Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181804
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1104 AM MST Mon Dec 18 2017

.UPDATE...
Issued at 1100 AM MST Mon Dec 18 2017

High winds have ended near Clark this morning and have expired the
High Wind Warning. Gusty winds will continue through the
afternoon.

&&

.SHORT TERM...Today through Wednesday
Issued at 222 AM MST Mon Dec 18 2017

High clouds are rapidly spilling up and over the flattening ridge
marking where the ridge is still buckling a bit over the Rockies
as the trough axis disappears to the east. This puts us underneath
the right front quad dynamics for strong winds along the
foothills east of the Continental Divide and along the Southern
wind Corridor from RKS to CPR attm. In addition, there is the
potential for gap flow in the Cody Foothills, given the
temperature gradient at 700mb between YNP and the Cody Foothills.
The first bout of high wind 8 miles south of Clark occurred at
6:15 pm earlier this evening where the winds gusted to 74 mph. The
winds around Clark are increasing once again along with a gust of
83 mph now being reported 8 miles south of Clark. This potential
should persist through 15Z this morning, so will keep the high
wind warning going for this zone through then.

The winds have already been gusting to 47 mph at Outer Drive south
of Casper. Mos guidance for CPR is indicating 29 knots this
afternoon. The gradient at 700mb from northeast to southwest across
Wyoming will be decameters. With that said, it appears that the
chronically windy areas like Outer Drive could see borderline high
winds today as will the higher elevations of the Green and
Rattlesnake Range. Will handle this with an SPS.

This westerly flow will produce periods of light snowfall in the
western mtns today through Tuesday night from almost purely upslope
flow. Weak isentropic lift in the far west today will aid in light
snowfall in the far west today. Currently a stratocu deck is in
place along and west of the divide.

Then on Tuesday night, the next high amplitude trough will be making
landfall onto the Pacific coast. As the trough approaches,
increasing snowfall in the far west will ensue Wednesday. As the
ACT Pacific cold front approaches from the northwest Wednesday,
significant convective bursts of snowfall appear likely in Northwest
and Northern Cy Wed afternoon with negative lifted indices.  A mid
level circulation will likely produce locally higher amounts of snow
Wednesday night with the models suggesting the placement of this low
in Southwest Cy but placement of this type of feature that far out
is a bit premature. Snowfall will spread to areas east of the divide
Wednesday night as the Canadian cold front pushes south across
Wyoming which will eventually pull down some of the coldest air of
the season. This front will come crashing through Wednesday
evening with 9mb to 11mb 3 hourly surface pressure rises behind
the front east of the divide. Strong post frontal north winds can
be expected with this front east of the divide by Wednesday
evening. More on the prospect of this cold wave will be addressed
by the long term forecaster.

.LONG TERM...Thursday through Sunday
Issued at 222 AM MST Mon Dec 18 2017

Snow continues into the morning Thursday across Central and
Southwest Wyoming before tapering off from nw to se in the
afternoon. Very possible/likely that advisories will be needed
for this event that begins Wed aftn/eve. Cold behind this system
Thursday night but that`s just a pre-cursor to our arctic outbreak
which starts with the passage of the arctic front Friday and
continues through the weekend and likely into Christmas day and
beyond. This arctic front could have quite the punch with cyclonic
nnw flow, left exit region jet streak dynamics and some decent qg
forcing to go along with strong n wind of 25-40 mph in quite a
few areas. Winter highlights will be likely with this system
Friday into Saturday for the snow, bitter cold air, strong wind
and bitter cold wind chills. Snowfall amounts east of the divide
will likely easily be in the advisory range (2-4, 3-6) with the
ratios possibly playing a big factor on final totals. By Saturday,
models are still generally showing H7 temps dropping into the -24
to -28C range for most areas with H5 temps -37 to -40C. General
trend in the past 24 hours is a continued slight wwd shift of the
overall pattern which continues to favor more snow than in a nly
flow with less dynamics. This arctic system will have more
potential than your `average` arctic front. The GEM continues to
be the most aggressive with this pattern trying to close off a mid
level low over sern ID/nrn UT late Friday into Saturday but even
the Euro has a hint of this. Will be worth watching with time as
this system will likely have a high impact due to the bitter cold,
snow and wind! There will likely be one more reinforcing of
bitter arctic air that arrives Christmas night into next Tuesday
as the Hudson Bay low moves swwd to near the U.S./MN border area
with another shortwave and strong upper high moving swd over us.
The 00z GFS is the anomaly here with a trough undercutting this
ridge from the west. This scenario seems like a low likelihood as
the mean ridge holds out near 130W. Time to prepare now for
another bout of snow at the end of the short range period into
Thursday leading to our arctic blast Friday into early next week
with more significant snow Friday into Saturday. Highs may
struggle into the single digits behind this for many areas over
the weekend with lows well below zero.

&&

.AVIATION...For the 18Z TAFS through 18Z Tuesday afternoon
Issued at 1100 AM MST Mon Dec 18 2017

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

Marginal MVFR conditions KJAC and KPNA with marginal LLWS at KJAC
will continue this afternoon. The LLWS is expected to subside this
evening, while stratus decks into low VFR/MVFR categories,
areas of -SN returning tonight. SCT-BKN clouds FL120-200 will be
in place from KJAC southward after 12Z Tuesday.

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

VFR conditions will prevail across the area through 12z Tuesday with
mainly broken high clouds. The potential for mountain wave
activity vicinity KCOD has decreased, but LLWS will persist through
the period. Surface southwest winds 20-30g40kts will occur
vicinity 50SM NE KRKS-KCPR through around sunset today.

&&

.FIRE WEATHER...
Issued AT 222 AM MST Mon Dec 18 2017

A weak high pressure ridge over Idaho and Montana will move east
of the western and central Wyoming with west northwest flow
increasing behind the ridge. Strong winds will increase over and
near the east slopes of the mountains with a high wind threat in
the Cody Foothills today. Winds have already gusted to 74 mph 8
miles south of Clark earlier this evening. Other areas will have
strong winds and local high winds will develop through the wind
corridor between Rock Springs and Casper. Non windy areas with
fresh snow cover will be cold with potential fog this morning
along the rivers and unfrozen lakes. Pacific moisture will
gradually increase in the flow through mid week keeping clouds and
areas of mountain snow with rather light accumulations each day.
Wind will gradually shift to the southwest tonight and Tuesday
with warming temperatures and increasingly gusty winds through
Wednesday. The colder air of the season is likely to occur toward
the end of the week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...LaVoie
FIRE WEATHER...Lipson


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