Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221704
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1204 PM CDT Fri Mar 22 2024

.KEY MESSAGES...

- Wintry mix of precipitation lingering across southeast
  portions of the forecast area early this morning, expected to
  exit east of Hwy 71 by 9-10 AM.

- Focus shifts to strong winter storm expected to impact the
  northern Plains this weekend and into early next week. Still
  uncertainty regarding distribution of precipitation types and
  eventual snowfall amounts, especially for areas along/south of
  the I-90 corridor.

- In addition to greater potential for heavy snowfall across our
  west and north, we will also have periods of strong winds to
  contend with which could produce blizzard conditions at times,
  especially while the snow is falling. Have issued a Winter
  Storm Watch for Sunday through Monday night where confidence
  is highest that these conditions may occur.

&&

.DISCUSSION...
Issued at 500 AM CDT Fri Mar 22 2024

THIS MORNING: Still seeing an area of light to moderate precip
across the Highway 20 corridor as of 4 AM, with reports of a
wintry mix (sleet and freezing rain) around the Sioux City area.
Also noted some convection in this area, with several lightning
strikes detected west through south of Sioux City over the past
hour or two. This precipitation shield has been making steady
progress east, and should exit our forecast area by 9-10 AM. The
morning commute could be on the slick side where roads have not
been treated, so allowing extra time would be advised.

THIS AFTERNOON-TONIGHT: The relative calm before the storm, as
they say. Sun may be slow to appear today as stratus extends
well off to the west and north. Overall a chilly day with highs
mostly in the 30s. If we see any clearing by late today, clouds
will quickly move back in this evening ahead of our impending
storm system. Could see some light snow develop into our far
western areas (west of the James River) after midnight with some
light accumulations possible by daybreak Saturday.

SATURDAY-SATURDAY NIGHT: Precipitation chances increase in the
warm advection wing ahead of the main storm system, and models
are showing decent consensus in this focusing near/north of I-90
by Saturday afternoon/evening, with a dry slot working into the
Missouri River corridor. Low level thermal profiles support all
snow with this portion of the system, with accumulations of 1-3
inches possible along/north of I-90 by daybreak Sunday, and
generally less than an inch south.

SUNDAY-MONDAY NIGHT: This will be the most impactful portion of
this early Spring storm system, especially by Sunday night into
Monday. Still a fair amount of uncertainty in how the low level
thermal profiles will evolve as the strengthening upper level
low ejects out of Colorado and swings northeastward across the
Plains. However, seeing a little better consistency in the warm
layer (aloft and surface) lifting at least as far north as I-90
in areas along and east of the lower James River Valley Sunday
afternoon/evening. Still some question how far west this warmer
air will build into south central South Dakota, and for now have
coordinating starting the Winter Storm Watch for Gregory/Charles
Mix/Douglas counties at 12Z Sunday. This may be adjusted if it
looks like the warmer air will be more dominant during the day
on Sunday.

Locations north of I-90 may also see a period of mixed precip
during the day Sunday, but this area remains the most consistent
with models showing a prolonged period of snow Sunday through
Monday, before slowly tapering off west to east Monday night.
Probabilities within the broader ensemble for greater than 6
inches of snow remain high (>80%) for our northern 2 tiers of
counties and down into the portion of I-90 near/west of
Mitchell. These areas also currently have moderate to high
(50-90%) probabilities of seeing more than a foot of snow
within this 48 hour period (on top of the aforementioned light
accumulations on Saturday/Saturday night). Given the increased
consistency and growing confidence, have issued a Winter Storm
Watch for these areas (along/north of Chamberlain/Mitchell to
Flandreau/Pipestone/Windom) for Sunday through Monday night.

On top of the snow, the intensely wound-up storm system will
produce periods of strong winds, especially Sunday night into
Monday night as the strengthening surface low swings from
eastern Nebraska, northeast through Iowa. Periods of wind gusts
35 to 45 mph seems reasonable given the very strong pressure
gradient on the northwest side of the surface low. Thus have
included a mention of possible blizzard conditions in all areas
of the watch.

It should be noted that further adjustments to the watch area
and timing are likely, as later model cycles better refine the
details of low level thermal profiles/precipitation types and
periods of the more intense precipitation and stronger winds.
At this time, the I-90 corridor around and east of Sioux Falls
is in the zone of highest uncertainty with regard to precip
types and eventual snowfall. We could end up with snowfall
amounts varying by as much as 4-6 inches or more from north to
south across this row of counties. Or that tight gradient could
still shift farther north or south, leading to an even larger
variation in potential snowfall amounts.

Regardless of what variety the precipitation falls, this should
bring beneficial moisture to much of the region. Latest data
still indicates very high (>90%) probabilities of >0.50" liquid
(or liquid equivalent for snow) through early Tuesday across the
forecast area with moderate to high (60-90+%) probability of
more than 1.00". Latest QPF forecast could approach or break
daily precipitation records on Sunday if the current numbers
pan out.

TUESDAY ONWARD: Uncertainty grows again through the longer
range of the forecast from Tuesday onward, especially with
regard to temperatures, as will now have a deep snowpack to
influence potential warming for at least the next several days.
Winds will remain breezy into Tuesday, which could continue to
produce some travel headaches if the snow is still movable. Dry
conditions look to dominate the midweek, but we could see
another round of active weather by the end of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1202 PM CDT Fri Mar 22 2024

Patchy MVFR ceilings will be posible across parts of northwest
IA and southwest MN through mid afternoon, otherwise ceilings
are lifting to VFR as well as some clearing around highway 14
into central SD. Later tonight through Saturday morning some
patchy light snow will be possible, the better chances in
central and southeast SD through 18z Saturday. Conditions may
fall to MVFR Saturday morning in these areas.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from late Saturday night through late
     Monday night for SDZ038>040-052>060.
     Winter Storm Watch from Sunday morning through Monday evening
     for SDZ050-063-064.
MN...Winter Storm Watch from Sunday morning through Tuesday morning
     for MNZ071-072-080-081-097.
IA...None.
NE...None.

&&

$$

DISCUSSION...JH
AVIATION...08


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