Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221756

Area Forecast Discussion
National Weather Service Riverton WY
1056 AM MST Tue Jan 22 2019

.SHORT TERM...Today through Thursday night

The winter storm that has effected western and central Wyoming
continues to slowly wind down this morning. We still have some snow
showers and some rough travel conditions as of midnight. As a
result, we will let the remaining highlights ride for now, likely
until expiration. A transitory area of high pressure will then build
across the area. A few snow showers will linger into the morning,
but all should be done by late morning with the afternoon dry for
all areas. We should also see slowly increasing amounts of sunshine
across the area. We did keep temperatures on the cool side of
guidance given the fresh snow cover though.

The weather will become active again later tonight as an Alberta
Clipper moves toward the western portion of the state. This will
bring a return of snow to the west late tonight and possibly moving
East of the Divide Wednesday into Wednesday night. If highlights are
needed, we favor advisories. The model snow ratios are usually WAY
too high, and this is the case here, with some as high as 30 to 1.
It will not be that cold and I hardly ever saw that ratio when I was
in Arctic Alaska let alone here. As a result, we had to lower them
about 30 percent from what loaded in, going no higher than 20 to 1.
In addition, the Thursday system looks much weaker on the latest
model runs and keeps the best energy and what moisture it has to
work with well to the east. The QPF from the previous shift looked
pretty good though with the main event from Wednesday into Wednesday
night. In addition, even though a few areas may see over a foot,
impacts would be minimal. One, it would fall over a 30 to 36 hour
period. Two, the highest accumulations would likely be limited to
the higher peaks where the impact would mainly be fresh powder on
the slopes and some good skiing. As for East of the Divide, the
models are more split. Most agree on northern Wyoming getting some
snow, but further south there is more uncertainty. In addition, the
snow is more likely to be banded and rather hard to pinpoint this
far out. We made few changes as a result. The best chance for snow
here would be Wednesday afternoon into Wednesday night with drier
conditions for Thursday.

The next concern is wind. Wind should gradually decrease today as
the storm moves away. However, with the clipper approaching, the
pressure gradient will tighten once again and bring the potential
for strong to high winds. First, we will focus on the Cody
Foothills. The models are split here with the NAM having 700
millibar winds approaching 60 knots on Wednesday morning. However,
the GFS is not as impressed with only 40 knots. Winds are also more
northwest rather then west oriented. At this point, there could be a
high winds gust around the crossroad of the universe, Clark but most
other areas probably not. Of greater concern is across southern
Wyoming, in the vicinity of Interstate 80. Both models do have some
50 knot wind barbs here. However, MOS guidance has backed off a bit
on the wind, now showing 30 and 33 knot peak winds rather than 36
knots. As a result, we will hold off on issuing any high wind
highlights to see if this trend continues. It will be a trash can
and toupee alert day though since both could go flying down the
street even if we do not reach high wind criteria. It will also
bring some areas of blowing snow around the area and could reduce
visibility; mainly from Rock Springs and northeastward toward
Casper as well.

.LONG TERM...Friday through Tuesday

A clipper system pulling southeast across the region Friday into
Friday night should give a quick shot of snow for the area. The
trend is for farther west track which will result in the entire
area seeing the possibility of some snow, but the best shot still
looks to be across the northwest and north. This system is not as
strong as the one on Wednesday with snowfall amounts are expected
to be less, although there is some left-exit region dynamics
pulling across the area. A moist northwest flow Saturday into
Saturday night could continue some snow showers across across the
northern mountains, while the rest of the area should remain dry.

Global models show the potential for a strong storm system diving
southward out of southwest Canada into the northern Rockies late
Sunday into Sunday night. By 00Z Tuesday the trough axis looks to
be roughly from Hudson Bay across the Upper Midwest southwest across
the Central Plains,and westward into the Four Corners area. This
storm system looks to be tapping in some colder air than previous
storm systems for our forecast area. Models are progging -16 to -20
Celsius at 700mb Monday. Currently, the cold front pushes into N.
Wyoming late Sunday into Sunday evening, before sweeping across
the forecast area Sunday night. Models are showing a good shot of
snowfall along and just behind the cold front with the potential
for another round of snow with some trailing energy Monday.
Plenty of time to watch. For now have trend PoPs slightly higher
and cooled temps. Tuesday looks to be mainly dry with temps trying
to slowly moderate.


.AVIATION...For the 18Z TAFS through 18Z Wednesday

West of the Continental Divide (KJAC, KRKS, KPNA, KBPI)

Snow showers continue to end over portions of Sweetwater County.
Skies will remain clear for a few hours, before mid to high level
clouds return over the northwest later this afternoon. Skies will
become overcast by 06Z Wed over the far west mountains, with
mountain obscurations and -SN. These clouds will spread across the rest
of the area by 12Z, with mountain obscurations and -SN. MVFR
conditions, with -SN, will impact KJAC and KPNA by 12Z Wed.

East of the Continental Divide (KCPR, KCOD, KRIW, KWRL, KLND)

VFR conditions will prevail at the terminal sites through 12Z
Wed. LLWS will develop at KCOD after 00Z Wed and continue through
the TAF period. Gusty southwest winds will also return to KCPR
airport after 03Z Wed. Isolated snow showers will be possible
vicinity KCOD-KWRL-KRIW between 14Z-18Z Wed.



Leftover snow showers should end by late morning with a balance of
the day and evening being mainly dry. Winds should slowly diminish
through the day as well. A weather system moving into the northwest
will bring another round of snow to the western mountains late
tonight into Wednesday. Winds will increase tonight in the Lee of
the Absarokas and from Rock Springs to Casper and may become strong
on Wednesday. Mixing and smoke dispersal will be generally fair to




SHORT TERM...Hattings
LONG TERM...Murrell
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