Area Forecast Discussion
Issued by NWS Riverton, WY

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542
FXUS65 KRIW 221935
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
135 PM MDT Mon May 22 2017

.SHORT TERM...(This evening through Thursday)
Issued at 135 PM MDT Mon May 22 2017

A cold Canadian low pressure trough and cold front
moving south across the area this afternoon will bring
areas of showers, with snow above 8500 feet, and isolated
thunderstorms with strong wind gusts and small hail. After
midnight, most of the showers will be across the south with
partial clearing across the north.  The cold air will keep
freezing temperatures across the snow pack tonight with some
lows in the mid to lower 20s in the northern and central
mountains and higher valleys.  Tuesday will have clearing
across the region with warming.  Dry and warmer conditions
will continue on Wednesday with some increase in the snow
melt but mountain temperatures will only warm to the 50s
with 40s above 10000 feet.  Another Canadian cold low pressure
trough will move south bringing increased showers to the
north and west mountains Wednesday night with showers increasing
through Thursday with isolated thunderstorms developing in the
afternoon. Temperatures will cool Wednesday night and Thursday
with a significant decrease in snow melt.

.LONG TERM...(Thursday night through Monday)
Issued at 135 PM MDT Mon May 22 2017

The beginning of the extended period will continue to see a long
wave trough over the Northern Rockies, bringing cooler air and
unsettled conditions to western and central WY at least through
Saturday night.  Both the GFS and the ECMWF are in fairly good
agreement from Thu night through Saturday with the placement and
movement of shortwave troughs around the larger low, which is
keeping high pressure far to the south over Texas.  The result
should be slightly cooler than normal temperatures along with a
chance of showers, with isolated thunderstorms in the afternoon and
early evening hours. There are hints that the northern part of the
forecast area will see a better chance of precipitation as well as
the western and northwest mountains.

The forecast guidance then diverges Saturday night into Sunday and
Monday, with the GFS being more bullish in keeping shortwave energy
from Canada continuing over the area.  The ECMWF however, moves the
trough eastward and allows ridging to build closer to the area
through the Great Basin.  Thus, the later part of the weekend and
into Memorial Day could either be very seasonal with mostly dry
conditions, or a continuation of below normal temperatures along
with a chance of showers and gusty north wind.  The operational GFS
is actually an outlier compared to the ECMWF, Canadian global, and
the parallel GFS.  For example, the GFS would be forecasting 60s to
about 70 on Monday, with the other models suggesting 70s to around
80.  Sunday and Monday as a result are more of a consensus forecast,
and will address the uncertainty with future guidance.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 135 PM MDT Mon May 22 2017

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...
Issued AT 135 PM MDT Mon May 22 2017

A cold Canadian low pressure trough and cold front
moving south across the area this afternoon will bring
areas of showers, with snow above 8500 feet, and isolated
thunderstorms with strong wind gusts and small hail. After
midnight, most of the showers will be across the south with
partial clearing across the north.  The cold air will keep
freezing temperatures across the snow pack tonight with some
lows in the mid to lower 20s in the northern and central
mountains and higher valleys.  Tuesday will have clearing
across the region with warming.  Dry and warmer conditions
will continue on Wednesday with some increase in the snow
melt but mountain temperatures will only warm to the 50s
with 40s above 10000 feet.  Another Canadian cold low pressure
trough will move south bringing increased showers to the
north and west mountains Wednesday night with showers increasing
through Thursday with isolated thunderstorms developing in the
afternoon. Temperatures will cool Wednesday night and Thursday
with a significant decrease in snow melt. Mixing heights will
increase in the afternoons with heating and breezy conditions
supporting good or better smoke dispersal on Tuesday and
Wednesday.

&&

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

$$

SHORT TERM...Baker
LONG TERM...McDonald
AVIATION...McDonald
FIRE WEATHER...Baker



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