Weather Service Report


795 
FXUS65 KRIW 231802
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1102 AM MST Sun Feb 23 2020

.SHORT TERM...Today through Tuesday night

All in all, today looks to be a quiet day with a mix of clouds and 
sunshine. A warming trend will continue in the basins as inversions 
continue to grow shallower and the air mass continues to modify. 
Still some question if the inversions can break though. 
Nevertheless, it should be the warmest day of the past several since 
most areas will be starting off warmer than previous days. Any snow 
in the west from the approaching Pacific system should hold off 
until after sunset. ANd here is where we will split the discussion 
into West and East of the Divide.

West of the Divide...That aforementioned system will move into the 
west and bring a period of snow tonight. At this point, the 
steadiest snow should occur between around 9 pm and 3 am. Since it 
is a fairly quick hitter and does not have a lot of moisture to work 
with, snowfall amounts should be below advisory levels. However, it 
could get nasty travel wise for a few hours either side of midnight 
as the trough axis moves through. It will become windy with that 
passage as well leading to areas of blowing and drifting snow. For 
now, we will go with a Special Weather Statement. The only places 
that look to get advisory level slow would be the top of the 
mountains where impacts would be negligible to even wanted in the 
case of the ski resorts. Strong winds will also be possible in 
Sweetwater County with the frontal passage. It is borderline for 
high wind though. Again, we will go with a special weather statement 
to cover for the wind.

Steady snow should end by sunrise. However, with northwest flow 
continuing there will be some snow showers with the orographic flow. 
There could even be a couple of those dastardly, pain the neck 
snowbands that develop. This would be localized though.We maintained 
chance to likely POPs but with lower QPF to cover for this. 

High pressure then builds in from the west Monday night and should 
shut off the snow by late Monday night. Tuesday at this point looks 
to be a dry day. Temperatures look to remain below normal but 
nothing unusual for late February.

East of the Divide...This system will press across the area later 
tonight and into Monday. There is still a question how much snow can 
make it across the Divide, especially given some downsloping flow. 
Most of the models give the best chance of snow to northern Wyoming 
but even here amounts do not look that high. In addition, the snow 
will be more banded and hard to pinpoint where showers will POP up 
as mesoscale models generally have no clue more than 12 hours out. 
For now, we have gone with slight chance to chance POPs but kept QPF 
low. There could be a bit more moisture in far eastern areas, 
including around Casper as the system may slow down a bit. Timing 
here may be a problem since it could be around the morning commute 
so this has to be watched. As the system passes, strong winds will 
likely develop in the favored northwest flow locations as well late 
tonight. This is especially so around northern Johnson County. Some 
guidance indicates high wind but others don't. This would be fairly 
brief though so no highlights here for now.

A second system, this one coming southward out of Canada will move 
into the area Monday night into Tuesday. Given the origin of the 
system from Canada, there is not a ton of moisture to work with. As 
a result, most areas would see only light snow. The exception may be 
the northern Bighorn Mountains where the favored northwest flow may 
allow for local advisory amounts of snow. The main threat will be 
strong wind. The NAM continues to show 700 millibar winds as high as 
60 knots Monday night in Johnson County. This looks to be the best 
chance of high winds of the period. Still some time to evaluate 
though. The snow should taper off later Tuesday as the system moves 
away to the south and east with winds gradually decreasing through 
the day. Tuesday night at this point looks to be dry and cold.

.LONG TERM...Wednesday through Sunday night

Northwest flow will be in place over the Cowboy State Wednesday, 
in the wake of the storm. Embedded shortwaves could bring another 
round of light snow to far western portions in the afternoon, 
spreading across northern and eastern portions Wednesday night. 
Gusty winds will once again be in place across the Wind Corridor 
Wednesday, as the surface gradient remains tight. Snow chances 
will continue over northwestern portions Thursday, with chances 
for snow over the Bighorns, Johnson and Natrona counties late 
Thursday afternoon into Thursday night. Gusty winds will likely 
continue Thursday, with the surface and 700mb gradients remaining 
tight. However, the jet is not particularly strong (70+kt) and 
700mb winds will range 30-40kt. The ridge axis will build and 
shift over the Northern Rockies Friday, pushing the jet eastward 
and warming 700mb temperatures. The ridge will remain in place 
Saturday, before shifting eastward over the Plains. A major 
longwave trough could develop over the western CONUS by Sunday. 
This could bring a moist, southwest flow to the far west through 
the day as the main low center digs over southern CA. 

Temperatures will slowly moderate through the rest of the week, with 
the warmest days likely occurring Friday and Saturday. However, 
overnight lows will remain cold with readings in the single digits 

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1053 AM MST Sun Feb 23 2020

VFR conditions will prevail through 00Z west of the divide and 
through 06Z east of the divide. A weather system will move into 
western WY after 00Z spreading some snow across the region with 
MVFR to IFR conditions occurring through 12Z. Expect some mountain
obscuration in the west after 00Z. East of the divide, scattered 
snow will develop after 06Z with areas of MVFR to IFR conditions 
and some mountain obscuration. A gusty wind will develop over many
areas tonight through 12Z Monday as the cold front moves through 
the region during this time. Breezy conditions will continue
through 18Z behind the front, but not as strong. One exception
will be KBYG where winds will likely continue gusting up to 45
knots.

&&

.FIRE WEATHER...

High pressure will bring another dry and cold day to much of the 
area today. Wind should remain light to moderate today with humidity 
remaining above critical levels. Mixing and smoke dispersal will be 
generally fair to poor. A system approaching from the west will 
bring some snow to the west tonight and areas further east from late 

&&

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

$$

SHORT TERM...Hattings
LONG TERM...LaVoie
AVIATION...Lipson
FIRE WEATHER...Hattings

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Western Regional Climate Center, wrcc@dri.edu