Area Forecast Discussion
Issued by NWS Riverton, WY

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101
FXUS65 KRIW 300910
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
310 AM MDT Thu Mar 30 2017

.SHORT TERM...Thursday through Saturday night

This is one of those forecasts days that gives meteorologists
premature hair loss and aging. One of those transitional season
forecasts where just 1 or 2 degrees of temperature or a few
hundred feet of elevation can make a huge difference in weather
and more importantly the impacts of the weather. The details are
below. In addition, with the differences in weather, we will
divide the discussion into West and East of the Divide.

West of the Divide...The radar of our neighbors to the west shows
some precipitation moving in from the west. The webcams have shown
some snow at Teton Pass. However, temperatures in the valleys are
warm enough to have mainly rain. There is a brief period this
morning when 700 millibar temperatures drop enough, down to minus 4
or so, when there could be some snow in the Valleys, mainly the
Jackson Valley. However, temperatures warm again in the afternoon so
lower elevations should switch back to rain, possible as high as
8000 feet. As for amounts, we agree with the day shift that although
there could be some isolated advisory amounts, they should be the
exception rather than the rule. Periods of rain and snow will
continue tonight as well. As for any potential highlights, there
could enough snow for some thing around Lincoln County but
temperatures may be marginal. It would not be until tonight at the
earliest though.

The next question is what happens in Sweetwater County, especially
Friday. As the pressure gradient really tightens across the area as
low pressure passes by and develops to the south. This will bring in
some rather strong winds. The GFS MOS is quite impressive and
showing 44 knots sustained around noon on Friday. The NAM is not
quite as impressed but still rather windy. This could turn out to
the be the main concern as some models show much of the area being
dry slotted. Not ready to pull the trigger on a watch though since
the models have been inconsistent on the dry slot and there could be
some snow. The day shift will have to consider it though.

Conditions should improve Friday night as low pressure moves away to
the east. Saturday should be a dry day with the exception of a few
lingering showers across eastern Sweetwater County.

East of the Divide...Here is where the forecast is giving us acid
reflux. Things a quiet right now. However, clouds will thicken and
lower today as a 700 millibar low dives south into the Great Basin
and turns flow toward the east and northeast. A few showers should
blow East of the Divide today but most areas should be dry most of
the time. Temperatures should remain mild with 50s and some 60s for
high temperatures.

However, things will begin to change tonight. As the east to
northeasterly flow deepens it will throw copious amounts of moisture
back into the area. With this flow, it will favor portions of
central Wyoming, especially areas the eastern slopes of the Wind
River Range, Lander Foothills, the Green and Rattlesnake Ranges and
to a lesser extent the Wind River Basin. There could also be decent
precipitation in areas like the Cody Foothills and the East Slopes
of the Big Horn Range although the moisture should not be as deep
here. The models have been consistent in keeping the bullseye of QPF
over areas around Lander for several runs now so we are confident
this is where the best precipitation will be.

However, the problem is precipitation type for some areas. There
remains some uncertainty for many areas. As for the mountains, the
East Slopes of the Wind River Mountains will almost certainly be the
bullseye for the snow. This will be a major snowfall with 1 to 2
feet a very good possibility with more possible in some localized
areas. However, the uncertainty will be in areas from around 5500 to
6500 feet. For the most part, through tonight, temperatures should
be warm enough for rain. However, during Friday morning 700 millibar
temperatures will drop to around minus 6c in some areas. This
would be enough for heavy wet snow in a place like Lander. This is
probably the area of greatest uncertainty as 1 or 2 degrees of
temperatures or a few feet of elevation can make a HUGE
difference. A lot depends on how much dynamic cooling there is as
well. It could be a situation where a place like Hudson has only a
slushy inch and the entrance to Sinks Canyon could have over a
foot. Or where, it could be mainly rain and a bit of snow in
Riverton meanwhile several inches of snow may fall at the WFO,
about 700 feet higher in elevation. For the most part, we made
only minor tweaks, mainly to raise snowfall amounts a bit,
especially in the Lander area. The NAM also brings the changeover
a bit slower, so this could effect amounts. Add in the strong late
March sun, and it could be a situation where there are only
slushy roads and a lot on the grass. As for the time of heaviest
precipitation, it looks like the window from around 4 am to 4 pm
at this time. Some lighter precipitation will continue into Friday
evening, maybe mainly snow as cooler air moves in. For other
areas, Casper looks to have much less QPF and somewhat warmer
temperatures although there could be a few inches of snow as
cooler air moves in.

Any lingering light snow should end by noon on Saturday with
increasing amounts of sunshine through the day. Another concern may
move in for Saturday if wet snow melts rapidly as it could raise
flooding concerns in some areas. Dry and seasonable cool weather is
expected for Saturday night.

.LONG TERM...Sunday through Thursday

Synopsis...An upper level disturbance moving into the northern High
Plains Sunday morning will bring isolated to scattered rain and snow
showers across the north and central.  The next Pacific storm system
will move across the Great Basin and central Rockies Monday and
Monday night with more widespread rain and snow expected across most
of the area on Monday.  Precipitation will taper off from northwest
to southeast on Tuesday.  A ridge of high pressure will bring dry
and milder weather to the area Wednesday and Thursday.

Discussion...Some spread at the outset between GFS and ECMWF with
GFS quicker to lift upper low out of the southern Rockies into the
Central Plains on Sunday.  GFS solution leads to northern stream
shortwave digging more across the northern High Plains on Sunday
morning while ECMWF keeps this shortwave moving east along the
U.S./Canadian border, keeping QPF north of Wyoming.  Gave some
weight toward GFS with some low PoPs across the northern 2/3rds of
the area, best chances from northern Bighorn Mtns and roughly along
I90.

Both ECMWF and GFS shows what looks to be the last in a series of
upper lows diving down across the Intermountain West into the
southern/central Rockies Monday/Monday night before synoptic pattern
is forecast to transition to troughs near both coasts with broad
ridge in between across Rockies and central U.S. late in the week.
GFS brings more shortwave energy and precipitation into the west
through the mean ridge on Thursday while ECMWF keeps it dry.
Forecast favors maintaining the ridge over the area at least through
Thursday; dry weather prevailing both Wednesday and Thursday.

&&

.AVIATION...12Z ISSUANCE

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

Scattered rain and mountain snow through the day. Areas of MVFR
conditions will occur along with some mountain obscuration. KRKS
will stay VFR through 00Z. In southwest WY around 00Z, rain will be
increasing in coverage along with MVFR conditions prevailing through
12Z Fri.

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

VFR conditions to prevail through 00Z. Isolated showers will occur
this afternoon. After 00Z, areas of MVFR cigs will develop along
with an increase in rain coverage. Around 06Z Fri, widespread rain
will occur along with MVFR to IFR conditions. Please see the
terminal forecasts for more details.
&&

.FIRE WEATHER...

An area of low pressure will spread valley rain and mountain snow in
the West today. A few showers will also move East of the Divide
today although most of this area should remain rain free most of the
time during the daylight hours. Wind should remain light to moderate
for the most part although a southwesterly breeze may develop from
Rock Springs to Casper. Mixing and smoke dispersal will be generally
poor in the west and fair to good east of the divide. There is a
good chance of a wetting rain and snow in many areas East of the
Divide on Friday.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through late Friday night
for WYZ014-015-019.

Winter Storm Watch from Friday morning through late Friday night
for WYZ018.

&&

$$

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



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