Area Forecast Discussion
Issued by NWS Riverton, WY

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651
FXUS65 KRIW 160500
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1100 PM MDT Tue Aug 15 2017

.SHORT TERM...(This evening through Friday)
Issued at 127 PM MDT Tue Aug 15 2017

Imagery shows stagnant longwave pattern across the CONUS with two
embedded shortwave trofs...one over the western CONUS and the
other down across the east coast with weak ridging between the
two. The wrn CONUS shortwave axis is currently located down over
WY`s wrn border with a local jet max ahead of the axis between
cntrl WY and ND. This jet is still producing quite an elongated
area of vorticity across this region with a nice baroclinic leaf
now showing ovr ern MT. SFC has general low pressure from the
Desert SW ewd into the Southern Plains with the deepest portion of
the low (heat induced) centered over the Desert SW. One front is
now south of the FA laying across nrn CO while another is west of
WY, draped from nrn MT to sern OR. Precipitation, showers and
isolated thunder, located from ecntrl to nern WY and then off into
the Plains States.

The rest of today, post frontal early with only modest high pressure
building across WY due to the upper shortwave trof axis still moving
over the FA. A second front is generating to the west as mentioned
above and should push through the FA later this afternoon with
another (final) round of showers/storms possible. Effective CAPE and
shear 500-800 j/kg and 15 to 25 kts respectively for most of the FA
except for ern Sweetwater, much of Natrona and southern Johnson
counties where CAPE from 900-1400 j/gk and shear 25-35 kts will be
present through the afternoon. These locations would be where to
expect the stronger storms of the day.

Wednesday and Thursday: Upper trof axis and SFC front through the
area with NW flow aloft returning to the FA will combine with a
modestly warming/drying atmosphere encroaching from the west as
monsoonal flow gets shunted well to the east and into the srn/cntrl
Plains. Wednesday, will see some weak shortwave ridging early and
generally increasing stability. This will be followed by an also
weak shortwave trof late in the afternoon/evening. This pattern
will likely keep any hope of precip associated with the day`s
convection mainly over a few higher peaks of the cntrl/nrn FA.
Thursday will be drying further through the boundary layer.
Additionally another weak upper wave will push through a weak
trof/frontal boundary in the afternoon...but only yielding very
low precip chances along/behind the front across nrn/ern WY.
Nothing special here. Friday, begins a transition to upper level
rising with continued warming and drying at the SFC as SFC flow
becomes wrly...with virtually no chance for precipitation anywhere
in the CWA.

.LONG TERM...(Friday night through Tuesday)
Issued at 127 PM MDT Tue Aug 15 2017

Transitory ridge is overhead to start the period but as the weekend
progresses, an upper level low will form over California with a
dirty (many small disturbances/vorts) southwest flow developing
across our area which continues through at least Monday. We start
out dry Friday night and most areas Saturday before a surge of
moisture moves into the west and southwest Sunday with an increase
in showers and thunderstorms which increases further Sunday night
and Monday. This moisture will also work east of the divide later
Sunday night into Monday with a decent chance of considerable
clouds and maybe even isolated morning showers on Monday. A weak
cold front will slide south later Sunday into Sunday east of the
divide as a shortwave passes off well to the ne over srn Canada.
This front may be a focus for some storms on Monday in the eastern
zones. Some divergence in the models for Tuesday as to whether
the low remains over California or gets kicked eastward. Either
way, will keep a fair amount of clouds around and at least some
showers/storms. Temperatures will start out in the 80s to lower
90s over the weekend west of the divide with 70s to around 80
west. Temperatures should cool a few degrees with cloud cover and
possible showers Monday and Tuesday but mid level temps are not
showing much change in the models with generally 12 to 15C each
afternoon (little cooler far north Sunday behind our
front/somewhat nearby disturbance).

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday Evening)
Issued at 1100 PM MDT Tue Aug 15 2017

VFR to prevail through the forecast period. Convection is weakening
across the southern-third of the forecast area late Tuesday evening.
Everything points to dry conditions after 08z/Wed. Lingering
moisture over eastern Idaho will drift east Wednesday and help to
initiate convection over the northwest mountains. Just enough
instability to assist development, although wind shear is weak. This
may limit convection to areas in and near the mountains. Have
continued VCTS forecast at KCOD after 21Z/Wed. KCOD represents the
terminal with perhaps the best chance at VCTS. Convection should
wane with the setting sun with little remaining after 03Z/Thu. Wind
speeds should generally be less than 10kts through Wednesday evening.

&&

.FIRE WEATHER...
Issued AT 127 PM MDT Tue Aug 15 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to a few mountain zones above 8000 feet across
western Wyoming. (Please see the Fire Weather Forecast product for
more information). An unseasonably strong upper level disturbance
along with another associated cold front will push through the
forecast area today giving a chance for precipitation to all areas
along and east of the Divide...a bit more hit and miss west of the
Divide. Locally heavy rain possible east of the Divide and across
south central Wyoming...with locally strong storms possible from
eastern portions of Sweetwater County - northeast through Natrona
County. Gustiest winds of the day are expected across southern WY
this afternoon and especially near stronger showers and storms.
Wednesday through Friday will see a generally warmer, drier and more
stable atmosphere with small precipitation chances mainly over the
higher peaks of the area Wednesday and Thursday. Winds will remain
seasonably light with RH values bottoming out in the mid teens to
mid 20s (percent) by Friday.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...CNJ
FIRE WEATHER...Braun



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