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Area forecast discussion 
National Weather Service Riverton Wyoming
344 am MDT Friday Apr 18 2014

Short through Sunday night

WV imagery shows complex but mostly tame upper levels...with low amplitude
trough ridge trough pattern across the northern Continental U.S....lying over top of sub
tropical based trough ridge trough. Weak closed low anchors the southern
portion of the West Coast trough. The northern branch of the upper levels is
moving faster than the southern Branch. Ridge currently over Wyoming will give
way to the quick moving trough to the west. However...moisture
associated with the western trough is modest for the most part due in part
to its quick movement with the best WV currently loaded to our west and
northwest...associated west/baroclinic Leaf in the Pacific northwest...starting to be
rapped into what will be a rapidly developing cyclone well to our north
near the northern Idaho/California border. Additionally...there is jet phasing going
on now as a precursor to rapid cyclogenesis. The surface is currently
represented by a broad low centered near north central Montana out in front of
the western trough...W/ a Lee side trough now extending down into our eastern
forecast area.

Today...the problem with this multi-part upper system is that instead
of focusing all dynamics for the development of one large deep
brutal system...the lower levels are going to be busy stretching and
trying to spin up at least three low pressure systems over the next
couple of days...from north of the border...S into the Southern Plains. The
available moisture coming in from the west will also get divvied up
with the best WV heading up north with the strongest developing
cyclone and the rest mostly missing US south on Saturday as the
other Lee cyclone develops. Our western mountains will for a short
period be in decent position this morning through middle afternoon for
orographically induced precipitation. By this afternoon...low levels will
warm enough...with surface highs east of The Divide...approaching or just
cresting the 70 degree mark in the basins and create some
instability late this afternoon and early evening. However...even
with best upper forcing from jet this afternoon/evening...meager
moisture will severely limit convective development for most areas.
The best chance for isolated thunder east of The Divide will all be
about the timing...and will take a front...some brief shallow
moisture pooling near near the front...and probably some nearby
terrain to lend a helping hand. At this time...the best location appears to
be in the Big Horn basin...with the far southern portion most
favorable...along The Owl creek and Bridger Mountains...north to
perhaps Thermopolis and/or Worland. Mostly dry adiabatic atmosphere
south and east of the the Big Horn basin will only be slightly
unstable even with relatively high lapse rates...and the window
short for convection to get through. Difficult to ascertain where the
NAM is/was getting the relatively high bl moisture in a SW flow
regime for Natrona County...generating rather large cape. It has
toned down some in tonights run...but still shows it to some
degree...albeit under fairly well capped skies. Overall...just
doesn't look all that promising. Still have low probability of precipitation for late
afternoon/evening just in case...with virga the mostly likely main event
for much of the area. 2 inches of wet snow at best in isolated
locations in the western mountains...with most locations getting half
that with seasonally warm conditions. Otherwise...the surface to middle level
p and T pattern will favor increased winds through the day. However it
still appears that only very isolated high wind conditions will
manifest...if at all. Just not enough confidence to warn for this.

Saturday will be somewhat cooler north of the front...but nothing
too drastic with high temperatures still reaching into the middle 60s for
many low elevation locals...and all with less wind than Friday. As a
weak pressure gradient fills in. Stalled front near and just north of
the Wyoming/Colorado border will allow for some continued convective
development...especially by middle day...and may push up to the very
southern reaches of Natrona County. Sunday afternoon will offer the next
best chance for isolated to widely scattered showers with some
thunder as low levels warm (near 70s again) and a weak upper
disturbance moves through while a weak clipper type front brushes
our eastern zones. Generally 30 percent of less probability of precipitation for this

Long term...Monday through Friday

Our main weather maker during this period will be an upper low
wobbling its way eastward along the Aleutian Islands early this
morning. This system will take the form of an upright trough along
130w by 00z Tuesday...pumping up a ridge across The Rockies. This
will bring dry and mild conditions on Monday.

