Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 090526
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1026 PM MST Fri Dec 8 2017

.SHORT TERM...(This evening through Monday)
Issued at 223 PM MST Fri Dec 8 2017

The clipper type weather system will continue to exit to our east
along with a relaxing pressure gradient and subsiding winds
through this evening. Currently, BYG is still gusting to 35 knots
from the northwest as 700mb 50 knot winds sfc there but these
winds should gradually diminish.

We will remain on the front side of the high amplitude ridge over
the Far Western United States through Monday. This means a
continuation of quiet weather along with well established
temperature inversions in the western and central basins and
valleys through Sunday morning. Tonight and Saturday morning look
more ideal for late night and morning fog formation than what we
saw this morning given the relatively drier air overlying the
temperature inversion tonight. Unseasonably mild temperatures will
continue through the weekend above the inversions and areas that
mix out. Temperatures have already reached 55 in BYG and 56 at
COD this afternoon, so we should easily see highs in the 50s
through the weekend at the lower elevations that mix out.

On Sunday night and Monday, the next clipper type system in the
northwest flow will likely produce increasing northwest winds once
again, especially in Northern Johnson County.

.LONG TERM...(Monday night through Friday)
Issued at 223 PM MST Fri Dec 8 2017

Mean ridge holds near the West Coast through the medium range
period. Main forecast challenge will continue to be temperature
inversions with the procedure continuing to warm up the
temperatures way too much, especially in the west. Will watch this
closely. Otherwise, models have been showing a piece of energy
currently just south of the west end of the Aleutians which rides
up and over the ridge and drops through us around Tuesday night.
12z GFS has shifted further east (ern WY) with this small but
tightly wound mid level low compared to its previous run. The 12z
Euro is actually farther west with the low (tracking across far
ern ID Wed). There is some weakening of the mean ridge with time
but it`s still quite strong up into swrn Canada. Overall trend
could allow one of these systems to track further west then they
have been so it`s worth watching closely in the next couple days.
Other than that, it`s a quiet period with inversions and some
patchy fog (mainly far west) to deal with. Temperatures could warm
a little better Tuesday ahead of this Tuesday night/Wed system,
especially in the north and east. Otherwise, generally upper 20s
and 30s far west to upper 30s to mid 40s east each day.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1025 PM MST Fri Dec 8 2017

The main forecast challenge will be the coverage and duration of fog
across the far western valleys including the KJAC terminal
Saturday morning. Some fog formed across the western valleys
Friday morning. Atmospheric conditions (i.e., stronger surface
inversion) are at or even better for fog formation Saturday morning
so will prevail fog at KJAC terminal with it dissipating by
midday. Elsewhere VFR conditions with only some high-level clouds
are expected.

&&

.FIRE WEATHER...
Issued AT 223 PM MST Fri Dec 8 2017

Winds will over the higher mountains and northern Johnson County
will finally subside by this evening. Temperature inversions will
remain in place through the rest of the week, mainly over the
central basins and western valleys. Expect poor mixing in these
areas. Patchy fog is expected in the far west valleys as well as
over portions of the Wind River Basin during the late night and
morning hours through the weekend. Considerable warming will
continue in the higher elevations and in the mixed out areas
through the weekend.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Murrell
FIRE WEATHER...Lipson



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