Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 301819
Area Forecast Discussion
National Weather Service Riverton WY
1219 PM MDT TUE AUG 30 2016
.SHORT TERM...Today through Thursday night
...Red Flag Warning for portions of Johnson county this afternoon and
Imagery shows upper ridging across most of the CONUS with the
primary axis up through the Rocky Mountain State including WY. Large
trof just coming into the PAC NW. Surface has weak high pressure
across the forecast area with lower pressure well off to the west.
Earlier isolated showers/storms have dissipated.
Today/tonight, upper level ridging begins and continues through
tonight while the surface changes from weak high pressure to weak
troffing across western WY...and high pressure builds across eastern
WY and over the plains by the end of the day with some increase in
low level moisture values as the low level wind tends out of the
east to southeast. Winds look to remain below critical levels for a
Red Flag Warning across most 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...especially across northern
portions of the county. A Fire Weather Watch has been upgraded to a
Flag Warning for Johnson county today. Otherwise, only very isolated
chances for showers/storms will be possible mainly over the higher
mountain terrain...with best chances over the Bighorn mountains.
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. Instability will be on the increase with
slightly better chances for showers/gusty storms mainly across the
western mountain group...with, of course, some chance for a couple
of possible new lightning starts. Additionally, with DCAPE values
between 500 and 1000 j/kg over this area of western WY, wind gusts
from these storms could blow in excess of 45 mph at times. Winds
across the far northeastern FA...mainly Johnson county and eastern
Natrona county...will also increase and be very gusty out of the
south once again. ATTM...it looks like RH values will increase just
high enough to keep a Red Flag Warning from being issued for.
However the increased southerly winds could/will affect the new Rest
Stop fire if it is still going then.
Thursday/Thursday night, the upper level ridge translates eastward
into/over the plains with the approach of the NW CONUS
trof...increasing southwest flow and instability while levels cool
aloft. However, moisture increase will be modest at best with storms
moving northeast at 20 to 30 mph spitting out gusty outflow winds to
45 mph or occasionally higher and small hail mainly west of the
Divide during the day with lesser chances east by late in the
day/evening. Best chances for measurable precip will occur over the
.LONG TERM...Friday through Tuesday
Friday looks very warm and breezy to windy ahead of an
approaching weather system and cold front. Fire weather concerns
in parts of the region, especially where existing wildfires
continue to burn. Most of the showers and storms will be in the
west and north with lesser coverage in the south and east. Cold
front moves through Friday night into Saturday. Mainly dry and
cooler Saturday with very isolated showers or storms. Per the
ECMWF model, another weather system moves through Montana Sunday
afternoon and night with a second cold front moving into WY. The
00Z GFS model does not show this but has a stronger weather system
diving into Idaho Sunday night and toward WY on Monday. Not much
faith in this scenario and 06Z GFS may not resemble the 00Z run.
Have a slight chance of showers and storms over the north Sunday
afternoon and night with lesser coverage elsewhere. Monday and
Tuesday will see a west northwest flow and will follow ECMWF
model. Isolated showers and storms over the north both days and
dry elsewhere. High temps will be mild Monday and Tuesday.
Isolated showers and thunderstorms will be possible over the
central and northern mountains, with some gusty winds potentially
reaching into the adjacent terminals. Smoke from wildfires will
leave hazy conditions across portions of the state, and may
partially to full obscure mountain ranges. Limited visibilities
are occurring periodically at terminals across northern Wyoming.
Otherwise, VFR conditions generally anticipated with light wind
and mostly clear skies through the TAF period.
...Red Flag Warning for portions of Johnson county this afternoon
and early evening...
Fire weather conditions elevated across the entire forecast
area...critical for portions of Johnson County...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 mid 20s in the mountains. Winds will generally blow/gust
below critical levels through the day for most areas except for
Johnson county this afternoon and evening where winds will gust out
of the south to around 25 mph at times...especially across the
northern portion of the county. These gusty south winds could affect
the Rest Stop fire by mid afternoon. RH values, look to range from
14 to 20 percent across most of Johnson county during this period.
Isolated showers/storms possible over the central and northern
mountains...with possible localized outflow gusts to 40 mph with
stronger convection. Wednesday should see a slight increase in
moisture across most of the forecast area, but still quite dry for
most lower elevations. Winds will gust once again out of the south
across the eastern fire zones, but RH values should increase above
critical levels over these same zones. Isolated showers/storms
possible over the central and western mountains...with possible
localized outflow gusts to 45 mph with stronger convection.
Red Flag Warning until 8 PM MDT this evening for WYZ281.