Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 222056
AFDFSD

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

.KEY MESSAGES...

- Saturday will see chances (50-70%) for light snowfall
  throughout the day, with amounts generally under an inch south
  of I-90, and in the 1-2 inch range north of I-90.

- High confidence for a strong storm system impacting the area
  Sunday through Tuesday, though precipitation types and thus
  amounts remain uncertain generally south of I-90.

- Areas along the highway 14 corridor have the highest chance of
  seeing at least a foot of snow (70-80%). Areas within the
  Winter Storm Watch all have a 50% or greater chance for seeing
  at least 6 inches of snow, with blizzard conditions also
  possible given strong winds while snow is actively falling.

- Probabilities for at least an inch of liquid QPF remains in
  the 70-95% range across the region, with the main question
  being how much of this will be converted to snow along and
  south of I-90.

&&

.DISCUSSION...
Issued at 349 PM CDT Fri Mar 22 2024

A ridge of surface high pressure noses into the area from the north
as the high pressure moves into northern MN overnight. This will
allow for some lighter winds and clear skies through part of the
night. A system to the southwest will begin to spread cloud cover
into the area later tonight into Saturday morning, which will limit
the lows in central SD. Currently thinking teens to lower 20s for
lows with the least cold temperatures in central SD.

Saturday will see some frontal forcing move into the area from
southwest to northeast through the day. The dendritic layer will
deepen a bit through the day to the north and east while becoming
higher and shallower to the south and west. This should bring a
decrease in snow potential from southwest to northeast through the
day. Overall the forcing and moisture not overly impressive so a
generally half an inch to an inch should be common with the better
chances near and north of I-90. Temperatures should only rise into
the lower to mid 30s in most locations.

Dry air will be introduced during the evening hours on the backside
of the warm frontal band, bringing a brief lull in precipitation
activity for areas south of I-90. For those along and north of the I-
90 corridor, could see see light snowfall continuing as low-mid
level moisture remains prevalent along with weak 700mb frontogenesis
(FGEN). Throughout the rest of the overnight hours, the surface low
out west will be strengthening as it ejects onto the central plains.
This will result in isentropic lift strengthening into the early
morning hours, along with some weak shortwaves aloft leading to our
next chance for precipitation. While there remains some uncertainty
on the thermal profiles, models are beginning to come into better
agreement and are showing snow mixed in with some freezing rain
and/or sleet will be pushing northwards throughout the morning
hours. In addition to light snowfall amounts, light icing will also
be possible.

As the surface low continues to move towards the region, warm air in
the inverted trough begins to move in and will bring temperatures in
the mid to upper 30s up towards the I-90 corridor by the late
morning hours, possibly getting past I-90 while remaining west of I-
29. This will bring a switch to rain for majority of the area, which
and will likely melt a decent amount of the snow that will have
fallen south of I-90. Warm air looks to continue filtering in
throughout the afternoon and evening hours, which will keep the
dominant precipitation type as rain across majority of the area. So,
areas along and south of I-90 may only see up to an inch of snow
throughout the day, while areas north of I-90 could see 2-6" as the
cooler temperatures reside longer in that area. That is the main
consensus amongst the models, though as alluded to earlier, there
exist other solutions where cooler temperatures keep the rain/snow
line further southeast, which would increase our snowfall totals
expected for Sunday.

Heading into Monday, confidence is increasing in storm track, though
there remains some room to wobble as models continue to provide
varying solutions. In general though, as the low pressure moves
towards the area, the deformation zone in combination with strong
vorticity advection from mid-level vorticity and being near the left-
exit region of the upper level jet will continue/increase our
precipitation chances and rates. Monday remains the day when we are
expecting the majority of the precipitation to fall, with ensembles
continuing to show 70-90% chances for an additional 0.5" of liquid
rain during this 24 hour period. This will continue chances for
accumulating snow, with probabilities for an additional 6 inches of
snow for areas along and north of I-90 ranging from 40-80%. This is
slightly lower than it was yesterday, largely because the rain/snow
line has shifted further north of I-90. Lastly, as this surface low
moves northwards while being just east of the area, a strong surface
pressure gradient (SPG) will form across the region, bringing wind
gusts of 35 to 45 mph, raising concern for lowered visibilities
while snow is actively falling.

Overnight into Tuesday, consensus is again rising that this is when
we`ll see a switch to snow area-wide, so some light snowfall
accumulation is expected to continue into Tuesday. While snowfall
rates will be decreasing throughout the day, the winds will continue
to gusts into at least the 30s, leading to continued concern for low
visibility conditions. Given the heavy snowfall and icing potential
expected throughout the weekend and into Tuesday, remain aware of
the forecast and change plans if needed.

Uncertainty grows as this system moves out of the area Tuesday into
Wednesday, as evident by the ensemble clusters which are quite
evenly split at 29/26/24/21% of ensemble members respectively. As
the previous discussion mentioned, how much snow falls and where
will have big impacts on temperatures for next week, so we`ll have
to see if we end up warming as much as the NBM is showing.
Otherwise, ensembles continue to show that another surface low may
bring renewed chances for winter weather, though again the spread in
ensemble clusters does not allow for much confidence at this
time.

&&

.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 Tuesday
     morning for SDZ038>040-052>061.
     Winter Storm Watch from Sunday morning through late Monday
     night for SDZ050-063>065-068.
MN...Winter Storm Watch from Sunday morning through Tuesday morning
     for MNZ071-072-080-081-097.
IA...None.
NE...None.

&&

$$

DISCUSSION...APT/08
AVIATION...08


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