Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210547

Area Forecast Discussion
National Weather Service Riverton WY
1047 PM MST Fri Jan 20 2017



.PREV DISCUSSION... /issued 125 PM MST Fri Jan 20 2017/

.SHORT TERM...Tonight through Monday

Light snow has ended across the forecast area, while remnants of
patchy dense fog persists across the I80 corridor. Otherwise,
partly clouds skies across central and western portions this
afternoon, with clearing conditions slowly occurring from eastern
Fremont County and the Big Horn Basin eastward. Have left fog out
of the forecast tonight, as clouds should move back into the SW
and W this evening. Only forecasting partial clearing across
eastern portions late tonight. SW winds of 10-15 mph are expected
to develop during this same time period across Natrona County.

EPAC upper low anchored off the WA/OR coast with the main branch
of the PFJ to continue to dig into the Desert Southwest and
Southern Rockies tonight and Saturday. Moisture associated with
this system will return to the far west and southwest portions
early Saturday morning. A transitory ridge will begin to build
over the Great Basin Saturday night, bringing additional moisture
to the western mountains through Sunday. Snowfall amounts will be
light, with less than an inch in the valleys and 2 to 4 inches in
the higher elevations. This ridge will be a result of the next
storm system making its way onshore over N CA Saturday night. This
system will be supported by a 160-170kt jet. This will bring
another round of precipitation to the west Sunday afternoon and
continue into Monday. A Winter Weather Watch may be needed for
areas west of the Divide, due to the possibility of the long
duration of this event. Will pass this decision to the night
shift, as amounts and duration become better focused.

.LONG TERM...Monday night through Friday

The models are in fair agreement involving spreading areas of snow
eastward from west of the divide into central and toward southeast
Wyoming through Monday night, and continuing into Tuesday.  Cold
advection on the northwest quadrant of the low to the south of the
forecast area will usher in colder temps by Monday night and into
Tuesday. Although there could continue to be some lingering mainly
light snow in the mountains and eastern sections on Tuesday night,
clearing skies and colder temps should occur elsewhere. Generally
dry and cold conditions are then expected through the end of next
week with temps remaining around 15 degrees or so below normal
through the extended forecast period.

.AVIATION.../06Z Issuance/

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

Mid-cloud deck arriving in southwest Wyoming may limit the amount of
fog formation the rest of tonight. Still left some TEMPO groups in
KBPI and KRKS until about 10Z/Sat, but believe that conditions will
generally remain VFR. KJAC and KPNA to remain VFR as well, although
a brief period of fog at KPNA is not out of the question. Shortwave
and associated moisture moves across the region mainly this
afternoon. Best chance for MVFR conditions will be at KJAC Saturday
afternoon, and perhaps for a brief period at KRKS late in the day.
Have included only VCSH at KBPI and KPNA as flow pattern is not that
favorable. Forcing is absent soon after 00Z/Sun, and conditions will
at least briefly improve to VFR. Expect a brief period of gusty west-
northwest wind at KBPI and KPNA toward 00Z/Sun, while KRKS has the
best chance of seeing west wind 15-25kts in the afternoon and early
evening Saturday. Mountain top obscurations will be frequent through
the forecast period.

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

Main concern will be fog until about 16Z/Sat. Enough melting of snow
Friday afternoon to moisten the boundary layer. With clouds
departing east, the clearing sky has allowed for good radiational
cooling and subsequent fog formation. Most likely candidates for IFR
conditions will be KRIW and KWRL, with the lowest visibilities
favored near sunrise. Still could see MVFR at KLND during the same
time frame. KCOD and KCPR look to remain fog free. Overall, after
about 16-17Z/Sat, sct-bkn mid-cloud decks will remain with little
chance of any precipitation.


Fire Danger low for all of WY through the next week. Below seasonal
to seasonal temperatures, a good existing snow pack and
increasing chances for measurable snowfall over the next week, will
keep fire danger very low. Heaviest snowfall of the period will
begin by Sunday night west of the Divide, expanding east of the
Divide by Monday. Smoke dispersion fair to good across the
forecast area today, except for the Basins which will remain
poor. Fair to good smoke dispersion west of the Divide Saturday
afternoon, poor to fair east of the Divide.


Winter Storm Watch from Sunday afternoon through Monday
afternoon for WYZ001-012>014-023>025-027.



LONG TERM...Team Riverton
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