Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 270908
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
308 AM MDT Sat May 27 2017

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

Imagery shows amplified ridge over the EPAC into the West Coast with
an amplified positively tilted trof stretching from cntrl
Canada through the nrn High Plains then across the cntrl Rockies and
into the Great Basin. The trof axis runs across the FA from ncntrl
through swrn WY. The SFC has a cold front through the FA and into
nrn CO with high P building across WY. A few isolated to widely
scattered showers are peppered around the region with the most
concentrated area of showers across srn/sern WY.

Today the upper level trof continues to exit the region to the east
while digging through the cntrl Plains...developing strong SFC low P
over the srn/scntrl Plains with a warm front extending up to the mid
MS Valley, working toward the OH Valley. This will most likely be
the area to watch for all sorts of severe WX on Sat. Central and
western WY, however, will be post frontal but with enough cold air
remaining overhead combining with a slowly warming SFC to keep the
atmosphere modestly unstable enough in the afternoon and evening to
get showers and a few T-storms...moving from north to south at 20 to
30 mph...mainly on or just south of good terrain features. Small
hail and gusty winds possible.

For the rest of the Memorial Weekend forecast (Sunday and Monday),
the FA will remain under NW flow aloft with enough instability in
the afternoons and evenings to get showers/thunder...with the better
day of the two being Sunday as a small disturbance is still forecast
to move across the ern FA in the afternoon. Precip will be much more
isolated Monday better coverage over the western Mountains. The main
concern from any stronger storms (no severe here) would be small
hail and gusty winds. Otherwise, slow but steady warming should be
in place through the holiday...nice, but no records...with
relatively light winds.

.LONG TERM...(Tuesday through Saturday)

Flooding concerns from snowmelt runoff increase late next week as
forecasted consecutive warm temperatures would cause rapid
mountain snowmelt.

A dirty ridge will be across the Intermountain West/Rockies
Tuesday and Wednesday ahead of an upper low/trough pushing into
the West Coast Tuesday night/Wednesday. Enough moisture and
instability for diurnal mountain convection decaying as it pushes
into the lower elevations.

The 00Z ECMWF and Canadian models show the trough deamplifying
and/or splitting as it pushes east northeast across the
Intermountain West/Rockies Wednesday night and Thursday with an
increase in chances of convection across the entire area. Those
models then show flat ridging Friday, before another Pacific
trough could impact the area next weekend. The 00Z GFS shows the
splitting trough pushing into the area Thursday, but then parts of
same, but different segments of the splitting trough remaining
over the area through next weekend. The operational GFS solution
is a new wrinkle tonight, but the hemispherical pattern continues
to show more of a chance of ridging over the west. Thus have
leaned toward the drier ECWMF and Canadian solutions. However
still would expect some diurnal convection Friday and Saturday
especially near the mountains and adjacent foothills downwind from
the prevailing mid-level flow.

Temperatures are expected to be slightly above average. These warm
temps will cause rapid mountain snowmelt, and will elevate
flooding concerns from middle of next week and beyond.

&&

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

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

Isolated to scattered rain and snow showers, mainly over the
mountains, will continue to dissipate with clouds scattering out
early this morning.  This will allow patchy fog to develop vicinity
KJAC until around 14z.  The exception to the clearing trend will be
KRKS where some lower MVFR ceilings may back into the area from the
north through around 13z. VFR conditions will then prevail from 14z
Saturday onward.  Slight instability will result in isolated showers
and thunderstorms developing in the vicinity of the Continental
Divide with this convection tracking off the southern end of the
Wind River Range but should dissipate before reaching vicinity KRKS.
This activity will dissipate with sunset, leaving prevailing SKC-FEW
FL130-150.

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

An upper trough will continue to push southeast across the area
early this morning with most shower activity expected to be south
and east of the area by 12z; patchy fog may partially obscure
terrain through 14z.

Slight instability Saturday afternoon will result in isolated
showers and thunderstorms developing along mountain valley/breeze
boundaries along the south and eastern slopes of mountain ranges and
moving SSE.  This activity mostly anchored to the high terrain and
not impact any terminals. KRIW-KCPR routes will be most vulnerable
to a few thunderstorms in the vicinity of the southern Bighorn
Range. Convection will dissipate with sunset, leaving SKC-FEW FL130-
150.

&&

.FIRE WEATHER...

Fire danger low across the forecast area through the holiday weekend
as all fuels are currently in green-up and much of the forecast area
has received some form of precipitation over the last few days.
Precip chances decrease over the next 3 days, being isolated to
widely scattered at best and most likely to occur over the mountains
and closely adjacent lower elevations. Additionally, high
temperatures and minimum RH values will remain at or below seasonal
and above 20 percent respectively. Winds too will remain relatively
light through the period. Smoke dispersion will be good to very good
most afternoons.

&&

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

&&

$$

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



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