Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
215 PM MDT Wednesday Mar 12 2014
Short term...tonight through Thursday
Cold and dry high pressure will be over the area tonight. Temperatures
will be warmer than last night but still on the cold side. Thursday
should be from 6 to 9 degrees warmer. Areas of southwest winds 15
to 20 miles per hour will develop from Rock Springs to Casper and along and
near the east slopes of the mountains.
Long term...Thursday night through Wednesday
Deep relatively dry atmosphere and significant model diff from Monday
onwards are the main highlights.
Current upper level set up has ridging western Continental U.S. Intruding over top of
exiting low in the SW...and large trough eastern Continental U.S.. this together with
WV and blended tpw shows large dry bullseye across most of the western
half of Continental U.S. With only very modest values for increase available
moisture waiting upstream to accompany any of the activity through the
weekend and possibly over the next week. This is not to say that we
will be completely benign through the next week...just that precipitation
amounts will tend towards the low end and probably well away from
even advisory criteria.
A weak and fleeting SW will be moving through the forecast area at the beginning
of this period...but will only be useful for pushing out thursday's
weak surface front and return S to west ll flow County Warning Area wide. There will be
virtually no moisture to do work on to produce precipitation during this
period. Moving forward in time to the next quote...significant...end
quote...SW in waiting (now located well upstream across the western
aleutians) on Sat...not expecting much in the way of precipitation or snow.
The mountains will be best suited to receive most of the measurable
precipitation with little elsewhere...even with decent to generous qg
forcing during the day on Sat. Light snow...light rain/snow...light
rain...flurries/sprinkles will all be possible through Sat too. Perhaps
the best low elevation area to expect anything better than a hundredth
or two of water will be across Johnson and Natrona counties with
better ll forcing from the surface front and slightly better moisture
mainly Sat morning. High temperatures will drop off into the 30s and 40s
for the most part...with only the highest elevations seeing the 20s.
Warming returns Sunday and Monday with highs back into the 40s and
50s. Through the day Monday...the lr models begin down a path of
significant deviation with the Euro returning to a faster lw
ridge/trough Continental U.S. Scenario and upper level northwest flow back over top of
US...bringing through a fast moving SW and clipperish type front Monday
night through Tuesday morning west/assorted precipitation...then keeping things dry
through the rest of the forecast. The GFS on the other hand is much slower
and wants to go zonal ahead of a much more significant SW trough some
24+ hours after the Euro. At this time...the Euro seems more logical in that
this has been the set-up for about 80 percent of the winter to date.
However...having phase and timing differences of this magnitude...even
in days 6 and 7...raises some doubt as to the soundness of either
model this far out. So have kept a bit of both in the current forecast
with Euro thinking for Tuesday and the GFS...although to a somewhat
lesser extent...for Wednesday.
VFR conditions with only scattered high clouds over the north
through 18z Thursday. Generally light winds with a southwest breeze
at krks and kcpr.
With mostly clear skies tonight mixing heights will be low with
poor smoke dispersal...cold temperatures and high relative
humidity. Thursday and Friday dry and warmer conditions are expected.
Afternoon mixing heights may rise high enough to combine with
breezy conditions for fair smoke dispersal. Relative humidity will
drop as temperatures warm and dry air prevails over the area.