Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 250822
Area Forecast Discussion
National Weather Service Riverton WY
222 AM MDT THU AUG 25 2016
.SHORT TERM...Thursday through Saturday night
Much cooler air has pressed across much of the area with a fall like
feel to the area. Thunderstorms have persisted well into the evening
with some dropping small hail, covering roads in some areas.
Coverage of storms should decrease through the overnight hours
though. There will still be a few showers around in the morning
though. Another shortwave will pass through the area today and bring
another round of isolated to scattered showers and thunderstorms.
The NAM has come in a lot drier today. The GFS continues to be the
wetter model. For now, we split the middle with mainly isolated on
the lower elevations and scattered activity in the mountains. The
main threat with the cool temperatures and low wet bulb zeros.And
speaking of the mountains, there is some snow falling in the
higher elevations. This will likely continue through the day
although accumulating snow will become less likely through the day
with the still strong August sun angle. Temperatures will average
well below normal as well with most areas remaining in the 60s.
The last of the three shortwaves then moves through on Friday. Again
there is disagreement in coverage of precipitation. Continuity
looked reasonable once again so we made only some tweaks. With more
sunshine expected temperatures should warm some, but still remain
below normal. Relative humidity should remain high enough so fire
weather concerns should remain low to moderate.
On Saturday ridging will re-establish itself across the Cowboy
State. As a result, temperatures will again rise back to normal
levels along with plenty of sunshine. And in addition, fire danger
will rise once again. At this point, critical fire weather looks
borderline with relative humidity falling into the teens in some
locations and a gusty wind developing West of the Divide and favored
areas east of the Divide.
.LONG TERM...Sunday through Thursday
Synopsis...A dry west to southwest flow will prevail through the
period with above normal temperatures.
Discussion...Synoptic pattern will change little through the
extended period, featuring a ridge over the east central Pacific,
trough along the west coast and broad ridge from the Four Corners
region across the southern tier of the U.S. Deterministic guidance
was mostly discounted; in the ECMWF`s case - shoving the west coast
trough inland mid-week, or in the GFS`s case - dislodging a cutoff
low over the Pacific...and into the northern Rockies on Wednesday.
Ensemble means continue to hold upper trough along the west coast
with a gradual backing of the flow. Temperatures will trend 5-10F
warmer on Sunday from Saturday, and then generally hold in the 80s
to lower 90s across the lower elevations through the rest of the
period. Winds look to be generally light Sunday through Tuesday, so
fire weather conditions will be elevated but below critical
thresholds for most areas. Gusty west to southwest wind may return
Wednesday and Thursday for possible critical fire weather
West of the Divide
Isolated showers through 18z in NW WY. Then another round of showers
and thunderstorms will occur over Western WY this afternoon into the
evening hours. Isolated showers will linger until 06z Fri.
VFR conditions will prevail with local MVFR conditions at times in
thunderstorms. Smoke from nearby wildfires will impact visibility in
NW WY through tonight.
East of the Divide
Scattered showers will occur this morning. Then another round of
showers and thunderstorms will develop during the afternoon
and continue into the evening hours. Isolated showers will linger
until 12z. Areas of MVFR conditions will occur today and tonight
due to lower cigs and with thunderstorms. Snow will occur in
the highest elevations of the mountains through tonight.
Fire weather should remain sub critical through Friday. A trough
over the area and a couple of upper level disturbances will bring
below normal temperatures along with a chance of showers and
thunderstorms. The best chance of rain will continue to be east of
the divide. Relative humidity and wind will remain below critical
thresholds. Mixing and smoke dispersal will be generally poor to
fair. Fire danger may begin to increase again on Saturday as warmer
and drier conditions return to Wyoming.