Blizzard and Winter Storm in South Dakota, December 25 to 26, 2023

Area Forecast Discussion
National Weather Service Sioux Falls SD
434 AM CST Mon Dec 25 2023

.KEY MESSAGES...

- Blizzard/Winter Storm Warning and Winter Weather Advisories
  remain in place this morning, but have shortened the length by
  6-12 hours due to trends showing a westward shift in the storm
  track tonight into Tuesday.

- Mixed precipitation will spread northwest today, becoming
  periods of heavy snow is it expands into much colder air along
  and west of the James River Valley. Strongest winds gusting to
  45 mph will coincide with the heavy snow, creating blizzard
  conditions.

- Mixed precipitation east of the James River will create
  holiday travel impacts due to ice and/or snow accumulations,
  but these areas may transition to rain by late afternoon into
  this evening before precipitation diminishes tonight.

&&

.DISCUSSION...
Issued at 430 AM CST Mon Dec 25 2023

Broad shield of precipitation across Nebraska/Iowa and the
southeast half of Minnesota to start this morning, in advance of
a strengthening mid-upper level low which will swing out of
Colorado into the Plains today. Broad lift within a developing
TroWAL north of the upper low will spread the precipitation west
across the forecast area today, with increasing intensity in
response to very strong warm advection at the nose of the
TroWAL.

Thermal profiles remain the most challenging part of this winter
storm forecast, though latest CAMs show decent agreement in the
precipitation type transition zones, as well as the timing of a
very pronounced mid-level dry slot which will serve to cut off,
or dramatically diminish the precipitation intensity as it
expands northwest across the forecast area late this afternoon
through tonight.

BLIZZARD WARNING AREA: This area has remained precipitation-free
much of the night, and contemplated delaying the start time of
the blizzard warning. However, regional radar shows snow is
filling in across Nebraska and this should lift quickly north
through 12Z-15Z. At the same time, winds which are sub-blizzard
criteria now will be on the increase, so expect conditions to
deteriorate rapidly as we progress just an hour or two beyond
sunrise. HREF probabilities for intense snowfall rates of 1" or
more per hour have increased to greater than 70% during the
afternoon hours, which isn't unreasonable given the very strong
lift seen in the models. These heavy snowfall rates combined
with winds gusting as high as 45-50 mph will create difficult,
if not impossible travel conditions west of the James River this
afternoon into tonight.

The greater challenge for this area is what to do on the back
end. With the westward trend in the track of the surface and
upper low, we begin to see warmer air wrap as far west as the
James River Valley (Huron-Mitchell-Yankton) as early as this
evening, changing snow to a wintry mix, and perhaps warm enough
for rain in some areas, before the dry slot moves in around or
shortly after 06Z. Have split the Blizzard Warning into 2 parts,
each transitioning down to a Winter Weather Advisory for the
period where winds taper off slightly, and a wintry mix becomes
more prevalent. For the James River Valley, the Blizzard Warning
now ends at midnight followed by an advisory until noon Tuesday.
West of the river valley, the warning will end at noon Tuesday
followed by an advisory until 6 pm Tuesday. These end times may
still be on the long side, but remained conservative for this
first round of changes.

WINTER STORM WARNING/WINTER WEATHER ADVISORY AREAS: Early
morning freezing line at the surface near the southeast edge of
our forecast area, and expect to see above freezing temps/dew
points pulled back into our far eastern counties through midday.
The warmer air continues to expand northwestward this afternoon,
largely remaining north of the Missouri River Valley. This will
allow the ongoing wintry mix (mostly freezing rain and/or sleet)
in northwest Iowa to gradually change to rain from east to west
as we progress through the day. Colder air wrapping around the
south side of the TroWAL will lock temperatures below freezing
through the Missouri River Valley, limiting the icing in this
area, but allowing for slightly higher snowfall amounts. All in
all, amounts did not change drastically with this forecast, so
existing types of headlines still seemed reasonable.

The main area of concern is along the higher elevations of the
Buffalo Ridge, especially this morning before the warmer air
spreads in. Unlike northwest Iowa, that area of southwest
Minnesota has seen surface air temperatures below freezing much
longer, so roads and especially elevated surfaces may be more
receptive to icing over. Ice accumulation calculations do show
areas of greater than 0.25", but these higher amounts are mainly
for flat surfaces such as roads and vehicles. The Freezing Rain
Accumulation Model (FRAM) does a separate calculation for line
(radial) surfaces, which factors in additional factors beyond
just the rainfall amount and QPF. One factor is the rainfall
rate, and higher rates have been shown to accumulate less on
these radial surfaces as more of the liquid is able to run off
before freezing. In this case for today, the line accumulation
is less than half of the flat surface accumulation, with peak
line ice accumulations near or less than 0.15". For that reason,
opted to stick with the Winter Weather Advisory across southwest
Minnesota, but will monitor conditions closely, and consider an
upgrade if conditions warrant.

Similar to our Blizzard Warning area, the earlier/farther west
influx of warmer air and dry slot prompted an adjustment to the
expiration times of the headlines east of the James River
Valley, with all headlines now expiring at midnight tonight.
Again, though, these may be cancelled even earlier if conditions
warrant.

TUESDAY AND BEYOND:
Dry slot looks to be in command of much of the forecast area
except perhaps our far west/north by daybreak Tuesday, though
cannot rule out additional light precipitation lingering into
Wednesday as the now stacked and very slow moving system spins
through northern Missouri and the mid-Mississippi Valley.

