Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 200745
Area Forecast Discussion
National Weather Service Riverton WY
145 AM MDT TUE SEP 20 2016
.SHORT TERM...Tuesday through Thursday night
There is a cornucopia of concerns this morning with fire weather,
thunderstorms, rain and even snow through the period. So we have so
little to talk about and so much time. Wait, strike that, reverse
The main concern today will be for fire weather. A ridge of high
pressure will be in place over the area today and bring dry and warm
weather to a vast majority of the area. As far as potential for fire
weather, relative humidity will certainly be low enough. The
criteria that is marginal is wind though. However, there should be
enough for some areas to see critical fire weather so we will
upgrade the Fire Weather Watches to Red Flag Warnings for Natrona
County and the Green and Rattlesnake Ranges. As for the potential of
precipitation, with limited moisture it will be limited to the far
northwest in vicinity of a stationary front and even here chances
look rather small with most areas rain free most of the time. The
GFS is doing it`s usual Amazon rain forecast thing over the Big Horn
Mountains so we will keep them dry.
Wednesday looks to be the transition day with a cold front and more
importantly an upper level low that will be moving into the Pacific
Northwest. This will turn flow southwesterly and usher in some
moisture to the area. The models have slowed a bit with the
progression however with the moisture. As a result, we removed most
POPS on Wednesday morning. Even in the afternoon, any convection
looks to be scattered at best and restricted to areas West of the
Divide. Meanwhile, areas East of the Divide will be mainly dry and
still warm, but a bit cooler than on Tuesday. There will be a bit
more cloud cover as well and with more moisture any fire concerns
should be reduced.
Thursday looks to be major Paine in the neck day. No, not the bad
90s movie starring Damon Wayans, but the weather could be influenced
by some moisture moving northward from what is left of Hurricane
Paine that is expected to move into Baja California. Deeper moisture
will begin to spread over the west late Wednesday night and into
Thursday morning. The models are showing some decent QPF as well
from Thursday into Thursday night as the upper level low approaches.
Pinpointing the heaviest precipitation is still difficult however
with the low, they are not called "Upper Level Devils" for nothing.
Cooler air will move in as well. As for the chance of snow, the
origin of the air is Pacific so it is not tremendously cold. The
mountain tops will likely be seeing snow by Thursday night and it
could get down close to pass level by early Friday morning.
Meanwhile, East of the Divide the concern is for thunderstorms. Much
of the area will be overcast with cool temperatures, some areas 20
to 25 degrees cooler than Tuesday. This should limit instability
with numerous showers ongoing. However, both the NAM and GFS are
showing warmer temperatures in far eastern portions of the County
Warning area. There is also a decent amount of directional shear
over the area in the afternoon. If some areas can mix out with some
sunshine, a few thunderstorms could get strong in the afternoon.
Still a lot of uncertainty on this but it needs to be watched.
.LONG TERM...Friday through Tuesday
The approaching area of low pressure crosses over the western
Wyoming border by Friday afternoon, with widespread shower activity
expected west of the divide. Cooler temperatures will accompany this
system, resulting in mountain snowfalls Friday, with some light
snowfall possible over the western basins Friday night.
Models still do not have significant agreement on the movement of
the low as it tracks through Wyoming, with the GFS remaining the
more aggressive solution sending the low into northeastern Wyoming
by Saturday morning. The slower ECMWF shows the core of the low
further south with the model also showing a slight opportunity for
splitting as it moves into eastern Wyoming.
Western Wyoming remains the most favorable to receive shower
activity from this system, though areas east of the divide will see
a decent opportunity by Saturday afternoon as the low progresses
eastward. Regardless, widespread cooler temperatures anticipated in
response to this system.
As the system departs on Saturday/Sunday, building north to
northwest flow will develop over the area, helping to weaken shower
coverage with the potential for widespread dry conditions by Sunday
afternoon. Additionally, this flow will help to bring slightly
warmer temperatures across the area on Sunday.
Dry and warming conditions then anticipated through Tuesday.
VFR conditions will occur through tonight. Very isolated showers may
occur over NW WY after 21Z until 06Z. Some wind in the far west and
south this afternoon with some gusts around 20 knots. The wind will
decrease after 01z.&&
...RED FLAG WARNING FOR NATRONA COUNTY AND THE GREEN MOUNTAINS THIS
A combination of low relative humidity, a gusty west to southwest
breeze and a Haines index of 6 will bring critical fire conditions
to portions of Central Wyoming. Elsewhere, elevated fire
conditions are possible with low humidity and breezy conditions in
the afternoon. Isolated showers will remain possible across
portions of northwestern Wyoming. Mixing and smoke dispersal will
range from good to excellent. Fire concerns will ease Wednesday
with higher humidity and showers moving into the west. Much of the
area will have a chance of a wetting rain from later Wednesday
night into Friday.
Red Flag Warning from noon today to 8 PM MDT this evening for