Trough becomes more negatively tilted as it pushes into the
intermountain west on Tuesday and then the fun begins. 18/00z European model (ecmwf)
is much more progressive than earlier runs...swinging the upper
trough across The Rockies on Wednesday and into the plains Wednesday
night...shortwave ridge building in Thursday morning. Though
GFS/gefs mean have also trended more progressive...the deterministic
continues briefly stall out a closed upper low over Montana on
Wednesday before pushing the whole messy trough out into the plains
Thursday night. These differences also translate into substantial
spread upstream with European model (ecmwf) bringing next Pacific system onshore on
Friday while GFS keeps the system off the coast. European model (ecmwf)
deterministic is in overall agreement with the ec EM...the GFS is on
the slow end of its ensemble members.

Given the ongoing run-to-run inconsistencies...the forecast gives
the most weight to continuity.

On Tuesday...very mild and windy conditions will develop across
central areas as Pacific cold front pushes across Idaho with
increasing chances of showers and thunderstorms across the
west...isolated storms spreading east late in the day. Snow levels
will remain quite high Tuesday ahead of the front...generally around
10kft and up. Rapid snow melt and significant rises in creeks and
streams will be a concern with the warm temperatures...especially
with the potential for rain near and below 10kft.

On Tuesday night and Wednesday...cold front will push across the
area spreading showers and isolated thunderstorms east of The Divide
with snow levels lowering to the valley floors in the west.
Precipitation looks to be mainly rain or a rain/snow mix across the
central basins through the day.

Thursday and Friday...forecast maintains chances of mountain snow
showers with slight chances of rain/snow showers in the valleys and
basins during this period with uncertainty on how fast this trough
will exit and the next one arrives.


Aviation.../12z issuance/

A storm system passing north of the cowboy state will result in
scattered to locally numerous mountain Snow/Valley rain
showers...mainly along and northwest of a kafo...kpna...kdub...kcod
line today. The best chance of convection in this area will be this
morning...with the precipitation shifting east and south with
time...but becoming less widespread during the afternoon. Kjac will
likely see MVFR conditions in rain showers this morning...possibly
lingering into the afternoon. East of The well as
Sweetwater County...the main concern will be the very windy westerly
winds in the krks-kcpr wind corridor as well as Lee side of
mountains and adjacent foothills. Some low level wind shear is
expected at kcod and klnd this morning...before stronger winds mix
to the surface by midday. Downsloping and a very dry atmosphere east
of The Divide will inhibit most if not all convection...with the
best shot at convection across northern Wyoming. Cannot rule out
some thunder...but timing of convection out west...and dry
conditions east should keep lightning isolated at best. A Pacific
cold front will push across the area later this afternoon and
evening...with a slight wind shift in most cases...with breezy winds
continuing...but more importantly improving conditions with
convection waning quickly between 00z and 03z Saturday. Also winds
will begin to weaken by late evening. Northwest/western mountains
will be obscured much of the time through early afternoon...with
improving conditions by late afternoon/early evening.


Fire weather...

Today...the basin zones will climb near or into the lower 70s...ahead of
the next Spring weather system due to push through the forecast area
through the Day. Mountain induced rain/snow chances will increase
through the morning and afternoon hours... lasting into the early
evening. snowfall accumulations look to remain on the
low side with an inch or two of wet snow at best. Winds will also
pick up to the breezy category across most of the forecast
zones...with the usual wind prone areas of the southern dispatch
zones...Mountain Ridge tops and northern Cody foothills possibly
getting some isolated high winds. In this afternoon
there will be an increasing chances for gusty light showers and
perhaps an isolated thunderstorm lasting into the evening period.
Saturday...high temperatures will be seasonal...and the winds more
tame...with isolated to widely scattered chances for showers or an
isolated thunderstorm Sunday afternoon...mainly east of The


Riw watches/warnings/advisories...


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