Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
251 am MDT Friday Jul 29 2016
Short term...Friday through Sunday night
Radar is mainly quiet early this morning and that should continue
into the morning hours. A cold front will continue to hang around
the area today before washing out. Good news on the fire front is
that relative humidity values should remain above critical values.
Winds should also remain light to moderate as well except across
southern areas where a gusty breeze could develop in the afternoon.
And like yesterday, there will be some isolated thunderstorms around
in the afternoon. However, most areas should not see one and even
areas that do so one, a vast majority of the day will be dry. The
exception would be favored trigger areas, like the Washakie Needles
area. This convection should die off by midnight with the late
night hours dry. Temperatures should be close to seasonal averages.
Things will turn hotter once again for Saturday as flow turns west
to southwest once again. Highs in the 90s will be fairly common for
lower elevations east of The Divide. As for fire danger, relative
humidity will fall to around 10 percent for many areas. However,
winds at this point look to be marginal. As a result, we will punt
the decision on any fire weather highlights back to the day shift.
With the drier airmass and rising heights, coverage of convection
will be less east of The Divide. Some moisture surging in from the
west could increase chances west of The Divide, but even here we
kept probability of precipitation in the isolated range. With the late approach of a
shortwave, this looks to be a late show with most storms likely
forming after around 6 PM Saturday.
On Sunday, slightly deeper moisture will spread across the area,
raising humidity and bit and lower temperatures a degree or two. How
much convection will develop is in question as the NAM is wetter as
a shortwave moves across the area. The GFS is drier however. For
now, we left continuity alone for the most part with slight chances
across most of the area.
Long term...Monday through Friday
Dry westerly flow is expected Monday between a subtropical ridge
over the southern rockies and an upper low moving into the Pacific
northwest. At the surface there could be a weak boundary east of The
Divide with light northerly/easterly winds, but moisture is
limited so any convection will be limited to the far south or
over the mountains decaying as they move into the lower
On Tuesday...the upper low is expected to push east along the
Canadian border into northwest Montana/S Alberta. This will result in
cyclogenesis/Lee trough across the northern High Plains. This will
result in breezy southwest/westerly surface flow developing in
typical areas. This combined with hot temperatures and low relative humidity
could result in fire weather highlights. Also enough instability
and moisture for even isolated convection across the forecast
The associated cold front with the aforementioned upper low is
expected to push south, east of The Divide Tuesday night. Expect
cooler temperatures across the entire area Wednesday with the most
cooling occurring over northern Wyoming east of The Divide where
temperatures could be 10 degrees cooler. In wake of the cold front
models are showing a mainly stable atmosphere with slight chances
for convection confined to the far south. Also, elevated fire
behavior could continue over the southwest.
Thursday and Friday, models are showing a very dry westerly flow
with slightly above average temperatures. Right now winds look
marginal for fire weather concerns, but relative humidity will be quite low in
the single digits into the teens.
West of the Continental Divide...kjac/kbpi/kpna/krks
Smoke from The Cliff creek fire near Bondurant may bring mfvr
visibilities to vcnty kbpi-kpna until 17z Friday, and areas of
smoke from the Lava Mountain fire will partially obscure terrain
over Togwotee Pass through Friday morning.
With the exception of obscurations from smoke, VFR conditions will
prevail through 12z Saturday with afternoon and evening thunderstorm
activity developing along and east of the Continental Divide Friday
afternoon and evening. Surface west to northwest wind at 15-25 kts
will develop across the southern wind corridor, kevw-krks-krwl,
Friday afternoon through early evening. A wind shift from the NE
will attempt to push south into vicinity krks Friday evening, but is
not expected to push much further south than Interstate 80.
East of the Continental Divide...kcod/kcpr/klnd/kriw/kwrl
Areas should be free of shower or thunderstorm activity through
Friday morning. Smoke from the larger cliff creek and Lava Mountain
will result in some reductions in visibility across the Wind River
basin, but visibilities are expected to remain 6 miles or
greater. A slightly unstable northwest flow will result in
isolated thunderstorms across the area Friday afternoon and early
evening, with most of this activity expected remain over the near
the mountains. Kcod and kwrl stand the best chance of seeing ts in
their vicinity. This activity should dissipate after 03z Saturday.
Fire conditions are expected to remain below critical today. A
westerly breeze will develop across southern Wyoming today. Relative
humidity may fall into the teens at times but should remain above
critical levels. Isolated thunderstorms are possible in the
afternoon and evening...mainly in the mountains and the adjacent
foothills. With southwest flow returning Saturday, hot temperatures
and lower relative humidity could increase fire danger.