Area Forecast Discussion
Issued by NWS Riverton, WY

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150
FXUS65 KRIW 182324
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
524 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.../00Z ISSUANCE/

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

Mid level moisture this afternoon is already leading to moderate
cumulus development over the mountains and surrounding valleys and
basins.  Scattered rain showers with isolated thunderstorms are
expected over the mountains with isolated showers and thunderstorms
impacting KJAC, KBPI, and KPNA from 00Z to 06Z. Possible rain
showers will move from NW to SE and impact KRKS from about 00Z to
06Z, as well as north of KJAC through 06Z. Higher terrain
certainly obscured this afternoon and evening. May see some gusty
wind of 20 mph from the showers. Southwest flow to increase after
15Z Thu as a line of convection from eastern ID moves info far
western WY Thu afternoon.

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

VFR conditions expected for the next 24 hours east of the divide.
Isolated showers possible late this afternoon and early evening at
KCOD and KLND as showers move off the mountains.  Otherwise mostly
clear skies with generally light wind, except for gusts near 20 mph
through 02Z at KCPR. Southwest flow will pick up after 15Z Thu
across central WY with gusts increasing to 30 mph Thu afternoon.

$$

.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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