Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 220943
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
243 AM MST Sun Jan 22 2017

.SHORT TERM...SUNDAY THROUGH TUESDAY NIGHT

Temperatures in the basins have been warming through yesterday
evening and early this morning, rising above the dew point in many
of these areas so fog should not be quite as extensive as it was 24
hours ago. A closed low off the Pacific Northwest will eject a weak
vort lobe across the region this morning which will be responsible
for some light snow in the far west this morning. After s short
respite from the snowfall, this closed low will eject a more
prominent shortwave toward Wyoming this afternoon. This shortwave
should track northeast across Wyoming this afternoon. As it does,
the steering flow will back to the southwest and become stronger and
more difluent ahead of the main low still to the west. During this
time frame, fairly strong isentropic lift will get underway in the
west resulting in more significant snowfall this afternoon, and
continuing through the night with the added influence of left front
quad dynamics. As this weather feature plows through tonight and
into Monday, the flow will back to a more south southwest direction
which is not the most favorable pattern for heavy snowfall in the
far west, especially in the Jackson area. Have left high end
advisory amounts for the far west for now and will keep the watch
going. By Monday afternoon, a second trough will eject eastward and
will close off in Idaho. The acct mid level circulation will track
east, right over Riverton late Monday. At this point, the snowfall
will spill over the divide. During this time frame, h7 temps will
rise to -5c in Central Wyoming. With the basin inversions still
likely in place, this might spell the potential for freezing rain.
However, without the classic pattern for this to happen which is
when a cold high pressure center off to the north and southerly wind
overlay a shallow stream of cold northerly winds, but this is not
the weather pattern this time around. If anything, there could be a
brief period of rain or rain mixed with snow east of the divide, but
the predominant precip type will be snow. With the close proximity
of the mid level circulation over central WY, increased the
qpf/snowfall somewhat for the Wind River Basin and Owl Creek/Bridger
Mtns for now through Monday evening. When the low propagates to
Southeast Wyoming, some weak wrap around isentropic lift as well as
low level upslope flow will develop with some resultant seeder
feeder action for an extended period of light snowfall later Monday
night and into Tuesday, ending from north to south. For now have low
end advisory amounts for areas east of the divide. Temperatures will
be colder Tuesday night behind this exiting system.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

Synopsis...Cool and unsettled northerly flow will keep scattered
snow showers over the western mountains and far west valleys on
Wednesday with snow showers diminishing on Thursday.  A ridge of
high pressure will build into the area Friday through Sunday
bringing dry conditions.  The mountains and east will see a warming
trend while the lower central basins and western valleys remain
trapped in shallow cold air with areas of morning fog Friday and
Saturday.

Discussion...Synoptic pattern Wednesday morning features a sharp
upper ridge along the west coast with broad downstream trough
stretching from the Interior West into the Great Lakes region. Upper
ridge will fold over across the northern Rockies Thursday and Friday
with stronger warm advection aloft with the upper ridge pushing
south across Wyoming on Friday.

Steep lapse rates and some embedded weak circulations within the
longwave trough will keep some snow showers over the western
mountains and far west valleys on Wednesday.  Gradual decrease in
moisture and increase in stability will shutoff most snow showers by
Thursday evening.   Strong warm advection with drying mid-levels and
weak flow aloft will be favorable for steep inversions and fog
development Friday and Saturday (especially Saturday morning) across
the central basins and western valleys.  Strong shortwave pushing
east across Montana will increase flow aloft with better mixing,
eroding of inversions on Sunday, so fog is not expected to be as
widespread.

&&

.AVIATION.../12Z ISSUANCE/

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

There could be continued stratus over the far western valleys this
morning between 020-040 KFT AGL, but soundings support this deck
scattering out between 12Z and 15Z. If this deck does scatter out
near sunrise there will be a chance for some low-lying fog, boundary
layer moisture is marginal. Otherwise VFR conditions are expected
through about 21Z with mainly mid/high level cloudiness. This will
quickly change between 21Z Sunday and 03Z Monday as snow breaks out
in a moist southwest flow ahead of a storm system. MVFR/IFR
conditions are likely tonight in low ceilings/visibilities across
much of the area. Mountains will be obscured much of the time after
21Z.

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

Once the mid-level deck of clouds push away from the area between
09Z and 13Z, patchy fog could occur with some IFR/MVFR restrictions.
Currently not expecting fog to be as widespread as yesterday, and
will only have VCFG during the morning for the basins. Otherwise,
VFR conditions with some mid-high level clouds. Windy southwest
surface flow will occur across SE Fremont into Natrona County
including KCPR terminal. Tonight, an approaching storm system will
rapidly increase mid-level cloudiness and have some low-level wind
shear in spots including KCPR and KLND. Some models have some
precipiation spilling east of the Divide tonight, but will keep the
precipitation along and west of the Divide for now.
&&

.FIRE WEATHER...

Basin and valley inversions will persist this morning for poor
smoke dispersal. Better smoke dispersal is expected along the
southern wind corridor from the Red Desert to Casper this
afternoon where southwest winds will increase. Expect better smoke
dispersal above the temperature inversions as well. Light snow
will taper off later this morning in the far west followed by a
short respite from the snowfall. Then this afternoon, snowfall
will increase significantly in the west and continue through
tonight. Significant snowfall accumulations are expected in the
west. The snowfall will spill over the Divide Monday with light to
moderate snow expected to continue Monday night and into Tuesday
east of the Divide. Expect temperatures to become colder Tuesday
night.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from this afternoon through Monday afternoon
for WYZ001-012>014-023>025-027.

&&

$$

SHORT TERM...Lipson
LONG TERM...AEM
AVIATION...WM
FIRE WEATHER...Lipson



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