Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1008 am MST sun Dec 28 2014
Short term...today through Monday night
issued at 330 am MST sun Dec 28 2014
Current surface analysis shows Arctic front from northern
Minnesota...through ND...and into northeast Montana...sliding slowly
southward. Upper level analysis shows low pressure anchoring
itself over southern Hudson Bay...with large scale trough stretching
southward across the central Continental U.S.. strong shortwave energy noted
on water vapor imagery is moving southeastward into the northern
rockies...with a very strong jet digging south and west of the
trough. Skies are clear to partly cloudy right now...but clouds are
beginning to increase across western and northern portions of the
County Warning Area. Temperatures are mostly in the teens across the forecast area with
Snow and Arctic air are on the way for later today into the first
part of the week. Brunt of Arctic air will hang around through
Tuesday night. Widespread snowfall is expected tonight...with 2 to 4
inches of accumulation across much of the area by Monday morning.
00z models are in pretty good agreement...with some minor
differences on timing and track of more persistent snowfall tonight
into early Monday. Confidence is high enough that a Winter Weather
Advisory has been issued for much of the County Warning Area from late today through
early Monday. The rest of the County Warning Area could be added later today if
there are some adjustments to the track of the system.
For today...the Arctic front will slide south and west across the
area during the morning and early afternoon hours. High temperatures
will likely be reached during the late morning and midday
hours...upper teens and 20s...with temperatures starting to fall in most
areas during the afternoon. Some light snow/flurries are possible
with the frontal passage...but the bulk of snowfall will accompany
the shortwave as it moves into the area later this afternoon and
evening. Most of the snowfall should fall at a lighter intensity as
the best frontogenesis and dynamics are to the west of the forecast
area tonight and available moisture will be minimized in the cold
air. However...snowfall ratios of 20-25:1 will produce a fluffy
snowfall that should accumulate rather easily. Snowfall will be
widespread across the County Warning Area tonight...with snow beginning to
taper off across northern portions of the County Warning Area late tonight.
Depending on the exact timing of the system...northern portions of
the advisory area could south-southeast snowfall tapering off by daybreak. Winds
will remain on the lighter side with this system...but some gusts to
20 miles per hour on the plains could produce patchy blowing snow tonight.
Temperatures will drop to near or below zero by early Monday...with wind
chill values zero to 20 below zero in the morning.
Snow will continue to taper off from northeast to southwest across
the County Warning Area on Monday. Little additional accumulation is expected from
late morning through the afternoon. Highs will range from zero to 10
above...coldest across northwest South Dakota. Wind chills will remain below
zero in most areas during the day. Patchy light snow and flurries
are possible into Monday night over northeast Wyoming and far southwest
South Dakota...but lingering snowfall should move out late Monday night. Some
breaks in the clouds are expected overnight. With fresh snowcover...
temperatures should drop to 5 below to 15 below zero. Winds up to 10 miles per hour
overnight should produce wind chills of 20 below to 30 below across
much of the plains. A Wind Chill Advisory will likely be needed for
a good portion of the County Warning Area later Monday night into Tuesday morning.
Long term...Tuesday through Saturday
issued at 330 am MST sun Dec 28 2014
Very cold on Tuesday as 1055mb high settles in across the area.
Even with relatively light winds Tuesday morning...wind chill
values will be -15f to -30f. Slow moderation in temperatures
Wednesday/Thursday. Reinforcing shot of Arctic air arrives Friday
with another round of light snow and very cold temperatures for
Aviation...(for the 18z tafs through 18z monday)
issued at 1005 am MST sun Dec 28 2014
Widespread snow will develop by middle afternoon across northestern Wyoming and northwestern
South Dakota...with the snow spreading quickly south/east this evening and
continuing tonight. Widespread IFR/MVFR ceilings and IFR/LIFR visibility
expected...with the worst conditions from northestern Wyoming through southwestern South Dakota.
Conditions will slowly improve from north to south Monday morning.
South Dakota...Winter Weather Advisory until 11 am MST Monday for sdz012-013-
Winter Weather Advisory from 6 PM MST /7 PM CST/ this evening to
11 am MST /noon CST/ Monday for sdz027-030-032-041>044-046-
Wyoming...Winter Weather Advisory until 11 am MST Monday for wyz054>058-