Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 161730

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1130 AM MDT Fri Mar 16 2018

Issued at 757 AM MDT Fri Mar 16 2018

Potent wrapped up low supporting impressive forcing and resultant
snowfall rates across the area. The heaviest snow is in a band
from far western SD into south central SD where 1-3 inch per hour
snowfall rates are present. Loaded a blend of the latest hires
models to get a better handle on convective influences and strong
LSA/UVM. This results in expanding the warning a row of zones
north and issuing a winter weather adv for far NW SD. All else in
shape for now.


.DISCUSSION...(Today Through Thursday)
Issued at 304 AM MDT Fri Mar 16 2018

The latest water vapor images showed a potent upper low moving
across western Kansas. This was supporting a large area of
precipitation across the central into the northern plains. A few
bands of thunderstorms also had developed from eastern Kansas into
north-central Nebraska. The main forecast challenges continue to
be precipitation type and snowfall amounts.

Given the convection over the central plains, it appears possible
that this will cause a readjustment in the low-level flow. Also,
with the southern position of the upper low, the Q-vector
convergence maximizes just to the south of our CWA. Finally,
relatively drier air was moving up through the central plains.
Considering all of this, current thinking is that snowfall will
not be quite as high as what models have been predicting,
especially for the more northern areas. Thus, have kept Rapid City
in an advisory, and have downgraded the central Black Hills and
Weston County to advisories. Expect that the heaviest snow will
setup southeast of Rapid City, even though there should be a good
burst of snow across west-central South Dakota this morning during
the commute. By midday, the snow should start to focus more to
the east of Rapid City into central South Dakota, with freezing
rain transitioning to snow over south-central South Dakota. Snow
will then end across the area this evening.

For precipitation type, it now appears that freezing rain and sleet
will be confined mostly to south-central South Dakota, consistent
with the latest thermal profiles and SREF precipitation type
probabilities. Rain was still falling in parts of northeastern
Wyoming as of 09z, but this should transition to snow (and not
freezing rain or sleet). This does pose a concern that not as much
snow will fall over Campbell County as previously forecast.

Given light winds and high boundary-layer RH tonight in the wake of
the precipitation, could see widespread areas of fog across the
CWA. Otherwise, it should be dry and warmer Saturday in advance of
the next system. This next system now appears that it will be
farther south than today`s storm, with relatively minimal impacts
as scattered rain/snow showers move across the area Sunday and

Another weak storm in northwest flow may bring isolated showers for
Tuesday, but then ridging will ensue for Wednesday and Thursday,
bringing drying and warming. Nevertheless, yet another storm will
bring more showers on Friday.


.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1122 AM MDT Fri Mar 16 2018

Heavy snow will continue across much of the area with IFR to LIFR
conditions through this afternoon. Freezing rain and ice
accumulations will continue across far south central SD. Snow
will taper off from west to east late this afternoon and early
evening, with ice changing to snow across far south central SD and
persisting there through midnight. Conditions will gradually
improve late tonight, but areas of MVFR/IFR cigs will persist
into tonight. Areas of fog are likely to develop late tonight
into Saturday morning across much of the area, especially western


SD...Winter Weather Advisory until 6 PM MDT this evening for SDZ001-

     Winter Storm Warning until 6 PM MDT /7 PM CDT/ this evening for

WY...Winter Weather Advisory until 6 PM MDT this evening for



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