Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170753

Area Forecast Discussion
National Weather Service Riverton WY
153 AM MDT Sat Mar 17 2018

.SHORT TERM...(Today through Monday night)

Imagery still shows large upper level closed low/trough across the
western CONUS with occluding cyclone spinning out over the east
Central Plains in to the upper mid-west...pinching off what`s left
of the ridge. A weak to moderate is jet moving through the base
of the trough and up across the southwest CONUS, into southwest
WY. The surface has developing low pressure across the Great
Basin, just starting to sneak into southwest WY. No precipitation
is currently falling over the CWA this morning. The nearest
precipitation is located to the west over northeast NV/southern
ID/northern UT associated with the developing low.

Today through Sunday night, portions of the FA will be under the
continuing influence of the large western CONUS trough to one extent
or another. The problem is that the system has been around for so
long, slowly expanding in size and well away from the northern
branch of the jet, that while its impact has been felt for a while
now, it weakens easily with respect to development and focus
since it is also well displaced from the southern branch of the
jet. Today, modest moisture return is expected from the
west/southwest as both the surface and upper portions of the
system over the Great Basin spin and slowly translate eastward
toward/into western WY. Early this morning, expect some light
mainly terrain forced precipitation to begin across the western
mountains. Then late this morning through the afternoon,
instability will increase along with vertical shear (low end speed
and directional) with an increase in number of stronger showers
and a few isolated thunderstorms. Tonight, showers decrease across
the west while a front moves into/across southern WY, increasing
precip chances. Snow accumulation today through tonight along and
west of the Divide will range from 1 to 4 inches in the mountains
(locally higher over the Tetons); a half inch to an inch and a
half in the western valleys; and from a trace to 2 inches across
southern WY including southern Lincoln and Sweetwater Counties.
Sunday, most of the western zones are relatively quiet. However,
the continued passing of the upper trough to the east will
reintroduce strong cyclogenesis over eastern CO with inverted
troughing reaching into central and eastern WY with frontogenesis
taking place to the west and north of the area. By late morning to
mid-day Sunday, cyclogenesis really winds up, pulling a front
down through the the FA, increasing chances for precipitation for
most of the CWA (mountains and the Wind Corridor best...basins
least). Again, as with the system that spun up over the last
couple of days, southeastern WY and northern CO will see the best
QPF values while areas to the north and west drop off
significantly in available moisture. Less than Advisory amounts of
new snowfall are expected Sunday and Sunday night...all areas.

Monday and Monday night, the whole kit and caboodle will (finally)
move out of the region to the east, offering up severe weather
chances across the Central and Southern Plains while WY remains post
frontal and cool with flat shortwave ridging aloft. A weak shortwave
trough will move out of southwest Canada and through MT/northwest-
northern WY Monday, keeping modest POPs in place through the end of
the forecast period. Again, less than Advisory level snowfall is
currently expected Monday through Monday night as moisture return
looks lean at this time.

.LONG TERM...(Tuesday through Saturday)

Overview...Unsettled northwest flow will linger over the area on
Tuesday with scattered snow showers over the western mountains and
far west valleys.  A warming trend will then precede the next
Pacific storm system on Wednesday and Thursday; unseasonably mild
and breezy to windy conditions are expected across central areas on
Thursday while chances of rain and snow showers increase across the
west.  A Pacific cold front is expected to push across the area on
Friday spreading chances of rain and snow showers east of the
Continental Divide along with gusty wind across much of the area.

Discussion...Upper level pattern on Tuesday morning to feature
fragments of the current western trough stretching from coast to
coast.  On the west extreme, orphan low off the NW coast is expected
to kick inland Monday and across the area on Tuesday.  GFS has
reversed from its solution 24 hours ago and now is much slower with
this feature than the ECMWF.  The more consistent ECMWF brings this
perturbation SE across Wyoming late Tuesday.  We continue to keep
PoPs confined to the western mountains and valleys with this system,
but may need to add chances east of the Divide when we have more
confidence in the timing and track of this system.

Upper trough is expected to merge/sharpen off the west coast on
Wednesday, pumping up a ridge downstream over the Rockies.  This
will bring mainly dry conditions across the area, except for a few
rain and mountain snow showers across the west.

Most of the area will be in warm sector ahead of a Pacific cold
front on Thursday, and this will be the mildest day of the period
with central basins surging into the 60s.

Medium range models continue to show poor consistency and a great
degree of spread handling the Pacific trough as it pushes across the
western U.S. Thursday night through Saturday.  There better overall
agreement at the surface, pushing a cold front across the area on


.AVIATION...(For the 12Z TAFS through 12Z Sunday night)

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

VFR conditions are forecasted at the terminal sites through the
period with the exception of KJAC. Isolated snow showers will
continue in the western mountains through the period. Mountain
obscurations will be common, especially in the afternoon and evening
when the most showers will be around. VCSH will be possible at KJAC
by 12Z with -sn or -rasn before 18Z Saturday and continuing through
around 06Z when local MVFR conditions are possible. Coverage of
showers decrease around 05Z.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

VFR conditions expected through the forecast period. Periods of mid
to high clouds will stream over the area . Gusty winds will occur
this afternoon, as a cold front moves through the area, then
decreasing after 03Z after the front passes and winds decouple.



Seasonal or below (west) to seasonal or slightly warmer (east)
temperatures expected across the the region today. Another upper
level disturbance will move into/across western Wyoming today
offering increasing chances for precipitation (snow or rain/snow
mixed showers and isolated thunderstorms). Precipitation chances
will then decrease across the west tonight while increasing over
southern Wyoming. Snow accumulation today through tonight along and
west of the Divide will range from 1 to 4 inches in the mountains
(locally higher over the Tetons); a half inch to an inch and a half
in the western valleys; and from a trace to 2 inches across southern
WY including southern Lincoln and Sweetwater Counties. Strongest
winds (gusting 25 to 40 mph out of the south/southwest) will be
found across the Wind Corridor from Sweetwater through Natrona
Counties this afternoon. Modest precipitation chances move east of
the Divide Sunday.




LONG TERM...Meunier
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