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fxus65 kriw 260821 
afdriw

Area forecast discussion
National Weather Service Riverton Wyoming
221 am MDT Wed Apr 26 2017

Short term...Wednesday through Friday night

We are currently in a relative lull across the area as the low that
brought the heavy snow to portions of the area moved away to the
east. There are still a few showers out there at this time, but
nothing of consequence.

However, this looks to change as yet another Pacific front and
trough approach the area. Another period of snow will break out in
the western mountains this afternoon. This system does look
somewhat warm at the onset, so valleys look to be largely rain
during the day. However, colder air will move in tonight and and
drop the snow levels to the valley floors. As of now, the winter
weather advisories will ride as is. An area like the Wind River
mountains will likely have to wait until later tonight or even
Thursday for any decent snow so we held off on any advisories there.
At this time, the best snow would fall tonight into early tomorrow
morning as the trough axis and some left front quadrant jet dynamics
enhance lift a bit.

Meanwhile, east of The Divide look for a drier and somewhat milder
day. There will still be a few showers around from time to time and
even a thunderstorm, but most areas will be rain free most of the
time. The steadier precipitation will likely get kicked across the
mountains but most of this would hold off until after midnight. The
best chance of this would be across northern Wyoming some could fall
as snow. The day shift may have to evaluate for winter weather
advisories, especially in the Bighorn range.

Steadier precipitaion will shift into central Wyoming Thursday as
flow turns northward and more upslope as low pressure moves to the
east. Some of this will likely fall as snow with 700 millibar
temperatures falling to minus 7 to minus 8. However, accumulations
may be tricky as much of the snow would fall during the day and we
will be fighting the almost may sun angle. It could be a situation
where grassy surfaces have a few inches and the roads are only wet.
There could also be some heavier bands that set up but these are
impossible to pinpoint this far out. Meanwhile, the west should see
a gradual drop in the coverage of showers although a few will
persist through the Thursday and into Thursday night. There will, of
course, be a better chance for snow to accumulate at night. And the
models have trended toward the best snow to be further east, across
Natrona and Johnson counties so this will have to be watched as
well. Needless to say, it will be a cloudy, chilly and cruddy day
with high temperatures continuing well below normal.

The unsettled and cool weather will likely stick around in Friday as
well. Again, the models keep the best quantitative precipitation forecast in eastern portions of the
area. However, there are some differences. The GFS throws more
moisture back into central Wyoming during the day while the NAM is
not as bullish. We should continue to have good north to
northeasterly upslope flow also. We split the middle at this point.
And again, it is cold enough for snow in most areas, even colder at
700 millibars with temperatures as low as minus 10. Although, once
again, the sun angle may keep roads mainly wet during the day. The
system should finally loosen it's grip on Friday night as it moves
away and a transitory high pressure area moves across the state.

Long term...(saturday through wednesday)

Medium range period begins with most of the guidance showing a
positively tilted upper trough/low near the 4-corners region. Most
of steady/significant pcpn that may fall with this storm system will
be already over by the start of this period. However, it will remain
cold and unsettled with another weak E-se movg shortwave on Saturday
to aid in increasing afternoon snow showers over and near the
mountains, especially wrn half, spreading east by the evening hours.
Northwest flow will dominate early next week before we finally get
some warm ridging developing for the second half of next week. Euro
is further north with these sewd moving shortwaves early next week
than the GFS and Gem. Big picture still supports more of the GFS/Gem
so will lean that way. Next shortwave on Sunday will produce a good
chance of mountain snow showers and foothill rain showers in the
west Sunday afternoon with a few showers spreading east Sunday
night, more numerous in the far north (especially the bighorns). The
biggest difference between the Euro and GFS is with the digging
shortwave Monday night and Tuesday. The GFS/Gem actually close this
system off over swrn Wyoming on Tuesday with snow likely starting in
the west and north later Monday into Monday night and then shifting
into central Wyoming later Monday night into Tuesday. Cold and
unsettled into Tuesday night before improving conds begin on
Wednesday. The Euro has trended further SW with this shortwave but
it is much weaker than the GFS and Gem. If the GFS/Gem idea is
right, temps will be way too warm east of The Divide as many areas
at least above 5k would likely be snow/snow showers on Tuesday (not
50 degrees). Wednesday will will be mostly dry but with a cold
start. However by the end of day Wednesday, it should rise into the
50s east of The Divide for most areas (even if its lower 50s).
Warmer weather is in store just beyond this period for at least a
couple days.

&&

Aviation...12z issuance

West of The Divide...kbpi/kjac/kpna/krks routes

The next Pacific storm system will begin to spread scattered rain
and snow showers into the western Wyoming 12z-18z Wednesday with
areas of MVFR ceilings and increasing mountain obscurations. Valley
rain and mountain snow showers will become more numerous Wednesday
afternoon with a few thunderstorms possible. Snow levels will lower
to near valley floors Wednesday evening as showers reach peak
coverage with MVFR conditions becoming widespread, areas of IFR
conditions also pushing into vicinity krks by 02z Thursday. These
conditions will persist across the area through 12z Thursday.

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

VFR conditions will prevail through Wednesday evening. Widely
scattered rain and snow showers will move off the Absaroka Range
into vicinity kcod-kwrl through 15z Wednesday before moving further
east and dissipating. The next Pacific system and associated cold
front will spread scattered showers and isolated thunderstorms off
the mountains into central Wyoming late Wednesday afternoon and
evening. Rain and snow showers will become more numerous along with
widespread MVFR/local IFR conditions vicinity and north of a kriw-
kcpr line 03z-12z Thursday.
&&

Fire weather...

A cool and wet pattern looks to persist for much of the week. Today
looks to have a bit of a lull but showers will still be possible for
many areas. The next front will move into the west and spread more
rain and snow for areas west of The Divide this afternoon and
evening. A few thunderstorms may fire east of The Divide this
afternoon and evening with a gusty wind developing across the
southwestern wind corridor. Mixing and smoke dispersal will range
from poor to fair across the north and west to very good in the
southeast. Steadier precipitation will spread east of The Divide
tonight with cool and damp weather expected from Thursday through
Friday.
&&

Riw watches/warnings/advisories...
Winter Weather Advisory from noon today to noon MDT Thursday for
wyz001-002-012-024.

Winter Weather Advisory from 6 PM this evening to noon MDT
Thursday for wyz013-023.

&&

$$

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