Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 211759
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1259 PM CDT Thu Mar 21 2024

.KEY MESSAGES...

- Light snow is possible mainly across the highway-14 corridor
  this morning. Accumulations will be minor at only around a
  tenth of an inch.

- Chances for precipitation increase this evening and night.
  Amounts from a few tenths to up to 1-2 inches is expected
  along and north of a Wessington Springs, South Dakota to Sioux
  Falls, South Dakota, to Spencer Iowa line. The highest amounts
  will fall across southwest Minnesota where isolated higher
  amounts are possible.

- Locations south of the Wessington Springs, South Dakota to
  Sioux Falls, South Dakota, to Spencer, Iowa line will see
  light rain with accumulations up to a few hundredths to up to
  around three tenths of an inch expected. The highest rainfall
  totals will fall across northwest Iowa.

- Confidence continues to increase in a strong storm system impacting
  the area Saturday through Tuesday. Specific details remain
  uncertain at this time so continue to check the latest
  forecast for the most up to date information.

&&

.DISCUSSION...
Issued at 414 AM CDT Thu Mar 21 2024

Mid level clouds with flurries to very light snow begin the day
today. Any snow that does fall is not expected to accumulate due to
the warm road temperatures. Should continue to see flurries to light
snow continue through most of the morning hours. Hi-res guidance
does show this so have kept slight chance to chance PoPs across
highway-14 for the morning hours. Any snow that does fall will be
very light only up to about a tenth of an inch. The majority of
today will be quiet as warm aid advection (WAA) strengthens out
ahead of an incoming mid level wave. This WAA will warm 850 mb
temperatures up to -6 C to just above 0 C, warmest across south
central South Dakota. Mixing this to the surface will result in high
temperatures up to upper 30s to mid 50s. Winds will turn to out of
the southeast and become a bit breezy with gusts up to around 20-25
mph. Dew points will start the day in the single digits but will
slowly warm thanks to the southeasterly winds. This will make for
afternoon relative humidity (RH) values to fall down to about 25%
across parts of the Missouri River Valley. Given how winds and RH
are just barely touching Red Flag criteria, have opted to not issue
since dew points will be increasing throughout the day and the warm
layer aloft should keep mixing in check. However, high fire danger
is still expected. Will monitor conditions throughout the day today.

The previously mentioned wave ejects into the Northern Plains this
evening which will bring the next chance for precipitation to the
area. WAA will further strengthen out ahead of the wave and tighten
the thermal gradient as well. This will strengthen the frontogenesis
(FGEN) between 850-700 mb. The best lift looks to occur along the
700 mb front which will pass northeast of our area across
southeastern North Dakota, northeast South Dakota, and western
Minnesota. This will keep the bulk of the snow tied to this area.
However, an inverted trough will the slide through the area tonight
which will have strong 850 mb FGEN along it. This area of forcing
will cool thermal profiles to saturation and result in precipitation
across the area. The dendritic growth zone (DGZ) will saturate and
with omega (upward motion) values up to 20 to 30 microbars per
second in the DGZ, snow is expected. Not all locations will see snow
though as those mainly north of a Wessington Springs, South Dakota
to Sioux Falls, South Dakota, to Spencer, Iowa line will see all
snow with this event. Locations along this line will see a rain/snow
mix and all rain south of the line. In terms of accumulations a few
tenths to up to 1-2 inches is expected with isolated higher amounts
possible. The highest snowfall totals will fall mainly across
southwest Minnesota. Have also kept a small amount of ice in the
forecast as soundings show the DGZ drying out on the backside of the
band of precipitation that comes through. Given the faster movement
of this band and warm road/surface temperatures, only a hundredth of
ice accumulation is expected, mainly across parts of northwest Iowa.
South of the line, locations can expect a few hundredths to up to
around three tenths of an inch of rain, highest over northwest Iowa.

Friday will be a quiet day as a surface ridge pushes through the
area. High temperatures will remain on the cool side thanks to
northerly flow flow within a cold advective regime. High
temperatures will only reach up to 30s and 40s with low temperatures
falling to the teens and 20s.

Attention turns to a potential powerful storm system set to impact
the region this weekend and into early next week. Starting on
Saturday, a strong thermal gradient will reside along the front
range of the Rockey Mountains which an incoming upper level wave
will interact with and begin lee cyclogenesis. The wave doesn`t
fully eject into the Plains until Sunday morning but WAA will begin
to strengthen across the area on Saturday. This may be strong enough
to produce some light snow across the area but without stronger
forcing, accumulations look to be minor at this time. As the wave
finally ejects on Sunday, precipitation is expected to expand across
the region. Uncertainty remains regarding the track and evolution of
this system as the deterministic GFS, Euro, and Canadian continue to
have different solutions and dynamics. Their ensemble counterparts
also show similar solutions to their deterministic correspondents.
The system looks to begin as a warm front dominant system on Sunday
before uncertainty increases on Monday. The Euro and Canadian
ensembles show the southern jet streak rounding the base of the
trough while the GFS keeps the jet mainly upstream of the trough,
keeping the evolution of the system and especially any potential
banding uncertain at this time. Precipitation types are also
uncertain as ensembles continue to vary on how warm or cold surface
temperatures will get throughout the duration of this system.
However, confidence continues to grow in terms of impactful
precipitation as the GFS, Euro, and Canadian ensembles all show
increasing probabilities for exceeding half an inch of QPF, now up
to a 70-90% chance. They also show probabilities continuing to
increase for exceeding an inch of liquid precipitation, now up to a
40-60% chance. The Euro Extreme Forecast Index (EFI) also supports
this as it shows 80-90% of its members point to an anomalous event
and has 1 to 2 shift of tails over the area for snowfall. Will
continue to closely monitor trends with this system.

After the system pulls away on Tuesday, Wednesday will be a dry day
with near to below average high temperatures in the 30s and 40s.
This may change depending upon the powerful systems snowfall. An
active pattern continues aloft which may bring renewed chances for
precipitation at the end of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1259 PM CDT Thu Mar 21 2024

VFR conditions expected into the evening hours, with breezy
southeasterly winds gusting into the teens and upper 20s also
continuing. By the late evening and early overnight hours, we`ll see
our next round of precipitation move across the area from the
northwest to the southeast, with MVFR/IFR cigs and MVFR visbys
accompanying the precipitation. Areas along and north of I-90 will
likely only see light snow, while areas south of I-90 will start off
with rain possibly turning into a light wintry mix
(rain/snow/freezing rain/sleet). Only expecting a trace of ice, with
temperatures warming throughout the the morning hours.
Precipitation chances quickly decrease after sunrise.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...Meyers
AVIATION...APT


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