Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 121033
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
433 AM MDT Thu Apr 12 2018

.DISCUSSION...(Today Through Wednesday)
Issued at 433 AM MDT Thu Apr 12 2018

Current surface analysis shows developing low pressure over
western WY, with a frontal boundary extending east-southeast into
the central Plains. Upper level analysis shows weakening ridge
over the plains, with large scale trough digging southeast across
the Pacific NW. Strong jet extends around the base of the trough
and into the northern Rockies and Plains. Clouds have increased
across much of the area early this morning, with jet energy
helping to produce mostly light rain showers from parts of
northeast Wyoming into the Black Hills. Temps are mild again this
morning, mostly in the upper 30s to mid 40s. Winds are mostly
light and variable.

Strengthening upper level system to the west will push across the
Rockies tonight and the Plains Friday into Saturday, bringing a
significant snow storm to much of MT, a good portion of the Dakotas,
as well as northern/eastern Nebraska and eventually the upper
midwest. Models in good agreement on main features of the system
crossing the region, but still some minor differences on track,
timing, and coverage of heavier pcpn/snowfall. Blend of models is
being used, with some adjustments made based on more consistent
ECMWF.

Shortwave energy in southwest flow ahead of the main system
will bring chances for rain showers today, along with a few
afternoon and evening thunderstorms, as east-southeast winds
increase to the north of strengthening low over WY that will move
southeast into southern NE/northern KS tonight. CAPE values will be
modest from northeast WY to southwest SD later today, so no strong
storms are expected. Highs will range from the 40s across northwest
SD to the 60s over parts of northeast WY and southwest SD.

Colder air will start to push south through the area tonight and
Friday morning as winds turn to the northeast and eventually north
with low pressure passing to the south. Main mid-level low moving
across MT today will bring increasing chances for pcpn this evening
across northwest SD into far northeast WY and the rest of western SD
overnight. The low will then track southeast across the area and
into the Central Plains later tonight and Friday. Rain will change
to snow from north to south late this evening through Friday morning
as temperatures fall. Pcpn should change over to all snow across
south central SD by midday. Models remain in good agreement on the
heaviest snowfall being across areas south and east of the Black
Hills Friday into Friday night, where 9 to 18 inches are likely. A
secondary maxima is likely across the northern Black Hills and
foothills as well, as a period of upslope enhanced snowfall develops
Friday morning. The strongest winds will be across portions of west
central to south central SD, where some gusts to around 50 mph are
expected Friday and Friday evening. Winter storm watches have been
replaced with a blizzard warning across much of the west central and
southwest SD plains and a winter storm warning for a good portion of
northwest SD, the Black Hills and foothills, as well as far
northeast WY. Went with a winter wx advisory for the rest of
northeast WY. Will keep the watch for south central SD for now, but
if trends hold it will likely be replaced with a blizzard warning
later today.

Conditions will improve gradually from northwest to southeast late
Friday and Friday night, with main snowfall ending across south
central SD by Saturday morning. Ridge behind exiting system will
bring drier weather for the weekend into early next week, but a slow
warmup is likely due to snowcover. Temperatures will hopefully be
back to near average levels in many areas Monday. The next system
will bring chances for rain and snow showers Tuesday.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1121 PM MDT Wed Apr 11 2018

Ahead of an upcoming storm system, chances for precipitation will
increase late tonight and persist into Thursday. Areas MVFR CIGS
will develop under the best -SHRA (-SN up high) mainly across far
northeast WY into northwest SD. Gusty east-southeast winds will
develop Thursday morning and persist into Thursday afternoon
which will feed moisture and create possible -TSRA late. Colder
air will push into the region Thursday night as the storm wraps
and and conditions deteriorate to IFR/LIFR due to snow and low
CIGS.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning from midnight tonight to 6 PM MDT Friday
     for SDZ001-002-012-024-028-029.

     Winter Storm Watch from Friday morning through late Friday night
     for SDZ044-046-047-049.

     Winter Storm Warning from 3 AM to 9 PM MDT Friday for SDZ025>027-
     041-072-074.

     Blizzard Warning from 6 AM Friday to 6 AM MDT Saturday for
     SDZ013-014-030>032-042-043-073.

WY...Winter Storm Warning from midnight tonight to 6 PM MDT Friday
     for WYZ056-057-071.

     Winter Weather Advisory from midnight tonight to 6 PM MDT Friday
     for WYZ054-055-058.

&&

$$

DISCUSSION...26
AVIATION...Helgeson



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