Area Forecast Discussion
Issued by NWS Riverton, WY

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372
FXUS65 KRIW 150848
AFDRIW

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

.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 12Z TAFS through 12Z Saturday morning)
Issued at 150 AM MST Fri Dec 15 2017

West of the Continental Divide...KJAC, KPNA, KBPI, and KRKS

Areas of fog spreading into northern Grand Teton National Park from
eastern Idaho will possibly advance south to vicinity KJAC by
sunrise. Fog will also likely continue to expand across the upper
Green River Basin, occasionally affecting KBPI and KPNA through the
morning. VFR conditions will then prevail through Saturday evening
with increasing mid and high clouds.  A cold front will rapidly
spread increasing snow, mountain obscurations, and widespread
MVFR/IFR conditions NW to SE across the area 06z-12z Saturday.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

VFR conditions will prevail across the area through 06z Saturday.
Surface southwest winds will increase to 25-35G40-45kts vicinity
50SM NE KRKS-KCPR 16z-19z Friday, while areas of LLWS spread down
the east slopes of the Divide, vicinity KCOD-KDUB-KLND.  A cold
front will plunge south to vicinity KCOD-KBYG line around 06z
Saturday and to a vicinity KRIW-KCPR line by 12z Saturday.  Areas of
MVFR-IFR vsbys and cigs in -SN will spread in behind the front, most
higher mountains will be obscured by 12z Saturday.

&&

.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...NONE.

&&

$$

SHORT TERM...Lipson
LONG TERM...Murrell
AVIATION...AEM
FIRE WEATHER...Lipson



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