Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170548 AAA

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1148 PM MDT TUE AUG 16 2016

.UPDATE...added 06z aviation discussion.



An upper low pressure system will drift east across the Great Basin
and fragment into West and Central Wyoming on Wednesday.  This
system will bring isolated showers into the west Wednesday morning.
Widely scattered showers and thunderstorms will blossom across the
region Wednesday afternoon.  The main hazard will be gusty outflow
winds 35-45kts, with thunderstorm outflow winds likely dominating
the wind field in the late afternoon and early evening. A cold front
will push south to vcnty the Montana/Wyoming border around 06z
Thursday and to a vcnty KRIW-KCPR line by 12z Thursday, accompanied
by a northerly wind shift.  Ceilings near FL150 and isolated showers
will linger 6z-12z Thursday across most of the area.


.PREV DISCUSSION... /issued 224 PM MDT TUE AUG 16 2016/

SHORT TERM...Tonight Through Friday

Subtle ridging with mid level warming should keep convection
confined to mainly the mountains this aftn and eve. There is a
potential very weak wave moving toward the nw mtns which may help
with a few evening storms moving off the Absarokas later on so left
the isold pops across the north. Also seeing an isold storm or two
in Sweetwater County attm. On Wednesday, we start to see a little
more mid level moisture begin to get drawn nwd ahead of low
currently in NOCAL. The initial surge of this mid level moisture
and possible weak wave may produce a few light showers/virga in
the sw by Wednesday mrng. By afternoon, a chance of mountain
showers and tstms exist with isold pops spreading from west to
east as the day wears on. Gusty wind will still remain the main
threat with a few of the stronger storms producing small hail like
yesterday. Things get more interesting Thursday as increased
moisture from the south and southwest combine with a rather
shallow front dropping swd with our first nrn shortwave.
Combination of upslope east of the divide, pw`s near or at an inch
and remnant energy from our low to the south will likely assist in
some stronger tstms, especially east of the divide but all areas
area showing at least some decent cape for us. Not a lot of shear
but decent buoyancy for some stronger cells with a couple possibly
briefly svr with marginal hail. Backing up just a little, it looks
like there will be enough of a moisture surge and possible weak
disturbances in the s-sw flow to keep a few showers or even a
stray thunderstorm around Wednesday night. The much colder system
is still to drop swd out of wrn MT Thursday Night into Friday with
a sharp cold front moving swd during the day. This front should
pack a pretty good punch with strong tstms ahead of it followed by
falling temps and gusty to strong n winds. Thursday`s highs will
start to cool off with the initial front in the north and east but
the real cooldown will be Friday with highs across the north
likely struggling through the 50s with only the lowest elevations
hitting the lower 60s. Highs in Central Wyoming will probably hit
60s in the morning before the front at least stabilizes temps or
even drops them some. Showers and some thunderstorms will be a
high likelihood with this system, especially for the Central
areas nwd. Some decent pcpn over half an inch is possible in the
mountains with a tenth to a quarter for the lower elevations the
north and central with some of the upslope and mtns getting
locally more. Definitely the first `fall` front of the season! The
mountains above 10k or so will likely see a little snow late
Thursday night and Friday. Some areas will likely set record low
max readings with some areas possibly experiencing one the coldest
August days on record.

LONG TERM...Friday Night Through Tuesday

Beginning of FCST, post frontal for most of the FA with more sharply
defined and stronger embedded SW moving down through the west side
of the original upper trof with its axis stretching from the
northern Dakotas down into northern UT. The Euro continues closing
off a low in the base of this SW Friday night...although not as
aggressively as in the 00Z run...and opening by early Saturday

Stratiform precip with embedded convective elements will be
ongoing...with areas of light snow expected across the higher
terrain (>8000 ft) of the northern and central mountains. by
Saturday morning, the upper embedded SW trof will pass into the
Plains as SFC high pressure builds into/across WY, clearing things
out until cold NW flow aloft gets things unstable enough to get some
convective activity going later in the afternoon over mainly the
northern and central mountains. Otherwise cool moist most locations
east of the Divide with drying starting to invade the western zones
once again. Seasonally cold again Saturday night with lows ranging
from the mid/upper 30s to low 40s across the lower elevations to the
lower and mid 20s in the mountains.

Sunday, low pressure builds back into the Great Basin once again
with at least a pseudo warm front lifting from the low to the
northeast across southeastern WY. ATTM, it looks like a mainly dry
forecast with most of the convective activity now held across
extreme southeastern WY near this boundary. Monday/Tuesday, modest
ridging takes place with Monday being mostly stable and dry. Tuesday
the ridge flattens in response to another approaching upper trof
from the west. This will allow fleeting disturbances to flow across
the FA as the upper flow begins to turn to the southwest ahead of
the trof...destabilizing the atmosphere and allowing slightly better
chances for showers/storms to least over the


VFR conditions will prevail through the forecast period with
generally light wind of 10 knots or less for most locations...
perhaps to 15 knots across some preferred areas in the foothills
Wednesday afternoon. Slight instability will result in isolated
showers or thunderstorms over the elevated terrain early this the vcnty of the mountains and closely adjacent
terminals of KPNA, KBPI, KCOD and KLND. However, even then there
will only be a remote chance of moving off the mountains and over
these terminals. However, any of these showers/storms could
produce erratic wind gusts of 25-35kt...even well away from the
storms themselves. Showers and storms return again Wednesday
afternoon, with vcnty TSRA at all TAF sites after 20z.


Fire weather conditions remain elevated through Wednesday with min
rh`s in the teens to lower 20s but wind should remain light to
moderate outside of gusty showers and thunderstorms. Isolated
showers and thunderstorms, over the mountains and northern Wyoming
will continue into the evening. The chance of mainly mountain
thunderstorms will increase Wednesday with much cooler and wetter
and much cooler weather from Thursday into and Friday, especially
the northern and central sections.


.RIW Watches/Warnings/Advisories...


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