Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210900
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
300 AM MDT Fri Apr 21 2017

.SHORT TERM...Friday through Sunday night

Another day of mashing of teeth with the uncertainty in the
forecast. Much like a couple of weeks ago, the key phrase of the day
will be 1 or 2 degrees; or in a similar vain, 100 or 200 feet. This
is all it will take in many areas to make the difference between
just a cold front and a heavy, wet snow. The radar continues to fill
in as low pressure moves across the area. What we do know is that it
will be a cloudy, cool and wet day for many areas East of the
Divide. However, since the precipitation looks to be banded in
nature for many areas, pinpointing the exact locations of the
heaviest QPF is difficult at best. As for the highlights, they look
reasonable so we will continue them as is. This will be the same
situation for the Special Weather Statements as well. 700 millibar
temperatures will be in that critical minus 5 to minus 7 range as
well. A lot may depend on precipitation rate with any possible
dynamic cooling. Complicating matters even further today will be the
high late April sun angle. This could be a situation in some areas
where there is several inches of snow on the grass and very little
on paved surfaces. To sum it up, we made very few changes to
continuity as this may be a situation where we are reacting to
heavier bands of snow that pop up, stay tuned. Meanwhile, out west
the steadier precipitation will end, although with the atmosphere
becoming unstable with daytime heating, snow showers will remain
possible.

As low pressure moves away tonight, there should be a lessening in
intensity and coverage from west to east, although rain and snow
should linger in the east much of the night. Transitory ridging
building across the area should then bring a return to dry weather
for all areas by noon Saturday along with a good deal of sunshine.
High temperatures could be a bit tricky though, depending on any
snow cover and heating going toward melting snow rather than heating
the air.

On Sunday, the next system will approach the west. However, the
models have trended a bit further north with the best energy. For
now, any showers during the day should be confined to areas West of
the Divide. Meanwhile, areas East of the Divide should see a nice
end to the weekend with mild temperatures and light to moderate
wind with sunshine mixing with high clouds. Some showers may spread
East of the Divide Sunday night although at this point it does not
look like a big event.

.LONG TERM...(Monday through Friday)
Issued at 154 AM MDT Fri Apr 21 2017

The GFS and ECMWF models are painting an unsettled weather pattern
for the last week of April. Differences in the pattern show up Wed
night and continue through Friday with respect to the timing of
weather systems in the westerly flow aloft. Areas along and west
of the continental divide look to see periods of rain and snow Mon
through Fri with mainly snow in the mountains above 7500 feet. The
potential exists for significant snow accumulations above 8500
feet in the western mountains which could lead to travel issues
over the passes. Will be keeping an eye on this pattern in the
coming days to better time the heavier snowfall events. Meanwhile,
east of the divide, the weather pattern looks to be showery at
times on any given day or night. Some places may be totally dry on
a given day while other locations could see some precipitation
each day. Again, will adjust the forecast as needed based on model
consistency. High temps will be on the cool side Mon through Fri
with near to below normal readings. Expect breezy to windy
conditions at times in the favored wind corridors.

&&

.AVIATION...12Z Issuance

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

Areas with MVFR ceilings are expected this morning over far western
Wyoming, otherwise isolated to scattered snow showers are expected
with brief MVFR/IFR conditions throughout the day. Windy northwest
or west wind will occur across the Upper Green River Basin and
Sweetwater County today. Tonight, decreasing cloudiness is expected
with some fog possible after midnight.

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

Widespread MVFR/IFR conditions will likely occur at most locations
through the day. Snow levels are expected to lower to around 5000
feet over the Big Horn, and Wind River Basins by 15Z with snow
levels higher farther east. The worst conditions are expected at
KCOD and KRIW where persistent IFR conditions could occur due to
reduced visibility in snow. Precipitation is expected to gradually
improve this evening from west to east, but could see some
precipitation linger across Johnson/Natrona Counties through much of
the night. Mountains will be obscured much of the time with IFR or
LIFR conditions expected.

&&

.FIRE WEATHER...

Low pressure moving through the area will bring rain and snow
showers to most areas today with the exception of the far southwest.
As a result, a wetting rain is likely for many areas. Snow levels
may reach valley and basin floors at times although pinpointing the
heaviest rain or snow is difficult. Mixing and smoke dispersal will
be generally fair to poor but except in southern Sweetwater county
where it will be fair to good. A gusty north wind will also blow at
times. Mainly dry weather will return Saturday as high pressure
builds across the state.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until midnight MDT tonight for WYZ002-
007-015.

Winter Storm Warning until 6 AM MDT Saturday for WYZ008-009.

Winter Weather Advisory until midnight MDT tonight for WYZ003.

&&

$$

SHORT TERM...Hattings
LONG TERM...AR
AVIATION...Murrell
FIRE WEATHER...Hattings


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