Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221743
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1140 AM MDT Mon May 22 2017

.SHORT TERM...Monday through Wednesday night

Most areas look fairly quiet as I write this. However, another cold
front and associated upper level disturbance will be dropping
southward across the state today and bring another chance of showers
and isolated thunderstorms. The models have been fairly consistent
in keeping the best upper level forcing and moisture to the east of
the county warning area though. In addition, the front is a fairly
fast mover so the precipitation will not be long lasting. The
heaviest precipitation will likely be across the Bighorns where
there will be some favored upslope flow across the western slopes.
The models are likely overdoing the QPF a bit though. Nevertheless,
there will likely be some snow with snow levels falling as low as
7500 feet. However, duration will not be long enough for any
advisories. In addition, with the very high sun angle any snow that
falls after around 8 am will have a very difficult time sticking on
paved surfaces. The chance of showers and a few thunderstorms will
shift southward across the state during the day as well. Nothing
really strong is expected though, mainly the usual gusty wind and
small hail type. These should end this evening as the front moves
away to the south and east. In all areas, temperatures will remain
on the cool side with the cold cover and cold advection.

The other story will be the wind. The pattern looks favorable for
some strong winds in areas favored by north to northwestern cold
advection following frontal passages. This means Johnson County
especially but possibly even the northern Big Horn Basin. The wind
at 700 millibars is not quite enough for any high wind highlights,
but wind gusts to 50 mph will certainly be possible in above
mentioned areas. Wind should decrease after sunset though as the
pressure gradient decreases.

We should then enter a quiet weather period for a while. Riding
will build across the area for Tuesday and bring a dry and pleasant
day. With more sunshine it will be warmer but possibly still a bit
below normal temperature wise. We did go above guidance though since
the models tend to have a cool bias in patterns like this. Wednesday
looks like a warm day with an increasing southwesterly breeze ahead
of an advancing cold front. This should increase snow melt and
possible lead to rises in the rivers. As for the chance of
precipitation, it looks rather limited during the day with the best
moisture and dynamics remaining over Montana so we have only
isolated POPS along the northern border. There could be increased
chance of showers across the south however as some moisture gets
some lift from the right rear quadrant of a jet streak so we
increased POPS across the south later Wednesday night.

.LONG TERM...

System moving through the Aleutians will be our weather maker to
start the medium range period. A system moving through Kamchatka
attm will help build a ridge along the west coast and up into wrn
Canada helping to slow down and linger this system from Thursday
through Saturday as several shortwaves drop down into the trough.
GFS lingers this trough the longest (through Saturday) while the
Euro and GEM bring in some shortwave ridging Saturday. Mean ridge
is starting to show in the GFS 5-wave which carries over into
Monday so we`ll see if maybe the Euro/GEM idea maybe wins out
eventually. For now, the model blend keeps pops everywhere through
Saturday so will have to leave that for now. Otherwise, the second
shortwave (on Fri) dropping down the backside of the initial
trough will likely bring the best shot of showers/few storms to
much of the area, especially in and downwind of the mountain ranges
in the nw flow. The initial front on Thursday will also likely have
some gusty north wind. Beyond the question mark for Saturday`s
weather, we do see some ridging building into the far west but
ripples moving through the ridge and frontal boundary likely hanging
around the east combined with daytime heating will produce daily
convection.

&&

.AVIATION...18Z Issuance

A weak low pressure system is moving southward in mid-level north to
northwest flow, currently over northern WY.  VFR conditions with
some local MVFR conditions due to ceiling heights with the rain
showers through 03Z/Tue from north to south.  Least impacted TAF
sites will be in western WY as the disturbance mostly slides down
the east side of the continental divide.  Some isolated
thunderstorms can be expected this afternoon, primarily around KCPR,
with gusts around 25 kts as well.  After 00Z/Tue showers should move
into SW WY with weak showers only through 06Z/Tue.  With lowered
ceilings, higher terrain partially obscured east of the divide
through 06Z/Tue. With gusty north wind diminishing after 03Z/Tue
patchy fog possible around KCPR and KRKS from about 10Z-14Z/Tue.
Tuesday expected to be mostly clear with generally light to breezy
wind, with some northwest gusts over 20kts possible.

&&

.FIRE WEATHER...

Unsettled weather will continue today. A cold front will bring
another chance of showers and isolated for today across many areas
with the best chance across eastern portions of the area. Snow will
fall in the higher elevations with a few inches of snow possible in
the Bighorn range. Relative humidity will remain well above critical
levels. Strong north to northwest winds will be possible across
Johnson County and the northern Bighorn Basin with breezy conditions
in many areas. Mixing and smoke dispersal will range from fair to
very good. Drier and warmer weather will return Tuesday and most of
Wednesday.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...McDonald
FIRE WEATHER...Hattings


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