Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170846
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
246 AM MDT Mon Apr 17 2017

.SHORT TERM...Monday through Wednesday night

Things are relatively quiet at this point across the area. However,
after a nice weekend things look to be turning more active. Radar is
showing a few echos across the west but these don`t look to be
reaching the ground at this point. With a Pacific trough and cold
front approaching the area there will be an increase in showers
across the western mountains, especially across the northwestern
mountains. The models continue to indicate the best dynamics moving
by to the north so although there could be some moderate
precipitation for a time, winter weather highlights do not look
likely at this point. As for snow levels, at this point they look to
run around 7000 to 7500 feet. As for East of the Divide, the chance
for showers looks slim with the area in the right front quadrant of
the jet. We kept a slight chance across the far north and the Big
Horns could see a few with upslope flow but most areas won`t.

However, this will lead to another problem. With the downward
momentum from the jet and the tightening of the pressure gradient
ahead of the front, it will turn windy for many areas today,
especially in the afternoon. And this applies to most areas, not
just the normally wind prone areas. At this time, they do not look
quite strong enough for high wind highlights though. It will be a
hold on to your hat afternoon for many areas before the front swings
across the area late today. Temperatures should remain mild ahead of
the frontal passage.

This front should move east of the area this evening with a relative
lull in the activity tonight. However, another more potent system
will move toward the west late tonight and especially Tuesday
morning and spread a better chance of precipitation toward the area.
This system looks warmer at the onset with 700 millibar temperatures
climbing to between minus 1 and minus 3 which would make the
precipitation largely rain in the western valleys. There could even
be a couple of embedded thunderstorms as well. The steadier
precipitation should continue into Tuesday night as an upper level
low passes across the area before tapering off on Wednesday.

However, the problem this morning is east of the Divide where the
forecast is causing much consternation, mashing of teeth and hair
pulling. That is about the details of what happens as the system
moves east and where the best dynamics set up. The NAM is further
south with the heaviest precipitation across areas mainly south of
Rock Springs to Casper line and keeps most of the organized
precipitation to the east of the area. Meanwhile, the GFS is
further north and brings the heaviest precipitation across much of
central Wyoming with an area of moderate to possibly heavy
precipitation across portions of Johnson and Natrona Counties
Tuesday night. It also lingers showers across much of the area
into Wednesday as well. For now, we made few changes but trended a
bit more on the pessimistic. We also lowered high temperatures a
bit as well with the cloud cover around. Things should dry out
fairly quickly Wednesday night as the system moves away.

.LONG TERM...Thursday through Monday

Synopsis...A Pacific storm system will spread valley rain and
mountain snow showers with isolated thunderstorms across the west
and south Thursday afternoon. This system will dig into the
Central and Southern Rockies Thursday night and move out into the
Central and Southern High Plains on Friday. Heavier, more
widespread precipitation is forecast to shift along and east of
the Divide Thursday night and Friday. An upper ridge will build in
Friday night and Saturday with showers decreasing from the north
and west, dry and milder conditions prevailing across most of the
area on Saturday. A quick-moving upper level disturbance will
spread chances of showers and isolated thunderstorms across the
area on Sunday with temperatures remaining seasonably mild. Dry
and mild conditions are expected Monday.

Discussion...Upper low over the eastern Pacific near 43N/147W will
be our main weather maker Thursday and Friday. ECMWF and GFS have
had difficulties handling the multiple vortices within the larger
circulation that slide down the front side of ridge across the
Intermountain West on Thursday. Latest GFS has trended toward
ECMWF or a more north and east track with this energy as it is
shoved across the Great Basin. In fact, both models but especially
the ECMWF now float a distinct mid-level circulation over east
central Wyoming Friday morning that is associated with a heavier
band of QPF running from southeast into central Wyoming.
Confidence in specific placement of these features is low at this
time with little run-to-run continuity established. However,
precipitation chances have been increased across most of the area,
especially northern zones to account for these overall trends, and
past behavior of these types of systems this Spring. The storm
system is expected to become more consolidated over the southern
and central High Plains Friday afternoon where focus for heavier
precipitation will shift. ECMWF shows more consistency than GFS
with a weaker shortwave moving across the area on Sunday, that
will spread a quick shot of showers and isolated thunderstorms
west to east mainly Sunday afternoon and Sunday evening. Overall,
precipitation amounts are expected to be light with this moisture-
starved system and snow levels should be around 7500 feet and up.
Models agree on flat ridge translating the area Monday and Monday
night.

&&

.AVIATION...12Z ISSUANCE

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

A weather system will spread rain and mountain snow across western
WY today. The rain and snow will be decreasing in areal coverage
after 00Z, but snow showers and lower ceilings will still occur at
times in the mountains through 12Z Tue. Expect MVFR/IFR conditions
near the mountains along with frequent mountain obscuration. KJAC
airport will be impacted by rain showers after 15Z through 00Z with
MVFR ceilings possible at times. Then improving conditions are
expected after 00Z. Most of southwest WY will stay dry today and
tonight. Will keep VCSH at KPNA airport this afternoon. Expect a
gusty wind at most of the terminal sites today with the wind
decreasing after 03Z Tue in most locations.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail at the terminal sites today along with a
gusty wind. Isolated rain showers will be possible over northern WY
this afternoon into early evening. The wind will decrease around 03Z
Tue in most locations. The northern Bighorn mountains will some
light rain and snow this afternoon into early evening with MVFR
conditions expected. Conditions there will improve after 03Z Tue.

&&

.FIRE WEATHER...

A cold front will spread some showers into Western Wyoming today.
although heavy amounts of precipitation are not expected. Meanwhile,
a vast majority of areas East of the Divide should see little or no
precipitation with this front except for possibly the mountains.
Strong winds will develop in many areas East of the Divide this
afternoon. With relative humidity falling into the teens in some
areas, fire danger may become elevated. Mixing and smoke dispersal
will be generally good to excellent for most areas. Another system
will bring a better chance of rain and snow for Tuesday.
&&

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

$$

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



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