Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 152050
AFDRIW

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

.SHORT TERM...Monday through Wednesday night

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

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...00Z Issuance

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

Lingering snow showers will occur through 03Z in the mountains. VFR
conditions will occur through the period. Mountain obscurations will
continue until late evening. Winds at KRKS slow in the evening and
gust again starting mid morning tomorrow.

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

Areas of snow continue in Northern and Central WY this evening. Snow
in Northern WY will come to a halt after 01Z. MVFR and occasional
IFR conditions will occur at the terminal sites. Snow in central
portions ends by 05Z with VFR conditions prevailing after 05Z.
&&

.FIRE WEATHER...

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
locations.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 2 PM MST this afternoon for WYZ001-
002-012.

&&

$$

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



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