Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 221729

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1029 AM MST Thu Feb 22 2018

.DISCUSSION...(Today Through Wednesday)
Issued at 342 AM MST Thu Feb 22 2018

Current surface analysis shows high pressure east of the region,
with low pressure across southern CO. Upper level analysis shows
large scale trough digging southeast across the western states,
with southwest flow ahead of the trough across the Rockies and
Plains. Shortwave is noted in the flow over CO, moving quickly to
the northeast. Skies are mainly clear across much of the area,
with some clouds clipping south central SD. A wide range of temps
across the area as usual this morning, ranging from the teens
below zero in the valleys of northwest SD to the teens above zero
over far southern SD where there is some wind to mix out the low
levels. Winds are light in most areas except for far southern SD
where southerly jet is producing winds of 10 to 20 mph.

Mainly quiet weather is expected for a good portion of the forecast
area in the near term, with the exception being across south central
SD. That area will see a good chance for snowfall from a couple of
systems over the next few days. The first disturbance over CO will
move quickly northeast across NE and into central and eastern
portions of SD during the day. Short range models in good agreement
on track and timing of snowfall, with just some minor differences
noted. Snow is expected to develop over much of south central SD
late this morning/midday then continue through the afternoon before
tapering and ending early in the evening. The snow could be moderate
to briefly heavy this afternoon, especially across Tripp Co.
Expecting 1 to 3 inches of snowfall for many areas east of a line
from Philip to Martin, highest amounts generally into south central
SD. Could see some amounts near 4 inches in Tripp County so have
decided to issue a winter wx advisory for there. Further west to the
Black Hills, chances for snow taper off considerably, so will keep
forecast dry for the Black Hills area, though a few flurries are
certainly possible.

The next system will be the trough over the west that will
eventually move east across the Rockies and into the Plains later
Friday through Saturday. Less confidence in the track of this system
right now, with most of the models at least trending a bit toward a
further north track across the plains, with the GFS further north
and slower than the EC. Again, the most impacted area with this
system should be south central SD, but there is potential for some
accumulating snow to develop at least a little further west and
north if models continue these trends. For now, have some lower end
pops over southern portions of northeast WY and the Black Hills on
Friday, expanding eastward into southwest and south central SD
Friday night. Will keep highest pops and some minor accumulations
limited to mostly south central SD. This system looks to be a quick
mover as well, with the bulk of snowfall across southeastern
portions of the CWA late Friday night and Saturday morning.

The rest of the forecast looks mainly dry right now. Latest models
have trended drier for at least the first half of next week as the
main storm track is now progged by the medium range models to shift
further south across the US. Will keep some low pops in across parts
of the area Tuesday and Wednesday, with the potential for weak
disturbances to pass through in the northern stream.

As far as temperatures go, chilly air will remain in place for the
end of the week, though still seeing some moderation in temperatures
each day. Highs today will mostly be in the teens to mid 20s, but
temperatures will again struggle to rise to the north and northeast
of the Black Hills. Weekend temperatures look to mostly be in the
lower 20s to lower 30s, with highs hopefully close to average levels
in most areas early next week.


.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1028 AM MST Thu Feb 22 2018

Light snow, and associated MVFR to IFR cigs/vsbys, will gradually
overspread much of the SD Plains through early afternoon. Lower
cigs may extend as far west as KRAP by late afternoon, but IFR
conditions are generally expected to remain east of the terminal.
VFR conditions should prevail at KGCC through the TAF period.


SD...Winter Weather Advisory until 9 PM CST this evening for SDZ049.



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