Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 161058
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
358 AM MST Fri Feb 16 2018

.DISCUSSION...(Today Through Thursday)
Issued at 326 AM MST Fri Feb 16 2018

Current surface analysis shows high pressure centered over
central SD, with warm front to the west, stretching from central
MT to eastern WY. Upper level analysis shows shortwave trough to
the south and east of the area, with northwesterly flow in place
from the Pacific NW into the Northern Plains. Skies are now clear
across the area. Winds are light and variable in most areas,
though west-southwest winds have developed behind the high across
northeast WY to parts of northwest SD, with speeds of 10 to 15 mph
in some areas. Temps in most areas right now are a few degrees
either side of zero. However, milder air is moving into northeast
WY and the Black Hills, where some teens are noted in a few areas.
Wind chill values across northwest SD range from 10 below to 20
below zero.

Milder air to the west will continue to push into the area this
morning and afternoon as the warm front moves east across the
Dakotas, with highs rebounding into the 30s in most areas. Some
lower 40s are likely along the northern and eastern slopes of the
Black Hills as west to southwest winds persist. Winds will become
breezy in some areas by the afternoon, especially the higher
elevations of northeast WY to northwest SD. Will see some increasing
clouds in the afternoon and evening from the north and west as a
weak shortwave disturbance and cold front cross the northern Rockies
and northern high plains. This disturbance will quickly cross the
area later tonight into Saturday morning, bringing areas of light
snow. The best chances for a light accumulation, mostly an inch or
less, will be across far northeast WY, the northern Black Hills, and
northwest SD. The front is forecast to stall over the area Saturday
into Saturday night. A wider range of temperatures is expected on
Saturday, with highs ranging from the lower 20s across far northwest
SD to the mid to upper 30s across southern parts of northeast WY to
southern SD.

Later Saturday night into early next week will bring unsettled and
much colder weather as a large scale upper trough digs southeast out
of western Canada and moves across the Rockies and Plains. A series
of disturbances within the trough will bring snow at times, with the
strongest of these disturbances possibly bringing significant
snowfall to portions of northeast WY and western SD later Sunday
into Monday. Medium range models differ some on the track of these
disturbances, so confidence is becoming increasingly higher on
significant snowfall somewhere across the central/southern forecast
area, but confidence remains fairly low on exactly where at this
point.

The first of these disturbances looks like it will mostly pass north
of the area later Saturday night into Sunday morning. However, the
latest GFS has the southern edge of the main snowfall clipping areas
along the ND border late Saturday night/early Sunday, so a light
snow accumulation is possible there, along with the possibility of
some mixed pcpn. This disturbance will help to push the arctic air
back south across the area Sunday and Sunday night. The stronger
disturbance within the trough will then overrun the arctic air at
the surface and tightening baroclinic zone across the area later
Sunday and Sunday night. Relatively high snow/liquid ratios are
expected Sunday night into Monday with cold air in place, 20-25 to
1. A heavier band or two of snowfall could certainly develop as
well. Right now, the latest models favor southern parts of northeast
WY, the Black Hills, and much of southwest into central SD with the
higher snowfall potential, several inches or more, with most models
showing a band of 6 to 12 inch potential somewhere across these
areas. Will hold off on winter storm watches for now as this system
is still 2.5 to 3 days out, but if latest trends hold with todays
model runs, at least portions of the forecast area will eventually
need one. This disturbance will move east of the area during the day
Monday, but another disturbance is likely to impact portions of
southwest into south central SD later Monday into Monday night with
additional accumulations likely there. The trough is then forecast
to weaken and moves south and east of the region sometime on
Tuesday, though the EC is slower with the exit of the trough.

Arctic air will remain entrenched across the area for the first half
of the week, with temperatures well below average through at least
Tuesday and likely Wednesday as well. Mainly dry weather is expected
for the rest of the week, with hopefully a warming trend to at least
near average temperatures by the end of the week.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 357 AM MST Fri Feb 16 2018

VFR conditions will dominate most places through the period. MVFR
CIGS might sneak into far northwest SD tonight as weak storm
system brings chances for light snow.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...26
AVIATION...Helgeson



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