Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 201730
Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT SAT AUG 20 2016
.SHORT TERM...Saturday through Monday night
The rain has stopped and clouds continue to break up at this point
as high pressure builds across the area. There are no weather
concerns today as dry northwest flow will predominate. For today
that means a September like day with below normal temperatures and
light winds. A few clouds may develop this afternoon, but
certainly no showers are expected.
We will see a transition back into August for Sunday as flow turns
more west to southwest and brings warmer air back into the region.
As a result, expect high temperatures about ten degrees warmer
than on Saturday. As for the chance of precipitation, the GFS is
doing it`s usual rain forest effect over the mountains. With a
dry air mass over the area and a lack of a trigger, we made the
forecast dry. With light to moderate winds expected, it looks like
a fairly nice and typical day for late August.
Some concerns to creep in for Monday though. First and foremost in
the return of elevated fire conditions. With southwest flow
increasing ahead of an advancing cold front, temperatures will
become hot once again with 90s in many of the lower elevations. In
addition, a tightening pressure gradient will increase the breeze
across the area. With relative humidity dropping into the lower
teens in many areas, fire danger will likely increase once again.
The other concern is the potential for convection. Even on this
day, it would be only a slight chance. The NAM is mainly dry as is
the European. The GFS is more active though. For now, continuity
looked reasonable so we made few changes with mainly high
elevation heat source thunderstorms. We did restrict POPS to less
then 30 percent however.
.LONG TERM...Tuesday through Saturday
A mainly dry, cold front associated with an upper low pushing east
across southern Saskatchewan should be across northern WY by 12Z
Tuesday. This front is expected to quickly reach the Divide by
midday, but stall out during the afternoon before shifting through
the entire forecast area Tuesday night.
In wake of the upper low pushing east across southern Canada,
models show a couple shortwave troughs diving south/southeast out
of Canada with reinforcing shots of cool air. Models are also
showing an increase in mean RH between 850-500mb especially east
of the Divide Thursday into Friday. Although instability remains
weak, forcing from upper level impulses should be enough for
isolated to locally scattered convection especially along and east
of the Divide.
By Saturday the ECMWF shows flat ridging and dry conditions while
the GFS still has the last in a series of shortwave trough in the
northwest pushing across the area. Will lean toward the drier
ECMWF solution with only slight mountain PoP for now.
Temperatures will be seasonal east to slightly above west on
Tuesday. This will be followed by below to well below average
temperatures East Wednesday thru Friday, and seasonal to slightly
below average west.
On the fire weather front...there could be elevated to extreme fire
weather conditions over the south/southwest and far west Tuesday.
Widespread VFR conditions again expected at all TAF sites for the
next 24 hours. Generally light to breezy wind expected this
afternoon and early evening with some terminals possibly seeing
gusts of 15 to 20 knots. NW flow expected for SW WY with N to NE
flow for east of the divide. Some high clouds floating over the
state today with some cumulus possible this afternoon over
Absarokas, Wind Rivers, and Bighorns, and downstream in NW flow.
Mild overnight with ridge axis moving into western and central WY on
Sunday. Wind will increase from the west over western, southern,
and central WY Sunday afternoon.
Fire concerns should remain below critical through the weekend.
Expect below normal temperatures and light winds today although
relative humidity will drop into the teens in some locations.
Mixing and smoke dispersal will be generally fair to poor. Warmer
weather is expected tomorrow but wind will continue to be light to
moderate. Fire danger will elevate Monday with a combination of
hot temperatures, low relative humidity and a gusty west to