Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 202036
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
236 PM MDT Fri Oct 20 2017

.SHORT TERM...(This evening through Monday) Issued at 233 PM MDT
Fri Oct 20 2017

Imagery shows a broad trof across the wrn 2/3rds of the CONUS and
the main storm track and nrn branch of the jet digging into the wrn
CONUS. Increasing wind speeds as well as divergence aloft noted.
First embedded upper wave (n/s oriented) also pushing through cntrl
WY. Strong SFC front(ogenesis) developing to the west of WY over
cntrl ID, racing to the east. This frontal area is attached to
deepening low pressure over MT. Light areas of precipitation just
now entering wrn WY ahead of the main trof axis under divergent
the flow aloft.

Rest of today and tonight, the FA continues under swrly and slightly
divergent flow aloft while the SFC and boundary layer P GRAD
increases rapidly ahead of the approaching and strengthening cold
front from the west. Winds continue to increase first across the
FA...and will continue to do so until FROPA this evening. Winds
gusting out of the sw could easily reach 40 to 60 mph (isolated
higher at the usual spots) in Sweetwater, Sublette, Fremont and
Natrona Counties. A High Wind Warning was issued earlier for these
conditions. The Clark area could also briefly gust to near high wind
levels late this afternoon/early evening especially if an elevated
inversion under WAA occurs near or just below ridge top level to the
west. Finally, northern Johnson could also see brief High Wind this
evening with frontal passage. Minimum RH looks to "only" bottom out
in the mid/upper teens and 20s mainly over Johnson County. Fire
weather conditions east of the Divide will still warrant "elevated"
status if nothing else because of the strong winds and Friday
evening wind shift with the front. Johnson County could see
occasional Red Flag conditions this afternoon. Otherwise, modest
EPAC moisture flows into wrn/nwrn WY from the west through the trof
this afternoon. A rain or rain snow mix across some valley locations
can be expected this afternoon with all snow generally above 7.5k
feet this afternoon...lowering all the way to the valley floors
later this evening. 24 hr QPF totals west of the Divide through
Saturday afternoon will range from a few hundredths across southwest
WY to perhaps 0.3 to 0.5 inches across portions of northwest WY.
Snow accumulation over the time frame will range from a few tenths
to perhaps a couple of inches in the western valleys (highest over
the northern Jackson Valley)...with anywhere from a half inch across
the southern mountains to as much as 6 inches over the higher
elevations (> 8500 feet) of the Teton and Gros Ventre Ranges
(localized 8 inches in the Tetons). The cold front will then move
east of the Divide late this afternoon through tonight...with low
chances for precip mainly over portions of the Cody Foothills and
Big Horn Basin.

Saturday will be post trof/frontal and cool with highs in the 40s
and 50s across the lower elevations...20s and 30s in the mountains
and a few lingering showers over the wrn/nwrn mountains. The upper
levels will become more wnw with the nose/exit region of a rather
strong jet (100 to 120kt) moving into/over WY...with strengthening
H7 flow nearly orthogonal to the terrain. By Saturday night, the jet
drifts somewhat north and east, repositioning to a more e/w
orientation mainly over MT with the nrn/nwrn FA continuing under the
better mid/upper level dynamic forcing. Increasing but modest EPAC
moisture will begin to push back into nwrn/wrn WY while the terrain
and jet orientation along with increased boundary layer flow (in
response to a growing P GRAD between high pressure over the Great
Basin and lee side troffing east of the Divide) add to the increased
flow aloft allowing stronger winds aloft to get to lower levels
under relatively strong boundary instability. All this will also
provide a good mechanism for getting precip to the surface at least
over the mountains west of the Divide. Colder mid/upper temps during
the day Saturday will be slowly replaced Saturday night by
increasing temps and slowly raising snow levels in a moderate WAA
regime. Snow levels likely end up rising from the valley floors
(what it would have been during the day Saturday) across wrn WY to
perhaps 6.5 to 7.5kt ft by Sunday morning. Again, it mainly looks
like sub-Advisory level snow except for a few higher elevations
mainly above 8500 feet which could see more than 6 inches by end of
day Sunday. Did raise winds again over those provided (the same as
in all other earlier periods) across the Wind River and Absaroka
Mountains/and area foothills Saturday night through Sunday and
Sunday night through Monday as a "typical" high wind pattern (or
near so) develops with good upper level wind speeds, direction and
possible mountain wave production in addition to good cross barrier
temp/pressure/omega gradients develops. A High Wind Watch has been
posted for forecast zones 2, 3 and 16. Also, if today pays off
across the Wind Corridor with respect to High Wind, the same area
may need to "Watched" for Sunday. Lingering light showers continue
across the western mountain Sunday night...ending overnight into
Monday morning as a brisk clipper front moves through the region
east of the Divide. This clipper passage could cause another brief
period of High Wind across Johnson County possible by Monday
afternoon.

