Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 141834 AAB
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1134 AM MST Tue Nov 14 2017

.SHORT TERM...Tuesday through Thursday night

It certainly did get windy around Clark last night with wind gusts
as high as 99 mph! As cold advection kicks in, these winds have
begun to decrease. For the rest of the discussion, given the
contrast in conditions we split it into West and East of the Divide.

Some snow will begin to spread into western Wyoming early this
morning. Like the previous systems, this looks to be a fairly quick
hitter and amounts should remain below advisory level. However,
there could be a burst of snow as the front comes through.
Therefore, we will issue a special weather statement for this
morning. Snow should then taper off this afternoon and end by the
evening hours. An area of high pressure will then build in for
tonight and Wednesday and bring around 24 hours of quiet weather.

The next system will then move in for Wednesday night through
Thursday night. The models still appear to be moving the trough and
front in too quickly, so we removed POPS for Wednesday afternoon,
and the steadiest precipitation would likely hold off until after
midnight. There looks to be a bit more upper level support with this
system. However, another key difference is that there will be more
southwest flow and therefore it will be a warmer system. On
Thursday, 700 millibar temperatures look to range anywhere from
minus 4 in the far north to plus 1 or 2 in the far south. This would
put snow levels from 6500 feet in the north to as high as 9500
feet in the south. As a result, places like the Jackson and Star
Valleys could see mainly rain or rain and snow mix through
Thursday that would really cut down accumulations. Snow will fall
in the higher elevations though. At this time, it looks more like
advisory amounts for most mountain areas. Forecaster builder snow
ratios looked too high and we had to adjust them downward given
the temperatures. Most precipitation should change over to snow
Thursday night, although the best moisture will gave shifted east
at that point.

East of the Divide...Here, once again the main concern shifts from
snow to wind. We have already mentioned the wind gusts near Clark,
although these have been decreasing slowly. We may cancel the
warning early, but that will be a game time decision. We will
once again see windy to perhaps strong winds across the favored
areas much of the day and even in most areas following the frontal
passage. At this time, most areas should remain below warning
level. There could be a warning level gust in a place like Fales
Rock or Camp Creek, but seeing how there are no roads there and no
one lives there, impacts would be minimal. Expect another mild
day with temperatures above average once again. The frontal
passage at this point looks to be a dry one with the only
precipitation in the mountains and even here it does not look like
much.

High pressure then builds in to bring a dry but somewhat cooler for
Wednesday. After that, the area will begin to feel the effects of
the front and trough that will be moving into western Wyoming. The
first effect will be the increasing wind. The first place to see the
effects will be, you guessed it, the Absarokas and Cody Foothills.
With the tell-tale 700 millibar winds increasing to 60 to 65 knots
and a tightening pressure gradient, there could  be another high
wind event Wednesday night into Thursday morning. The difference
this time will be a chance of high wind across areas like Casper
where impacts would be greater. This would be on Thursday as the
front approaches and tightens the pressure gradient ahead of the
frontal passage. There could be some high wind watches issued
sometime over the next day or two, especially for the Green
Mountains and Natrona County. All areas will have a gusty wind
however. In addition, it will be a mild day with some highs in the
60s possible in the areas that mix the best. At this point, this
front looks to have a better chance of spreading some precipitation
eastward across the area, although it would hold off until Thursday
night. The models have differing solutions on where the best
precipitation would set up though. There is a chance, however,
that some areas could see some snow or rain changing to snow later
Thursday night as cooler air moves in behind the front.

.LONG TERM...Friday through Tuesday
Overview...A strong cold front will continue to push across the
area Friday morning with areas of light to moderate snow
continuing in the west and south through the day, some rain and
snow spreading into central Wyoming. Snow will taper off northwest
to southeast late Friday-Friday night. Dry and cooler conditions
will prevail on Saturday. A warming trend will follow on Sunday
through early next week as a ridge of high pressure builds over
the western U.S. Some moisture from the Pacific may bring
occasional chances of light snow to the far west early next week.

Discussion...Our weather maker for Thursday and Friday is an
upper low currently off the B.C. coast. This system makes up the
eastern flank of a Rex Block centered over the central Pacific.
Models continue to show poor agreement with the progression of
this system across the western U.S., with the ECMWF on the slow
side, GFS faster, with an overall trend toward a slower solution.
These two models are in good agreement timing-wise with initial
shortwave ejecting NE into central Canada Thursday night, so the
slower ECMWF is showing a greater degree of a split as its
southern stream portion of the trough lags across the Great Basin
on Friday while GFS is already pushing this energy out of the
central Rockies into the High Plains. These differences affect
possible impacts to the area with the GFS showing an overall
improving trend on Friday while ECMWF split system would impact
the I-80 corridor to a much greater into Friday evening. The
slower trend is favored, so chances of precipitation were weighted
toward the ECMWF with a focus on greater impacts (possible winter
weather advisories for wind and snow) across southern Wyoming.
After this trough clears the area Friday night, the Rex Block
pattern over the Pacific is expected to retrograde slightly with
deepening trough along 140W Monday and Tuesday, with ridge
building downstream over the western U.S.. This will result in
temperatures recovering to above normal levels Sunday through
Tuesday. A Pacific system coming onshore Monday is expected to
split into/around the western ridge with poor model agreement on
the track through the ridge. Mean ridge should hold over the
western U.S. for most of next week, so odds favor above normal
temperatures and below normal precipitation for most of the area
into Thanksgiving weekend.

&&

.AVIATION...18Z Issuance

KJAC airport will see some rain and snow late this morning as a
weather system and its cold front pass by. For now, there will be
MVFR conditions in the TAF through 20Z. VFR conditions should
return after 20Z with decreasing clouds toward 00Z. Snow in the
western mountains will occur through 21Z, then decrease in
coverage. Expect some mountain obscuration.

The cold front is expected to be in the central and northern
basins by 18Z. In the wake of this system, clouds will be
decreasing during the afternoon. A gusty west-to-northwest wind
will prevail through much of the afternoon, favoring prone
terminals like KBPI, KPNA, KRIW, and KRKS. Wind will decrease
around 00Z in most locations.

&&

.FIRE WEATHER...

A front and upper level disturbance will move into western Wyoming
and bring mainly light snow to areas West of the Divide. Meanwhile,
East of the Divide, look for another dry and mild day. A gusty to
possibly strong wind will develop in many areas as the front moves
through. However, relative humidity should remain above critical
levels. Mixing and smoke dispersal will range from poor in portions
of the Big Horn Basin to very good from the Red Desert through
Natrona County.&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 AM MST early this morning for WYZ002-
003.

&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Ross/Lipson
FIRE WEATHER...Hattings



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