Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
1127 PM MDT sun Jul 5 2015
Short term...tonight through Monday
A winter like storm will intensify over western and central
Wyoming this evening. Highs across the north were 30 to 35 degrees
colder today from yesterday. This cooling will continue. Showers
have become widespread with most thunderstorm activity in the
south but some have also occurred in the Bighorn Mountains.
A middle level cyclonic circulations is expected to develop in the
Muddy Gap area and this will cause rapid intensification of
precipitation rates. Freezing levels will lower to 9500 to 9000
feet and snowfall of 3 to 6 inches...locally much higher will
occur above 9500 feet tonight continuing through Monday as
temperatures will warm little and in some areas will continue
to fall or at best remain steady tonight through Monday.
Some areas around the Wind River...absaroka and Bighorn Mountains
have the potential for 1.5 to 2 inches of rain...this also
includes Casper Mountain. The nam12 puts 3.5 inches over the
highest elevations of the Wind River mountains. Thankfully river
levels have fallen as the snow melt runoff is all but over...but
significant river rises are possible through Monday.
Long term...Monday night through Sunday
Similar solutions again offered by medium-range forecast models
regarding overall evolution of the synoptic weather pattern this
coming week and weekend. GFS and European model (ecmwf) solutions both depict middle-
level circulation over southwest/southcentral Wyoming early Tuesday.
Suspect that showers will persist overnight and prior to onset of
deeper convection Tuesday afternoon. Have at least tried to trend in
that direction with precipitation forecast across southern Wyoming from 12z-
18z/Tuesday. This circulation will slowly drift east Tuesday
afternoon and evening...and therefore have increased precipitation chances
generally along and east of a line from Rock Springs to Casper.
Instability parameters look better for more widespread convection
through Tuesday evening...but wind profiles remain unimpressive for
anything more than typical Summer thunderstorms. Weak shortwave to
our north will nudge a weak cold front south across the northern and
central zones Wednesday morning. This boundary could be the focus
for afternoon convection...particularly across the Big Horn basin
and Johnson. Best dynamics will also be across the north where
shortwave energy is expected to be in play. Speed shear profiles
will still remain unimpressive...but directional shear will be more
favorable with northerly to easterly lower level flow. Overall...
Wednesday looks to be a typical Summer day in Wyoming with
convection being scattered and diurnally driven. An upper low will
begin to move onshore in California Thursday causing the upper flow
to back more to the southwest. Thus...it looks to be drier and
warmer for Thursday. Best chance for showers and storms will be
across the west both Thursday and Friday. The lower atmosphere will
dry across southwest Wyoming Friday as gusty southwest wind
increases in response to the upper low tracking across the Great
Basin. Friday should be the warmest day of the week. Warmer middle-
level temperatures combined with the gusty wind and drying
atmosphere could lead to warmer temperatures than what are currently
forecast. Saturday will be just a bit cooler with a better chance of
convection across northwest Wyoming as the upper low moves across
the western forecast area. Both the GFS and European model (ecmwf) solutions offered
today are quicker with the progression of this upper low.
Therefore...Saturday now appears cooler than what it did with the
model runs from yesterday. Potentially next Sunday could be a
pleasant day in the wake of the departing upper low.
East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes
Widespread IFR/MVFR ceilings and light fog with periods of light
rain are expected to prevail through the rest of the night. Ceilings
are expected to improve to low VFR and visibility improving to p6sm in
most locations between 17z and 20z. Areas of rain are expected to
decrease in coverage quickly after 12z. Additional showers and
possibly a thunderstorm could develop in the afternoon and last
through the evening...mainly areas along and near The Divide. Late
Monday night...some fog and low clouds are expected to return.
Mountains will be obscured much of the time.
West of The Divide...kbpi/kjac/kpna/krks routes
Areas with MVFR ceilings and possibly some fog are expected...but
not as widespread as what occurs east of The Divide Monday morning.
There will be isolated rain showers activity Monday morning...but with
daytime heating expect scattered showers and thunderstorms to break
out and will have thunderstorms in the vicinity at all terminals in the afternoon and
evening. The convection could linger well into Monday night.
Unseasonably cool conditions with widespread showers...and some
snow above 9000 feet is in store for tonight and Monday. Above
9500 feet snow accumulations of 3 to 6 inches is possible.
Very high relative humidity with low mixing heights and poor
to fair smoke dispersal is expected.