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Area forecast discussion 
National Weather Service Riverton Wyoming
300 PM MST Friday Feb 27 2015

Short term...tonight through Saturday

Middle-afternoon satellite imagery showing an upper low over the
Pacific northwest which will be the primary driver of our sensible
weather through Saturday. Another vorticity center was spinning east
across eastern Montana. The tail end of this system has been
dragging across the northwest portion of the state today. This has
kept light snow ongoing in that corner with infrared imagery even showing
some enhancement the last couple of hours. Colder air in association
with this vorticity lobe is currently pushing south through central
Montana. Shallow cold dome will be moving in to northern Wyoming
this evening.

Pacific northwest upper low will move to the Great Basin by Saturday
afternoon. This will keep weak northwest flow across the forecast
area with just enough embedded moisture to keep light snow going in
the northwest mountains through tonight. Cold air surge will be
pushing through the Big Horn basin and Johnson County this evening.
Cloud cover will increase behind this surge but little is expected
in the way of snow. The cold air will wring out what little moisture
it can...but right now have amounts of one inch or less. Tweaked
temperatures for tonight down just a hair and did the same for high
temperatures on Saturday as sunshine will be limited. Saturday
maximum temperatures will remain well below normal...on the order of
15 to 20 degrees...north of a Lander to Casper line. The southwest
will be about 5 to 10 degrees below normal. Initial gusty northerly
wind will accompany the surge of cold air...but pressure rises do
not appear to be that impressive. East-northeast wind will be
strongest across eastern Sweetwater County and nearby areas. Speeds
in these locations will be 15 to 30 miles per hour. Moisture in west-southwest
flow aloft will eject from the Great Basin low and track toward
southwest Wyoming. Earlier model runs...especially the NAM...had
good 850-700mb layer frontogenetical lift some potential vorticity
and jet energy to assist snowfall development across Sweetwater
County. However... newest model runs have been bringing surface high
farther it looks less likely that light snow will get as
far north as I-80 by 00z/sun. Tried to indicate this trend as best
as possible during the 18z/Sat-00z/sun timeframe.

Long term...Saturday night through Friday

There are a couple of conundrums in the forecast today. The first is
what will happen Saturday night and early Sunday along the
Interstate 80 corridor. The models continue to be split. The GFS and
the new European show the cold front that will be the focus for any
snow pushing far enough to the south to keep the bulk of the
precipitation over Colorado. The NAM keeps things a bit farther
north. However...the new 18z NAM has backed off as well. At this
time...we trimmed probability of precipitation and quantitative precipitation forecast a little but did not go all the way.
The evening shift should take another look at this to see if probability of precipitation
could be lowered some more. Sunday should feature improving
conditions as the front sags southward out of the area. Temperatures
will remain rather cold however.

The next question is for Monday and into Tuesday. The question is
what will happen with the cold front dropping southward out of
Canada. The previous shift mentioned that the system does not look
as impressive and we agree. There continues to be not great upper
level support with the front. There is a bit of left front quadrant jet
energy feeding the system late Monday night into Tuesday morning so
at this time that looks to be the time of the worst conditions. The
models are also fairly consistent with timing. This will be a fairly
quick hitter as well with most of the precipitation over by Tuesday
evening. And this leads to the next cold will it get
Tuesday night. The air mass moving into the area will be bitterly
cold with flow coming straight out of the Arctic. In addition...
fresh snow cover should provide good albedo. 700 millibar
temperatures Tuesday night look as cold as -25 c. The wildcard is
how much the sky will clear though. The temperatures in continuity
looked reasonable so we made few changes. They are still well below
the guidance which is smart since MOS usually does a poor job with
Arctic intrusions. High temperatures will be tricky for the rest of
next week as well. Wednesday looks to be a cold day definitely. Some
moderation should begin Thursday as the core of the Arctic air
retreats. The question is inversions in the basins and valleys. If
it was January...they would hold. However...with the stronger March
sun there is a better chance of mixing out. So...few changes were
made. As for the potential for another weather maker for the end of
the week...the new European has backed off of the system while the
GFS continues to keep things dry. With the GFS continuing to be more
consistent...we kept the forecast dry with near normal temperatures.


Aviation.../00z issuance/

Frequent mountain obscurations will persist along and west of the
Continental Divide through the forecast period. Obscurations east of
the Continental Divide will become more widespread beginning in the
north around 03z/Sat and central sections from klnd to kcpr by
around 09z/Sat.

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

Terminals will be VFR through this evening. Surge of colder air over
central Montana will push into the far northern Big Horn basin by
03z/Sat. Lower cloud decks of MVFR or low VFR ceilings along with
some very light snow will push to central Wyoming by 06z-09z/Sat.
This airmass is fairly dry so the snow that falls will just be wrung
out from the little bit of moisture in this unseasonably cold
airmass. Klnd may be the best bet for a period of reduced visibility
late tonight due to weak upslope near the Wind River Range.
Conditions may improve slightly late Saturday afternoon.

West of The Divide...kbpi/kjac/kpna/krks routes

The tail end of an upper level disturbance over northeast Montana
has kept light snow going across the northwest mountains tonight.
Limited moisture in northwest flow will drag across northwest
Wyoming on Saturday. Tough to discern when there would be a
prolonged period of MVFR in kjac. For now have kept conditions
hovering in the low VFR range through 00z/sun. Elsewhere...expect
VFR conditions at the other terminals through most of the forecast
period. There is a chance for light snow to begin around krks with
gusty east-northeast wind after 22z/Sat. However...confidence is not
that high and believe the better chance comes after 00z/sun.


Fire weather...

Continued northwest flow aloft will allow for light snow showers
across northwest Wyoming tonight and Saturday. Moisture is amounts will be light. Another surge of colder air will
push into the northern Big Horn basin and Johnson County late this
evening. This surge will reach central Wyoming early Saturday and
southwest Wyoming around sunrise. Expect colder temperatures...more
cloud cover...and very light snow with this airmass. High
temperatures will be 10 to 20 degrees below normal. Drier air will
filter into the state from the north Saturday night and Sunday. This
will allow for more sunshine and slightly warmer temperatures...
although it will remain colder than normal. The next chance for
widespread snow comes Tuesday and Wednesday as another cold system
drops in from the north.


Riw watches/warnings/advisories...none.



Short term...cnj
long term...hattings
fire weather...cnj

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