Area Forecast Discussion
Issued by NWS Riverton, WY

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856
FXUS65 KRIW 191748 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1048 AM MST Fri Jan 19 2018

.SHORT TERM...Friday through Sunday night

Again, with the contrast in the weather pattern, we will split the
discussion into West and East of the Divide once again.

West of the Divide...

So far, things have been a bit slow in evolving with only some light
snow across the higher elevations so far. Radar to the west is
beginning to fill in to the west however so snow should be
widespread by daybreak. At this point, things still look to be on
track with the heaviest snow expected during the morning hours which
will coincide with the best jet dynamics and frontal passage. Things
are rather warm in the Jackson Valley this morning, so there could
be some rain at the onset of the precipitation. The models do show a
period of heavy snow there this morning. Given that it would likely
occur around the morning commute, we will hoist a winter weather
advisory for the Jackson Valley. All other highlights will remain as
is at this time. The heaviest snow in the northwest should shift out
of the area shortly after noon and move to the south and east. Light
snow will continue tonight into Saturday but any accumulations are
expected to minor.

And this brings the next problem as the heaviest precipiation shifts
across southern Wyoming. At this time, it looks as though the almost
all the precipitation would hold off until after around 5 pm. Snow
then spreads into the area from southern Lincoln County to
Sweetwater County. At this point, the heaviest snow looks to fall
in Lincoln County tonight and then move into Sweetwater County late
tonight into Saturday in conjunction with the best dynamics. The
models have backed off on QPF just a bit though. However, snow looks
to be a near certainty late tonight into Saturday and Saturday night
across the I-80 corridor with the highest snowfall amounts the
further south and east you go into the county warning are. If there
is somewhat of a silver lining it is that with the low further away
the pressure gradient will not be that great. There will still be
some northeast wind but not as bad as with some previous systems.
Snow should end here from northwest to southeast Saturday night into
early Sunday as the low moves into the Plains states.

As of now most of Sunday into Sunday evening looks dry. Another
system will approach the western mountains later Sunday night with
another chance of snow. However, we kept POPS low since the models
have a bias to rush systems in too quickly in this time frame.

East of the Divide...

Dry weather will hold through today and also into most of this
evening. The main concern today looks to be wind. Windy conditions
will continue through the morning across the favored areas from the
Red Desert through Casper, although so far most wind has been below
warning criteria. 700 millibar winds actually decrease through the
day so wind should subside a bit. Temperatures will also be the
warmest of the work week, especially in the basins where inversions
should finally break.

Snow will spread across the area late tonight as a front moves
across the area. The main feature with this storm is that the snow
looks to be mainly driven by jet dynamics and frontogenesis rather
than by upslope as flow is more easterly than northeasterly most of
the time. As a result, much of the snow will fall in a band.
Continuity has this band from Sweetwater County through the Green
Mountains into Natrona County with amounts decreasing to the north
and west. At this point, this looked reasonable so we made only
minor tweaks to the forecast. One note of caution however, a small
shift in this feature could bring heavier snow further north and
west so still some uncertainty. For now, we will let the watch ride
as is. Warnings and advisories will almost certainty be issued this
afternoon though. As for the timing of the heaviest snow, across the
Wind River and southern Big Horn Basin, more than likely mid morning
into afternoon then tapering off in the evening. Further east, where
the region will have a longer influence of the right rear quadrant
of the jet streak, steadier and heavier snow will likely persist
into the evening, hence the higher snowfall amounts. Again, if there
is something good about this system, it will be the wind. By that, I
the relative lack off. When lee cyclogenesis takes place, the low
will move more easterly, keeping the tighter pressure gradient away
from the area. There will still be a breeze, but not as much wind as
many other systems.

Snow will taper off from northwest to southeast Saturday night and
should be out of the area by mid morning Sunday. Mainly dry weather
should then continue through Sunday night. Lows Sunday night may be
tricky. There will be fresh snow cover. However, some clouds may
linger. In addition, the origin of the air mass in Pacific, not
Canadian or Arctic, so it will likely be cold but not frigid.

.LONG TERM...Monday through Friday

A quick moving shortwave trough will push across the northern
Rockies Monday with a good shot of some snowfall over the west
possibly spilling into northern part of the forecast area late
Monday into Monday evening. Models show another transitory/flat
ridge Tuesday/Tuesday night with mainly dry conditions except
perhaps a slight chance for some snow over the northwestern
mountains.

By late Tuesday night/Wednesday, global models show a trough
pushing into the Pacific Northwest/N. California. Some difluent
flow and a moist westerly isentropic upglide flow ahead of this
trough should result in snow breaking out across western Wyoming
late Wednesday into Wednesday night. Windy to very windy
conditions are expected in typical wind corridors Wednesday. This
aforementioned trough is then expected to dig, and become a broad
trough across much of the western U.S. Thursday into Friday.
Global models are in decent agreement with this, but have a good
amount of disagreement on timing/track on where the main energy
will occur with this longwave trough. For now will broad brush
mainly slight valley/chance mountain PoPs on Thursday with an
emphasis across the west where the potential for some significant
snowfall could occur, and then slight chance across the entire
area for Friday.

Temperatures in long term will be mainly seasonal to slightly
below average.

&&

.AVIATION...18Z Issuance

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

A storm system will spread snow to northwest Wyoming today. The snow
gradually develops over southwest Wyoming through 00Z. The snow
will not arrive at KRKS until after 06Z Saturday. MVFR to IFR
conditions will prevail in northwest Wyoming today into tonight
with mountains frequently obscured. These conditions will develop
over southwest Wyoming this afternoon and tonight.

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

VFR conditions will occur through 06Z Saturday. Snow will be
developing after 06Z Saturday with MVFR to IFR conditions becoming
widespread by 12Z Saturday. KCPR will see a gusty wind today with
the wind decreasing after 01Z Saturday. LLWS will occur in some
locations today.

Additionally, the Aviation Weather Center will have the latest
information on icing and turbulence forecasts.

&&

.FIRE WEATHER...

Snow will spread across the west this morning and then East of the
Divide tonight and continue into Saturday. Windy conditions will
continue from the Red Desert through Natrona County today. Colder
temperatures will spread into the area on Saturday and continue into
the first part of next week. Mixing and smoke dispersal will
generally be fair to poor.&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through late Saturday night
for WYZ019-020-022-028>030.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ001-
002-012.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ013.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Ross/lipson
FIRE WEATHER...Hattings



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