Area forecast discussion
National Weather Service Riverton Wyoming
323 am MST Monday Dec 9 2013
Short term...today through Wednesday night
an Alberta clipper is rapidly sweeping southeast across the County Warning Area. The
thickest clouds have already exited to extreme southeast Sweetwater
County. The light snowfall is so fine from this feature (albeit
grainy high density snow) that there is quite the lag time for this
snowfall to reach the ground. It is still snowing at lnd/riw in
spite of the fact that the sky is mostly clear. Temperatures should take a
nose dive once clearing becomes established well before sunrise this
Yet another Alberta clipper is already diving south across
British Columbia/Alberta at this time and will rapidly track across the area late this
afternoon/early evening with most of the vorticity energy remaining off
to our northeast over the northern plains. This will translate into
light snowfall over the northern mountains this afternoon through
tonight. Then another in a series of Alberta clippers will follow
behind it and track across the region about 24 hours later...with
the axis tracking through around 00z Wednesday. This next clipper
will be less well defined but broader resulting in slightly more
significant snowfall in the mountains...especially on Tuesday.
Snowfall will end Tuesday night.
Temperatures will remain below normal through Atlas Monday. Then
on Tuesday...temperatures will warm up at the higher elevations and in
areas that experience southwest winds from rks to cpr.
Basins/valleys will likely remain trapped for at least a couple of
more days through Wednesday. A snow free day is expected in the mountains
as flattening high pressure moves into the area Wednesday.
As for the wind...this afternoon and night appear to be the best
time frames for stronger borderline high winds. Met MOS guidance
still insists on 30 to 31 knot sustained winds between 21z this
afternoon and 00z this evening for rks as well as 31 knots for
Casper 15z Tuesday. In addition...a 16mb to 18mb surface pressure
gradient between gcc and rks...6 height lines spaced every 3
decameters over Wyoming prognosticated by the NAM...as well as the right front
quadrant of the 115 knot jet maximum overhead all build a good argument for
wind along the southern wind corridor and along The East Slope and
foothills of the Continental Divide this afternoon and night. Will
issue an Special Weather Statement rather than issue a hilite for now.
Long term...Thursday through Monday
models remain reasonably consistent in the medium range period
showing a Flat Ridge on Thursday followed by a weak splitting trough
on Friday. The weekend starts with a Flat Ridge which turns into a
fast northwest flow with embedded disturbances for Sunday and Monday.
Flow increases late Thursday or Thursday night as the ridge axis
flattens and shifts east of US. Increased mountain top flow and Lee
troughing will likely lead to increase SW flow in the wind corridor
and Cody foothills. A weak splitting trough is still forecast to
move through late Friday with a quick shot light snow to mountains
and possibly a few flurries elsewhere. Another Flat Ridge on
Saturday before a fast northwest flow develops. Strong jet moves across
Montana Saturday night and Sunday for the possibility of strong
winds in the Cody foothills and higher mountains of the north.
Temperatures may also warm Saturday night with some wind potential.
Flow may be tight enough for at least partially mix out the basins
Sunday and Monday. Probably not fully but with some weak cold
advection at times and fairly strong northwest flow...we at least have a
shot to see some mixing.
West of The Divide...kjac-kbpi-kpna-krks routes.
Ceilings will gradually lower from north-S during the day today with
areas of MVFR/IFR will spread into the northwest mountains to vicinity
kjac through this afternoon. IFR/MVFR -sn is expected to prevail
from Yellowstone National Park through vicinity kjac-kafo by this evening
and continue through the overnight. These conditions are expected
to be more localized further south from vicinity kbpi-krks. West wind
will increase along the Interstate 80 corridor this morning with
25-35kt winds common by 18z today along with areas of MVFR blsn.
East of The Divide...kcod-kcpr-klnd-kriw-kwrl routes
Winds will increase along the east slopes of The Divide this morning
with areas of low level wind shear developing vicinity klnd-kcod and continuing
through tonight. Winds will mix down to the surface vicinity krks-
kcpr with 25-35kt winds common this afternoon along with areas of
MVFR blsn. An upper level disturbance will spread local areas of
MVFR/IFR -sn primarily vicinity and north of Interstate 90 late
this afternoon and evening.
30 to 40 miles per hour winds with gusts to 50 miles per hour are expected from Rock
Springs to Casper and along The East Slope and adjacent foothills of
The Divide this afternoon and night. Temperatures will recover to
milder readings at the higher elevations and along the southern wind
corridor from Rock Springs to Casper Tuesday and at the
basins/valleys by Thursday. Light snowfall is expected in the
mountains through Tuesday.