Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
540 PM MDT Friday Jul 31 2015
Short term...tonight through Saturday
The upper level ridge is over the western US today and looks to
remain for the next seven days. However...the details are a bit
messy with embedded moisture and weak short wave troughs. For
tonight and Saturday...Wyoming will be in the northwest sector of the
ridge. Isolated convective activity late this afternoon may
persist into the evening hours over the absarokas and
bighorns...potentially impacting northern Johnson County. Guidance
indicates the best action over eastern Wyoming as some storms have
already developed around Cheyenne. As surface low pressure
develops over Utah overnight and surface high pressure moves into
Montana...low level winds are expected to shift to the
northeast...with some higher gusts possible in Sweetwater County.
Low level drying is expected from north to south on
Saturday...keeping convective activity tomorrow well capped. The
only area with a weak potential is around southern Sweetwater
County where weak instability and increased moisture may help a
couple of storms to develop. Otherwise similar temperatures and
winds are expected on Saturday.
Long term...Saturday night through Friday
Continued high pressure will leave dry and warm conditions across
western and central Wyoming through Sunday morning. A wave of low
pressure will move across the Desert Southwest Sunday
afternoon...and begin to move into southern Wyoming Sunday
afternoon and evening. This system looks to have decent potential
to introduce showers and thunderstorms to the area as it moves
through. The wave will be slower to move through the area...
spreading shower activity as it moves through...along with cooler
temperatures. Shower and thunderstorm coverage expected to
increase across much of the state by Monday afternoon.
The axis of the ridge of high pressure will move across much of
the state by Tuesday morning...bringing a southwest flow to the
area. Additional pushes of moisture will then impact the area...keeping
afternoon shower and thunderstorm activity present.
Models diverge on their agreement with the potential for another
wave of low pressure to impact the area Wednesday into Thursday.
The GFS is more aggressive with arrival of this system pushing
through by Thursday. The European model (ecmwf) is slower with the progress of this
system...having it move through by Friday morning. Depending on
which solution then is accurate... Friday will either be under the
influence of a transitory ridging pattern...or in a near zonal
pattern ahead of the next approaching area of low pressure.
Relatively quiet pattern leads to VFR conditions through 00z Sunday.
Light winds expected at taf sites with some gusts 15 to 20 knots
over the higher terrain Friday night. Some weak convection across
northern and eastern Wyoming will linger to 03z to 06z as well. Saturday
will be mostly clear with similar light winds as Friday. Next
storminess expected over far southern Wyoming Saturday afternoon and then
increasing over SW Wyoming mountains Sunday afternoon and evening.
Generally quiet weather pattern through Sunday morning over the
area with near seasonal temperatures and low relative humidity at
the surface. Afternoon relative humidity values in the low teens and upper single
digits will continue into Saturday...with poor overnight recovery
except in the valleys of western Wyoming. Winds will remain generally
light...except for some enhanced northeast flow over southern
Wyoming both Friday night and Saturday night due to surface low
pressure developing over the Great Basin. Saturday looks to be
mostly clear...while Sunday should see increasing cloudiness from
the southwest...with showers and isolated thunderstorms moving in
Sunday evening. Sunday should see above normal temperatures in the
middle 90s in the central basins.