Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 261753 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1154 AM MDT Fri May 26 2017

.SHORT TERM...Friday through Sunday night

As a lot of long time Wyoming residents know, Memorial Day weekend`s
weather can be quite temperamental. And it looks like Friday will
be no exception. There is not a ton going on right now, but this
will change as a cold upper level low moves across the state. Now
for the lower elevations, where most people live, this will mean a
cloudy, chilly and damp day with some rain showers. However, in
the mountains it will likely mean something else. And yes, I am
talking about the dreaded s-word, snow. This system also has some
decent jet dynamics with it so some of the snowfall is likely to
be heavy. And there could even be some thundersnow in the western
mountains as there is decent instability. As for snow levels,
right now they look to average around 7500 to 8000 feet, at least
in areas where there are more traveled mountain passes. As a
result, we will continue the winter weather advisories as is.
There will likely not be that much snow as indicated in the
advisories on the ground at one time, especially on the roads
given the very strong late May sun though. The models have also
sped up a bit in moving the low out a bit quicker so much of the
snow may end by midnight. And in addition, the Storm Prediction
Center has put parts of Sweetwater County in a marginal risk for
severe thunderstorms so a few storms there could get feisty. The
models still show some snow in the Bighorns with northwest flow,
but most of the models are notoriously too wet with this pattern,
especially with the jet dynamics exiting. Most of the steadier
precipitation should end by Saturday morning.

More settled weather should then move in for Saturday and Sunday. On
Saturday, temperatures should moderate somewhat with more
sunshine. There will still be some northwest flow and cyclonic
curvature over the area however, so most areas will have a chance
of mainly afternoon showers and thunderstorms. However, coverage
at this point looks to be scattered at best with most areas mainly
dry most of the time. Temperatures will also warm as well,
although remain below normal. The main threat from any storms
would be small hail given the cool temperatures and low wet bulb
zero temperatures. A drying trend will continue for Sunday as
ridging slowly builds across Wyoming. This means temperatures
getting close to seasonal levels and less coverage of
thunderstorms. Any storms that do form will likely be mainly over
the mountains and the adjacent foothills.

.LONG TERM...Monday through Friday

A relatively moist northwest flow will be across the area through
Tuesday between an upper ridge over the West Coast/Intermountain
West, and a longwave trough over the central/eastern U.S. Models
show enough moisture and instability with peak heating for daily
showers and thunderstorms forming over the mountains and trying
to push southeast into the basins/valleys. This activity could be
enhanced at times with some subtle waves in the flow and local
convergence zones. Another concern is for convection to put out
gusty outflows well away from their parent convection which could
cause choppy lakes and reservoirs from distant thunderstorms.

By Wednesday models generally show weak ridging shifting over the
Northern Rockies as models show some Pacific energy shifting into
the West Coast. This energy over the west is progged to split as
it pushes across the Intermountain West/Rockies Thursday and
Friday. As typical with day 7 and 8, models have varying
solutions, but generally show a better potential for isolated to
scattered convection late next week. Also, 00Z runs are showing
some low-level easterly flow developing over the eastern zones
Thursday and Friday which could result in some stronger convection
over far east if it indeed develops.

Models have trended slightly warmer overall, with seasonal to
slightly above average temperatures through the extended which
will once again elevate flooding concerns across the area.

&&

.AVIATION...18Z Issuance

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals

An upper level low pressure system will drift east across western
Wyoming this afternoon.  Showers will become more numerous vicinity
and north of a KEVW-KPNA line with mountains becoming mostly
obscured with snow above 8000 feet; widespread MVFR conditions
developing across the valleys.  The atmosphere will destabilize in
the afternoon with some lifting of ceilings, isolated thunderstorms
developing with erratic wind gusts and small hail. Brief IFR/LIFR
conditions will be possible if a thunderstorm hits one of the
terminals.  A cold front with a line of showers and thunderstorms
will move across KRKS area 19z-22z.  Drier air will push into
western Wyoming this evening with lingering rain and snow showers
mainly across the northwest mountains or vicinity and north of a
KJAC-KDUB line after 02z.  Partial clearing after 06z Saturday will
result in some patchy fog partially obscuring terrain vicinity KJAC
through 15z Saturday.


East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

An upper level low pressure system will drift east across western
Wyoming this afternoon and across north central Wyoming this
evening. Rain and mountain snow with snow levels around 7000 feet
will be widespread vicinity and north of South Pass-to northeast
Sweetwater County through 22z.  MVFR conditions will be widespread
across the area through most of the afternoon. Most precipitation
will shift into the High Plains this evening, with gradual
improvement from northwest to southeast overnight into Saturday
morning.

&&

.FIRE WEATHER...

Expect a cloud, cool and damp day across most of the area as low
pressure moves across Wyoming. Most areas will see showers and
possibly a spotty thunderstorm. In the mountains above around 8000
feet, snow will fell and several inches are possible in the higher
elevations. Relative humidity should remain well above critical
levels. Mixing and smoke dispersal will be generally fair although
good to very good in southern Sweetwater County.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM this morning to midnight MDT
tonight for WYZ002-012-014-015.

Winter Weather Advisory from noon today to 6 AM MDT Saturday for
WYZ008-009.

&&

$$

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



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