Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 240558
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1158 PM MDT Sun Apr 23 2017

.UPDATE...
Issued at 1131 PM MDT Sun Apr 23 2017

Winter Weather Advisories have been issued for the western
and northern mountains for Monday and Tuesday.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 218 PM MDT Sun Apr 23 2017

A series of Pacific low pressure troughs will bring widespread
showers with a threat of isolated afternoon and early evening
thunderstorms to the area through the week with periods of heavy
wet snow in the mountains. Snow levels will hang around 7500 feet
dropping to the valley floors at times during the late night
hours. Relatively strong and gusty winds are expected at ridgetops
and pass levels. Cloud cover and precipitation will cool
temperatures increasing the chance of snow into the lower
elevations Tuesday night and Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 218 PM MDT Sun Apr 23 2017

Beginning of fcst, general longwave trof continues across the CONUS
with an embedded minor/flat sw ridge across the Great Basin and wrn
WY. A strong jet will be moving through the general flow out of the
EPAC and through the PAC NW states, to the se into the cntrl/srn
Rockies, ending across the srn Plains. Weak/weakening flow aloft
across most of WY except for the far southwest...remaining through
Saturday. However, EPAC moisture will be open to the nw CONUS with
copious amounts of moisture remaining available to the FA through at
least Saturday.

Overall, the MR models remain in decent agreement concerning the big
picture general pattern. The main difference at this point is that
the ECMWF is trending further north than the GFS, especially with
regard to the development of the upper low and subsequent cyclone by
the time it develops over the Plains...which would keep
precipitation around the FA a bit longer in the Saturday/Sunday
timeframe.

From Wednesday night through Saturday; The general theme will be
persistent cool and wet. With the jet dropping south of the FA and
overall weak or weakening forcing aloft across WY, the terrain, near
surface fronts/boundaries (convergence zones) and weak/modest
convective instability will provide most of the impetus for getting
precip on the ground. As an embedded sw moves across/near/or just
south of WY Thursday, a better overall chance for frontogenesis
over/near the FA will occur...while weak cyclogenesis occurs across
sern WY or nern CO. This will likely be the best period of
precipitation over the FA with some return of moisture from the
Plains making its way back into the forecast area. Otherwise, the
entire CWA will share decent chances for periodic precipitation
through Saturday and the region will just have to wait and see just
where the ll fronts/boundaries will actually set up and move to
before placing too much emphasis on any one area. With Thursday
being the nearest period to be affected significantly in this
forecast, it currently looks as though ecntrl to ern WY will have
the best chances for precipitation. Snow levels look to remain
seasonally high through Wed night for much of the area with good
indications behind the prospective cold front on Thursday that snow
will make it to valley levels west of the Divide by Thursday
afternoon...with rain changing to or mixing with snow east of the
Divide by Friday morning. A possible brief period of significant
morning snowfall currently exists across ern Fremont and all of
Natrona County Friday morning as H7 temps drop to -7 - -10 deg C.

Friday night and Saturday: The main longwave trof axis begins to
pass through and east of the FA...with areas west of the Divide
clearing through Saturday morning (other than terrain forced
isolated light showers) with locations east of the Divide clearing
and high P building by Saturday evening. Sunday will generally be
dry (or drying across the ern counties if the Euro`s timing pans
out) awaiting the arrival of the next upper sw within increasingly
nw flow.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1131 PM MDT Sun Apr 23 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

An area of partial clearing will occur overnight between weather
systems.  The next more powerful storm system will spread
widespread MVFR areas of IFR valley rain and mountain snow into
the far west valleys and western mountains 15z-18z Monday,
mountains becoming mostly obscured by 18z. An associated cold
front with this system will move east across western Wyoming 21z
Monday-00z Tuesday with a line of heavier showers and a few
thunderstorms possible along the front, pushing east into vicinity
KRKS. Gusty west to northwest surface winds and a drier airmass
will follow the cold front across southwest Wyoming Monday
evening, while northwest Wyoming and the western mountains will
continue to see scattered to numerous valley rain and mountain
snow showers, snow levels expected to lower to most valley floors
including KJAC 6z-12z Tuesday.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

Lingering rain showers with an upper level disturbance will be
mostly east of  Interstate 25 by 09z. Partial clearing, VFR
conditions will prevail through 18z Monday. The next more powerful
Pacific storm system will push a cold front east across the area
around 00z Tuesday, with a low pressure system spinning up in the
vicinity of the Bighorn Mountains Monday night. Scattered showers
and isolated thunderstorms will develop ahead of the cold front
mainly after 21z Monday, with an area of more widespread MVFR/IFR
valley rain and mountain snow showers expected to develop vicinity
and north of a KTHP-50SM north KCPR line 3z-12z Tuesday while
gusty west to northwest winds and a drier airmass prevail south of
this line. A colder northerly flow on the backside of the low
pressure system will lower snow levels to near 5000 feet across
the northern Bighorn Basin including KCOD 8z-12z Tuesday.

&&

.FIRE WEATHER...
Issued AT 218 PM MDT Sun Apr 23 2017

A series of Pacific low pressure troughs will bring widespread
showers with a threat of isolated afternoon and early evening
thunderstorms to the area through the week with periods of heavy
wet snow in the mountains. Snow levels will hang around 7500 feet
dropping to the valley floors at times during the late night
hours. Relatively strong and gusty winds are expected at ridgetops
and pass levels. Cloud cover and precipitation will cool
temperatures increasing the chance of snow into the lower
elevations Tuesday night and Wednesday. Areas with low cloud cover
and more widespread precipitation will have poor smoke dispersal
through Wednesday.  The central basins have the best chance for
good afternoon smoke dispersal Monday and Tuesday with most areas
having poor smoke dispersal on Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 AM Monday to noon MDT Tuesday for
WYZ001-002-012-014-015-024.

Winter Weather Advisory from 6 PM Monday to 6 PM MDT Tuesday for
WYZ008-009.

&&

$$

UPDATE...Meunier
SHORT TERM...Baker
LONG TERM...Braun
AVIATION...Meunier
FIRE WEATHER...Baker


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