. A wintry mix of rain, freezing rain, sleet, and snow
will spread northward into the Missouri Valley around daybreak
Friday, then spread across the region through midday Friday. North
of the I-90 corridor precipitation will be primarily snow, however
along and south of I-90 the wintry mix - including freezing rain
and sleet - will continue through the end of the TAF period,
impacting KFSD and KSUX TAFs. Portions of northeast Nebraska and
west central Iowa, including KSUX may see a bit more liquid rain
with slightly warmer temperatures, while south central SD may see
the biggest impacts of icing and snowfall. Regardless, MVFR and
IFR ceilings and visibility will possible for all TAF sites.


SD...Winter Weather Advisory from 10 AM Friday to 1 AM CDT Saturday
     for SDZ052-053-059-060-066-070-071.

     Winter Storm Warning from 5 AM Friday to 1 AM CDT Saturday for

     Winter Weather Advisory from 1 PM Friday to 1 AM CDT Saturday
     for SDZ038-039-054>056-061-062-067.

MN...Winter Weather Advisory from 1 PM Friday to 1 AM CDT Saturday
     for MNZ089-090-098.

IA...Winter Weather Advisory from 10 AM Friday to 1 AM CDT Saturday
     for IAZ020-031-032.

     Winter Weather Advisory from 1 PM Friday to 1 AM CDT Saturday
     for IAZ001>003-012>014-021-022.

NE...Winter Weather Advisory from 10 AM Friday to 1 AM CDT Saturday
     for NEZ013-014.



SHORT TERM...VandenBoogart
LONG TERM...VandenBoogart
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