Area Forecast Discussion
Issued by NWS Riverton, WY

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

FXUS65 KRIW 192227

Area Forecast Discussion
National Weather Service Riverton WY
427 PM MDT Wed Apr 19 2017

.SHORT TERM...(This evening through Saturday)

Instability from the cold pool aloft revealed itself once the back
edge of the stratiform cloudiness exited the region to the east
northeast asct with the mid level low. Currently, the strongest
convection has progressed east to Eastern Fremont County and is
poised for Natrona Co attm. Lighter, mainly high elevation snow
showers are evident in the west. Once the sun sets, instability
will decrease and skies will clear. Expect some valley fog to form
in the far west tonight and Thursday morning as well as areas
that received substantial rainfall east of the divide.

Then after a very short respite from the precipitation, the long
advertised amplifying negatively tilted splitting trough will
approach the CWA Thursday. Mountain snow and valley rain showers
along with some isold t storms and small hail will develop during
the day Thursday with the far west getting an early start to the
convection in the late morning. Then as the trough nears, a mid
level low will become established within the trough over southern
Wyoming and will linger and wobble between Eastern Sweetwater and
Natrona Counties from Thursday night through Friday night. It
appears, as with most well defined mid level lows, that very
substantial precipitation could result, especially east of the
Divide. With h7 temps eventually lowering to -8c, the Absaroka,
Wind River, and Big Horn Mountains stand a good chance of seeing
warning amounts of snow with snow levels low enough to potentially
cause impacts to the major passes throughout the CWA. The GFS is
not showing quite a much precip as the runs of 24 hours ago. The
advertised bulls eye precip over the Big Horn Mtns has dropped
from 4.5 inches of total precip to 2.5. For now have indicated up
to 21 inches of snow over the highest Big Horn Mtns, with up to
17 inches over the Absarokas, and up to a foot over the Wind
Rivers between Thursday night and Saturday Night. Issued a winter
storm watch addressing this with the Big Horn Mtns staggered 6
hours ahead for valid times. The snowfall should extend into
Friday night before tapering off, in spite of the fact that the
mid level low will lose its definition and the bulk of the
southern stream energy will be way out in Eastern Kansas by then,
with streams of vorticity streaming down the front side of our
extended trough.

Saturday is shaping up to be a milder but still coolish and drier
day with highs in the 50s east at the lower elevations and 40s

.LONG TERM...(Saturday night through Wednesday)

The mid range models continue to indicate that an upper ridge will
build into the area by Saturday night with improving conditions
expected. On the heels of the Friday through early Saturday system
will be another progressive Pacific system that will move rapidly
from the northwest U.S. by late Saturday, into the western portions
of the forecast area by Sunday night. Valley rain and mountain snow
showers, isolated thunderstorms, will increase in the west ahead of
this system by Sunday afternoon, with lowering chances of rain and
mountain snow showers across central areas with the frontal passage
by Sunday night. Both GFS and ECMWF models indicate that a more
flatter ridge axis will build across the eastern Pacific Tuesday and
Wednesday with strong Pacific jet carving out broad trough across
the western and central U.S. by mid-week.

Due to the progressive mid to upper level flow, it will be more
difficult to time the movement of the short waves during the latter
part of the forecast periods. The larger upper trough is expected to
continue to deepen over the area Tuesday and Wednesday, with more
stronger short wave energy moving through the trough by the latter
part of the extended forecast periods. The result will be cooler
than normal temperatures setting in by Wednesday with chances of
mainly valley rain and mountain snow showers from Tuesday onward.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)


Scattered showers through 02z, mainly in the northwest including the
KJAC terminal. Breezy to windy northwest to west wind are expected
in the Upper Green River Basin and Sweetwater County through the
afternoon. Tonight, the main concern will be the possibility of fog
formation, but increasing high cloudiness should limit chances for
fog. A new storm system will bring in an increasing chance of
showers and isold tstms late Thursday afternoon into the far west.


Most terminals should be low VFR conditions with bkn-ovc
stratocumulus deck between 035-050 kft agl through 03z with sct
showers/isold thunderstorms centered mainly around KLND, KRIW and
KCPR with more isold showers elsewhere. There could also be
isolated convection in the afternoon, as there is some weak
instability, but not enough to include in area terminals for now.
The stratocu deck is expected to quickly dissipate by around dusk
with mainly SKC conditions expected. Also winds are expected to
weaken around 00Z Thursday. Northern mountains, especially the
Bighorn Mountains will be obscured much of the time today with
improving conditions this evening. VFR conds will prevail Thursday
but an approaching storm system will bring showers and isold
tstms to the area by Thursday evening.



Fire danger low across the forecast area through the rest of the work
week. All fuels currently in green-up. Periods of rain and high
country snow will occur periodically throughout the week. Isolated
Thunderstorms will also be possible Thursday afternoon and


Winter Storm Watch from Thursday evening through Friday evening
for WYZ002-014-015.

Winter Storm Watch from late Thursday night through late Friday
night for WYZ008-009.



LONG TERM...Troutman
FIRE WEATHER...Lipson is the U.S. government's official web portal to all federal, state and local government web resources and services.