Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus65 kriw 201747 
afdriw

Area forecast discussion
National Weather Service Riverton Wyoming
1147 am MDT sun Oct 20 2019

Short term...(today through tuesday)
issued at 415 am MDT sun Oct 20 2019

Satellite imagery early this morning shows the upper low over the
central/southern Bighorn Mountains. Radar imagery early this
morning shows persistent snow showers in the northwest flow across
western Wyoming, while an area of rain and snow expanding in
coverage near the upper low. There was even a lightning strike or
two across northeast Fremont County around 2 am. This upper low is
progged to push into the Nebraska Panhandle by 18z, and then to
north-central Nebraska by 00z Monday.

The activity over the west will continue to decrease in coverage
and intensity as the morning progresses. No changes to winter
highlights over the west.

The best dynamics will be over the far eastern sections of the
forecast area associated with the upper low before it and the
dynamics pull away around midday. In the afternoon, despite the
bulk of the dynamics pushing away, models continue to show
instability isolated to scattered snow showers pushing southeast
across the area mainly east of The Divide. The high resolution
models continue to impact the Casper area with this shower
activity. Snow could be heavy in these showers. Despite
temperatures in the mid to upper 30s early this morning, expect
any rain to change quickly to snow from dynamical cooling and a
low wet-bulb zero temps. Thus with snowfall, possibly heavy at
times, in combination with strong wind have elected to issue a
Winter Weather Advisory until 6 PM today for Johnson and Natrona
counties. Once the precipitation changes to snow, expect all snow
through the event. Road impacts remain in question due temps in
the 30s, but suspect the rate of the snowfall this morning should
make become slick this morning, before likely improving in the
afternoon. However, the heavy snow showers this afternoon could
result in wet roads quickly become slick and snow covered again.
The High Wind Warning remains in effect for Johnson County.

Tonight, models show a shortwave in the strong northwest flow
pulling across the area. There is also some upper jet forcing
which will allow for widespread snowfall over the far
west/northwest last tonight into Monday morning. Snow
accumulations should be light, but main concern in this pattern is
for grand targhee and possibly the alpine area getting heavier
snow than what model precipitation amounts would suggest.

By midday Monday into the afternoon, the forecast area begins to
be on the anti-cyclonic side of the upper flow which should
decrease the coverage of snow significantly.

Tuesday, persistent moist northwest flow will continue periods of
snow over the northwest.

Long term...(tuesday night through saturday)
issued at 415 am MDT sun Oct 20 2019

A cold front will move into the state on Wednesday. With this
system, the showers in the northwest will push east of The Divide,
spreading further south into portions of central Wyoming. The
showers will be in the region through Thursday. Gusty winds will
also accompany this next system across much of the area. Thursday
looks to be the coldest morning with the models showing 700 mb
temperatures at -8 to -14 c for surface temps in the teens and low
to mid 20s. The showers will diminish later in the day for drier
conditions.

Friday will see similar morning lows, however it will be a fairly
dry day. Models are indicating a new system moving into the north on
Saturday, but it there is too much uncertainty this early in the
forecast period.

&&

Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1140 am MDT sun Oct 20 2019

Snow continues to fall at cpr, and is expected to continue for a
couple more hours before decreasing in intensity. Light snow also
continues in the vicinity of jac, which is expected to continue to
bring brief light snow showers through the rest of the afternoon.
Snow has ended for remainder of sites for the afternoon, with VFR
cloud decks to continue for all sites through the rest of the day.
Expect gusty winds for most areas this afternoon, especially
around byg, with decreasing winds overnight. Jac will see an
increasing threat for low clouds and snow again overnight tonight
into tomorrow morning.



&&

Fire weather...
issued at 415 am MDT sun Oct 20 2019

Unseasonably cold and blustery today and Monday. The snow will end
across much of the west by midday, but some significant snowfall
and strong wind is expected across Johnson and Natrona counties
today. The snow over the east will end by early evening, but
another round of snow will impact the west beginning around
midnight lasting through Monday morning. However accumulations
will be lighter than what occurred with the past system. Another
strong storm system will impact the area Tuesday night and
Wednesday with the bulk of the snowfall across northern and
central Wyoming.

&&

Riw watches/warnings/advisories...
Winter Weather Advisory until 6 PM MDT this evening for
wyz008>011-020-022.

High Wind Warning until 6 PM MDT this evening for wyz010.

&&

$$

National Weather Service Glossary of Abbreviations