Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200500

Area Forecast Discussion
National Weather Service Riverton WY
1100 PM MDT Sat Aug 19 2017

.SHORT TERM...(This evening through Sunday)
Issued at 121 PM MDT Sat Aug 19 2017

Shortwave passing across the north and associated frontal boundary
has concentrated the wrn MT smoke layer with vsby restrictions
around BYG today. Otherwise, just warm with some associated clouds
up north. On Sunday, the ridge to our sern expands nwd bringing a
weak disturbance or two into the southwest with limited mid level
moisture. A few storms may fire in the southwest Sunday afternoon
with some gusty wind potential. The next shortwave moves into
Montana Sunday night and exists the nern part of our area late
Monday afternoon. A band of clouds will be associated with this
sewd moving shortwave while another band (deformation) of mid/high
clouds will try to be drawn newd from out of UT into swrn Wyoming
and then across areas east of the divide during the morning/early
afternoon. For the eclipse, it looks like the Jackson area may
start to see clearing by totality (20-30 percent sky cover) while
the deformation axis clouds decrease or shift/shear ewd in the mid
to late morning to around the noon hour. If anything the trend is
for less cloud coverage during the critical time, especially the
western half but possibly the entire area. Will watch this very
closely for Sunday. For now, will go mostly sunny around Jackson
and partly cloudy east of the divide. Monday looks dry now with
faster timing and have all but ended convection chances Monday.
Ridge returns Tuesday for another very warm and dry day with only
a slim chance of a thunderstorm over swrn Sweetwater County.

.LONG TERM...(Sunday night through Saturday)
Issued at 121 PM MDT Sat Aug 19 2017

On Tuesday night and Wednesday, the long wave high pressure ridge
will be setting up overhead from the west resulting in one of the
hottest days of next week in many areas with the Big Horn Basin
seeing high temperatures in the low 90s and around 80 at the lower
elevations of the far west. Also by Wednesday, instability will
increase along with an increase in PW resulting in a slight chance
of late day thunderstorms Wednesday as well as on Thursday. Then by
Friday, the models diverge in how they handle the next upstream
approaching shortwave. The Canadian and the GFS drive the shortwave
axis across the CWA around 06Z Saturday of next weekend, whereas the
Euro keeps the blocking high over the Western United States, keeping
this shortwave weaker and much farther north up in Canada. For now
will go with the Euro since the 5 wave shows the high clearly
anchored over the Western United States. Will therefor go drier
Friday and Saturday with Saturday seeing another spike in


.AVIATION...(For the 06Z TAFS through 06Z Sunday evening)
Issued at 1100 PM MDT Sat Aug 19 2017

Conditions at all terminals will be VFR through the forecast period.
Wildfire smoke has been pushing into the state from the north and
west Saturday. The smoke is most dense across far northern Wyoming
and near KBYG. Northerly surface flow behind a weak front has nudged
smoke into KCPR and KCOD where visibility has gone to 6SM FU. This
should continue off-and-on during the night before improving mid-
morning Sunday. Have left out MVFR conditions for now, but do expect
occasional mountain obscurations. Some additional smoke working into
western Wyoming late Saturday, which may begin to settle in some of
the western valleys by sunrise Sunday. Scattered mid-cloud deck will
slowly push into southwest Wyoming early Sunday. This moisture will
combine with weak instability to trigger some late day convection
Sunday across this part of the forecast area. Vertical shear is
limited and don`t expect convection to be of great intensity. Bigger
hazard would be outflow wind, even from weak showers, that could
approach 45kts with dry sub-cloud layer. Convection should wane
between 03Z-05Z/Mon. Another round of afternoon westerly wind 10-
20kts in the southwest terminals and KRIW, while KCPR looks to stay
around 10-12kts. These winds will be diurnally driven, so expect
decreasing speeds around 01Z/Mon.


Issued AT 121 PM MDT Sat Aug 19 2017

...Elevated Fire Weather Concerns This Weekend Due To Very Dry
and Warm Conditions with Breezy Areas...

Fuels at critical levels for most of the low elevation forecast
zones in addition to a few mountain zones above 8000 feet across
western Wyoming. (Please see the Fire Weather Forecast product for
more information). Elevated fire weather concerns for the entire
forecast area through the weekend due mainly to very low RH values
each afternoon and evening. Very dry conditions with minimum RH
values dropping into the low to mid teens across the lower
elevations, upper teens in the foothills, and 20 to 30 percent in
the mountains. Locally breezy conditions will occur at times over
the weekend increasing the fire weather concerns, especially across
the southern and western sections and higher terrain east of the





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