Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221721

Area Forecast Discussion
National Weather Service Sioux Falls SD
1221 PM CDT Thu Mar 22 2018

.SHORT TERM...(Today)
Issued at 433 AM CDT Thu Mar 22 2018

One last mild day before a cooler and wetter pattern sets up into
early next week.

Today will start out mostly sunny for most areas. The exception
will be across portions of southwestern Minnesota and in the
Missouri Valley into west central Iowa. With falling pressures
over the Rockies, the pressure gradient will tighten with a
gradual increase in southeasterly winds through the day. Even with
the wind, temperatures will warm into the 40s and 50s...with 60
degrees possible around Sioux City.

.LONG TERM...(Tonight through Wednesday)
Issued at 433 AM CDT Thu Mar 22 2018

The primary forecast concern will be the upcoming early spring
storm which will bring rain and snow to the forecast area starting
late tonight and continuing into Saturday morning.

There has been little change in model solutions in the large
scale. All models show a strong wave moving from southern Wyoming
and across Nebraska on Friday and Friday night and then moving
southeast on Saturday morning. The ECMWF continues to be the
slowest of the models with the wave primarily affecting the
eastern Plains Friday night while the NAM/GFS and Canadian show
heavier precipitation beginning by Friday afternoon. For this
forecast used the timing of the majority.

Clouds will quickly increase overnight as cirrus overspreads the
region and large scale ascent increases low level and mid-level
clouds. After 06Z the influence of the upper level wave will
increase lift across central South Dakota and expect that light
precipitation will begin around HON and 9V9 by sunrise.
Temperatures aloft begin well above freezing and, fortunately,
with winds increasing to over 25 mph overnight west of the James
River, temperatures at the surface will also remain above
freezing. Therefore expecting precipitation to begin as rain.
Farther east, temperatures aloft and at the surface will be
cooler. At this time, it looks like any places where surface
temperatures will be around freezing will also have temperatures
aloft below freezing. Therefore, if any light precipitation moves
as far east as I-29 before daybreak it would be either rain or

On Friday, both the wave and strong warm advection will move east
through the day. This will allow temperatures aloft to rise above
freezing across all of southeast South Dakota and northwest Iowa.
Along and east of the Buffalo Ridge in southwestern Minnesota,
maximum temperatures aloft with be closer to freezing. In
addition, strong easterly flow, with gusts over 30 mph, will
continue to bring drier air with dew points in the upper 20s to
around 30 into southwestern Minnesota and adjacent areas of SD
and IA. At the same time, an area of 850 mb frontogenesis is
expected to set up from northeastern SD into southwestern MN by
late afternoon. Putting this all together, precipitation will move
east through the morning and organize itself into a band of
heavier precipitation later in the afternoon and evening from
northeastern South Dakota into southwestern Minnesota. As always,
the exact location of the band is uncertain but the most likely
area would be from Watertown SD to Spencer Iowa. With some hints
of instability seen in model soundings, there could be a period of
heavy precipitation during the late afternoon and evening. As the
upper wave moves to the southeast later Friday night, the band of
precipitation will move southeast leaving light snow or even
freezing drizzle late Friday night into Saturday morning.
Precipitation amounts are expected to be from 0.75 to 1 inch. West
of I29, generally a quarter to half inch rain is expected.

Precipitation type is the most uncertain portion of the forecast.
Basically northeast of a line De Smet, SD to Sioux Falls to
Spencer Iowa, temperatures in the lowest 10000 ft will be within 2
or 3 degrees of freezing. For Sioux Falls and Spencer, it is
likely temperatures will remain above freezing until later Friday
night keeping precipitation almost all rain. However, along and
east of the Buffalo Ridge - from Watertown, SD to Jackson, MN -
there is a high probability that temperatures may begin above
freezing but with continued inflow of dry air at the surface and
strong ascent aloft, temperatures will fall to around or even
below freezing. This includes Marshall, Slayton and Windom.
Precipitation will begin as rain in the late morning or early
afternoon with the exception of the higher elevation on the
Buffalo Ridge where a rain and snow mix is possible. As
temperature cool in the afternoon, there is the possibility of a
brief period of freezing rain or sleet as temperatures cool during
the afternoon. However, sometime late Friday afternoon or early
Friday evening, precipitation is expected to change to all snow.
The aforementioned instability, moderate to heavy snow will be
possible in the evening with the potential of over 6" of snow
northeast of a Watertown to Jackson line. Therefore have expanded
the winter storm watch to include Murray and Jackson Counties.
Areas west of this line, from Brookings to Spencer IA, while not
in the watch, could see a couple of inches of wet snow late Friday
afternoon and Friday night

Significant uncertainty exists with the timing and amount of snow
expected with this system. Should temperatures cool earlier more
snow would fall than currently forecast impacting travel Friday
afternoon. On the other hand, if temperatures can remain above
freezing into late Friday evening, lower snowfall totals would
occur. Persons planning travel across southwestern Minnesota
Friday afternoon and evening should be prepared for conditions
that could rapidly deteriorate with water covered roads becoming
snow covered as rain changes over to snow.

Two other system will impact the area into the weekend. On late
Saturday night and Sunday morning, light precipitation is expected
in central and eastern SD. This looks to be a mix of rain and
snow changing to rain after sunrise Sunday. The best chance for
precipitation will be west of the James River in South Dakota.
Another more significant system is expected late Sunday night and
Monday. The ECMWF is slower and stronger with this system. Once
again, precipitation type will be an issue as rain or snow will
begin on Sunday night and continue into Monday. There is also the
possibility of sleet or freezing rain with this system but that
will depend upon when precipitation begins and how cold it gets
Sunday night. At this point, it appears temperatures will remain
above freezing in much of southeastern South Dakota into
northwestern Iowa with cooler temperatures in southwestern
Minnesota. It also appears temperatures will warm on Monday
changing all the precipitation to rain during the morning. As this
system approaches, more details will be provided on the impact of
this system.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1212 PM CDT Thu Mar 22 2018

Quiet weather and VFR conditions are expected to continue for the
first half of the TAF period. Mid and high level clouds will be on
the increase by the evening hours ahead of an incoming storm
system. Rain and MVFR/IFR ceilings will spread into the region
from west to east through the morning hours. Rain could mix with,
or change to, snow over southwest Minnesota, especially into the
afternoon hours after this TAF period.


MN...Winter Storm Watch from Friday morning through Saturday morning
     for MNZ071-072-080-081-090.



SHORT TERM...Schumacher
LONG TERM...Schumacher
AVIATION...Kalin is the U.S. government's official web portal to all federal, state and local government web resources and services.