Area Forecast Discussion
Issued by NWS Riverton, WY

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763
FXUS65 KRIW 151746
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1045 AM MST Fri Dec 15 2017

.UPDATE...
Issued at 558 AM MST Fri Dec 15 2017

A site about 8 miles south of Clark along highway 120 measured
sustained winds of 50 mph with a gust of 64 mph. High resolution
models show the potential of these winds continuing at times
through much of today. These high winds should shut off pretty
quickly as cold advection arrives this evening. Thus have issued a
high wind warning for the Cody Foothills through 6 PM today.

Updated grids and other products have been sent.

&&

.SHORT TERM...(Today through Sunday night)
Issued at 150 AM MST Fri Dec 15 2017

Today will feature the approach of a flattening ridge along with
a developing surface lee trough. The result will be increasing
winds along the southern wind corridor from RKS to CPR and over
the higher elevations as the gradient increases. 30 knot sustained
winds are showing up in the MET MOS guidance for CPR for this
afternoon but the gradient and forecast wind speeds at 700mb are
not impressive. Then tonight, the trough axis now off the Pacific
coast will approach the area, and as it does it will begin to
split. Snowfall will break out in the northwest portion of the
state by this evening and will then spread southeast across the
rest of the CWA. By Saturday, the southern stream energy will
split all the way down to the Desert Southwest and will pull cold
air down over Wyoming, resulting in frontogenesis resulting in an
upslope flow east of the divide. As this trough splits, a vort max
will track south across Idaho which is a favorable pattern for
Lander to receive snowfall. With all that said and considering
that this is a splitting system, for now it appears that this will
be a low end advisory event for much of the CWA tonight through
Saturday before tapering off Saturday night as the snow exits off
to the southeast. The focus of the most significant snowfall from
west to east of the divide will occur around 12Z Saturday for the
reasons stated above. The Upper Green River Basin will be in the
snow shadow given the predicted northerly downslope winds after
the post frontal winds slide down the west side of the Wind River
Mtns. Will continue to handle this weather event with an SPS for
now. Another flat ridge approaches on Sunday along with an
embedded shortwave which will increase the gradient once again for
a potential for an elevated wind event for Sunday night.

.LONG TERM...(Monday through Friday)
Issued at 150 AM MST Fri Dec 15 2017

On Monday, strong northwest flow is expected across the forecast
area. Enough moisture in the flow for snow showers over the west
and north, but snowfall amounts should be on the light side as the
forecast area is on the anti-cyclonic side of jet, and the main
energy with any fast moving shortwave remaining north of the
forecast area. Could also see near high wind in the Cody Foothills
Monday morning with help of a decent temp gradient from
Yellowstone to Cody Foothills, and the right exit region of an
upper jet pulling across the region.

Some ridging is expected to develop Tuesday as the flow buckles
ahead of a strong trough/upper low pushing into the West Coast.
This should result in mainly dry conditions, but still enough of
moisture for slight chances of snow across the extreme
northwest/northern mountains.

Global models are a tad faster with the aforementioned trough and
associated cold front pulling across the Rockies in the
Wednesday/Thursday time frame. The strong cold front is expected
pull across much of the area Wednesday afternoon and evening which
is about 6 to 12 hours faster than 00Z runs yesterday. The best
dynamics are the forecast area appear to be Wednesday and
Wednesday night with decreasing forcing as the trough axis pulls
into the Plains on Thursday. Have increased PoPs Wednesday, with
the overall best chance of snowfall occurring Wednesday night in
most locations. Have lingered PoPs into Thursday in case trough is
bit slower than advertised currently. High temperatures will be
tricky on Wednesday due to timing of cold front. However highs
look to be around 10 degrees cooler over the west, and 15 to 20
degrees colder east of the Divide Thursday in wake of the cold
front. Highs on Thursday could struggle to get into the 20s. Even
colder air could arrive by next weekend.

&&

.AVIATION...For the 18Z TAFS through 18Z Saturday
Issued at 1045 AM MST Fri Dec 15 2017

IFR conditions at KJAC are quickly scattering out so VFR should be
predominant across the area the rest of the day through late
tonight. Winds will continue to be gusty at KCOD/KRKS/KCPR until
early evening. The next frontal system will enter the area
overnight, bringing snow showers and MVFR/IFR conditions across the
area. The Western Mountains and Valleys will start around 08Z
Saturday morning, including KJAC, and spread east and south.  By 14Z
all terminals will be in MVFR to IFR conditions lasting the rest of
the period.  West of the divide, accumulations will be around 1-1.5
inch with the exception of KJAC which looks like 2-3 inches are
likely. East of the divide, accumulations will be 2-4 inches with
KCPR seeing 4-5 inches.

&&

.FIRE WEATHER...
Issued AT 150 AM MST Fri Dec 15 2017

Today will be dry with areas of valley fog mainly in the west.
Clouds will increase in the northwest Friday afternoon with strong
winds along the east slopes of the Absaroka mountains and Cody
foothills and brisk winds along the southern wind corridor from R.
Rock Springs Springs to Casper. A Pacific low pressure trough
will move southeast from British Columbia and the Pacific
Northwest bringing snow and colder temperatures to the western
Mountains and valleys to night with snow becoming widespread by
late evening. The heaviest snowfall will be in west central and
northwest areas especially before midnight with lighter amounts to
the south in Lincoln and Sublette counties. Snowfall will spread
east of the Continental Divide late tonight, becoming more
widespread by Saturday morning. Snowfall will continue in the west
Saturday and spread into central and south areas east of the
Continental Divide with the heaviest snowfall in Natrona and
Fremont counties, tapering off in the west Saturday evening and in
the east Sunday morning when colder Canadian air moves in with
northerly winds.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM MST this evening for WYZ003.

&&

$$

UPDATE...Murrell
SHORT TERM...Lipson
LONG TERM...Murrell
AVIATION...Hulme
FIRE WEATHER...Lipson



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