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Area forecast discussion 
National Weather Service Riverton Wyoming
1109 am MDT Tuesday Sep 16 2014

Short term...today through Thursday night
if weak convection can form on a day with weather features as
benign like Monday...given the weak convection that formed over
eastern Sweetwater and southern Natrona counties...than some
convection should form across northwest Wyoming this afternoon. Some limited
high and middle level moisture riding up and over the ridge over the
far western United States combined with negative lifted indices
will result in isolated afternoon thunderstorms with widely scattered
coverage in the mountains today. Also...the next upstream trough off
the Pacific coast now out at 140w has ejected a shortwave that
will deliver this moisture and weak instability. On
Wednesday...more moisture originating from the subtropical
Pacific...negative lifted indices and fairly high convective available potential energy for this
time of year will produce scattered to widely scattered late day
thunderstorms over mainly western Wyoming. Then the aforementioned
trough now at 140w will approach The Rockies Thursday and will
increase the chances of afternoon and evening thunderstorms in the
west with probability of precipitation between 50 and 60 in the western mountains and isolated
east. Some of this moisture will originate from Odile. The ridge
will continue to build in from the west today and Wednesday along
with continued warming temperatures. Highs today should range from the
low 80s at the lower elevations east of The Divide and 75 to 80
west. By Wednesday...highs in the middle 80s east and around 80 west.
The models want to continue to warm US up Thursday with high temperatures
near 90 in the Big Horn basin but increasing clouds will likely
counteract any warming...especially with the lowering sun angle of
middle September. Showers will likely last well into Thursday night
as an associated cold front moves across the area. West winds will
begin to increase somewhat by this afternoon as the aforementioned
Pacific trough approaches from the west...tightening the gradient.

Long term...Friday through Tuesday
splitting trough moves across the area on Friday. Southern piece
impacts the far south with some increased moisture and support as
front moves east across the area. Higher precipitable water values
of .75 to .90 are forecast by the GFS in the far north and east. The
cold front moves down from the north and backs up near The Divide
on Friday with some modest convective available potential energy possible in the higher precipitable water air.
Southeastward moving shortwave may also aid in thunderstorm development.
Otherwise...looks like a dry medium range period with ridge
building into the West. Wyoming remains in a somewhat cooler/mild
and dry northwest flow. Looks like a spectacular start to fall early next
week with temperatures throughout much of the period in the 70s
across the lower elevation with 50s and 60s in the high country.
About the only thing to watch early next week will be where the
southern piece of our remnant low will be. The European model (ecmwf) is much further
south tonight while the GFS is across southern Utah with moisture pushing
northward to near the southern Wyo border on Monday. Going dry right now but
it's Worth watching to see if any moisture can make it into at
least Sweetwater County with time. Otherwise...dry with no
significant winds and overall plenty of sunshine once we get by
our Friday system.

&&

Aviation.../18z issuance/
VFR conditions will prevail through 12z Wednesday. Isolated showers
and storms will form over western Wyoming after 18z and continue until
about 04z Wednesday. Gusty outflow wind to 35 knots is likely near storms.
The terminal sites west of the Continental Divide may be impacted by
showers or storms. More showers and thunderstorms are possible on
Wednesday but the vast majority should hold off until after 18z
Wednesday.

&&

Fire weather...
a building area of high pressure will remain over the area for the
next few days. The high pressure system will keep warming
temperatures and mainly dry conditions today. A bit of breeze may
develop this afternoon across the southwestern wind corridor but
should main below critical levels. The chances of afternoon
showers or thunderstorms will increase for the latter half of the
week beginning Wednesday afternoon as the next upstream trough
approaches. Winds will increase tonight or Wednesday as well.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...lipson
long term...skrbac
aviation...hattings
fire weather...lipson

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