Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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850
FXUS63 KFSD 212152
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
352 PM CST Wed Feb 21 2018

.SHORT TERM...(This evening through Thursday night)
Issued at 352 PM CST Wed Feb 21 2018

Main forecast concern for the short-term is the snow and freezing
drizzle expected Thursday.

Skies will start off clear to partly cloudy tonight and with
light winds, temperatures will quickly fall after sunset with lows
being reached by midnight. As southeast winds slowly increase
above 5 mph and clouds increase, temperatures will then become
steady or even slowly rise through the remainder of the night.

A strong 300 mb upper level jet will extend from west central
Minnesota into southern Canada putting the region in the right
entrance region of the jet. At the same time, strong 850-700 mb
warm air advection will quickly move north along with an 850 mb
front. The combination of low level temperature advection and
upper level divergence with the jet should result in increased
ascent and rapid saturation through the column. Expect snow will
reach the Missouri Valley and SUX area around 12Z - I-90 around
mid-morning and Hwy 14 around noon.

The jet will lift off to the northeast during the late morning.
This will leave a period with weaker ascent - especially south of
a YKN to MJQ line. Am thinking this will allow for a period of
freezing drizzle as the ice bearing layer dries out. This period
could range from an hour or two near the Iowa and Minnesota border
to 4-6 hours around SUX. If drizzle can persist the entire
period, could see up to a tenth of inch of ice near Hwy 20 east of
SUX. There remains uncertainty how far north the freezing drizzle
will extend. In the worst case scenario, it could be as far north
as Sioux Falls, Slayton and even Tracy Minnesota. For this
forecast, kept the freezing drizzle primarily confined to far
southeast South Dakota, northwest Iowa and to Jackson and Windom
in Minnesota.

A second upper level wave will approach from western Nebraska
during the late afternoon. This will enhance warm advection and
snowfall starting in south central SD and spreading along I-90
into southeast and east central SD and into southwest Minnesota.
This area of precipitation will gradually expand south through the
evening into the remainder of southeastern South Dakota and
northwestern Iowa. This will change any precipitation to snow.
The wave will move to the northeast during the evening with snow
ending west of Highway 75 by midnight and by 4 or 5 am in the
remainder of the southwestern Minnesota and northwestern Iowa.
There could be a brief period of freezing drizzle after midnight
east of I-29 but amounts would be minimal and would only last an
hour at most.

With a longer period of saturation expected, have raised QPF of
0.2 to 0.3" is expected for most areas. Snow ratios are more
uncertain but settled around 15:1. This results in snowfall from 3
to 4 inches north of a Yankton to Jackson line. If temperatures
in the lower layers are warmer, ratios could be closer to 10:1
which would produce snowfall closer 2 to 3 inches.

.LONG TERM...(Friday through Wednesday) Issued at 241 PM CST Wed
Feb 21 2018

For the long term, the primary concern is the potential for heavy
snow on Saturday.

Friday will be between systems. However, with relatively weak
flow, it will be difficult to remove the clouds from the area. So
skies will be mostly cloudy with temperatures again in the 20s. On
Friday night a much stronger wave will move northeastward from the
Colorado Rockies. As it does a 700 mb front will rapidly move from
northern Nebraska into southeastern South Dakota. With strong
dynamics and weakly stable air above the front, a band of snow is
expected to rapidly develop late Friday night and into Saturday
morning. As the wave moves northeast, this band will gradually
shift southeastward through the day before it lifts east and
northeast of the area Saturday evening. This band could produce 4
or more inches of snow over portions of the area. Given this storm
is still 3 days in the future, there remains significant
uncertainty as to where the frontal band will set up. The front
could be anywhere from eastern South Dakota into northwestern
Iowa. The second area of uncertainty is due to a likely tight
gradient in snowfall - especially on the north side of the band.
At this point, the least likely location to see heavy snow is is
in central South Dakota - such as Chamberlain - with the most
likely area from far southeast South Dakota, northwestern Iowa,
and portions of southwestern Minnesota.

Because of the active weather in the next 3 days used a consensus
forecast for Sunday through Wednesday. During this period,
temperatures are expected to be e near seasonal normal from the
upper 20s to upper 30s for highs and 10 to 20 degrees for lows.
The next chance of precipitation, primarily snow, would come
Tuesday night and Wednesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1141 AM CST Wed Feb 21 2018

Primary forecast concern for aviation will be the snow and low
clouds expected primarily on Thursday morning. As high pressure
moves east today, southeasterly flow will increase into tonight.
This will bring moisture northward with ceilings gradually
lowering through night...reaching 3000 ft in KSUX by 12Z and at
KFSD and KHON by 15Z. Ceilings will settle at around 1000 ft by
late morning at all locations. Snow is expected to move into KSUX
around 12Z and reach KFSD and KHON after 15Z. As snow increases,
visibilities will fall below 2SM and could be lower than 1SM for
an hour or two although have not lowered the visibility below 1 SM
at this time. Other concern is that precipitation could become
light and mix with or change to freezing drizzle around KSUX by
18Z. With GFS and NAM both showing this potential did put a mix of
snow and freezing drizzle in the KSUX TAF. There could even be a
mix with freezing drizzle as far north as KFSD in the early
afternoon which subsequent forecasts will need to examine.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from 6 AM Thursday to midnight CST
     Thursday night for SDZ068>071.

     Winter Weather Advisory from 9 AM Thursday to midnight CST
     Thursday night for SDZ038>040-052>062-064>067.

MN...Winter Weather Advisory from 9 AM Thursday to midnight CST
     Thursday night for MNZ071-072-080-081-089-090-097-098.

IA...Winter Weather Advisory from 6 AM Thursday to midnight CST
     Thursday night for IAZ020-031-032.

     Winter Weather Advisory from 9 AM Thursday to midnight CST
     Thursday night for IAZ001>003-012>014-021-022.

NE...Winter Weather Advisory from 6 AM Thursday to midnight CST
     Thursday night for NEZ013-014.

&&

$$

SHORT TERM...Schumacher
LONG TERM...Schumacher
AVIATION...Schumacher



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