Area Forecast Discussion
Issued by NWS Riverton, WY

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251
FXUS65 KRIW 222017
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
217 PM MDT THU SEP 22 2016

.SHORT TERM...Tonight Through Sunday

Large...fairly cold...upper level low is currently moving into Nrn
NV and it will move slowly into Nrn UT Friday morning. Main trend
since yesterday is a slower movement with the upper low slowly
lifting nne across the state Friday through Friday night, not
exiting totally until Saturday morning. Main forecast remains on
track with significant rainfall for much of the area (as anomalous pw
air lifts nwd across the area from the remnants of Hurricane
Paine) with rain amounts generally increasing the further west you
go in our forecast area. Approaching cyclonic nose of the
jet...combined with some heating to produce some modest capes with
lcl decent shear will likely result in a few stronger storms in
the southwest half mainly. Flash flooding is still possible over
and near the mountains of the west...especially around the recent
burn scar areas. Heaviest rain still looks to be this afternoon
through tonight but with slower movement...we may see some
significant rain continuing Friday into Friday night across the
west and north especially. Flash flood watch yet but this will be
something to consider tonight. Another speed max rotates through
the base of the trough later this evening and lifts across Wrn WY
with an increase in jet support once again later this evening and
into Friday morning. Friday looks cool and unsettled as the upper
low moves across the area. Widespread showers and embedded tstms
are expected across the mountains with more scattered coverage in
most of the lower elevations. More significant rain is expected on
Friday over the northwest mountains. This is the area that may
need the continued flash flood watch. Will monitor through the
day. Snow levels will start dropping later tonight through Friday
as the upper low moves overhead with h7 temps falling to between
0C and -2C in the southwest and probably +1C to +2C in the central
parts. Other concern will be snow levels and potential for
significant snow at pass level in the mountains...such as
togwotee, teton and south pass. May need to lower temps further to
indicate more snow down to 8-9k even during the day Friday due to
convective nature of the pcpn lowering snow levels more than just
the h7 temps would indicate (and even potential for this system to
be a little colder than some of the guidance is indicating).
Shower chances will gradually decrease from south to north on
Saturday. The northern mountains will see the best chance for
foothill rain and mountain snow continuing Saturday. Precipitation
ends Saturday night for all areas with a chilly Sunday morning on
store for most areas as the sky clears. Will have to look at the
fog potential also for Sunday morning. Lowered temps quite a bit
too but may need to go even further with time. Continued quite
cool though in the cyclonic nw flow even through Sunday with
mainly 50s east of the divide with 30s and 40s in the mountains.

.LONG TERM...Sunday Night Through Thursday

By Sunday evening, the strong, almost vertically stacked low will
have moved northeast to the North Dakota/Minnesota/Sask triple point
with a lingering tight northwest flow overhead behind the exiting
low, keeping temperatures relatively cool but slowly moderating
temperatures nevertheless. Somewhat brisk northwest winds will
linger in Johnson County through Sunday night, Monday, and into
Monday evening. Lighter winds and milder temperatures are expected
Tuesday with the upper level high progged to be overhead by Tuesday
night. By midweek, we will be seeing high temperatures in the 70s at
the lower elevations east of the divide and upper 60s to low 70s
west. By Thursday some spotty shower activity may occur over the
northwest mountains as the next trailing shortwave trough approaches
behind the backside of the aforementioned ridge with temperatures
more or less leveling off.

&&

.AVIATION.../00Z ISSUANCE/

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

Several waves of showers and thunderstorms with periods of
MVFR/IFR conditions will continue. A cold front set up along the
Divide this morning has lifted northward as a warm front with the
surface flow having veered to the southeast, or south in most
areas. Low ceilings are lifting and trending toward the VFR
category. Models have been relatively consistent in developing a
strong band of showers and thunderstorms which have actually
developed in the KRKS-KRIW-KWRL corridor early this afternoon. The
heaviest of this band of showers have shifted east a bit and now
extend from eastern Sweetwater County through eastern Fremont
County, and into northwest Natrona County. The showers within the
band are tracking north northeast. These showers will continue
going through the rest of the afternoon into the early evening.
Outside this area, models continue to paint scattered showers and
thunderstorms. Confidence on exact location of convective banding
is low, and will only have VCTS for now. Scattered showers and
even an isolated thunderstorm will continue through the entire
night. On Friday, expect more frequent showers but the rainfall
rate will be less intense. Frequent mountain obscuration will
occur.

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS ROUTES

Several waves of showers and thunderstorms with periods of
MVFR/IFR conditions will continue. A second wave of showers and
thunderstorms will begin to impact south/southwest WY lifting
northward from Idaho and Utah with continued development through
the rest of this afternoon. Between 03Z and 06Z shower and
thunderstorm coverage will decrease, except over northwest Wyoming
where rainfall will continue all through tonight and Friday,
including the vicinity of KJAC, but scattered convection will
continue through the night elsewhere. Frequent mountain
obscuration will occur.

&&

.FIRE WEATHER...

Fire concerns should remain low for the next few days as significant
rain falls across most of the area. A bigger concern will be
potential for flash flooding around the recent burn scars where mud
and debris flows are possible. It will be windy at times across the
eastern zones through at least Friday but rh`s should remain well
above critical levels and temperatures will be cooling off.

&&

.RIW Watches/Warnings/Advisories...
Flash Flood Watch through late Friday night for WYZ001-002-012.

Flash Flood Watch through late tonight for WYZ014-015-024.

&&

$$

SHORT TERM...Skrbac
LONG TERM...Lipson
AVIATION...Lipson
FIRE WEATHER...Skrbac



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