Area Forecast Discussion
Issued by NWS Riverton, WY

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289
FXUS65 KRIW 300905
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
305 AM MDT Sun Apr 30 2017

.SHORT TERM...(Today through Tuesday night)
Issued at 116 AM MDT Sun Apr 30 2017

Imagery shows longwave trof across the CONUS with closed low in base
of trof centered over the Texas Panhandle. Upstream sw trof/jet just
now entering the PAC NW. Strong 120 to 160 kt jet associated with
the LW trof stretches from the cntrl west coast down through the
Desert SW...across the srn Rockies then out through the cntrl
Plains/nrn Plains/north of the Great Lakes/then east to New England.
Wind speeds are 20 to 50 kts at jet level over WY. SFC has general
high P building over WY with lower P off to the nw ahead of the
approaching sw trof coming into the PAC NW. No precip currently
falling over WY this morning.

These benign conditions will last into this afternoon before the
upstream sw trof rides through the nw flow aloft and into wrn WY
with showers breaking out west of the Divide mainly over the
mountains. An associated cold front will move into the western FA
this evening...then east of the Divide before sun-up Monday morning.
Overall, light non-highlight precip/snow amounts expected with this
first system into early Monday afternoon.

The next and somewhat more significant Wx system will then follow on
the heals of Monday`s system...sparked by the approach of yet
another sw trof (with jet and associated cold front to follow just
beyond this fcst period) entering the FA from the NW later Monday
night. Precip chances will increase accordingly west of the
Divide...especially over nwrn WY...with light snow expected to begin
around midnight Monday, spreading to the south and east through.
ATTM, frontogenesis looks to develop across cntrl/srn WY Tuesday as
the main portion of the upper sw trof and exit region of an
associated jet move into/over the FA. Forecast H7 temps look to
range from -6 to -8 deg C east of the Divide with convective snow
showers and a few embedded thunderstorm (with snow) possible by
Tuesday afternoon and into the evening. Heaviest precipitation with
this second system looks to fall across the northern and central FA.
Overall, QPF will remain heaviest over the Absarokas and Bighorns
with much lighter amounts across the lower elevations east of the
Divide. While generally only a trace (lower elevations) to 2 or 3
inches (mountains) of snowfall expected for any one location, the
afternoon/early evening convective nature of the precip will have to
allow for localized/isolated 2 inch + snowfall/small graupel in
short order from a few heavier showers/storms over the lower
elevations...especially later Tuesday afternoon/evening. This wx
system will be moving out of the area Tuesday night

.LONG TERM...(Wednesday through Sunday)
Issued at 116 AM MDT Sun Apr 30 2017

Mainly dry and slightly below average temperatures are expected in
wake of the shortwave trough Tuesday. Model consensus is strong
ridging building across the Rockies Thursday and Friday ahead of a
trough/upper low off approaching the West Coast. This will result
in sharply warmer temperatures Thursday and Friday. The upper-level
ridge axis appears to shift into the High Plains late Friday which
could allow for a few showers possible over far western Wyoming
in the afternoon.

Models continue to struggle with the evolution of the trough/upper
low over the Western U.S. next weekend, but overall they our
slower and try to develop a cut-off upper low in the California/S.
Great Basin region. Either way, looks like the area will be in the
warm sector, and have nudged temperatures warmer. Models continue
to produce some daily convection across the forecast area
Saturday and Sunday. Both the ECMWF and GFS show weak instability
and steep lapse rates, but marginal moisture and probably some
capping inversion should keep convection to a minimum. 700mb
temperatures look to warm to around +8C to +11C Friday and
Saturday with slightly cooler temps Sunday as the mean trough
pushes closer. Some of the lower elevations east of the Divide
could see the 80 degree mark.

Depending upon the evolution of the trough, the forecast area
could see 2 to 4 days with temperatures 10 to 15 degrees above
average. This will need to be watched for rapid low mountain (<8500
feet) snowmelt, and any related flooding issues this runoff will
have on any small streams in the foothills or the western valleys.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 116 AM MDT Sun Apr 30 2017

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

VFR conditions will occur at most of the terminal sites through 12Z
Monday. Rain and mountain snow showers will develop over NW WY after
19Z and continue through tonight. Expect some mountain obscuration
and MVFR to IFR conditions in the mountains. At KJAC airport, VFR
conditions will prevail through 00Z, then VCSH will be in the
terminal forecast through 12Z Monday with the potential for MVFR
ceilings.

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

Patchy fog may form in the Wind River Basin overnight with local
MVFR/IFR conditions. VFR conditions will occur through 12Z Monday at
the terminal sites. Snow showers will develop in the Northern WY
mountains around 00Z and continue through 12Z Monday. Expect some
mountain obscuration and local MVFR conditions. Isolated rain or
snow showers will occur at the lower elevations after 06Z Monday.
Will have VCSH at a few terminals around or after 06Z Monday.

&&

.FIRE WEATHER...
Issued AT 116 AM MDT Sun Apr 30 2017

Fire danger low across the forecast area through the next week. All
fuels currently in green-up. Periods of isolated to widely scattered
light rain and/or snow showers will occur this afternoon into Monday
afternoon...and again later Monday night through Tuesday
evening...with heaviest and most widespread precipitation occurring
Tuesday afternoon and evening. Winds will be relatively light for
most locations over the next several days...with brief gusty periods
near any frontal passage (along with a wind shift) or near heavier
convective showers/thunderstorms on Tuesday. Smoke dispersion good
to excellent today through Tuesday (afternoons).

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Ross
FIRE WEATHER...Braun



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