Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
1120 PM MDT Thursday Jul 10 2014
Short term...tonight through Friday
Isolated showers and thunderstorms have finally begun to form this
afternoon. Most of this activity has formed over central
portions...with some showers moving over the Star Valley into the
northern portions of the Upper Green river basin. Needed
ingredients remain in place...namely ample moisture.
However...extensive cloud cover from this morning has kept cape
values low...as well as middle level temperatures relatively warm.
This will limit any widespread convective development for the rest
of this afternoon and evening.
Any nocturnal development...especially across eastern Wyoming...will
impact the convective potential tomorrow. A weak cold front that
stalled across central portions this morning...will surge
southward later tonight...allowing for easterly winds at the
surface across the eastern half of the forecast area. Most of the
storms are expected to form across northern and eastern portions
Friday afternoon. Cape values and instability look to be maximized
in these areas...while a weak short wave moves northward from Utah
into western portions. This should help generate showers and
storms across the west...although instability still looks to be
marginal. With that said...any storm development Friday will
depend on the cloud cover. There could be a repeat of today if its
as extensive as it was Thursday morning.
Long term...Friday night through Thursday night
The dirty ridge over The Rockies and Great Basin will continue into
the weekend...with ample moisture in the middle levels combined with
weak forcing for scattered showers and thunderstorms from Friday
night into Saturday night. Although the GFS and European model (ecmwf) appear to be
in good agreement on the large scale pattern...the European model (ecmwf) is much
more bullish for storm activity Saturday from northwest to southeast Wyoming along
a weak frontal boundary. The GFS indicates a similar boundary...but
with a stronger ridge over Nevada and Utah...it shows limited storm
activity. Isolated showers and thunderstorms are possible again
Sunday...yet all indications are for lower chances...along The
Divide and to the east in central Wyoming. Sunday looks to be the drier
day of the weekend...with temperatures east of The Divide near or
below normal due to the influence of the large longwave trough over
the Great Lakes.
With the western US ridge amplifying on Sunday...both global models
show a small shortwave from the Pacific moving from the Pacific northwest into
the northern rockies...then dropping south through Wyoming into the
central rockies. The influence of this feature is likely to start
impacting northwest Wyoming by Monday evening. The GFS is slightly more
progressive with the trough from Tuesday into Wednesday...with the
European model (ecmwf) holding it back in northern Wyoming Wednesday night.
Regardless...all indications are that the trough on the backside of
the ridge along with low level easterly flow pushing up to The
Divide...should provide a pattern of a good chance of precipitation
across the area from Tuesday into Wednesday. Have hedged the
forecast more toward the GFS solution at this time rather than
keeping precipitation around like the European model (ecmwf) desires in the middle of
the week. Would not be surprised to see some areas receive around
an inch total precipitation in 48 hours with this pattern.
Overall...the pattern from Saturday through next Thursday looks to
promote temperatures near or slightly below normal for much of the
area...with SW Wyoming likely the warmest. Sunday and Thursday looks to
be the warmest days...with Saturday...and Monday through Wednesday
having decent chances of precipitation. With relatively weak winds
from the boundary layer through the middle levels...along with
relatively high precipitable water values...storms that form will
move slowly and likely produce heavy rain at times.
Conditions will remain VFR through the period. The exception will be
possible MVFR visibilities in the heaviest rain showers or
thunderstorms Friday afternoon. Moisture is plentiful with area
soundings from Thursday evening showing precipitable water values
around one inch from kriw south through Utah and western Colorado.
This airmass will remain over the forecast area late tonight and
Friday. Embedded shortwave energy will impact areas mainly south and
east of a line from kevw to kbyg Friday. Strong convection will be
diurnally driven. Weak middle-level convergence will lead to best
convection in across Sweetwater County. 700mb flow north of this
boundary will be 5-10kts from the east-NE with 500 mb flow out of the
southwest at 10kts. Storms will be slower movers and brief heavy
rain will be the main hazard. Krks and kcpr most likely to be
impacted by thunderstorms. Terminals near the foothills...kcod...
klnd...and kpna also possible candidates with kriw and kwrl the
least likely. Have gone with vcsh at the latter two sites.
Periods of showers and thunderstorms will occur again Friday and
Saturday. Although lightning and gusty winds will be possible with
any thunderstorm...the main threat with these storms will be heavy
rain. A building ridge of high pressure will impact the state by
Sunday...bringing some drier conditions. Isolated to scattered
showers and thunderstorms will still remain possible...especially
over the northern half of the forecast area. Temperatures will
generally cool through the period...though some warming
temperatures are noted on Saturday and Sunday over southwestern