Area Forecast Discussion
Issued by NWS Riverton, WY

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204
FXUS65 KRIW 211724
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1024 AM MST Wed Feb 21 2018

.SHORT TERM...(Today through Friday night)
Issued at 126 AM MST Wed Feb 21 2018

Imagery continues to show a large deep trough across most of the
CONUS with some amplifying ridging over the East Coast. An upstream
trough axis cuts through northwest MT-western WY-through OR and
into/through northern California. One portion of the northern branch
of the jet continues to dive down the West Coast,  with the
downstream piece extending to the northeast from the TX Panhandle
across the Great Lakes. The surface has a large area of post frontal
high pressure enveloping the Plains/High Plains and northern Rockies
including Wyoming.

Today, extremely cold temperatures solidly in place across all of
Wyoming under the post frontal high pressure. With all the new
fallen snow and H7 temps running -20 to -25 deg C under clear skies,
morning low temperatures will range from -5 to -30 deg F while only
rising into the upper single digits and teens during the day. May be
the coldest morning of the week. The coldest air looks to be trapped
in the forecast area`s Basins and across Yellowstone NP. With the
extreme cold temperatures aloft still in place, a few isolated light
snow showers continue occasionally across the west.

Tonight through Thursday night, the main upper level trough
continues expanding across the western CONUS as the West Coast jet
and associated embedded shortwave dig deep down along the coast.
This general troughiness will prevail through most of the rest of
the forecast period with the cold air aloft providing for modest
lapse rates at times and continued small chances for light
snow showers remaining in the forecast across the western zones.
Additionally, by Thursday, the approach of the western trough with
an embedded weak shortwave running out ahead of the trough will
allow for the development a surface clipper type front to push south
east of the Divide and into south central and southern Wyoming.
Meanwhile, low pressure development across the Great Basin will push
southerly winds and a possible weak warm front into southern
Wyoming. These conditions will provide some modest convergence
across portions of southern/central Wyoming Thursday night and the
impetus for the areas of light snow to develop and continue through
the day Friday. There will also be some potential for gusty west to
southwest winds to return to the mountains and foothills along and
east of the Divide later Friday. The Clark area may get occasional
high wind gusts by Friday night...although it looks localized.
Finally, temperatures will remain well below seasonal through the
end of the forecast period...and even longer!

.LONG TERM...(Saturday through Wednesday)
Issued at 126 AM MST Wed Feb 21 2018

A longwave trough will remain in place across the western U.S. for
the extended. Temperatures are expected to below average to much
below average as a result. A shortwave trough is expected to dive
southeast out of southwest Canada across the Rockies in the
Saturday/Saturday night time frame. This should give a period of
light to moderate snowfall mainly across far western Wyoming, while
areas east of the Divide should remain mainly dry. Isolated areas
over the far Western mountain could approach advisory amounts. Also
breezy to windy southwest winds in the wind corridor from RKS-CPR
could result in a decent amount of blowing snow from the recent
snowfall. Medium range models then show a transitory shortwave
ridging Sunday. Despite this ridging, far western area could
continue to get light snow/flurries on Sunday.

Models show a strong trough digging into the West Monday and
Tuesday, and the possibly pushing east into the Rockies Wednesday.
Global models have significant differences on how this trough will
evolve, but consensus is to have a deep trough developing over the
Western U.S. Looks like the west would be impacted on Monday with
snow spilling east of the Divide Tuesday and Wednesday as the
associated cold front pulls south across the area in the Tuesday
time frame. Models are showing a signal of isentropic lift over the
cold front on Tuesday/Wednesday time frame. This system has the
potential for some significant snowfall depending upon how
everything evolves.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday)
Issued at 1024 AM MST Wed Feb 21 2018

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

VFR conditions to prevail at all terminals through 18Z/Thu with the
exception of KJAC. Flurries and light snow continue at KJAC with
just enough instability and available lingering moisture. MVFR to
continue intermittently into Wednesday afternoon. Conditions should
improve to VFR by this evening with mid-cloud decks through the
remainder of the period. Mountain tops will frequently be obscured.
Surface winds at all terminals will be less than 10kts.

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

Conditions will be VFR at all terminals through the 18Z/Thu with
only mid and high clouds pushing northeast across the area. Some
local morning MVFR fog in the lower basins but not expected at the
terminals. Southwest surface wind of 10-20kts anticipated at KCPR
this afternoon before decreasing with the setting sun. KCOD can
expect intermittent SSW wind 12kts Wednesday afternoon. Other
terminals will see surface wind less than 10kts throughout the
period.

&&

.FIRE WEATHER...
Issued AT 126 AM MST Wed Feb 21 2018

Extremely cold conditions continue across the region in the wake of
the exiting cold front. Winds are relatively quiet except for near
Rattlesnake, Green and Casper Mountains where moderately gusty winds
are keeping temperatures elevated a bit. Still, most locations will
see brutally cold morning lows of 5 to 30 below zero. Smoke
dispersion will be poor to fair (east of the Divide) to good (west
of the divide) both today and Thursday.

&&

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

&&

$$

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



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