Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 252151 CCA
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
322 PM MDT THU AUG 25 2016

.SHORT TERM...TONIGHT THROUGH FRIDAY

The next round of convective development has already gotten
underway over far western Wyoming as well as across the higher
terrain over northwest areas east of the divide, namely across
the Absarokas. A northern stream feature moving southeast out of
northwest Montana and a more developed southern stream shortwave
rounding the trough across Nevada will be responsible for this
second round of convection developing out ahead of these features
through this evening. Convection will not be that impressive, but
with low wet bulb zeros and with dew points in the 30s and 40s
coupled with temperatures in the 60s, these cells should be
capable of producing locally heavy rainfall, and at least soft
small hail, much like last night, where three tenths to an
estimated 8 tenths of an inch of rainfall fell in local areas
across central wyoming yesterday evening. PW values are greater
today than what we had to work with yesterday. A hint of a weak mid
level circulation over south central Wyoming could also enhance
convective activity through this evening as well. On Friday, we can
expect a few lingering light rain showers during the wee hours of
the morning before round number three of convection associated with
the final shortwave within the broader trough occurs Friday
afternoon. Just like today, the rain showers that fall tonight
should lay much of the ground work for moisture contribution for the
afternoon and evening thunderstorms Friday. Temperatures will be
about 5 to 10 degrees milder friday than today. RH values will be
well above critical values Friday.

.LONG TERM...FRIDAY NIGHT THROUGH THURSDAY

As the secondary wave leaves Wyoming for the northern plains Friday
evening, residual showers and isolated thunderstorms will linger
through about midnight east of the divide, particularly around
Johnson and Natrona counties. Skies will then clear Saturday morning
and afternoon as drier and warmer air moves into western and central
Wyoming from the west southwest. West wind will increase around noon
Saturday into the early evening hours, with sustained around 12 to
15 mph with gusts over 25 mph possible. Afternoon humidities will
drop to the 15 to 20 percent range, making for marginal fire weather
conditions.  Sunday looks to be dry again through the southern two-
thirds of Wyoming with the wind being more west northwest.  A bit of
moisture dragging along southern Montana will like increase
cloudiness and bring a slight chance of showers and thunderstorms in
the far NW corner of the state.  Afternoon humidities may be a bit
drier on Sunday, but the wind does not appear to be as strong with a
weaker surface pressure gradient. Afternoon temperatures in the
central basins will return to the mid to upper 80s Sunday and remain
that way through mid week.

With shortwave troughs setting up off the west coast, the flow from
the Great Basin into the northern and central Rockies will be
southwest to start off the work week.  The flow will be warm and
relatively dry for Monday and Tuesday, although not too strong.  As
a trough moves from northern California into Idaho late Tuesday into
Wednesday, the instability and mid level moisture will bring some
showers and thunderstorms to the northwest part of the state from
mid day Wednesday into the evening hours.  Breezy to windy south to
southwest flow can be expected across southwest Wyoming as a result.
A draping boundary will likely sweep into northern Wyoming to bring
some late evening activity to the Bighorn mountain area. The long
wave ridge then looks to strengthen to the east of Wyoming, allowing
moisture to move into and weak instability to develop along much of
the central and southern Rockies.  Thursday looks to be warm again
with isolated showers and thunderstorms, mainly over the mountains.
The GFS and ECMWF are actually is fairly decent agreement through
this period, although some timing and strength differences arise
with the Wednesday trough.  In addition, can`t totally rule out very
isolated storms over the higher terrain Monday and Tuesday.

&&

.AVIATION.../00Z ISSUANCE/

West of the Divide

Another round of showers and thunderstorms will continue to
develop over Western WY for the remainder of this afternoon and
continue into the evening hours. Isolated showers will linger
until 06z Fri. VFR conditions will prevail with local MVFR
conditions at times in thunderstorms. Smoke from nearby wildfires
will impact visibility in NW WY through tonight. One more round of
thunderstorms is expected after 18z Friday.

East of the Divide

Another round of showers and thunderstorms will develop during the
remainder of this afternoon...mainly over the mountains and the
northwest and then work their way southeast and into the lower
elevations this evening. Isolated showers will linger until 12z.
Areas of MVFR conditions will occur today and tonight due to lower
cigs and with thunderstorms. Snow will occur in the highest
elevations of the mountains through tonight. One more round of
thunderstorms is expected after 18z Friday.

&&

.FIRE WEATHER...

Fire weather should remain sub critical through Friday. A trough
over the area and a couple of upper level disturbances will bring
below normal temperatures along with a chance of showers and
thunderstorms. The best chance of rain will continue to be east of
the divide. Relative humidity and wind will remain below critical
thresholds. Mixing and smoke dispersal will be generally poor to
fair. Fire danger will begin to increase again on Saturday as
warmer and drier conditions return to Wyoming as the general flow
becomes increasingly westerly.

&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Lipson
LONG TERM...McDonald
AVIATION...Lipson
FIRE WEATHER...Lipson


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