Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 222027
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
226 PM MDT Sat Apr 22 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 143 PM MDT Sat Apr 22 2017

A series of pacific low pressure troughs will move through west
flow aloft into western Wyoming Sunday through Tuesday bringing
rather cool windy conditions with periods of showers with the snow
line fluctuating from 7500 to 8000 feet. Isolated afternoon and
early evening thunderstorms will be possible starting Sunday.
Much of the area will experience precipitation each day with
locally heavy precipitation in deeper convective cells. The rather
cool temperatures will limit snow melt above 8000 feet with daily
additional accumulations expected.

.LONG TERM...(Tuesday night through Saturday)
Issued at 143 PM MDT Sat Apr 22 2017

Beginning of fcst, general longwave trof across the CONUS with an
embedded minor/flat sw ridge across the Great Basin and WY. A strong
jet will be riding through the general flow out of the EPAC and
through the PAC NW states, to the se over the srn Rockies and
eventually across the srn Plains with weakening flow aloft through
most of the fcst period over WY. However, even with this weak flow
aloft, the door to the EPAC will open up and copious amounts of
moisture will remain available to the FA...again through much of the
fcst period.

While the MR models remain in agreement concerning the big picture
general pattern, there are some differences regionally in the
overall evolution of the weather pattern...especially Friday and
Saturday with the ECMWF being more vigorous and baroclinic in
developing a strong cold core low across the cntrl and srn Rockies
Friday night/Saturday morning almost without regard to the mountains
themselves. The GFS starts development earlier (Friday morning) but
is somewhat less robust through the cycle...especially with regard
to mountain crossing (and seems more reasonable). (Since this was
first being written, the ECMWF has fully come in an is now more in
line with the GFS before bombing out over the Plains Saturday
night...which looks reasonable).

From Tuesday night through Friday, the general theme will be
persistent cool and damp/wet. With the jet well south and overall
weak forcing aloft across the FA, the terrain, weak near surface
fronts/boundaries (convergence zones)  and modest convective
instability will provide most of the impetus for getting precip on
the ground. The entire CWA will share decent chances for periodic
precipitation through Friday...but will have to wait and see just
where the ll fronts/boundaries actually set up and move to before
placing too much emphasis on any one area. Snow levels look to
remain seasonally high through Wed night with some indications
behind a prospective cold front that snow could make it to
basin/valley levels by Thursday morning...keeping it around or mixed
with rain through Friday.

Saturday, the main longwave trof axis passes east of WY...with
somewhat of a reprieve from precip arriving for a day or so...ahead
of the transition to NW flow aloft and upper level stronger forcing.
This part of the story to be continued tomorrow.

&&

.AVIATION...(For the 00Z TAFS this evening through 00Z Sunday
evening) Issued at 143 PM MDT Sat Apr 22 2017

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

VFR conditions will prevail through the forecast period for most
locations except for possibly KJAC after 12z. After 12z Sunday
across west central and northwestern WY, a fleeting relatively weak
upper level disturbance will move across the area, obscuring some
terrain above 3500 feet AGL with cloud and allowing for possible
brief MVFR conditions in lower CIGs, VCSH. Ppossible VCTS west
of the Divide after 19z.

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

VFR conditions will generally prevail for most locations through the
the forecast period. Wind shear possible near both KCPR and KCOD
this evening  through Sunday morning.

&&

.FIRE WEATHER...
Issued AT 143 PM MDT Sat Apr 22 2017

A series of pacific low pressure troughs will move through west
flow aloft into western Wyoming Sunday through Tuesday bringing
rather cool windy conditions with periods of showers with the snow
line fluctuating from 7500 to 8000 feet. Isolated afternoon and
early evening thunderstorms will be possible starting Sunday.
Much of the area will experience precipitation each day with
locally heavy precipitation in deeper convective cells. The rather
cool temperatures will limit snow melt above 8000 feet with daily
additional accumulations expected. Afternoon mixing is expected to
combine with brisk transport winds for good or better smoke
dispersal. Areas with heavy convection will experience poor smoke
dispersal with very low mixing heights and low level saturation.

&&

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

$$

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



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