.LONG TERM...(Monday night through Friday) Issued at 233 PM MDT
Fri Oct 20 2017

Overview...A ridge of high pressure will build Tuesday and provide
much warmer weather Tuesday and Wednesday. Windy, warm and dry
conditions will elevate fire weather conditions on Wednesday. A
Canadian cold front will plunge south across the area Wednesday
evening into early Thursday morning. This will bring a chance of
rain and snow to areas mainly along and east of the Continental
Divide. A persistent and strong northerly flow aloft will keep
temperatures cool Friday and likely into the weekend.

Discussion...The GFS and ECMWF remain quite similar in projecting
the overall pattern evolution this coming week. Heights build
Tuesday in the wake of the Monday system that will dig across the
Midwest and Lower Ohio Valley. Tuesday will see above normal
temperatures and surface wind will be tame by Wyoming standards.
The next trough moves into the Northern Rockies Wednesday with
Wyoming remaining in the warmer air ahead of this system. Lee side
trough and approach of a Canadian cold front will lead to increasing
wind. Have increased maximum temperatures Tuesday and Wednesday and
also increased Wednesday wind speeds/gusts as blended models appear
to be underplaying both. Temperatures Wednesday could be still
warmer than currently forecast as the gusty wind is likely to boost
temperatures even more. The GFS is slightly faster in bringing the
Canadian front into the north by 00Z/Thu. ECMWF doesn`t bring the
front into the north until 3-6 hours later. Regardless, this front
should have no problem traversing the forecast area by 12Z/Thu.
Could be a rain/snow mix in the Big Horn Basin much of Wednesday
night with surface temperatures cold enough to support snow in the
central basins after 09Z/Thu. It would appear right now that this
front and associated precipitation will clear the area later
Thursday. Also increased wind speeds/gusts across the north
Wednesday night and Thursday. If current trends hold, these speeds
will need to be boosted even more as confidence grows. Strong
northerly flow aloft will likely remain in place Friday into the
weekend with temperatures only slowly rebounding.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 233 PM MDT Fri Oct 20 2017

Strong cold front will continue to progress east across the forecast
Friday afternoon. Front was approaching central terminals at mid-
afternoon and will reach KCPR between 23Z/Fri and 00Z/Sat. Strongest
sustained winds of 30-40kts will occur with frontal passage and for
a few hours after. However, as the wind direction veers to the west-
northwest it will favor gusty wind later into the evening at KRIW,
KRKS, KBPI, and KPNA. Overall, wind speeds will decrease at most
terminals between 03Z-06Z/Sat. Precipitation has been and will
mainly continue to be confined to areas along and west of the
Continental Divide. Light rain has been in the vicinity of KJAC, and
this terminal will remain the most likely to see precipitation
Friday afternoon and evening. Brief MVFR or perhaps IFR conditions
are still possible late Friday afternoon, but it may remain too warm
to mix with the snow. Precipitation will wane by 22Z or 23Z at KJAC.
Mountains tops will be obscured along and west of the Continental
Divide through about 06Z/Sat. Other terminals will be VFR with
little chance of persistent precipitation. As for Saturday, wind
will again be the main hazard after 18Z. Speeds will be less than
Friday and more from the west-to-northwest.

&&

.FIRE WEATHER...
Issued AT 233 PM MDT Fri Oct 20 2017

Fuels remain at critical levels across over Johnson and Natrona
Counties. (Please see the Fire Weather Forecast product for more
information).

...Elevated Fire Conditions Across Portions of Central and
Northern Wyoming this Afternoon...

...High Wind Warning for portions of Central Wyoming this
Afternoon...

Today will continue with above seasonal temperatures east of the
Divide and be relatively dry with minimum RH values ranging from the
upper teens to 25 percent across the lower elevations east of the
Divide. Winds across most of central Wyoming will gust out of the
west to southwest 25 to 50 mph...locally higher at times. Light to
occasionally moderate rain and snow expected west of the
Divide...little to none east of the Divide. Due to stated
conditions, a brief period of Red Flag conditions could present
itself across either Natrona or Johnson County this afternoon.
Saturday, a slight improvement in relative humidity is expected for
most locations. Although, with RH values still ranging into the
upper teens and 20s (percent) for portions of central Wyoming once
again, fire weather will likely remain elevated for another day
east of the Divide as winds blow out of the west to northwest 10 to
30 mph for most locations with some gusts to 35 mph possible. High
Winds again possible later Saturday night and into Sunday for
preferred locations east of the Divide.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late Saturday night through Sunday evening
for WYZ002-003.

High Wind Warning until 6 PM MDT this evening for WYZ019-020.

High Wind Watch from late Saturday night through Sunday morning
for WYZ016.

&&

$$

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


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