Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
335 PM MDT Monday Aug 31 2015
Short term...today through Tuesday
A subsidence inversion surfaced...drastically dropping relative
humidities this afternoon with westerly winds increasing to above
25 miles per hour in many areas...
A few thunderstorms will linger across the south this evening as a
plume of moisture and Theta-E ridge to our south and east
maintains an unstable atmosphere into the evening hours. Some of
the storms east of Natrona County could kick some moist outflow
boundaries westward into northern and central Natrona
County...this could kick off a few storms in this area too.
Otherwise...the best chance for a thunderstorm in Natrona County
this evening will be south of Casper with dry and breezy
Drier air from the northwest advances to the southeast for Tuesday
morning but another push of middle level moisture arrives into the
west later Tuesday afternoon which could help a few thunderstorms
develop over the mountains. There is not a lot of precipitable water with this
system so not a lot of rain is expected with these
storms...however there could be wind gusts over 40 miles per hour that travel
far from the parent storm. There is some slightly better moisture
across the south so expect a few more storms to have some rain
with them in Sweetwater and Lincoln counties...but at the same
time...storms could be more numerous.
Long term...Tuesday night through Monday...
With a long wave trough over the Pacific northwest and the ridge
over the central US from Tuesday through Saturday...Wyoming will be
in the southwest flow sector through much of the period. Wednesday
should see fairly dry flow over the state as the moisture remains
either over the southern rockies or farther northwest. The pressure
gradient and surface low over eastern Montana will allow windy
conditions to develop during the afternoon and early evening hours
over much of the area. Combined with low relative humidities of 15
percent or lower...fire weather concerns will certainly be
As the trough slowly migrates slightly southeastward Thursday...some
moisture and instability will give rise to isolated showers and
thunderstorms over ynp and the northwest mountains. Breezy conditions will
continue through southern to central Wyoming with low humidities as
well...with a bit of some relief across northern Wyoming.
Through Friday...the GFS and European model (ecmwf) are in fairly good
agreement...although the GFS looks to bring more moisture into
southern Wyoming. As the trough moves more over the northern rockies
(per gfs) or solidifies over Oregon (ecmwf and gem)...both models
increase the moisture and precipitation all day Saturday. With the
faster GFS track...a cold front is likely to move into northern
Wyoming later on Saturday to enhance precipitation across that area.
So...focus of activity remains to be seen with the overall pattern
of the low pressure system...with Saturday having the highest chance
of precipitation through the period. 700mb temperatures will drop
considerably as the core of the low moves over the area Saturday
night into Sunday...with light snow likely in the highest elevations
across northwest Wyoming.
Sunday looks to be cool regardless...with either clearing behind the
system...or continued showers. Monday looks to warm up slightly
over Sunday with some showers and thunderstorms across southern
Westerly wind with gusts to 30 miles per hour possible into the evening with
occasional low level wind shear tonight as winds go aloft. Isolated to widely
scattered thunderstorms will develop this afternoon vicinity of a
krks-kcpr line. Thunderstorm activity will mostly end across the
south by 3z Tuesday. Expect thunderstorms to return after 18z Tuesday
over the western mountains and across the south with strong
outflow winds possible.
..elevated fire danger for much of the week...
There will be dry and breezy periods throughout the week with
occasional lightning and cold fronts later in the week as another
large upper trough approaches. Westerly breezes will continue
into the evening with a few spots exceeding 30 miles per hour...especially
near storms and over higher terrain. Relative humidity will
improve on Tuesday afternoon but it will still dip into the middle
teens in many areas. A large trough will approach on
Wednesday...increasing flow from the southwest for Wednesday and
Thursday with widespread low humidity and gusty winds.
Fire Weather Watch from Wednesday morning through Thursday
evening for wyz275-277-279-414-415.
Red flag warning until 9 PM MDT Wednesday for wyz276-280>283-285-