Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 130950
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
350 AM MDT Fri Apr 13 2018

.DISCUSSION...(Today Through Thursday)
Issued at 350 AM MDT Fri Apr 13 2018

Current surface analysis shows main area of low pressure centered
over north central KS, with a secondary low weakening over
southeast WY. Frontal boundary extends from the KS low into WY,
with colder air slowly pushing south across the forecast area.
Upper level analysis shows large trough axis centered over the
Rockies now, with weakening 500 mb low over south central MT. A
new midlevel low is starting to develop much further south within
the trough across CO and will move east across the Central Plains
later today and tonight. Winds are mostly northeasterly right now,
helping to bring colder air in, as well as abundant moisture.
Shortwave energy on the east side of the trough is bringing pcpn
to far northeast WY and much of western SD at this time. Rain has
changed to snow across northwest SD, the Black Hills, the higher
elevations of northeast WY, and portions of the west central SD
plains, including the Rapid City area in the last couple of hours.
Temps range from around 30 across far northwest SD and the higher
Black Hills to the lower 40s in the Winner area. Winds have
started to increase a bit across the area, but will increase in
earnest later this morning as winds shift to a northerly
direction.

Forecast is still on track, with only minor adjustments for the most
part. No changes to headlines. The 00z runs still show relatively
good agreement on main features with the system over the next 12 to
24 hours. There was a slight adjustment to the west with the more
significant pcpn and snowfall today, especially with the EC and NAM,
as the low is projected to be a bit slower in crossing the Central
Plains. This change along with a bit quicker changeover across parts
of western SD has resulted in a bump up of snowfall totals for much
of the Black Hills eastward into the adjacent plains. Winds were
also lowered slightly in most areas later today into tonight, as the
surface low takes a slightly more southerly track. It will still be
quite windy across much of the southwest and south central SD plains
today, with the strongest winds generally over south central SD this
afternoon and evening.

The change over to snow will continue to progress south and east
across the forecast area through the morning hours, with south
central SD changing over to snow by midday or so. A bit of freezing
rain and sleet could also mix in across parts of south central SD
before changing over completely by midday. The heaviest snowfall
this morning will generally be across the Black Hills area and
southwest SD, gradually transitioning to south central SD in the
afternoon. The northern and eastern slopes of the Black Hills, the
foothills areas from Spearfish to Rapid City, and the adjacent
plains will likely see some upslope enhanced snowfall later this
morning, with some heavy snowfall at times. Winds will increase by
late morning, with gusts of 40 to 50 mph on the SD plains east of
the Black Hills, producing blizzard conditions for much of the day
and into tonight. Travel will be very difficult to impossible across
these areas, with some power outages likely as well.

Snow will taper off from northwest to southeast across much of the
forecast area late today and tonight, along with decreasing winds,
as the now stacked low pressure system moves into the east-central
plains. The exception will be across south central SD, where the
heaviest snow will continue tonight, tapering off Saturday morning.
Snowfall totals will range from up to a few inches across a good
portion of northeast WY to 1 to 2 feet east and southeast of the
Black Hills into south central SD. Much of the area from the
northern Black Hills and Spearfish to the Rapid City area is
expected to receive 9 to 14 inches.

The weather for the rest of the forecast period looks relatively dry
as ridge develops behind exiting system for the rest of the weekend
and the first part of next week. Temperatures will likely struggle
to warm in most areas with the new snow cover, with northeast WY
likely seeing the warmest temperatures in the near term. A shortwave
disturbance is progged to move quickly across the Northern Plains
Tuesday with a chance for rain and snow showers, along with gusty
winds.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Friday Night)
Issued At 1118 PM MDT Thu Apr 12 2018

A strong storm system will bring rain changing to snow tonight
into Friday morning. The result will be deteriorating conditions
with widespread IFR/LIFR conditions Friday. Wind gusts above 40kts
will occur on the SD plains causing blizzard conditions and VLIFR
VSBYS.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until 6 PM MDT this evening for SDZ001-002-
     012-024-025-028-029-072.

     Blizzard Warning from 9 AM MDT /10 AM CDT/ this morning to noon
     MDT /1 PM CDT/ Saturday for SDZ044-046-047-049.

     Winter Storm Warning until 9 PM MDT this evening for SDZ027-041.

     Blizzard Warning until 6 AM MDT Saturday for SDZ013-014-026-
     030>032-042-043-073-074.

WY...Winter Storm Warning until 6 PM MDT this evening for WYZ056-057-
     071.

     Winter Weather Advisory until 6 PM MDT this evening for WYZ054-
     055-058.

&&

$$

DISCUSSION...26
AVIATION...Helgeson



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