Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 251736
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1137 AM MDT Fri May 25 2018

.SHORT TERM...Friday through Sunday night

Over the past few days, the weather has reminded me of something
from high school geometry, a sine wave. I`m mainly relating this
to the activeness of the weather. Today we will be in the trough
of the wave, and hence the driest day of the week. Ridging will
be at it`s peak so only very isolated activity is expected, and
mainly across the mountains and possibly the Big Horn Basin and
near Buffalo. Over 95 percent of the area will see nothing though.
With the large temperature / dew point spreads, this has the
potential for those dreaded little green blobs on radar to produce
50 to 55 mph winds if anything does pop. Temperatures will also
be on the rise with most of the lower elevations seeing highs in
the 80s. And showers should end before midnight with mainly dry
conditions from then into Saturday morning.

We then start climbing again on Saturday as the pattern turns more
active. However, maybe not as active as previously thought. The
models have trended a bit slower and drier and keep the best
forcing in Montana. The best chance of any convection would be in
the western mountains and the northern border but even here it
does not look widespread and would be late and most likely mainly
in the evening. The other story will be the temperatures. Saturday
at this point looks to be the hottest day of the year so far with
some of the warmer spots likely reaching the 90s. This could also
increase snow melt in the higher elevations that could increase
river levels.

However like the super group Asia said back in the early 80s, it
will be a "Heat Of the Moment" rather than a "Heat Wave" like
Martha and the Vandellas and later Linda Ronstadt said. With more
cloud cover and low pressure moving over the area, temperatures
will cool anywhere from 10 to 15 degrees. This is not the main
story however. The big story will be another shot of rain and
thunderstorms. And this could be an earlier show, especially
across the southern half of the forecast area as decent dynamics
and upper level divergence move northward across the area. But at
this point, it again looks like the hardest hit area may be the
northern half of the state. There the storms could be strong or
maybe even severe. The big X factor may be if there can be any
sunshine in the morning that could increase surface instability at
bit more. This has to be watched closely over the next couple of
days. Most of the storms should end by midnight as drier air
pushes in from south through Sunday night.

.LONG TERM...Memorial Day through Friday

An upper low will be nearly stationary over the northern Great Basin
on Sunday before ejecting out northeast across Wyoming into the
northern High Plains late Monday and Tuesday.  A band of heavier
precipitation should be north of the area across Montana by sunrise
Monday, and the better instability across the Plains ahead of a
surface trough Monday afternoon.  So we are expected scattered
showers and afternoon thunderstorms to remain over much of the area
Monday, but overall precipitation amounts will be on the light side.
The remnants of the Great Basin system will be an open trough near
the eastern Wyoming border by 00z Wednesday with shortwave ridging
over the west.   Lingering moisture and slight instability will
result in scattered showers and thunderstorms over the western
mountains and far north Tuesday afternoon and evening.  Upper flow
will back SW Wednesday and Thursday as next Pacific trough moves
onto the West Coast.  This will bring a drier and warmer airmass
into the area, with a few afternoon thunderstorms developing over
the high terrain and along favored mountain/valley breeze
boundaries.  The impacts of this next trough remain uncertain at the
tail end of the extended period into next weekend.  The GFS shows a
split flow with more of the same, upper low lifting out of the Great
Basin across the area late Friday-Saturday.  ECMWF shows ridge
strengthening over the southern Plains with most of this trough`s
energy shearing into the northern Rockies.

&&

.AVIATION...For the 18Z TAFS through 18Z Saturday

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

Very isolated showers or storms will occur over the NW WY mountains
after 23Z through 03Z. VFR conditions are forecast at the terminal
sites through the period with no VCSH/VCTS expected.

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

VFR conditions will occur through the forecast period. Dry weather
is forecast at the terminal sites.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...

Expect a warm and mainly dry day with high pressure over the area.
Isolated thunderstorm may form in the northern mountains and drift
into the foothills late today and this evening but the vast
majority of area should remain dry. Relative humidity will fall
into the teens in many areas. A gusty southwest will develop
across southern Wyoming at times. Mixing and smoke dispersal will
range from fair in some of the basins to excellent in Sweetwater
County. A combination of hot temperatures, low humidity and a
gusty southwesterly breeze may bring elevated fire conditions at
times in southern Wyoming Saturday afternoon.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Hulme
FIRE WEATHER...Hattings



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