Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 232142

Area Forecast Discussion
National Weather Service Riverton WY
242 PM MST Thu Jan 23 2020

.SHORT TERM...(Tonight through Friday Night)
Issued at 159 PM MST Thu Jan 23 2020

Persistent cloudiness in northwest flow aloft has finally given way
to a bit more sunshine Thursday afternoon. Light snow with little
accumulation will continue over the northwest mountains through the
night with weak isentropic lift prevailing. Areas to the east can
expect continued mid and high cloudiness, but it will be dry. By
sunrise, much of this cloud cover should be over the I-25 corridor
and points east as a shortwave ridge axis begins to traverse the
forecast area. This ridge will reach central Wyoming by 18Z/Friday
as the next trough begins to impact western Wyoming.

Moisture will deepen over the far west between 15Z-18Z/Friday with
isentropic and orographic lift ramping up by midday. The best chance
for snow appears to be from 16Z/Friday to 01Z/Saturday west of the
Continental Divide. Widespread mountain accumulations of two to four
inches appear likely with up to six inches in the Teton Range and
southwest Yellowstone National Park. The Jackson and Star Valleys
could see up to an inch, but suspect that surface temperatures in
the lower 30s will hinder daytime accumulation to between a skiff
and one-half inch. Locations east of the Divide will be dry Friday
and Friday night. The surface pressure gradient favors gusty,
southwest 20 to 40 mph wind in central Wyoming from late Friday
morning until just before sunset. Localized, favored areas around
Casper such as Wyoming Boulevard should see the higher speeds
persist Friday night.

Shortwave energy within the trough will begin to slide to our
southwest across eastern Utah and western Colorado by 00Z/Saturday.
This will set the stage for increasing chances of light snow in
Sweetwater, Sublette, and south Lincoln counties late Friday
afternoon and early evening. Accumulations will again be tough to
come by until after sunset Friday. However, there could be a few
hours Friday evening where local road conditions could become slick
with only one-half inch of snow. Forecast has been collaborated and
adjusted accordingly. The chance of snow will decrease rapidly from
west-to-east between 02Z-06Z/Saturday with northwest flow in its
wake. Expect a weak frontal boundary to slide south into far
northern Wyoming and along the I-25 corridor from Buffalo to Casper
early Saturday morning. Have made adjustments as the combination of
northwest flow and the boundary could initiate a three- to six-hour
period of light snow with little accumulation.

As for temperatures, 700mb temperatures will rebound to around -2C
to -4C east of the Divide Friday. This will enable snow-free
locations in northern Wyoming to climb into the mid 40s to around
50F. This warmer air and cloud cover will probably not enable the
central and southwest basins to mix-out from the inversions.
Temperatures, however, will be seasonal Friday afternoon in those
valleys and basins. Cloud cover will help keep temperatures up
Friday night in the east with most areas above normal in the teens
and 20s. A gusty westerly wind will also help warm locations in
Sweetwater and Natrona counties. A clearing sky may lead to colder
valley temperatures across the far west and the cloud trends will
need to be watched.

.LONG TERM...(Saturday through Wednesday)
Issued at 159 PM MST Thu Jan 23 2020

An active and progressive weather pattern is shaping up in the
extended forecast periods. By 12Z Saturday, a weak short wave will
move into western sections of the forecast area, with light snow
expected west of the Continental Divide. The models are in decent
agreement involving a progressive mid to upper level ridge shifting
eastward across the region between Saturday night and Sunday
morning. Dry conditions will prevail at most locations with the
exception of the western mountains, with very light snow/flurries
expected to continue.

By Sunday, the models indicate that a series of short waves will
eject from the progressive western trough into especially the
western sections of the forecast area through the middle of next
week. There remains low confidence involving impacts east of the
Continental Divide overall, through the end of the extended forecast
period. A split mid to upper level flow will result in more of the
same pattern involving the western sections of the forecast area
expected to see more snow potential through next Wednesday. The most
significant system that could provide more snow than the other
multiple systems should be the system that provides snow to the west
on Sunday night and into Monday. It is possible that some snow could
spill east of the Continental Divide on Monday with this system,
especially in north/northwest and into southwest Wyoming. We will
continue to monitor the models and base forecast on trending.  The
highest precipitation chances will be with the first system with a
gradual trend downward due to increasing uncertainty further out in
the latter part of the extended forecast.

Overall temperatures should be seasonal to above average for much of
the extended forecast periods due to more of a zonal and Pacific
origin of these systems.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

Will continue with lower range VFR all locations with the exception
of KJAC, which will keep predominantly MVFR through the forecast
period. A weather system will move into the western areas by shortly
after 18Z Friday and have adjusted downward visibilities/ceilings
into the MVFR range for KBPI and KJAC and have added in a mention of
VCSH and lowering ceilings to KPNA and KRKS after 20Z Friday
afternoon. Have also made some adjustments downward involving west
to southwest winds at KRKS.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will occur through the forecast period. Mid and high
level clouds will continue at most sites.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.


Issued AT 159 PM MST Thu Jan 23 2020

After a quiet and generally dry Thursday night, snow will begin to
increase across western Wyoming late Friday morning. The heaviest
snow will fall across the Teton and Yellowstone dispatch areas
Friday afternoon. The moisture will spread across Sweetwater,
Sublette, and south Lincoln counties late Friday afternoon and
evening. Valley locations could see a quick one-half to one inch of
snow during this time. Other locations east of the Continental
Divide will be dry through Friday evening. A weak frontal
boundary could produce some light snow across the far north and
along the I-25 corridor late Friday night. Again, any
accumulations would be light. Conditions will dry out rapidly
Saturday morning with a dry, seasonal day in store. The dry
conditions prevail through about midday Sunday when the next storm
system will begin to spread snow across areas west of the Divide.
Southwest Wyoming along with areas east of the Divide will be dry
Sunday. Gusty west to southwest wind will blow from eastern
Sweetwater County northeast through Natrona County Friday, Friday
night, and Saturday. The strongest winds of 20 to 40 mph are most
likely in zones 280 and 289 Friday. This will produce the best
chance for smoke dispersal to reach the fair category.




LONG TERM...Troutman
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