Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 140834

Area Forecast Discussion
National Weather Service Riverton WY
234 AM MDT Wed Mar 14 2018

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

Imagery shows shows amplified ridge/trough (west/east) pattern
across the CONUS with another large upstream negatively tilting
trough/closed low/occluding surface low lying offshore the PAC NW
and starting to affect the PAC NW. The ridge axis over the western
CONUS currently cuts south from southwest Canada through MT/western
WY/CO-UT and MN-AZ borders. A modest WCB lies across the southern
CONUS from Baja through NM and over TX. The surface has high
pressure across the Northern/Central/High Plains with a couple of
developing cold fronts developing under the influence of the large
western trough. Precipitation across the west extends roughly west
of a Spokane to Salt Lake City to Los Angeles line.

The large offshore trough continues to tilt eastward and move slowly
inland. From late this afternoon through the end of the forecast
period Wyoming will be affected by this trough one way or another.
It`s still tough to make too much (absolute) with regard to the
placement, extent and strength of the isentropic lift, frontogenesis
(eastern/northeastern WY), and cyclogenesis (eastern CO) and even
eastern placement of the eastward translating ridge (important to
just where the heavier EPAC moisture will set up). Still lots of
question marks continue even as the models come to some better

West of the Divide today, rain/snow showers with a few thunderstorms
will all be possible. Snow levels will rise to above 8k ft by this
afternoon through this evening before falling to valley floor levels
between around sunrise Thursday. At the moment, new snow amounts
west of the Divide from this afternoon through the day Thursday look
to remain near Advisory levels for most mountain locations above
8500 feet (perhaps 8 to 12 wet inches of snow in portions of the
Tetons). Thunderstorms possible west of the Divide today (where low
level heating and cooling aloft increase lapse rates to near 8deg
C/km) will have some chance for graupel/small hail (<= 0.5
inch)...nothing too large and likely on the "soft" side. Near
straight-line hodographs suggest gusty winds possible with the
stronger showers and storms...even with a relatively moist boundary
layer. Will end these low topped thunderstorm chances by early this
evening so not much chance for most the lower elevation areas east
of the Divide to see thunder...just showers.

Models set up vigorous return moisture from the Gulf of Mexico
around a developing cyclone over eastern CO on Thursday that is
probably a little too ambitious with regard to both liquid content
and extent of reach back to the west given the speed of development
and subsequent quick movement into the Central Plains. Still betting
that most of the return moisture east of the Divide will not make it
back too far into central WY with the regard to the aforementioned
rapid cyclogenesis that is expected to take place from the High
Plains of CO into the Central Plains from Wednesday night through
the day Thursday. Have reduced QPF values a bit and placed/kept the
heavier QPF (to a half inch or more/6hrs) mostly east of this FA.
Accumulating snow east of the Divide will not begin to fall until
late Thursday night...taking its time to stick on the ground for
many locations across central/northern Wyoming with the rather warm
pre-storm temperatures keeping temperatures up on the roadways
(grassy areas accumulate first). Best areas for accumulating
snowfall east of the Divide from Thursday night through Friday will
occur north of an intruding cold front - including portions of the
Big Horn Basin, Cody Foothills, Johnson County, and Natrona County
in addition to the Bighorn and Casper Mountains. Most of these areas
will see less than Advisory level snowfall...but near/marginal
Advisory amounts will be possible in the Bighorn Mountains, Casper
Mountains, Cody Foothills, southeast Bighorn Basin and northern
Johnson County. Still plenty of time to fine tune any highlights

Friday night the weather system exits the forecast area to the north
and east while the next embedded shortwave within the main, and
still large, trough across the western CONUS heads into
western/southwestern WY, increasing precip chances across that area
once again.

.LONG TERM...(Saturday through Wednesday)

Overview...A low pressure system will slowly move east across the
Great Basin and Rockies through the weekend.  This system will
spread another round of rain and snow showers east across Wyoming on
Saturday, with cooler and unsettled conditions lingering Sunday and
Monday.  Dry and slightly milder conditions are expected on Tuesday,
and then the next Pacific storm system will begin to spread chances
of showers into the west on Wednesday.

Discussion...Upper level pattern on Saturday is expected to feature
complex, fragmented low over the Intermountain West with
downstream shortwave undercutting a weakening ridge over the
Plains. Intermountain West low is expected to become increasingly
fragmented as it translates the Great Basin and Rockies over the
weekend. The best moisture, lift and instability with this system
lines up over the area on Saturday as a ~90 knot jet streak lifts
across the area, LI`s decrease to -1c to -2c across much of the
area. Rain and snow showers should spread SW-NE across the area
during the day, and although temperatures will remain fairly mild
across central Wyoming, strong lift and instability will likely
result in some snow pellet showers (graupel). There may be a few
claps of thunder, but probability is too low at this time to
include mention in the forecast. ECMWF and GFS show differing
scenarios on how this system will emerge out of the Rockies on
Sunday with the GFS lifting more energy into the northern High
Plains, the ECMWF showing a more robust shortwave moving out of
the Four Corners across KS/OK. The GFS scenario would result in
better coverage of snow showers across the north on Sunday. Either
way, an H5 -30c cold pool over area should keep mid-level lapse
rates steep and chances of snow showers across the entire area.
Most of this fragmented system will be out over the Plains on
Monday with a cool, unsettled NW flow with a few weaker
perturbations embedded. Upper ridge is expected to build over the
Great Basin on Tuesday between exiting trough over the
central/eastern U.S. and next Pacific system approaching the west
coast. ECMWF is quicker to bring this system onshore midweek than
GFS. Ensemble means, GFS 5-wave show transition to a more zonal
flow across the lower 48 mid to late week, favoring a quicker,
weaker storm track.


.AVIATION...(For the 12Z TAFS through 12Z Thursday)

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

Scattered to numerous valley rain/mountain snow showers are expected
to develop over far western Wyoming between 18Z and 21Z Wednesday,
and gradually expand eastward with time through the late afternoon
and evening. A few thunderstorms are also possible. Periods with
MVFR conditions are likely at KJAC and KPNA. Thunderstorms could
produce small hail and gusty, erratic winds. The bulk of the shower
activity should exit the area after 06Z, with the some fog
development possible. Also the rain showers left could change over
to snow in the far western valleys after 06Z Thursday.

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

VFR conditions are expected with increasing mid-level cloudiness
today. After about 00Z-03Z Thursday, some valley rain/mountain snow
showers are expected to push north, northeast into the Wind River
Basin, and then possibly across the Big Horn Basin. Brief MVFR
conditions could occur with the shower activity. By 09Z this
activity should be lifting away from the area with decreasing
cloudiness. Some fog could form depending upon how much rain falls.



Seasonal (west) to above seasonal (east) temperatures expected
across the the region today. The effects of the next spring/winter
weather system begin to be felt west of the Continental Divide today
with a little everything - from rain and snow to thunderstorms with
small hail possible across southwestern Wyoming. 6 to 10 inches of
new wet snowfall will be possible above 8500 feet through the day
Thursday...with rain/snow mixes and much less accumulation below
7500 feet. Cool and showery most locations east of the Divide from
later tonight through Friday. Gustiest winds most probable across
the Wind Corridor from eastern Sweetwater through Natrona Counties
both this and Thursday afternoons.




LONG TERM...Meunier
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