Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 111825 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1225 PM MDT Fri Aug 11 2017

.SHORT TERM...Friday through Sunday night

Thunderstorms have ended early this morning and for now the radar is
quiet. As for today, we expect another round of diurnal
thunderstorms. As for coverage, the best chance would be across the
northern half of the state with a shortwave moving across Montana.
However, much like the previous few days, most of the day will be
rain free with mainly clear skies to start and then increasing
clouds in the afternoon. The driest area will likely be across
southern Wyoming where the higher heights will be. Temperatures
should run fairly similar to yesterday.

The most active day at this point looks to be Saturday as another
shortwave moves through the area. However, on this day lapse rates
look to be a bit steeper than today with more instability. In
addition, there will be better directional shear that will likely
enhance tilted updrafts to allow storms to strengthen more. A
marginal risk is already around for Johnson County and even a slight
risk across far eastern Wyoming. So this looks to the chance of
stronger storms with hail and gusty winds. A locally severe storms
will even be possible in Johnson and possibly Natrona Counties. With
a bit more cloud cover temperatures will be a few degrees cooler. As
for Sunday, it still looks like more of the same with another
shortwave tracking through the state and bringing another round of
mainly afternoon and evening convection. Temperatures should again
average near to slightly below normal.

.LONG TERM...Monday through Friday

Overview...Pacific disturbances in southwest flow will bring slight
chances of showers and thunderstorms across mainly the north and
west on Monday into Tuesday. A drier westerly flow is expected to
prevail by mid-week through Friday. Temperatures will generally
be within a few degrees of normal values.

Discussion...Upper air pattern on Monday features amplified ridge
across central Canada and the Northern High Plains flanked by
troughs across the NW and NE U.S.  At the surface, high pressure is
centered across the upper midwest with return S-SE flow across the
High Plains, while a Pacific cold front moves across western Montana
to near the NW corner of Wyoming.  This will put most of the area in
the warm sector with lee side trough across eastern Wyoming.
Shortwaves in SW flow will shear out across Idaho into SW Montana
along the front, bringing isolated to scattered showers and
thunderstorms across the north and west.  Main upper low is expected
to lift across west central Canada on Tuesday bringing a westerly
flow across the area Tuesday, with the main shortwaves in this flow
expected to clear the area by Tuesday evening.  Both GFS and ECMWF
generally show a flat, relatively dry and stable westerly flow
prevailing across the area through Friday.

Looking at the trends just beyond the extended period toward Eclipse
Day, the GFS forecast of the upper air pattern has trended a bit
more amplified with broad ridge from the southern Rockies across
the central U.S. with trough over the eastern Pacific - a
favorable pattern for pulling monsoonal moisture north across the
Great Basin around the backside of the ridge. We will see if this
trend becomes more established (or not) over the next few days.

&&

.AVIATION...18 Issuance

A weak disturbance will move southeastward into Montana and NE WY in
the afternoon hours Friday.  West-NW breezy wind expected across SW
WY, while a northerly push will come down east of the divide. The
weak front should focus shower and thunderstorm activity north of a
line from YNP to the Wind River Basin and over to the KCPR area.
Significant thunderstorms, some likely containing small hail, have already
formed along and moved off of the Big Horn Mountains, tracking east across
the Interstate 25 corridor in Natrona and Fremont Counties early this afternoon.
KCOD, KBYG, and KCPR should see the biggest impact from the storms this
afternoon. Most of the storminess will diminish by 02Z/Sat, with some
residual showers into tonight. The storms are likely to produce brief
heavy rain and wind gusts over 30 kts.

&&

.FIRE WEATHER...

Fire conditions are expected to remain below critical for all areas
today. An upper level disturbance will cross the area and bring
isolated to widely scattered showers and thunderstorms with the best
coverage across northern Wyoming. Relative humidity should remain
above 15 percent with mainly light to moderate winds except in and
around any shower or thunderstorm. Mixing and smoke dispersal will
be generally good to very good.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...McDonald
FIRE WEATHER...Hattings



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