Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
1135 am MST sun Nov 23 2014
Short term...Sunday through Tuesday night
Have dropped the High Wind Warning for northern Johnson County
today. High winds are no longer anticipated.
Updated to expire winter storm warnings in the far west.
With the differences in the weather across the area we will once
again Divide the discussion into west and east of The Divide.
West of The Divide...the final shortwave with current system is
currently bringing some light to moderate snow to the areas with the
warnings continuing. With the echoes continuing we will continue the
highlights until they end at 6 am this morning. The remainder of the
day looks to be mostly cloudy with lingering snow showers although
any further accumulations are expected to be minor.
Another shortwave then drops in from the north and west tonight with
the another chance of snow. This one looks a little weaker and does
not have as much upper level support. Nevertheless...it looks like a
good chance for more accumulating snow. At this time...continuity
looked reasonable with around 3 to 6 inches in the mountains with
the highest amounts in the locations that favor northwest orographic
flow. The valleys look to have 1 to 3 inches of new snow. So...at
this time it looks to remain under advisory criteria. The best lift
will shift east of the area by Monday morning with more upslope snow
showers occurring through the day on Monday into Monday evening.
The next in the series of systems then moves in for Tuesday from the
north and west. This one looks to have a bit more punch then the
Monday system. The models do begin to diverge at this point. The GFS
and European models start bringing in precipitation later Monday
night while the NAM is slower...holding off the precipitation until
Tuesday. For now...we compromised in bringing precipitation in for
Monday night but holding off the heaviest until Tuesday. The GFS
also has better lift over the area. The NAM is a bit weaker with the
jet and keeps the best lift across Montana. For now...the majority
rules so we will keep the wetter solution. However...there is a fair
amount of uncertainty.
East of The Divide...the radar is mainly quiet following the passage
of the cold front. The main concern here shifts from snow to wind.
The models have trended down with the wind across portions of the
Wind River basin. 700 millibar winds that were around 50 to 55 knots
yesterday have trended downward to 35 knots so canceling the high
wind watch looks like a good call at this point. However...there
will still be a gusty northwest wind with gusts to near 50 miles per hour
possible. We will continue the High Wind Warning for northern
Johnson County with a combination of 50 knot 700 millibar winds...a
tight pressure gradient and a favorable northwest flow being the
factors for it. Some light snow will fall across the bighorns but
nothing of consequence. Temperatures will be colder than
yesterday...but nothing unusual for November in Wyoming.
The next system will try to impact the area on Monday.
However...much like the last system...down sloping flow will likely
take most of the moisture. There is some concern that some could
sneak east of The Divide since winds are not as strong as the
previous systems. As a result we have isolated probability of precipitation in for most
areas. It will still be breezy as well...although not as breezy as
the previous couple of days. The exception again may be northern
Johnson County which may have to be watched for higher winds. Then
expect a lull in the precipitation on Monday night as the area will
be in between systems.
We have the same dilemma for east of The Divide as west of The
Divide for Tuesday. The models are still split as for coverage of
precipitation with the NAM dry and the GFS and European wetter. For
now...we have left continuity alone for the most part with slight
chance to chance probability of precipitation across northern areas where the best lift will
be. The other concern will be for wind on Tuesday. All the models
strengthen 700 millibar winds to over 50 knots across much of the
area Tuesday afternoon and Tuesday evening. This...along with the
right front quadrant of a potent jet maximum passing to the north may
enhance downward forcing and bring gusty to possibly strong winds
for Tuesday ahead of the trough and associated front...especially to
areas that favor strong winds with southwest flow. This means from
Rock Springs through Casper. Nevertheless...a lot to watch over the
next few days.
Long term...Wednesday through Sunday
Challenging medium range period with models struggling on how much
energy will dive southward out of Canada and to some extent when. The
GFS is the most optimistic if you want a quiet/dry Thanksgiving.
The European model (ecmwf) and Gem have a series of southeastward moving shortwaves
combining with a surface high that forms on the back side of a southward
moving shortwave on the west side of the Hudson Bay low. The European model (ecmwf)
and Gem are sharply colder Thursday and Friday across the north
and east. The Gem is fastest with the first shortwave with a
chance of snow just about everywhere Wednesday night into
Thanksgiving day with 700 mb temperatures falling to as cold as -15c
to -17c across the north during the day. The European model (ecmwf) is a little
slower with coldest air filtering in Thursday and Thursday night
with 700 mb temperatures of -15 to -19c by later Thursday night and Friday in
the north. The front makes it down to most if not all areas east
of The Divide on Thursday but the coldest middle level air arrives a
little later. On the other hand...the GFS has a mild Thanksgiving
day with above 0c 700 mb air in most of the area with highs likely to
be in the 50s east of The Divide with at least 30s and 40s west.
Just one example is the mex guidance giving cpr 53 on Thursday
while the Euro guidance has 27. On Friday...the mex has 53 again
for cpr while the Euro has 12. The European model (ecmwf) ensembles support the op
European model (ecmwf) with a little less amplification while most of the GFS
ensemble members favoring the op GFS...but not all. The GFS
finally brings in a strong cold front across the north and east
but not until some time Friday or Friday night. Right now leaning
towards an initial shallow first system across at least the north
on Thanksgiving with the second system bringing down the coldest
air when combined with a shortwave moving into the northern High Plains
that develops a strong surface high. The combination of the
shortwave from the b.C. Area diving southeast over US with this strong
surface high will likely lead to fairly strong upslope east of The
Divide with sharply colder air and some light snow. Ahead of this
system...strong northwest flow will keep some light to moderate snow
going in the northern mountains on Wednesday. Turning colder then
Thanksgiving day into Friday with a chance of snow...especially
north and east. By the weekend...some Pacific moisture gets drawn
into the west as a upper low moves towards or onto the West Coast
while shortwaves continue to dive southeast into the northern High Plains
keeping a cold front in the vicinity of our northern and eastern zones. So
a busy extended period with the potential for travel to be
impacted at times. Nothing pointing to a major snowstorm but just
enough weather and cold temperatures at times to potentially hamper
travel at times.
East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes
VFR conditions will prevail at the airports through 18z Monday.
An upper level disturbance will spread -shsn and IFR/MVFR ceilings
into the Bighorn Mountains with areas of mountain obscurations. Northwest
surface winds will increase with this system middle to late Sunday
morning with wind gusts over 50kts expected along and north of the
i90 corridor. Further west...surface northwest winds at 20-30kts with gusts
to 40kts will occur across the Wind River basin...vicinity kriw.
Winds will decrease but remain gusty in these areas Sunday night.
West of The Divide...kbpi/kjac/kpna/krks routes
VFR conditions are expected to prevail at the airports through
18z Monday. Cold northwest flow aloft will keep areas of -shsn
and MVFR/IFR ceilings over the mountains with areas of mountain
Temperatures will be colder today with higher humidity and low
mixing heights with poor smoke dispersal. Lingering snow showers
will fall across western Wyoming today. Strong winds are possible
across portions of northern Johnson County. Another system moving in
from the west will bring additional snow showers tonight into early