Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 270641 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1141 PM MST Sun Feb 26 2017

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

An upper trough over the Intermountain West will move east across
the Continental Divide around 00z Tuesday.  Widespread MVFR, areas
of IFR conditions in light snow, frequent mountain obscurations,
will precede this system and its associated cold front vicinity and
north of a KEVW-90SM S KPNA line through 15z Monday.  Snow showers
will increase further south along the Interstate 80 corridor after
15z with prevailing MVFR conditions in light snow expected into KRKS
by the afternoon.  Drier air will move in behind the upper trough
passage Monday evening with scattered snow showers mainly confined
to the northwest mountains after 03z Tuesday.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail through 15z Monday.  Scattered MVFR/IFR
snow showers will develop from the south to a vicinity KRIW-KCPR
line 15z-18z Monday with more organized snow bands possible,
especially along KRKS-KCPR routes Monday afternoon.  Snow shower
coverage will be less over north central Wyoming; however, brief
MVFR/IFR conditions will be possible vicinity KWRL and KCOD Monday
afternoon and evening.  Drier air will push in behind a cold front
along with gusty NW winds, especially vicinity KDUB-KRIW 06z-12z
Tuesday

&&

.PREV DISCUSSION... /issued 220 PM MST Sun Feb 26 2017/

Short Term...Tonight Through Wednesday

Sufficient low and mid level moisture in west flow aloft to
support continued light snowfall over western mountains Tonight.
Flow aloft slowly shifts to the west southwest with increasing
moisture through Monday.  Variable clouds will spread east of
the mountains with little precipitation spilling over the
Continental Divide. Some light snow is possible near the
foothills. Otherwise rather brisk and gusty winds will continue
over and near the east slopes of the mountains and through the
wind corridor from Rock Springs to Casper.  Windy areas will
see some warming while the valleys and basins with light wind
will see little change in temperatures. In the west Snowfall of 3
to 6 inches every 24 hours with local 8 inch amounts in the
Tetons and Southwest Yellowstone National Park continues through
Wednesday. Temperatures will be rather steady through Mid Week.
Brisk winds at ridgetop and pass levels will continue with a
gradual increase Tuesday.

Long Term...Wednesday Night Through Sunday

Wednesday night through Thursday night: Post frontal with general
high pressure at the SFC, cool NW flow aloft, and some lee troffing
trying to get started east of the Continental Divide. Modest
moisture will also continue to flow into the region from the PAC NW.
This set-up will keep the northwest/northern portion of the FA
slightly unstable, and with a little extra mountain lift, will keep
the mountains across northern WY under conditional POPs.
Temperatures at all locations through Thursday will also remain
largely below seasonal so snow melt should remain in check.

Friday through Sunday: Weak ridging/upper level NW flow will begin
to flatten in response to an approaching embedded upper level SW
trof/jet streak that will arrive near western WY Friday night. The
transition to nearly zonal flow on Friday will allow for increased
lee troffing potential...and the wind potential that comes with it.
Some strong gusty winds, bordering on `High,` will be in-store
periodically for the usual suspect areas (lee side foothills east of
the Divide and across the Wind Corridor) from Friday through much of
the day Sunday. Additionally, relatively light snowfall will
continue in some fashion over northwestern WY until Saturday morning
and the arrival of the nose of the jet when precipitation will
increase spatially and in intensity across the western mountains. If
position and timing of this system remain on track, highlights will
surely be needed over the last couple days of the forecast period.
Otherwise, temperatures will respond positively to the lee side
troffing/wind set-up east of the Divide from Friday through Sunday.
High temperatures will rise back to near seasonal norms, starting
another period of melt-off...but hopefully not to an extreme.

Fire Weather...

Sufficient low and mid level moisture in west flow aloft to
support continued light snowfall over western mountains Tonight.
Flow aloft slowly shifts to the west southwest with increasing
moisture through Monday.  Variable clouds will spread east of
the mountains with little precipitation spilling over the
Continental Divide. Some light snow is possible near the
foothills. Otherwise rather brisk and gusty winds will continue
over and near the east slopes of the mountains and through the
wind corridor from Rock Springs to Casper.  Windy areas will
see some warming while the valleys and basins with light wind
will see little change in temperatures. In the west Snowfall of 3
to 6 inches every 24 hours with local 8 inch amounts in the
Tetons and Southwest Yellowstone National Park continues through
Wednesday. Temperatures will be rather steady through Mid Week.
Brisk winds at ridgetop and pass levels will continue with a
gradual increase Tuesday.  Poor smoke dispersal is expected through
Wednesday in the valleys. The windy areas over and near the
mountains and through the wind corridor may experience fair to
good smoke dispersal in the afternoons through Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...C.Baker
LONG TERM...Braun
AVIATION...AEM
FIRE WEATHER...C.Baker


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