Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241725
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1025 AM MST Sat Feb 24 2018

.SHORT TERM...Saturday through Monday night
Issued at 225 AM MST Sat Feb 24 2018

With the contrast in the weather, we will split the discussion into
West and East of the Divide today.

West of the Divide...Active weather looks to continue through the
period as a series of waves continue to cross the area. The first
will approach the area today with the steadiest snow expected this
evening as the trough axis moves through. As for amounts, there is
not a tremendous amount of moisture to work with. As a result, most
snow amounts should remain sub advisory. A few higher spots could
see a bit over 6 inches but impacts would be limited to putting more
fresh powder on the slopes. This wave will shift to the east later
Saturday night with a lull in the snow from late Saturday night into
Sunday morning.

Another wave will move toward the area for Sunday afternoon and
especially Sunday night and into Monday. Like the previous wave,
there is not a tremendous amount of forcing with it. However, this
one will be slower moving and of a longer duration. There could be
some highlights needed for this one but likely nothing more than
advisories. This wave shifts eastward Monday night with mainly dry
conditions possibly returning by later Monday night.

East of the Divide and Sweetwater County...Here, our concern shifts
from the white stuff that falls from the sky to stuff that moves
when the atmosphere moves. Or, as described in layman`s terms,
wind. Gusty winds are already occurring near Rock Springs and will
increase through the day as the pressure gradient tightens ahead of
the approaching shortwave. At this point, 700 millibar winds do not
look strong enough to support high wind warnings. However, we could
see some 50 mph gusts, so we will issue a Special Weather
Statement to account for this. At this point, the basins will
likely remain cold with the snow cover. But with the higher sun
angle and no reinforcing cold air, temperatures should continue to
slowly moderate, with 20s possibly returning to some areas. Areas
that get the wind will likely see temperatures warm a bit though.

Sunday may be the day when many areas finally mix out and
temperatures rebound a good bit, although remain below normal. The
price will be some windy conditions again. Mid level winds look even
stronger this day, over 50 knots in many areas. This should warm the
basins a bit. However, we have some concerns for high winds. The
main culprits would be the usual suspects like the Absarokas and
Cody Foothills, but strong winds could even get into places like
Dubois with the flow. Not enough confidence for highlights at this
point though, some will let the day shift make the call on any
watches. The strongest winds look to be Sunday night ahead of the
approaching wave, especially in the Lee of the Absarokas. Dry
weather is expected for all areas East of the Divide through the
weekend.

By Monday the trough will be crossing the Divide along with a weak
cold front. The big question here is will there be any snow with it.
It depends on the model at this point. The GFS is fairly dry.
However, the NAM has a band of snow on Monday evening in association
with some right rear quadrant jet dynamics exiting the area. With
the differences, we kept POPS low for now. It is a quick hitter and
would be gone by Tuesday morning, but could bring a brief period of
moderate to heavy snow if things set up correctly.

.LONG TERM...Tuesday through Saturday
Issued at 225 AM MST Sat Feb 24 2018

Dry weather for most places Tuesday. There may be some lingering
light snow in Sweetwater county Tuesday and will keep a low
chance in there. GFS model spreads moisture back into NW WY Tue
night through Wednesday with a fast moving weather system in the
flow. The ECMWF model shows this feature as well, but is lacking
much in the way of precip with it. Will side with GFS model and
keep a low chance of snow for the NW WY mountains and the northern
Bighorn mountains. Dry for the rest of the region Wednesday. The
models develop a large trough over the Pacific NW Thursday through
Friday and then move this trough closer to our area next
Saturday. As a result, a SW flow aloft Thurs through Saturday.
Periods of snow for the western mountains looks reasonable. The
700MB temps on both models would support snow in the valleys of
the west each day. East of the divide, mostly dry weather Thursday
and Friday with breezy areas. Both the GFS and ECMWF models show
a chance of precip east of the divide next Saturday and have added
a low chance for now. Plenty of time to watch how this trough
evolves late next week. As for temperatures, highs will be in the
20s and 30s for the most part with lows in the single digits and
teens. If some places melt their current snow cover, then some 40s
would occur later next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1020 AM MST Sat Feb 24 2018

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

We can expect areas of MVFR conditions to move back in to the TAF
sites at KJAC, KPNA and KRKS after 20Z this afternoon as the next
storm system approaches western Wyoming. Conditions are expected
to improve across the TAF sites overnight after 04Z-07Z from
KJAC-KRKS with VFR conditions expected through the end of the
forecast period. Have adjusted winds upward at KJAC and KRKS after
15z Sunday.

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

VFR conditions are expected through the forecast period at all TAF
sites. Have made adjustments to southwest winds at KRIW, KCOD and
KCPR. Have bumped up wind speeds at KRIW until around 03Z Sunday
and have made a slight upward adjustment to winds at KCOD and KCPR
after 15z Sunday.

&&

.FIRE WEATHER...
Issued at 225 AM MST Sat Feb 24 2018

An upper level disturbance will bring some snow to areas West of the
Divide, especially from later this afternoon into this evening. Dry
conditions will be the rule East of the Divide. From Sweetwater into
Natrona County as well as the lee of the Absarokas, it will be
breezy to windy. Inversions will hold in deeper portions of the
valleys and basins. Mixing and smoke dispersal will range from poor
in the basins to good in the west and from Rock Springs to Casper.

&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Troutman
FIRE WEATHER...Hattings


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