Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 291737
Area Forecast Discussion
National Weather Service Riverton WY
1137 AM MDT FRI JUL 29 2016
.FIRE WEATHER...Have hoisted a fire weather watch for some
western fire zones for Saturday afternoon and evening. Ongoing
large fires and expected very warm temperatures, low RH readings,
Haines of 6, and possible isolated dry lightning support the
current watch. This Fire Weather Watch is in effect for fire
zones 283, 288, 414, 415, and 416 from 1300 MDT to 2100 MDT.
.AVIATION...See updated 18Z Friday discussion below.
.PREV DISCUSSION... /issued 251 AM MDT FRI 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 POPS 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
NW. 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 NW 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 RH
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 RH will be quite low in
the single digits into the teens.
Conditions will be VFR through the forecast period. Best chance for
convection today will be over the Absaroka and Bighorn ranges and
points downstream. Have VCTS for KCOD and VCSH for KCPR and KWRL.
Confidence is not as high for KWRL. Showers and storms will be
moving southeast off the higher terrain. Diurnal convection will
wane by 03Z/Sat leaving a mostly clear sky. Surface wind will be
west-northwest 12-24kts between about 20Z/Fri and 02Z/Sat at KBPI,
KPNA, and KRKS, with lesser speeds at KJAC. Weak boundary will keep
northeast surface wind at KRIW and KCPR through much of the evening.
Overnight and Saturday morning conditions will be benign.
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.
Fire Weather Watch from Saturday afternoon through Saturday
evening for WYZ283-288-414>416.