Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 291751
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1151 AM MDT Wed Mar 29 2017

.SHORT TERM...Wednesday through Friday night

Today the forecast reminds me of playing a video game. It starts
off rather easy and then the difficulty level ramps up as you
progress through it. It will explain in the body of the
discussion.

Today looks to be the training missions of the game, the easiest
part. A flat ridge of high pressure will control the weather across
the area and bring mainly dry conditions and fairly seasonal
temperatures. Although we can not rule out a few sprinkles or
flurries in the far northwest, the chance is small enough so we kept
them out. Wind should remain light to moderate for the most part.

Things get more difficult as a Pacific trough approaches the area
from the west tonight and Thursday. Some lighter precipitation will
likely break out tonight in the west. However, the heaviest will
likely hold off until very late tonight and into Thursday when the
trough axis and upper level support provided by the left front
quadrant of a jet streak. And temperatures, as expected this time of
year, will be a factor. 700 millibar temperatures climb as high as
minus 1 to minus 2 which would lift snow levels to 7500 to 8000 feet
at times. As for snow amounts in the mountains, it looks more like
an advisory level event than warning at this time, but it will have
to be watched. Meanwhile, East of the Divide it will be warmer day
with increasing and thickening clouds. There could be some rain or
showers late, but most areas will be dry most of the time during the
daylight hours. Winds will become gusty at times ahead of the
trough, but mid level winds do not look strong enough to support any
wind highlights though.

The missions get harder on Thursday night. The models are
consistent in showing an upper level low moving out of the Great
Basin and passing south of Wyoming. This will turn flow to the
favored northeast upslope direction. There are also good amounts
of moisture associated with this system, with both the NAM and GFS
zeroing in on the Wind River Basin, Lander Foothills and East
Slopes of the Wind River Range as the most likely location for the
heaviest QPF, both showing over an inch. As far as type, 700
millibar temperatures look to stay at minus 5 or higher which
should keep snow levels above 6000 to 6500 feet through Thursday
night. The final and most difficult mission will be on Friday,
however. Temperatures continue to cool though so there could be a
window for snow on Friday in the lower elevations. It is
concerning that the GFS has trended cooler as well and keeps high
temperatures in the 30s. A lot will depend on how much dynamic
cooling there is. Meanwhile, the Wind River Mountains could see a
LOT of snow, over a foot is a very good possibility at this time.
The lower elevations may once again come down to a 1 or 2 degree
difference. This could make a difference as to whether the main
concern is snow or possible flooding. The NAM MOS is already
showing possible high wind for places like Rock Springs on Friday.
If there is snow falling, there could be blizzard conditions at
times. However, there will be a gusty northeast wind in many
places that will make Friday just a plain ugly day.

The ending credits should be Friday night, when conditions should
begin to improve Friday night as the low moves away and a ridge
builds in from the west.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

Synopsis...A mainly dry and milder pattern will prevail over the
weekend between storm systems. The next Pacific storm system will
dig into the Great Basin on Monday and move across the central
Rockies into the central High Plains on Tuesday. This system will
spread another round rain and higher elevation snow showers across
the area Monday and Tuesday with some thunderstorms possible on
Monday. Most precipitation will shift east of the area on
Wednesday, April 5th.

Discussion...Synoptic pattern on Saturday morning features closed low
over NW New Mexico, upper ridge extending from the sub-tropical
eastern Pacific across the northern Rockies, shortwave trough (our
Mon/Tue system) moving across the NE Pacific. Heavier
precipitation shifts from the southern Plains into the Front Range
of Colorado on Saturday as upper low wobbles slowly east across
New Mexico. Some low clouds/fog and light rain or snow showers
will likely linger roughly along and south of a Rock Springs-
Riverton-Casper line Saturday morning. Upper ridge over the
northern Rockies will then clear most of the area out in the
afternoon.

GFS and ECMWF in better agreement on latest 29/00z runs bringing
next system across the west Monday and Tuesday. Earlier ECMWF run
was progressive and open with this system with latest run trending
much slower, showing an upper low closing off over northern Utah
18z Monday. Overall ECMWF shows a further southward and quicker
track across the Great Basin and central Rockies than GFS Monday
and Tuesday. Though flow is split, both models show some
interaction with the northern stream over the Rockies, which is
highly suspect especially in GFS case. Persistence is likely the
best way to go here with more weight given toward ECMWF main upper
low track diving across the central Rockies, into SE Colorado/NE
New Mexico 18z Tuesday. Upper difluent flow and unstable airmass
ahead of system will likely lead to showers and a few
thunderstorms increasing across the west Monday morning and
spreading east of the Divide through the day. As with previous
systems, frontogenesis/northerly flow will usher in colder air
Tuesday dropping snow levels to around 6000 feet on Tuesday. Upper
low is expected to move across the Central Plains on Tuesday
night with precipitation ending across most of the area by
Wednesday morning.

&&

.AVIATION...18Z Issuance

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

Most of the morning low clouds and fog have dissipated. Mid and high-
level clouds will increase this afternoon and evening with some
lowering around KJAC by 06Z/Thu. Have included VCSH in the KJAC
terminal by 05Z/Thu as atmosphere begins to moisten ahead of the
next wave approaching from the west. Mountain tops will be obscured
between 06Z-18Z/Thu as moisture significantly increases by 12Z/Thu.
MVFR and local IFR conditions/mtn obscn at KJAC just prior to
12Z/Thu and continuing through the day Thursday. expect southwest
winds of 10 to 20 mph at KRKS this afternoon.

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

Most of the morning low clouds and fog have dissipated. Conditions
will be VFR through the forecast period. General trend will be for
increasing mid and high-level cloudiness during the late afternoon
and nighttime hours.

&&

.FIRE WEATHER...

A ridge of high pressure will bring mainly dry weather today with
light to moderate winds, although a gusty breeze is possible at
times across the Red Desert. Relative humidity should remain above
critical levels. Mixing and smoke dispersal will generally range
from fair to poor may possibly become good in areas where there is a
breeze. A Pacific cold front will move toward the west tonight and
spread some snow and rain to that region with the heaviest expected
Thursday. A wetting rain is possible in many areas East of the
Divide Thursday night and Friday.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Jones/Lipson
FIRE WEATHER...Hattings



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