Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 141800
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1059 AM MST Thu Dec 14 2017

.SHORT TERM...(Today through Saturday night)
Issued at 250 AM MST Thu Dec 14 2017

Imagery continues to show a large scale amplified ridge/broad trof
pattern (west/east) in place. However, the ridge continues to slowly
migrate wwd so the that the trof now encompasses about 80 percent of
the ern CONUS with the ridge now only influencing the w Coast. An
embedded shortwave is currently crossing WY with its axis from
ecntrl through swrn WY. The SFC, however, continues with strong high
pressure still entrenched over most of the wrn/nwrn CONUS and
centered over WA/OR/nrn ID. Most of the snow that the FA experienced
earlier has moved through/out of the region with only a few light
lingering areas of light snow/flurries falling across cntrl/ern/sern
WY this morning.

This morning, the upper shortwave will continue out of the area with
only a few isolated lingering areas of light snow/flurries scattered
about east of the Divide. A fast moving clipper front will move
into/through ern WY today with some increased gusty northwest wind
continuing into the afternoon, especially across Johnson County.
Later tonight and Friday, the wrn CONUS ridge begins to flatten as a
stronger shortwave trof moves through the ridge across BC/WA while
the ridge retreats further south and west, out over the EPAC. This
will aid in removing the n/nw flow over WY that has been so
persistent recently, (re)setting a new flow/storm trajectory from
the w/nw through the PAC NW and across WY by Friday evening. The SFC
pattern will develop a more typical winter type pattern with high
pressure west/lower pressure east/lee side and a tightening P/T Grad
between the two areas. Gusty west/southwest winds will then begin to
increase by Friday morning with localized High Wind conditions
possible Friday afternoon and evening over the northern Cody
Foothills as flow orthogonal to the Absaroka Mountains develops
while lee cyclogenesis proceeds further southward across ern CO.
With mountain ridge top winds generally blowing 35 to 45 kts and the
stronger inversion laying (only) some 2.5 to 3.5k ft AGL, this looks
to be an isolated high wind maker mainly for the Clark area...so not
planning on an NPW/HWW at this time as an SPS should be sufficient.

Then Friday night, the upstream shortwave trof mentioned above along
with a broader area of EPAC moisture will quickly move into western
WY, digging south as it does (slowly down the ewd progression in the
process). An associated cold front will also move into/through nrn
WY during the same time frame...increasing SFC winds ahead of the
front across the Wind Corridor. Snowfall will first begin out west
by midnight with the Teton/Gros Ventre/wrn Wind River/srn Absaroka
Mountains receiving the heavier amounts through Saturday morning.
East of the Divide, as the upper trof axis moves close to the ID/WY
border Saturday morning and the upper jet orientation moves to a
favorable position, a rather widespread area of snowfall can be
expected to breakout along and behind the front, continuing (at
least) through the evening period as the trof continues to dig well
to the south towards the area previously occupied by a subtropical
upper low and jet. Currently, the overall speed of the system
combining with decent winter moisture (PWs ranging from 0.35 to 0.45
inch) should give areas of Advisory level snowfall across most of
the FA except for the nrn Big Horn Basin and the lower elevations of
swrn WY. If any of the storm`s processes slow up its overall
progression, however, warning amounts would certainly be possible
across some lower elevations east of the Divide where low level
convergence and upper level divergence/forcing (and some mesoscale
banding) occurs superimposed (central WY e.g.). Still plenty of time
to evaluate.

.LONG TERM...(Sunday through Thursday)
Issued at 250 AM MST Thu Dec 14 2017

There could be some ongoing snow early Sunday morning from the
storm system Saturday, but any snowfall should quickly come to an
end Sunday morning. Otherwise a mainly dry day with seasonable
cold temperatures, but colder than what has been occurring. A
strong west northwest flow is expected Monday and Tuesday with a
couple of quick moving systems in the flow possibly giving the
northwest/northern mountains some snowfall at times. This activity
could spill into the adjacent lower elevations at times, but will
continue to keep those areas dry for now. Temperatures will be
above average in the mountains and mixed areas, but trapped basins
will remain near seasonal cold.

Global models in decent agreement with a strong upper low/trough
pushing into the West Coast Tuesday night. This trough is then
progged to push east into the Intermountain West/Rockies Wednesday
and Thursday. Models are showing a relatively mild and windy day
Wednesday with a strong cold front in the Wednesday night/Thursday
time frame with chances of snow across the entire area. This looks
to bring the coldest air this season. However looks like some
truly Arctic air slides south into the area next weekend with
highs possibly next getting out of the single digits. However this
is still over a week out.

&&

.AVIATION...For the 18Z TAFS through 18Z Friday
Issued at 1059 AM MST Thu Dec 14 2017

IFR conditions are hanging around for a couple more hours at KPNA,
and MVFR conditions are likely to persist at KRKS until late
afternoon.  Everywhere else is looking at VFR conditions for the
rest of today through early morning Friday when patchy fog is once
again possible.  I am not completely sold for fog at airfields yet
as models indicating the saturation layers are very shallow, so
vicinity fog is the current forecast. Winds will also be gusty
Friday morning at KCOD/KCPR/KRKS ahead of the next weather system.
See TAFs for further details.

&&

.FIRE WEATHER...
Issued AT 250 AM MST Thu Dec 14 2017

Fuels remain at critical levels across over portions of Fremont,
Johnson and Natrona Counties.

Seasonally cold and more moist today with only a few areas of
lingering light snow or flurries this morning east of the Divide.
Winds will be light across most of the forecast area except for
gusty north/northwest winds over portions of Johnson County today.
Good chances for significant precipitation/snowfall will arrive late
Friday night and continue through Saturday evening for most of
Wyoming. Smoke dispersion will be poor west of the Divide but fair
to good east of the Divide through early afternoon...becoming poor
all locations for an extended period beginning late this afternoon.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Hulme
FIRE WEATHER...Braun


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