Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 222340
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
540 PM MDT Fri Mar 22 2024

.KEY MESSAGES...
233 PM MDT Fri Mar 22 2024

- Areas of light snow are expected today through tonight, with pockets
  of freezing drizzle possible.

- A strong storm is expected to bring heavy snow and mixed
  precipitation to the region late Saturday through at least early
  Monday, with blowing snow expected to persist through the day
  on Monday. Blizzard conditions will be possible over portions of
  the western and central South Dakota plains.

&&

.DISCUSSION...(This Evening Through Friday)
Issued at 233 PM MDT Fri Mar 22 2024

Water vapor imagery this afternoon shows a robust upper-
level low approaching the Pacific Northwest coast. Plentiful deep
moisture and ascent is evident downstream of the low, spreading over
the central and northern High Plains in westerly/northwesterly flow
aloft. Day cloud phase RGB imagery depicts several layers of clouds
over the region, with higher clouds filtering eastward atop low
stratus/fog. In areas void of low cloud cover, some recent snowpack
is evident across northwestern SD. Flow area-wide (outside of the
Black Hills) has a considerable easterly/upslope component. The
latest surface analysis shows a baroclinic/frontal zone stretching
northwestward from northeastern CO through western MT, with ridging
nosing from central CN southward through the central Plains. Close
to home, temperatures are chilly, with readings generally ranging
from the lower 20s to lower 30s.

As an upper-level jet streak passes north of the area late today
into tonight, the baroclinic zone to our southwest will begin to
lift northeastward. Low-level frontogenesis will ramp up as it does
so, with deep isentropic ascent further contributing to gradual
moistening. Forcing is not intense, but it should suffice for the
development of light precipitation beginning in the vicinity of the
Black Hills and lifting northeastward with time, consistent with the
motion of this forcing for ascent. Thermodynamic profiles during
this time mainly favor snow; however, as deeper ascent/moisture
lifts northeastward, lingering low-level moisture may support some
areas of freezing drizzle. Confidence is low in this potential.
Model consensus suggests fairly light QPF with this initial wave of
precipitation, with LREF probabilities of 0.01" or more of QPF over
80% in spots, but probabilities of 0.05" or more limited to around
20%. Basically, we`re talking liquid amounts in the hundredths of an
inch.

A strong shortwave ejects from the broader trough over the West
Coast tomorrow into Sunday, spreading robust ascent over the central
Rockies and High Plains regions and supporting lee cyclogenesis from
central WY through eastern CO. Broad, strong midlevel to upper-level
Q-vector convergence should support the development of widespread
precipitation. However, as the low-level/midlevel cyclone
strengthens late overnight Saturday into Sunday morning, a dry slot
will likely nose into southwestern SD/northeastern WY, eroding deep
moisture and potentially contributing to a cessation of
precipitation or facilitating a transition to freezing drizzle. A
secondary possibility is for sufficient steepening of lapse rates
such that some heavier convective showers may occur, which may be
associated with frozen or mixed precipitation. Later on Sunday, two
primary corridors of precipitation will emerge: One close to the
cyclone tied to deep Q-vector convergence and frontogenesis and
another over northwestern SD associated with an upper jet entrance
region and isentropic ascent. These two areas will likely correspond
to the most widespread and impactful precipitation. More localized
persistent precipitation, some of which could be heavy, is probable
along the northern, northeastern, and potentially eastern foothills
as conditions favorable for upslope enhancement emerge late Sunday
afternoon into the evening.

Cyclogenesis continues late Sunday into Sunday night, with an
intense surface cyclone projected to shift from northeastern CO
toward the Upper Midwest late Sunday through Monday. Strengthening
pressure rises and a tightening pressure gradient will combine to
produce strong northerly winds, particularly over the south central
to west central SD plains. In these areas, blizzard conditions may
develop given the combination of expected snow and winds.

For now, opted to maintain the current Winter Storm Watch, with the
only change being to extend the segment of the watch covering south
central SD to 06z Tuesday given the potential for blowing snow to
persist into Monday night. Upgrades will likely be needed tonight or
early tomorrow.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 538 PM MDT Fri Mar 22 2024

MVFR/Localized IFR conditions predominately associated with low
stratus and patchy fog will persist over northeastern WY, the
Black Hills, and northwestern SD through early this afternoon.
This includes the KRAP and KGCC terminals. There will be gradual
improvement to low VFR for most of the area tonight with some
lingering MVFR cigs over the Black Hills.

A storm system will begin moving through the region early Saturday
with MVFR/IFR conditions associated with -SN primarily over
northwestern into south central SD on Saturday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Saturday evening through Monday morning
     for SDZ001-002-078.
     Winter Storm Watch from late Saturday night through Monday
     morning for SDZ012>014-024>026-031-032-072-073.
     Winter Storm Watch from Sunday morning through Monday evening
     for SDZ043-044-046-047-049-077.
WY...Winter Storm Watch from late Saturday night through Monday
     morning for WYZ057-060.
&&

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...Dye


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