Area Forecast Discussion
Issued by NWS Riverton, WY

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205
FXUS65 KRIW 220945
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
245 AM MST Thu Feb 22 2018

.SHORT TERM...(Today through Saturday night)
Issued at 240 AM MST Thu Feb 22 2018

Imagery shows a large digging broad trough across the western CONUS
with ridging continuing to expand westward from the Atlantic. A
relatively strong embedded developing shortwave trough is now
located over WA/OR with a weak lead trough just out in front of the
main trough from ID to AZ. One portion of the northern branch of the
jet continues to dive down the West Coast on the west side of the
main upper shortwave, while a developing jet streak/southern branch
is just now moving through TX and headed toward the very strong jet
now over the upper midwest and Great Lakes. The surface has
weakening high pressure over WY with developing low pressure across
the Great Basin. Other than a couple of isolated snow showers
present over the western mountains of Wyoming, most precipitation is
falling in WA/OR/ID.

Extremely cold temperatures still in place this morning. However,
with some modest warming and mixing aloft, moderation of the very
cold temperatures will begin later today. Still, with lots of snow
on the ground, low temps this morning will range from -5 to -20 deg
C under clear skies. The FA should be able to rise into the teens
and 20s today and then only fall back into the the single digits
either side of zero Friday morning as clouds start to build across
the region with the approach of the PAC NW shortwave and lead trough
described above. Although warming slowly, temperatures will still be
cold enough aloft for some modest instability to remain over the
mountains of the west. This together with some weak orographic lift
will allow for continued periods of isolated showers today mainly
west of the Divide.

Tonight, the approach of the embedded strong shortwave trough and
its lead trof will allow for the development a surface low over
eastern CO that will pull down a clipper type front east of the
Divide and into south central and southern Wyoming. Meanwhile, the
main upper shortwave will induce another area of low pressure across
the Great Basin, pushing southerly boundary layer winds and a
possible weak warm front into southern Wyoming late Thursday/early
Friday. These conditions will provide for modest convergence across
portions of southern/central Wyoming - and the impetus for the
development and maintenance of areas of light snow over this area
through the day Friday. There will also be some potential for gusty
west to southwest winds to return to the mountains and foothills
along and east of the Divide later Friday/Friday night. The Clark
area may get occasional high wind gusts by Friday night...although
it looks localized. Temperatures, although much warmer than the last
few days, will still remain below seasonal through the end of the
forecast period. Saturday will see a brief period of shortwave
ridging with only a few showers across the west once again. By
Saturday night, however, the next embedded upper shortwave trough
will quickly follow up the ridge and increase precipitation chances
across the western mountains once again.

.LONG TERM...(Sunday through Thursday)
Issued at 240 AM MST Thu Feb 22 2018

A longwave trough will remain in place across the western U.S. for
the extended. Temperatures are expected to below average to much
below average as a result. On Sunday, a brief transitory ridge will
push across the forecast area. Despite this, some light snow could
continue over the far west while the rest of the area will be dry. A
large trough is then expected to dig into the West Coast Sunday
night/Monday. A moist west/southwest flow out ahead of the trough
should result in snow breaking out across the western portions of
the forecast area Sunday night and lasting into the day Monday. The
00Z model runs tonight have this trough splitting Monday
night/Tuesday with the main energy pushing into central/southern
California while the weaker northern stream pulls across the
Northern Rockies. Models are much weaker tonight with the associated
cold front Tuesday and the possible isentropic lift over it Tuesday
night and Wednesday than last night`s 00Z model runs. Another storm
system than could impact the area late next week. Most confident on
snow across the west Sunday night/Monday with low confidence on
timing/location of snowfall after Monday due to continued model run
to run variability.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 240 AM MST Thu Feb 22 2018

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

All terminals will be VFR through 00Z. Will keep VCSH at KJAC
through 14Z. Then will keep VCSH at KJAC, KBPI, and KPNA after 20Z
as moisture increases in the west. Snow will develop after 00Z in SW
WY with areas of MVFR conditions occurring through 12Z Fri.

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

All terminals will see VFR conditions through 06Z Fri. Mid and high
level clouds will increase this afternoon and evening. After 06Z
Fri, scattered light snow develops from KLND to KCPR.

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

&&

.FIRE WEATHER...
Issued AT 240 AM MST Thu Feb 22 2018

Extremely cold conditions continue across the region fro another
day. Winds will also remain relatively quiet, not increase again
across the usual locations until later Friday. Still, most locations
will see quite cold morning lows of 5 to 20 below zero. While slow
warming is expected over the next few days, temperatures will still
be below seasonal through the weekend. Smoke dispersion will be poor
to fair east of the Divide and fair to good west of the Divide both
today and Friday.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Ross
FIRE WEATHER...Braun



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