Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250636 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1136 PM MST Fri Feb 24 2017

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Snow showers will continue to decrease in coverage overnight
across the northwest mountains, vicinity KJAC. An upper level
disturbance will bring another round of snow showers across the
western mountains and vicinity KJAC Saturday afternoon, with
mountains becoming frequently obscured and prevailing MVFR
conditions expected at KJAC through the early evening. Drier
conditions with surface west wind 20-30 knots will prevail across
the southwest along the I-80 corridor Saturday afternoon. Snow
showers will decrease in coverage across the mountains late
Saturday evening through Sunday morning.

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

Lingering snow showers will continue to decrease across the north tonight,
with some clearing and patchy valley fog developing, partially
obscuring terrain through 16z Saturday. A lee side surface trough
will strengthen Saturday morning with southwest wind 20-30 knots,
gusts near 40 knots, prevailing from vicinity 50SM NE KRKS-KCPR
through early Saturday evening. Gusty west winds are also expected
to develop along the eastern foothills from vicinity KCOD-KDUB-
KLND. Areas of blowing snow and partial terrain obscurations will
be likely with these winds. An upper level disturbance will spread
isolated to scattered snow showers vicinity and north of a KCOD-
KBYG line Saturday afternoon with the Bighorn Mountains becoming
mostly obscure north of U.S. Highway 16. This activity should
clear the north 3z-6z Sunday leaving prevailing VFR conditions
across the area.

&&

.PREV DISCUSSION... /issued 225 PM MST Fri Feb 24 2017/

SHORT TERM...Tonight through Monday

Imagery showing an upper level broad, deep, and somewhat flat
longwave trof across most of the CONUS with WY under weak flow aloft
with the main jet being directed around two book-end closed
lows/embedded trofs within the main LW trof...one over OR/WA and the
other moving into toward the wrn Great Lakes. Modest mid to upper
level moisture is still fairly evenly placed over WY today. The SFC
will show general high pressure around the region with the stronger
high pressure out over the Central/High Plains and also the
central/southern Rockies/ern Great Basin.

The rest of today though Saturday morning, mid to upper level
moisture will be modest but fairly evenly placed over the FA. The
SFC will show general high pressure around the region with the
stronger high(s) out over the Central/High Plains and central
Rockies/ern Great Basin with hints of (relative) lee-side trof
forming over northern WY near the Cody Foot hills. A modest P GRAD
will also begin to build along the Continental divide with west to
southwest winds gusting 20 to 30 mph across the higher ridge-lines,
the eastern foothills and the Wind Corridor by sun-up Saturday
morning. Cold air aloft is keeping H85 to H5 lapse rates hovering
between 7.5 and 9 deg C/km with isolated to widely scattered snow
showers continuing to be possible around the entire FA today...then
mainly across the western and northwestern mountains tonight and
Saturday morning.

Saturday afternoon and night, the western upper SW trof, mentioned
above, swings over the FA increasing convective parameters and
subsequent snow shower activity west of the Divide. 24 to 30 hr
total snow fall from Friday night through Saturday night looks to
range from 1 to 4 inches in the Teton, Gros Ventre, and Bighorn
mountains, with lesser amounts most other mountain locations. Gusty
west to southwest winds will continue across the Wind Corridor
through the day Saturday...with some blowing snow possible between
Rock Springs, South Pass and Casper. Winds mixing with snow will be
hard to measure in light of visibility restrictions at this point as
most of the snow that fell was relatively heavy with an inch or two
of light stuff on top (in some areas). It does not present as a
situation that will cause too many problems...but worth watching all
the same, with the South Pass area the most likely to have blowing
snow problems if they do arise.

Sunday, after the passage of the SW trof, modest ridging builds
across WY with weak SFC high pressure and relatively light wind for
the most part. The only chance for precip will come across the
western/northwestern mountains as mildly convective afternoon
showers occur ahead of the next embedded SW trof moving through the
western portion of the main longwave trof...into western WY by
Monday morning. Snowfall potential west of the Divide will begin to
increase accordingly late overnight Sunday and Monday as upper level
flow and forcing also increases. Trof passage with this system will
also take some time...perhaps until Tuesday night. These
circumstances together with continued modest moisture may lead to
(at least) WWx Advisory conditions beginning across some locations
out west.

LONG TERM...Monday Night through Friday

A series of short waves will be ejected from the main upper trough
across the Pacific northwest through Monday and Tuesday. This will
result in a continued unsettled weather pattern for the west with
periods of snow possible. Scattered light snow remains a possibility
to the east of the divide. Temps will remain slightly below normal
through the period. The models keep the forecast area in a northwest
flow aloft from Wednesday through Friday. This will result in some
light snow at times in the northwest mountains and the northern
Bighorn mountains.

The rest of the region will remain dry with albeit slow, moderating
temps from the middle to latter part of next week. A cold front may
brush the north and northeast zones Wednesday night. The cooling
will not be that much, as warmer air comes back quick on Thursday
afternoon to those areas. Temps will be a little milder Thurs and
Fri for most places. Low temps will be tricky due to the new
snowfall in parts of the region. Will continue to monitor the
expected breezy to windy conditions in some areas from Wednesday to
Friday. This will also impact temps, keeping them milder than the
valleys that do not see any mixing.

AVIATION.../00Z ISSUANCE/

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

VFR conditions will continue through the forecast period for the
sites. Expect that as per current trending that clearing will
continue tonight with a few snow showers mainly confined to the
extreme northwest corner of Wyoming 6z-12z Saturday. Expect VFR
conditions and lighter winds for the area after 12z Saturday through
the end of the forecast period.

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

Expect VFR conditions to prevail through the forecast period for the
locations east of the divide. The low clouds are expected to lift
and scatter out across the area by tonight. Southwest winds will
increase to 15-25 knots vicinity KCPR-50 SM NE KRKS 6z-12z Saturday
and continue through toward the end of the forecast period.

FIRE WEATHER...

Fire Danger low for all of WY through the weekend as a good
existing snow pack combines with seasonal to below seasonal
temperatures. Gusty west to southwest can be expected at times
across portions of central and southern WY. Smoke dispersion will be
fair to very good across most of the forecast area each afternoon
through at least Monday.

&&

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

$$

SHORT TERM...Braun
LONG TERM...Troutman
AVIATION...AEM
FIRE WEATHER...Braun



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