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Area forecast discussion 
National Weather Service Riverton Wyoming
1130 am MDT Wednesday Apr 16 2014

Short through Friday night

Sat imagery shows very progressive upper pattern west/SW trough axis
moving rapidly across the state from west-northwest to east-southeast...with the best
lift now over the eastern zones of Johnson and Natrona counties. At the
surface...low p quickly spinning up to our east over the western South Dakota/NE
region...dragging surface front through much of the County Warning Area at this
point...with the exception of southern Fremont western Natrona and
Sweetwater counties as the front/convergence zone stalls
out...with this area of convergence continuing to produce light
snow and sleet. Upstream and just off shore is an epac high with a
jet now moving down the east side of the ridge and orv WA/OR. Even
further upstream lay another stronger jet to the south of the
Aleutian low...digging into the west side of that region's trough.

Today...SW and associated surface front continue out of the
area...leaving only a cold pool aloft under northwest flow. The combination
of relatively cool lower levels under the cold pool will yield only
low/moderate instability at best and will likely need the mountains
to accomplish the task of putting anything on the surface for the most
part. Spittle for the most part at the lower elevations if that.
Cool today...but the good news is that middle and lower levels will not
generate much in the way of wind. Tonight...weak surface pattern with
upper levels beginning to slowly transition to ridging and drying.
Warming will commence on Thursday...with 70s again within reach by
Friday as the ridge centers overhead. However...this will be
somewhat short-lived as a weak Great Basin low gets the kick it
needs from the Aleutian jet energy/trough mentioned above...spins up
and races through the forecast area...dragging some epac and SW Continental U.S. Moisture
into the region along with a developing middle/surface front...increasing
and expanding precipitation chances Friday....especially afternoon and
overnight. Does not look all that special at this time...with most of the
precipitation coming down in liquid form outside of the high mountains.
Will have to watch our southern and perhaps far southeastern zones as these
look like the areas that will most likely be able to take
advantage of the best moisture combo along west/the best frontal
forcing. Will also look for at least isolated chance for thunder with
this activity.

Long term...Saturday through Wednesday

North dries out behind surface cold front...northern stream trough
on Saturday while fragmented upper level system lingers in The Four
Corners region. This latter system will continue to push moisture
and slight instability up to a stalled frontal boundary across the
south with chances of rain showers and isolated thunderstorms mainly
in the vicinity and south of a Rock Springs to Casper line...the
best chances near the Colorado border. Otherwise...high temperatures
on Saturday will be only a few degrees cooler than Friday behind the
cold front...and still several degrees above average.

For Sunday through Wednesday...the main weather
maker...eventually...will be an upper low currently near the western
end of the Aleutian chain. European model (ecmwf) and GFS are in reasonable
agreement with the timing of this system progressing onto the West
Coast Tuesday and into the Great Basin Tuesday night. Any agreement
ends there with the GFS stalling out a stacked low over the eastern
Great Basin Wednesday while the European model (ecmwf) keeps an open trough with lead
shortwave shearing out into the northern High Plains Wednesday. A
scenario closer to the European model (ecmwf) seems more likely with this system
bottoming out near the West Coast before lifting out across The

The upper flow will gradually back ahead of this system Sunday
through Tuesday bringing the warm before the storm...especially on
Tuesday across central and eastern Wyoming. The central basins are
likely to see lower 70s Sunday and Monday. Very mild and
breezy/windy conditions are possible on Tuesday in the warm sector
across central Wyoming as the Pacific cold front pushes east toward
the western border. The Bighorn range could see rapid snowmelt and
significant rises in creeks and rivers in this setup. Higher convective available potential energy
may also push back as far west as the Bighorn range which could
result in a few strong storms developing off the east slopes.

Cooler conditions on Wednesday with upper trough over The
Rockies...chances of mountain snow and either rain or a wet snow in
the valleys.


Aviation.../18z issuance/

The overnight disturbance is moving southeast out of Wyoming
today...with leftover moisture and northwest flow...mainly impacting
eastern and central Wyoming. MVFR conditions with lowered ceilings
expected this afternoon from kcpr to kbyg with mountain obscurations
throughout. VFR conditions are expected elsewhere throughout
western and central Wyoming.

Isolated showers are expected over the mountains of northern Wyoming
this afternoon before dissipating early this evening. Wind speeds
are likely around 8-10 knots with some gusts in the northwest flow
around 20 knots. High pressure will move in Thursday...with partly
cloudy skies and increasing southwest wind across southern and
central Wyoming in the afternoon and early evening.


Fire weather...

With the front through and a weak surface pattern setting
will be cool with less wind. Only the mountains can expect isolated
to scattered snow showers. A warming trend will commence by
Thursday...with many of the lower basins getting near the 70
degree mark by Friday...ahead of the next Spring weather system
due to arrive by Friday afternoon. Winds will pick up to the
breezy category across the southern dispatch zones Friday
afternoon...and there will be an increasing chance for shower and
isolated thunder Friday afternoon and evening...with better rain
chances over the southern zones again after midnight.


Riw watches/warnings/advisories...


Short term...Braun
long term...aem
fire weather...Braun