Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190532
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1132 PM MDT WED MAY 18 2016

.Short Term...Tonight Through Saturday

Building instability will keep the potential for a few thunderstorms
along and west of the divide into the evening. After this evening,
the forecast remains on track with large upper low dropping into the
Pacific Northwest tomorrow and then down into the Great Basin Friday
into Saturday. Main trend in most of the guidance is a little faster
eastward movement of the upper low by Saturday. If this occurs, it
will start to cool some east of the divide on Saturday already,
especially in the central zones while the far ern sections remain
similar to Friday. In the west, approaching cold front late in the
day Thursday into Thursday evening will coincide with the nose of
cyclonic jet to produce a strong band of thunderstorms ovr Eastern
Idaho which has a good chance of moving into the west later in the
afternoon into the evening hours with strong wind, heavy rain and
marginally severe hail potential. The proximity of the front may
keep the threat of some stronger storms and heavy rain into Friday
at least across the west, especially in the mountains. East of the
divide, it will become warm and windy with increased runoff.
However, with the potential for cooler temps by Saturday already, we
only get a couple warm days before we start cooling off again.

.Long Term...Saturday Night Through Wednesday

Large scale trough over the western US will be pushing the brunt
of its strength into the Rockies Saturday afternoon through
Sunday. With the elongated trough somewhat split into a stronger
northern piece over western MT and a weaker southern piece over
the Las Vegas area, western and central WY will see residual
convective activity Saturday evening as freezing heights continue
to gradually lower from west to east to about 7000 feet Sunday
morning. A follow up shortwave from Oregon will move into eastern
Idaho Sunday evening and produce more precipitation into western
WY. More of a zonal flow during this time frame looks to focus
precipitation across the northern half of the state into Monday.
A follow on shortwave trough from the eastern Pacific will keep
the large scale trough in place Monday evening into Tuesday. As
the southwest flow develops again across the central Rockies,
Tuesday and Wednesday should be better convective days across
western WY as the trough digs into Nevada. With 700mb temperatures
hovering between +4C and -2C for much of the long term, surface
temperatures should be below normal. There will be periods of
strong and gusty southwest and west wind as well. Still lots of
details uncertain at this time given the complexity of the pattern
as well as differences in the model guidance. For the most part,
the GFS takes more energy to the south and the ECMWF keeps more
zonal flow.

&&

.AVIATION.../06Z ISSUANCE/

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Airports

Scattered mountain rain showers will decrease gradually into the
morning with local mvfr ceilings above 9000 feet.  VFR conditions
are expected to prevail at the airports through 18Z.  A short wave
low pressure trough ahead of a large Pacific trough moving across
the Pacific Northwest will bring a large area of showers and
embedded thunderstorms across the western mountains with mvfr to
local ifr ceilings obscuring the mountains.  Thunderstorms will
likely have hail up to 3/4 inch and strong wind gusts.  Mvfr
ceilings and visibility reduced in heavy rain will be most probable
at KJAC and KPNA from 19Z through 03Z. Widespread clouds and showers
will continue from 03Z through 06Z with higher mountains mostly
obscured.

East of the Continental Divide...KCOD/KCPR/LND/KRIW/KWRL Airports

VFR conditions are expected to prevail at the airports through the
period.  Areas of southwest winds 15kts to 20kts with gusts to 30kts
will develop after 18z over and near the east slopes of the
mountains and through the wind corridor from KRKS to KCPR.  After
19z widespread showers with embedded thunderstorms will obscure the
mountains above 9000 feet along and east of the Continental Divide.
scattered showers and isolated thunderstorms will weaken as they
move east but some should hold together across the Bighorn and Wind
River basins.  Brief mvfr ceilings may occur in heavier showers
vicinity KLND and KCOD.  Thunderstorms will have small hail and
strong wind gusts. Showers and thunderstorms will decrease from 02Z
to 06Z but will linger over and along the Continental Divide.

$$

.FIRE WEATHER...

Scattered showers and a few thunderstorms will continue through this
evening, mainly across the higher mountains west of the divide. A
new storm system and associated cold front will bring an increased
threat of thunderstorms to the far west Thursday afternoon and
evening. East of the divide, it will be mainly dry, warm and breezy.
The threat of thunderstorms will persist out west Friday and
Saturday while east of the divide, it will continue to warm along
with increasing wind and lowering rh`s into the upper teens and
20s.

&&

.RIW Watches/Warnings/Advisories...None.

&&

$$

SHORT TERM...Skrbac
LONG TERM...McDonald
AVIATION...C.Baker
FIRE WEATHER...Skrbac



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