Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 280521
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1021 PM MST Sun Nov 27 2016

.SHORT TERM...Tonight through Wednesday (Issued at 215 PM MST)

Imagery shows large broad deep trof across the wrn CONUS...ridging
ern CONUS and modest shortwave ridging trying to make its way into
the PAC NW. Next upstream embedded shortwave trof currently located
just on the west side of the shortwave ridge over the
EPAC...and moving very quickly through the flow. Main upper trof axis
currently negatively tilted and passing right through WY. SFC has
two areas of low pressure...one over northeastern WY and the other,
and stronger low, over northeastern CO. Cold front is located from
the low in northeastern WY down through eastern CO and into NM. Warm
front extends from the low in northeastern CO east along/near the
NE/KS border. Current precip/snow is light and located mainly west
of the Divide, south to north.

The idea today/tonight is that the area of low pressure and upper
trof spawning all of this will begin to enter a rapid spin-up into
occlusion process over the western Central plains of NE/SD by this
evening with TROWAL formation over SD beginning overnight tonight.
This will keep rather brisk west to northwest winds behind the front
and across the FA combining with light snow east of the Divide while
cold air aloft keep the area west of the Divide somewhat
convectively unstable with light snow showers continuing until the
next upstream disturbance, as mentioned above, approaches and begins
to move through western WY Monday morning.

Monday through Wednesday, the now deep stacked low continues well to
the east over MN while the upper shortwave passing over WY brings
some additional moisture to the western mountains/valleys, develops
a sfc lee trof along the foothills just east of the Divide, and
evolves and pushes a secondary cold front through northern and
central WY by day`s end on Monday...continuing to CO overnight. What
this means to the CWA will be increased snow chances across western
WY as the combination of terrain and additional upper level lift
from the shortwave shake out a modest increases in moisture and give
the west another chance for accumulating snow. Monday night while
the FA goes post frontal and the upper wave moves east of WY the
entire FA will likely see chances for light snow showers or flurries
as the area remains somewhat convectively unstable through much of
the day Tuesday. Additionally, as the SFC P GRAD tightens
considerably Monday afternoon into Tuesday morning and winds aloft
increase around the west side of the main cyclone, northwest SFC
winds are expected to increase over Johnson county, particularly
from around midnight onward. Winds could reach warning criteria over
portions of northern Johnson county particularly through the Tuesday
morning hours. Snowfall combining with the winds looks to be minimal
ATTM. While there are a few detractors from this scenario, it`s
worth monitoring for the possibility of a warning or possibly a WWA.
Otherwise, by Tuesday evening and through the day Wednesday, winds
will decrease east of the Divide, mountain showers will linger,
surface high pressure will build across the FA and breezy west to
southwest winds will return to the southern zones by Wednesday
afternoon.

.LONG TERM...Wednesday night through Sunday

Beginning of the forecast, longwave trof in place across the CONUS
with WY under NW flow and an embedded upstream shortwave trof
located over the PAC NW and expanding into the northern Great
Basin/Rockies. While some differences do exist between MR models at
this time and just how to handle this diving shortwave, they end up
being rather insignificant in the whole scheme of things with regard
to the forecast across WY. Generally this trof will bring additional
unsettle WX to the western portions of WY with low end chances for
precip across the mountains and to a much lesser extent the valleys.
ATTM it appears to of relatively low impact with snowfall remaining
well below advisory levels (although this will be subject to change
if the track of the trof takes it a little further east before it
plunges south). After the passage of this system, upper northwest
flow continues with flat ridging trying to move into
southwestern/western WY...keeping the storm tack at bay to the
north...with only small chances for precip through the end of the
forecast period over the northern portion of WY. Temperatures to
remain rather seasonal through the period too...with a slight warm
up from beginning to end.

&&

.AVIATION.../06Z ISSUANCE/

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

Snow will be common in the mountains and around the KJAC
airport through 00Z Tue. Expect MVFR to IFR conditions to
prevail along with frequent mountain obscuration. Conditions
will improve in these areas after 00Z Tue with snow decreasing
and ceilings rising to VFR by 06Z Tue. In southwest WY, snow
showers will be scattered overnight through Monday night. MVFR
to IFR conditions will occur at times due to lower visibility and
lower ceilings.

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

VFR conditions will prevail at the terminal sites through Monday
night. Scattered snow showers will develop after 21Z across the
region, continuing until 06Z Tue. Areas of MVFR/IFR can be
expected where snow showers occur along with occasional mountain
obscuration. Will have VCSH at most of the terminal sites Monday
afternoon into Monday night.

&&

.FIRE WEATHER...

Fire Danger low through the week as unsettled conditions with the
passage of several upper level disturbances and associated surface
fronts keeps the forecast area seasonally cool, RH levels high with
modest moisture around and at least small chances for precipitation
around for the next few days. Gusty, sometimes strong, winds are the
only real impact to fire weather concerns with west to northwest
wind blowing 10 to 30 mph and gusting 40 to 50 mph at
times...especially across portions of Johnson county Monday night
through Tuesday. Smoke dispersion and will, however, range from fair
to very good for most of the forecast area during the daytime hours
through Tuesday except in the central and northern basins while will
tend to poor with low mixing heights. Wednesday will see a return to
poor to fair smoke dispersion across WY with low mixing heights and
more stagnant conditions.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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



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