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Area forecast discussion 
National Weather Service Riverton Wyoming
311 am MDT Tuesday Sep 1 2015

Short through Thursday night

Presently...a monsoon plume is drawing up moisture from the
southwest and just barely skirting far eastern Sweetwater County and
the southeast half of Natrona County. Any lightning activity is in
the deeper moisture over the Central Plains to the Desert Southwest.
Most of this high and middle level moisture will continue to slide off
to the south and east as we become more influenced by the eastern
Pacific trough and the Pacific moisture. We are looking at scattered T
storms mainly in the northwest mountains this afternoon as the prominent
eastern Pacific trough ejects a weak vorticity tracking east northeast
across the northern rockies with limited Pacific moisture.
Eventually only very isolated convection can be expected elsewhere.
Otherwise...warm dry southwest flow will continue through
Thursday...increasing and backing slightly during the week as the
closed low within the strong Pacific trough along the British Columbia coast
slowly slides southeast to the Pacific northwest through
Thursday...tightening the gradient. The southwest winds will
increase noticeably by Wednesday...thus increasing the critical
fire weather activity up another notch. By the
Pacific trough nears...qg forcing on the front side of the Pacific
trough will be in place well to our northwest over northern Idaho
and western Montana...but enough Pacific moisture and just enough
instability should get some scattered thunderstorms going in ynp late
Thursday. Temperatures should remain above average today through

Long term...Friday through Tuesday

Cold trough currently over eastern Gulf over into northern b.C. Will continue
to dig southeastward in amplifying flow upstream. End result is a major
trough over the west the second half of the week which looks to even
get cut-off from the flow slowing its eastward progress. The GFS has
slowed considerably since last night as suspected and is now in much
better agreement with the European model (ecmwf). Not fully yet but much better.
Bottom line is still for a significant cold trough/upper low to dig
into the Oregon/Idaho/nrn Nevada region on Saturday before moving east
and NE Saturday night into Sunday morning into our area. System then
starts to exit the region Sunday night with a very dry westerly flow on
Monday. Another trough may dig into the pacnw next Tuesday with the
potential for increasing SW wind across the area along with warmer
and continued dry conds. Ahead of the trough Friday...we should see
some monsoonal moisture clipping our southeastern zones with the leading
edge of the trough assisting with storms in the northwestern zones. Frontal
boundary will be hovering near the northern zones with cooler conds
behind it. On Saturday...digging trough into the northern Great Basin may
allow this cold front to sag further southward into central Wyo increasing
the chances of showers/thunderstorms there and of course in the west. Cool
if not cold upper low then lifts northeastward across our northwestern zones Saturday
night into Sunday morning with widespread showers and some thunderstorms
along its path with lowering snow levels. Campers/hikers and other
outdoor enthusiasts should be prepared for this dramatic change to
cold and wet weather with lowering snow levels. Current models are
showing -2c to -3c in ynp early Sunday. Snow levels may drop all the
way down to the low spots in the part with several inches of snow in
the mountains of the northwest. Strong SW wind ahead of this system will
likely blow across the mountains and southern parts. A chill day Sunday in the
west and cool to mild at best in the east. A gradual warming trend
Monday into Tuesday with dry low levels. Increasing wind again
Tuesday ahead of the next trough.


Aviation.../12z issuance/

VFR conditions will prevail through Tuesday evening. Isolated
showers and storms will develop over the west after 19z. This
activity will develop east of the Continental Divide after 22z.
Outflow wind gusts to 30 knots will be possible near any showers
or storms. The shower and storm activity will end in the west by
04z Wednesday and in the east around 09z Wednesday.


Fire weather...

..elevated fire danger for much of the week...

There will be dry and breezy periods throughout the week with
occasional lightning and cold fronts later in the week as another
large upper trough approaches. Westerly breezes will continue
into the evening with a few spots exceeding 30 miles per hour...especially
near storms and over higher terrain. Relative humidity will
improve on Tuesday afternoon but it will still dip into the middle
teens in many areas. A large trough will approach on
Wednesday...increasing flow from the southwest for Wednesday and
Thursday with widespread low humidity and gusty winds.


Riw watches/warnings/advisories...
Fire Weather Watch from Wednesday morning through Thursday
evening for wyz275-277-279-414-415.

Red flag warning until 9 PM MDT Wednesday for wyz276-280>283-285-



Short term...lipson
fire weather...lipson

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