Area Forecast Discussion
Issued by NWS Riverton, WY

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966
FXUS65 KRIW 160510
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1010 PM MST Thu Feb 15 2018

.SHORT TERM...(Tonight through Sunday)
Issued at 1010 PM MST Thu Feb 15 2018

Somewhat active this afternoon as we have some of those pain in the
neck snowbands around the area that are very impossible to nail down
exactly as a trough axis moves across the area. Snow across the
west is coming to an end though, so we will the advisory expire. The
models are in fairly good agreement in continuing the snow in
portions of the area East of the Divide but then ending it fairly
quickly around sunset. At this point, no highlights East of the
Divide but this may change if snow persists over a certain area. For
now, we will stick to special weather statements. We did make the
west completely dry late tonight. Although some flurries can`t ne
ruled out, these will be operationally insignificant and people
likely won`t care much.

The next in the series of shortwaves will move toward the west on
Friday, although any significant snow would likely hold off until
the afternoon into the evening followed by a lessening of intensity
after midnight as the trough axis shifts to the east. Meanwhile,
east of the divide shifts to the possibility of high
winds with the models showing 700 millibar winds rising to 50 to 60
knots in many areas. This wouldn`t be until Friday night, so no need
for any highlights yet. At this point, the best chance would be
across Fremont and Natrona Counties and in the Lee of the Absarokas,
especially in the vicinity of Clark. We did remove the POPS around
the Bighorn Range until later Friday night since the models tend to
over do precipitation there and rush it in WAY too quickly. Some
snow should then spread across northern Wyoming late at night and
early Saturday although amounts look light at this time. This
increased wind may fall mix areas and warm temperatures somewhat,
especially in the favored breezier areas.

The next and more significant system will then move in for Saturday
night into Sunday ahead of an Arctic Front dropping southward. The
snow will likely become steadier and heavier in the west Saturday
night into Sunday. Snow then spreads East of the Divide late
Saturday night into Sunday. The 12Z model runs did come in with some
timing differences however, with the NAM about 6 to 12 hours slower
than the GFS. It also sets up the heavier snowband further north.
This far out, we made few changes to see if there is further
agreement closer to the event. This will be critical with high
temperatures as well. Usually, the Arctic air does come in faster
than indicated although the snow is usually a bit slower than
indicated.

.LONG TERM...(Sunday night through Thursday)
Issued at 1010 PM MST Thu Feb 15 2018

Upper air pattern on Sunday night to feature high amplitude ridge
extending from the eastern Pacific into Alaska, deep Hudson Bay low
with a broad trough across the northern U.S., and strong Bermuda
high further downstream.  Shortwave digging into the northwest U.S.
will carve out deep positively tilted longwave trough extending from
Baja California to Hudson Bay by early next week, that will be
squeezed between eastern Pacific and Bermuda Highs. Medium range
models in good agreement on the development of this blocking pattern
but show increasing spread midweek breaking it down.  Preference is
for slower changes to this high amplitude pattern once it sets up,
and the GFS shows this persistence moreso than the ECMWF this
morning.  However, we gave the ECMWF`s colder temperatures more
weight on Monday and Tuesday.

The main travel impacts will be on Sunday with the heaviest snow
expected along the Arctic front.  Both GFS and ECMWF still show
deceleration and possible stalling of this front from Interstate 80
across Sweetwater County to near Casper. By Monday morning, mid-
levels become too cold to hold any significant amount of moisture,
so snow should become very light but will continue into Monday.
700mb temps with this system are in the -20 to -25 range so expect
sub-zero temperatures Tuesday morning. However, cloud cover should
remain extensive into Monday night which will keep temperatures from
bottoming out too far. Skies should clear Tuesday night to allow
better radiational cooling and more single digit temperatures in the
morning Wednesday. More mid-February sunshine and westerly flow will
allow temperatures to recover a bit on Wednesday and Thursday.&&

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday)
Issued at 1010 PM MST Thu Feb 15 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

VFR conditions will be over the valleys through 18z. Increasing
MVFR, areas of IFR, ceilings and visibility reduced in areas of snow
over the western mountains in Teton, Lincoln and Sublette counties
will obscure mountains above 9000 feet through 18z through 06z.
After 18z MVFR Ceilings and visibilities reduced in snow and
mist most probable at KJAC and KAFO, with brief IFR conditions
possible.  After 20z KPNA and KBPI may also experience MVFR
ceilings at  times especially from 02z to 06z.  Increasing winds
at mountain top level will result in areas of LLWS. Surface west
winds at  20-25kts with gusts of 35  kts  will prevail across the
I80 corridor.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Lower elevations will have prevailing VFR conditions. Over and east
of the Continental Divide down to 8500 feet in the Absaroka and
Wind River mountains after 20z will have areas of MVFR, local IFR,
ceilings and visibility reduced in snow obscuring mountains above
9000 feet through 06z. Increasing winds at mountain top level will
also result in areas of LLWS.  Areas of LLWS will develop vicinity
along the east slopes of the Divide, vicinity KCOD-KDUB-KLND.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 1010 PM MST Thu Feb 15 2018

Snow will end this evening from northwest to southeast with mainly
dry conditions from late tonight through early Friday. Winds will
slowly decrease through tonight as snow comes to an end. Another
system will approach the west tomorrow with another chance of snow,
mainly from Friday afternoon into Friday night. Any snow East of the
Divide would be restricted to northern Wyoming and be mainly light
from Friday night into early Saturday. Winds will increase Thursday
afternoon and possibly become strong Friday night in the normally
windier locations. Mixing and smoke dispersal on Friday will range
from poor in the basins to good in the west and normally windier

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Hulme
AVIATION...Baker
FIRE WEATHER...Hattings



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