Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 291633
Area Forecast Discussion
National Weather Service Riverton WY
1033 AM MDT MON AUG 29 2016
.SHORT TERM...Today through Wednesday night
Imagery shows broad low amplitude upper ridge across the CONUS with
the center of the upper high across the southern CONUS and weak low
pressure over the southern Rockies and another trof offshore PAC NW.
The surface has weak low pressure across the central/northern plains
with high pressure off to the west/northwest from southern CA down
into western WY with a weak front/trof separating the two
into/across northeastern WY.
Today through Tuesday night: Upper level ridging begins and
continues through Tuesday night while the surface morphs from weak
high pressure today to weak troffing across western WY...and high
pressure building across eastern WY and over the plains by the end
of the day Tuesday with some increase in low level moisture values
as the low level wind tends out of the east to southeast. Most lower
elevations will remain quite dry through Monday, however, before
some moisture increase is felt across the eastern zones on Tuesday.
Winds look to remain below critical levels for a Red Flag Warning
across zones where RH values do bottom out below 15 percent...except
for Johnson county where winds could gust out of the south to 25
mph at times. Otherwise, only very isolated chances for
showers/storms will be possible mainly over the higher mountain
terrain...with best chances over the Bighorn mountains on Tuesday.
Wednesday/Wednesday night, upper ridging continues with dry air
still mostly in place across the CWA. However, a little stronger
upper level forcing looks to make its way into/over western WY in
the afternoon and evening. Nothing particularly special here, but
instability will be on the increase with increasing chances for
showers/gusty storms mainly across the western mountain
group...with, of course, some chance for a couple of new lightning
starts. With DCAPE values between 500 and 1000 j/kg, wind gusts from
these storms could blow in excess of 45 mph at times. Winds across
the far northeastern FA...mainly Johnson county...will increase and
be very gusty out of the south. ATTM...it seems RH values will
remain high enough to discount a Red Flag warning for now. However
the increased southerly winds could/will affect the new Rest Stop
fire if it is still going then.
.LONG TERM...Thursday Through Monday
WY will remain in a southwest flow aloft Thurs and Fri. It will be
breezy to locally windy both days with warm temps in most places.
A cold front is forecast to move through Friday night leading to a
cooler day Saturday. Sunday looks mild with continued breezy to
windy areas. Another cold front may pass through the area Sunday
night leading to cooler temps on next Monday.
As for the coverage of showers and storms, Thursday looks like it
could see more development as upper level jet is in the area which
would aid lift in the west. Thus higher precip chances out west
seem reasonable Thurs afternoon and evening. Some showers and
storms should accompany the frontal boundary Fri afternoon and
night but pwats are not all that impressive. Saturday looks mostly
dry with very isolated showers/storms over the north. Sunday may
see a little better coverage of showers/storms ahead of the next
front. Monday looks mostly dry with very isolated showers/storms
over NW WY.
An increase in wind is expected on Thurs and Fri with very warm
temps in places and low relative humidity. This puts fire weather
concerns on the front burner once again, especially with the dry
fuels and ongoing wildfires.
Very isolated showers or virga are possible in the mountains along
the Divide and in the Bighorn Mountains this afternoon and evening.
The main hazard from this activity if it develops would be gusty
winds. Otherwise...VFR conditions with some high-based cumulus and
cirrus. Winds will be light. There will also be smokey skies from
wildfires, but any VSBY restrictions to MVFR would be brief. Little
change is expected for Tuesday.
Fire weather conditions elevated across the entire forecast area
today. Warm and dry conditions will continue to allow RH values to
drop again today, ranging from near 10 percent across the
southwest corner of Wyoming to the upper 20s in the mountains. Winds
will generally blow/gust below critical levels through the day
Tuesday for most areas except for Johnson county Tuesday afternoon
and evening where winds will gust out of the south to around 25 mph
at times and could affect the Rest Stop fire if not out by then. RH
values, however, look to remain above 15 percent for most of Johnson
county during this period.