Given the focus on the complex storm in the near term, did not
spend much time looking at the longer range today. However,
broad consensus looks storm-free with temperatures holding much
closer to seasonal normals for the last week of 2023.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1047 PM CST Sun Dec 24 2023

Expect IFR conditions to prevail through the period, with LIFR
conditions during the day Monday. Precipitation increases in
coverage and spreads west during the morning hours. Snow
prevails west of the James River, with a mix of precip types
toward the I-29 corridor. Rain and freezing rain linger the
longest east of I-29, especially across northwestern IA. Toward
the end of the period, some non-snow precipitation types is
expected to mix in west of the James River. Visibility will be
greatly reduced in falling snow due to strong winds and briefly
heavy snowfall rates, with additional visibility restrictions
due to blowing snow.

Strong northwesterly winds continue through the period. Gusts at
times along and west of the James River near 40 knots especially
during the afternoon and evening Monday. Winds will be lighter
but still breezy across northwestern IA, with some shift in
direction east of I-29 due to the meandering low pressure Monday
evening.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until midnight CST tonight for SDZ055-
     062-067-071.
     Winter Weather Advisory until midnight CST tonight for SDZ040-
     056.
     Winter Storm Warning until midnight CST tonight for SDZ039-054-
     061-066-070.
     Blizzard Warning until midnight CST tonight for SDZ038-053-059-
     060-065-068-069.
     Winter Weather Advisory from midnight tonight to noon CST
     Tuesday for SDZ038-053-059-060-065-068-069.
     Blizzard Warning until noon CST Tuesday for SDZ050-052-057-058-
     063-064.
     Winter Weather Advisory from noon to 6 PM CST Tuesday for
     SDZ050-052-057-058-063-064.
MN...Winter Weather Advisory until midnight CST tonight for MNZ098.
     Winter Weather Advisory until midnight CST tonight for MNZ071-
     072-080-081-089-090-097.
IA...Winter Weather Advisory until midnight CST tonight for IAZ001-
     012-020-031.
     Winter Weather Advisory until midnight CST tonight for IAZ002-
     003-013.
NE...Winter Weather Advisory until midnight CST tonight for NEZ014.
     Winter Storm Warning until midnight CST tonight for NEZ013.

&&

DISCUSSION...JH
AVIATION...SG

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
633 PM CST Sun Dec 24 2023

IAZ001-012-020-031-MNZ098-NEZ013-014-SDZ039-054-055-061-062-066-067-
070-071-260045-
Lyon-Sioux-Plymouth-Woodbury-Rock-Dixon-Dakota-Kingsbury-Miner-Lake-
McCook-Minnehaha-Turner-Lincoln-Clay-Union-
633 PM CST Sun Dec 24 2023

This Hazardous Weather Outlook is for northwest Iowa, west central
Iowa, southwest Minnesota, northeast Nebraska, east central South
Dakota and southeast South Dakota.

.DAY ONE...Tonight.

Rain transitions to a light wintry mix tonight, with periodic drizzle
and freezing drizzle. Minor snow and ice accumulations expected
overnight.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

A second wave of precipitation is expected Christmas Day and
continues through Tuesday. Snow accumulations of 2 to 8 inches. Ice
accumulations of one to two tenths of an inch are expected, highest
from a Brookings to Le Mars. In addition to the wintry
precipitation, widespread wind gusts to 45 mph anticipated late
Monday and Tuesday. If you have holiday travel plans, it is vital you
continue to monitor the latest forecasts. Be prepared to alter or
cancel plans.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

SDZ038-050-052-053-057>060-063>065-068-069-260045-
Beadle-Gregory-Jerauld-Sanborn-Brule-Aurora-Davison-Hanson-
Charles Mix-Douglas-Hutchinson-Bon Homme-Yankton-
633 PM CST Sun Dec 24 2023

This Hazardous Weather Outlook is for central South Dakota, east
central South Dakota, south central South Dakota and southeast
South Dakota.

.DAY ONE...Tonight.

Snow continues tonight, and some areas may see a couple inches of
snow by tomorrow morning. Minor ice accumulations possible.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Additional precipitation is expected Christmas Day and continues
through Tuesday. Heavy snow with accumulations 7 to 18 inches
expected Monday morning through Tuesday, with higher amounts across
south central South Dakota. Ice accumulations less than one tenth of
an inch. In addition to the wintry precipitation, widespread wind
gusts to 50 mph expected late Monday and Tuesday will lead to
blizzard conditions. If you have holiday travel plans, it is vital
you continue to monitor the latest forecasts. Be prepared to alter or
cancel plans.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

IAZ002-003-013-014-021-022-032-MNZ071-072-080-081-089-090-097-SDZ040-
056-260045-
Osceola-Dickinson-O'Brien-Clay-Cherokee-Buena Vista-Ida-Lincoln-Lyon-
Murray-Cottonwood-Nobles-Jackson-Pipestone-Brookings-Moody-
633 PM CST Sun Dec 24 2023

This Hazardous Weather Outlook is for northwest Iowa, west central
Iowa, southwest Minnesota and east central South Dakota.

.DAY ONE...Tonight.

Rain continues tonight, transitioning to freezing rain from north to
south. Some snow is expected to mix in. Ice accumulations around one
tenth of an inch with little to no snow accumulations overnight.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Additional precipitation is expected Christmas Day and continues
through Tuesday. Snow accumulations of less than two inches, highest
across eastern South Dakota. Additional ice accumulations of a tenth
of an inch or less, greatest for southwestern Minnesota. In addition
to the wintry precipitation, widespread wind gusts to 45 mph expected
late Monday and Tuesday. If you have holiday travel plans, it is
vital you continue to monitor the latest forecasts. Be prepared to
alter or cancel plans.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.