Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 242015
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
215 PM MDT Sun Mar 24 2024

.KEY MESSAGES...
213 PM MDT Sun Mar 24 2024

- Complex spring storm system continues to impact the area tonight
  through Monday.

- Scattered snow and mixed precipitation showers this afternoon
  will yield highly variable snow amounts. Additional snow
  expected across south-central SD later tonight through Monday
  where a Blizzard Warning remains in effect.

- Strong north-northwesterly winds will develop tonight into
  Monday. Gusts up to 50 mph are possible across the western SD
  plains into south-central SD. Blowing and drifting snow is
  likely; reducing visibility and resulting in hazardous travel
  conditions through the day Monday.

&&

.DISCUSSION...(This Evening Through Sunday)
Issued at 213 PM MDT Sun Mar 24 2024

Upper-and-mid-level water vapor bands show three distinct areas
to watch: a strong shortwave trough ejecting into the south-central
Plains, a second shortwave ejecting out of the Four Corners region
tied to the upper trough axis, and a broad area of subsidence and
drying pushing through the Pacific NW into the Northern Rockies. A
prominent dry slot/deeper layer of dry air is evident on the
low-level WV band (and in the upper/mid bands, although less obvious)
where little to no precipitation is occurring across south-central SD
early this afternoon. The surface baroclinic zone has sagged
southward, stretching from southern NE into southeastern WY as
surface cyclogenesis is ongoing across southeastern CO.

Upper level ascent continues to overspread the region (as manifest
by Q-vector convergence) along with isentropic lift + theta-e adv.
Scattered show/mixed precip showers continue to surge northward
through NE WY into WNW SD this afternoon. These showers have been
convective in nature and heavier precipitation has been spotty -
leading to high variability in snow/precipitation amounts this
afternoon and evening.

As the upper trough ejects into the central Plains later tonight into
Monday, the low-level/mid-level cyclone will further develop
southeast of the forecast area. The area of subsidence further
upstream will quickly push into the CWA early Monday morning,
limiting overall synoptic ascent. Recent deterministic runs (NAM,
CAMs) place the most focused area of low-to-mid-level frontogenetic
forcing along and immediately south of Tripp County. The result:
portions of the southeastern CWA lay beneath the cool side/descending
branch of the frontogenetic circulation. Snow amounts have trended
downward across NW SD and the eastern half of the forecast area
given the best fgen and synoptic forcing is shunted to the southeast
combined with drier air aloft (forecast profiles show an shallower
moist layer) and meager lift+RH in the DGZ. Probabilistic ensemble
data reflect the less favorable synoptic and mesoscale environment.
Both the LREF and the HREF QPF/snow probabilities have fallen; e.g.,
the LREF shows a very tight gradient of ~30-40% probability of >4" of
snow across far eastern Mellette and Todd counties to near 80% in
central/eastern Tripp county through Monday evening. Ensemble
probabilities of warning criteria snow (>6) are near 50% and confined
mainly to Tripp county and areas east-southeast. Despite the downward
trend in snow amounts, strong north-northwesterly winds will develop
tonight into Monday, resulting in areas of blizzard conditions across
portions of south-central SD and low visibility/very poor travel
conditions. Despite the lower snow totals, will continue with current
winter weather headlines given locally higher snow amount potential,
strong winds, and travel impacts expected tonight into
tomorrow.

Cold air will linger through mid-week as upper troughing persists
across the central portion of the country. Upper level ridging will
slowly build from west to east for the latter half of the week with
drier, southwesterly flow developing to end the week. Ensembles and
cluster analysis highlight potential for more active weather next
weekend into early next week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1035 AM MDT Sun Mar 24 2024

Widespread IFR CIGS will be located over the region this afternoon
through tonight. Areas of -SN will continue across most of
western SD into NE WY. Mixed rain, freezing rain and snow are
possible along and south of I-90 today. Gusty winds will develop
today, increasing over western South Dakota tonight, IFR/LCL LIFR
conditions in falling and blowing snow are expected tonight. -SN
will begin to taper off from west to east late in the TAF period
though MVFR/IFR conditions with locally IFR conditions will
persist for most of the region through the end of the TAF period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until 9 AM MDT Monday for SDZ001-002-013-
     078.
     Winter Weather Advisory until 6 PM MDT Monday for SDZ012-024>026-
     031-042-072-073-076.
     Winter Storm Warning until 6 PM MDT Monday for SDZ014-032-043.
     Winter Weather Advisory until 6 PM MDT /7 PM CDT/ this evening
     for SDZ044-046-047-049-077.
     Winter Storm Warning from 6 PM this evening to 6 PM MDT Monday
     for SDZ044-077.
     Blizzard Warning from 7 PM this evening to 1 AM CDT Tuesday for
     SDZ046-047-049.
WY...Winter Weather Advisory until 6 PM MDT Monday for WYZ057-060.
&&

$$

DISCUSSION/KEY MESSAGES...SE
AVIATION...Wong


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