Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212118
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
318 PM MDT SUN AUG 21 2016

.SHORT TERM...TONIGHT THROUGH WEDNESDAY

...Critical fire weather over the next couple of days the main
concern...

Imagery shows modest ridging across the western CONUS with a weak
embedded shortwave over the northern Great Basin ahead of the next
trof near the PAC NW...and a deep trof across the eastern CONUS.
Clear skies abound across WY while the surface shows high pressure
in control of the region with only a weak lee side trof across
eastern WY and a weak front over the northwest CONUS.

Monday, southwest flow aloft and quite unstable with dry adiabatic
lapse rates 10 to 11.5 up to at least h5 and winds of 25 kts to
40kts in this layer. So, warming a drying with more wind is on tap
at the surface with RH values dropping back into the single digits
and teens at lower elevations with lower 20s across some mountain
locations. Enhanced to critical fire danger everywhere Monday and
Tuesday. Critical conditions conditions all areas west of the Divide
with mountain adjacent foothills and Basins east of the Divide also
in the mix. Red Flag Warning now posted for these areas. Only a
couple of area may see an isolated shower/storm or two...one over
the extreme south/southwest and the other near the extreme
northwest.

Front sweep through the northern FA Monday night then Tuesday will
return to similar conditions to that of Monday, but with perhaps
slightly increased RH values and the gustier portion of the wind
field shifted further south and perhaps a bit less aggressive.
Will wait through another forecast period or two to make that
determination...with the southwestern FA quad the most likely to go
critical again. Only very low chances for a couple of isolated
showers/storms across the far south.

Tuesday night another front moves through the FA east of the Divide
with Wednesday seeing a change in the weather...at least east of the
Divide...as the area is post frontal and the PAC NW trof begins to
influence the CWA with increased moisture and yet another front
moving into the northern zones in the afternoon. There will be
increased chances for precipitation across the north and east of the
Divide...with only modest increase in moisture and most of the
showers/storms providing little rainfall and gusty winds.

.LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY

Cold front is well entrenched along and east of the divide to start
this period. With large scale ridge off the PACNW coast, and
positively tilted trough axis just to our west, we will be
susceptible to continued shortwaves rounding the trough and moving e
across us through at least Thursday and possible Friday. Would think
that there`s a good chance of showers Wednesday night with one
shortwave of decent strength. The next wave on Thursday looks weaker
and showers may be more confined to the mountains and foothills with
lesser chances elsewhere but still some showers around. Thursday
still looks cool east (60s mainly) of the divide before we start to
rebound Friday. Models diverge quite a bit later Thursday through at
least Friday with where the next energy will head. The GFS takes a
swd moving shortwave and forms a closed low over Central CA and
Western NV by late Friday which eventually opens up and moves across
us Sat aftn and evening with increasing shower chances. The GEM has
a slow swd moving vort that is just to our west at 12z Friday and
then swings across us during the day Friday. The Euro has a weaker
swd moving shortwave that develops a weak trough over the far nrn
Great Basin that shears/moves over us on Saturday. The bigger
pattern would favor something holding to the west through Friday at
least but whether it`s closer to the GEM or a piece dropping down
and folder over on the front side of the ridge is a tough call. Will
lean towards some energy holding to the west through Friday which
will keep the potential for some showers and a few thunderstorms,
especially around the mountains. On Saturday, will give a little
weight to the gfs with potential for some showers developing over
the west and spreading east late in the day. Another wave may come
through in the wsw/sw flow on Sunday with at least a few showers and
thunderstorms around the area, especially mountains.

&&

.AVIATION.../00Z ISSUANCE/

VFR conditions will occur through Monday. It will be breezy across
much of the area into the early evening. Wind speeds of 12 to 18
knots will be common at most terminal sites with gusts up to 22
knots. Monday will see even more wind...especially along and west of
the divide. Areas of smoke from wildfires in NW WY may impact
visibility downwind of the fires.

&&

.FIRE WEATHER...

...Critical fire weather conditions across northwest Wyoming today
and across all of the area west of the Divide on Monday...

Fire weather conditions elevated to critical across the forecast
area through Tuesday. With a warm and very dry transition period
between last week`s cold front and the next system due to arrive
Tuesday night, RH values will crash and range from the single digits
to lower 20s (percent) for most locations. Winds gusting upwards of
25 mph will combine with these dry conditions and critically dry
fuels to give extreme fire weather over portions of northwest
Wyoming today; all areas west of the divide in addition to portions
of the foothills and basins in northern and central Wyoming on
Monday; and across much of southwest Wyoming on Tuesday.

&&

.RIW Watches/Warnings/Advisories...
Red Flag Warning from noon to 9 PM MDT Monday for WYZ140-276>280-
283-286-288-289-300-414>416.

Red Flag Warning until 9 PM MDT this evening for WYZ140-286-415.

&&

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Braun


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