Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 232042
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
242 PM MDT Wed Aug 23 2017

.SHORT TERM...(This evening through Thursday)
Issued at 242 PM MDT Wed Aug 23 2017

Morning cloud cover and light precipitation has hindered the
development of afternoon convection across southwest Wyoming. Cloud
cover has waned over the past few hours enabling deeper convection
to ignite over this portion of the forecast area. Precipitable water
values were approaching one inch over northern Utah at 12Z/Wed. This
airmass is slowly moving into southwest Wyoming, placing the
precipitable water values at 125-150 percent of normal. This moist
airmass coupled with light steering winds will lead to potentially
heavy rain with the stronger convection through at least early
this evening. Hi-res models continue to trend toward this
convection reaching southern portions of Fremont and Natrona
counties between 00Z-03Z/Thu. Have mentioned isolated showers and
storms through this time frame. Convection will diminish with the
setting sun, but would not be surprised if showers lingered along
the western border areas through the night. Shortwave spinning to
our west will begin to track east on Thursday ahead of an
approaching trough set to swing through the Intermountain West.
This shortwave will begin to traverse northwest Wyoming Thursday
and eventually across the northern forecast area late Thursday and
Thursday night. Jet maxima of 60kts should provide a bit of a
boost to keep the convection going Thursday night. Not as much
available moisture, but still enough to provide some brief heavy
rain, particularly over the northwest mountains and Yellowstone
National Park. Showers and storms will also move faster Thursday
as steering flow increases in westerly flow aloft. The north is
likely to remain the favored area for Friday, but additional areas
of the southwest and central will get in on the act. Southerly
moisture will swing north through Utah and then east through the
forecast area Friday as high pressure sets-up near the Four
Corners. A weak cold front will push south through areas east of
the Continental Divide on Friday. Expect some afternoon 10-20 mph
winds in areas prone to northwest flow. This front will also drop
temperatures at least a few degrees across the north-central and
central areas. The high pressure will slide slowly west Saturday
placing Wyoming in drier west-to-northwest flow aloft. This
building area of high pressure will also limit the northward push
of the moisture to our south. Am concerned about northwest flow
producing isolated weak convection across the eastern-third of
the forecast area Saturday afternoon. However, tough to find too
many reasons to add convection to the forecast other than the
ridge looks a little dirty and northwest flow cannot always be
trusted. Overall, warmer and drier weather to greet the start of
the weekend.

.LONG TERM...(Thursday night through Wednesday)
Issued at 242 PM MDT Wed Aug 23 2017

Synopsis...A longwave ridge, centered over the Great Basin, will
build early in the period and envelop much of the Intermountain
West, before strengthening over western Canada by the end of the
period. The forecast area will be on the east side of this ridge,
keeping upper level flow from the NW. This may keep skies smoky
from fires in western MT through the period.

Discussion...Dry and warm conditions will occur through the period
with very isolated showers and thunderstorms possible through
Wednesday. The shortwave mentioned from the last shift now appears
outside of the forecast period in today`s 12Z runs. However,
thunderstorm chances increase over the NW and the mountains on
Wednesday. Highs will be in the 80s to the low 90s in the Big Horn
Basin throughout the period.

&&

.AVIATION...00Z Issuance

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

SKC skies continue over KWRL, as convection builds off the
Absarokas near KCOD. SCT-BKN mid to high level clouds also
continue over KRIW-KCPR. Isolated convection will be possible
near KRIW and KCPR late this afternoon, but will end between 03Z
and 06Z. The SCT-BKN band of clouds will continue its movement
toward the NE tonight, leaving a band of mid level SCT to BKN by
12Z Thu. These clouds will thin to FEW-SCT, before isolated
thunderstorms form off the east sides of the Absarokas and Wind
River mountains by 20Z Thu.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

The band of SCT-BKN mid-high level clouds continues its slow
progression northward over the area. Isolated showers also
continue to occur over the area, with thunderstorms beginning to
develop near the Tetons, Gros Ventres and vicinity KBPI-KPNA.
This convection is expected to wane/decrease in coverage between
03Z- 06Z Thu, as the cloud band continues moving northward.
Except for brief MVFR/IFR conditions in convection, VFR conditions
are expected. As convection clears tonight, only SCT mid-level
clouds will remain into Thu morning, becoming FEW after 15Z Thu.
Convection will be isolated again Thu afternoon, mainly impacting
KJAC.

&&

.FIRE WEATHER...
Issued AT 242 PM MDT Wed Aug 23 2017

Showers and thunderstorms will be most prevalent this evening across
southwest Wyoming as a weather disturbance pushes north across Idaho
and western Wyoming. This disturbance will track across the north
half of Wyoming Thursday with cooler and wetter conditions
anticipated in those areas as a result. The best chance for
showers and storms will be Thursday afternoon and evening across
the northern-third of Wyoming. Some areas of northwest Wyoming
will see a better than 30 percent chance of wetting rain, while
drier conditions prevail as we move south. Showers and a
thunderstorm or two will linger through Thursday night. Drier air
begins to push north into Wyoming Friday, and eventually east
across the state Saturday. There will be a general trend toward
warmer and drier conditions beginning late Friday and continuing
through Sunday. The exception will be north-central Wyoming where
a weak cold front will drop south and cool conditions for Friday
and Friday night. Smoke dispersal will improve Thursday as
westerly transport winds will increase. Many areas will see very
good to excellent smoke dispersal Friday with high mixing heights
along with west-northwest transport wind.

&&

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

&&

$$

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



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