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Area forecast discussion 
National Weather Service Riverton Wyoming
1131 am MST sun Jan 25 2015

Short through Tuesday night

Northwest flow continues over Wyoming with ridge west and trough east. Small SW over
western Montana and another further upstream across southern British Columbia...both headed this
way. North/S jet across eastern Wyoming...nosing into the Front Range of Colorado. advertised for the last several days...a nearly col over the
epac west of Baja California...which still looks to have consequences across
portions of Wyoming by middle week. Surface shows hi p west with a decent
lowering p gradient to the east...providing some mixed areas with gusty
winds...especially across the southern forecast zones.

Beginning of forecast has some areas of patchy fog in the usual spots.
Some modest moisture still hanging around the northern/northwestern County Warning Area will
translate S and east today. This together with the disturbances
mentioned above will keep some low end probability of precipitation around the western and northern
zones through at least mid-day...without much in the way of substance.

Monday...skies clear...inversions set back in for another morning
starting with patchy fog...but clearing/sunny skies otherwise...
under an increasing influence of the western ridge of hi p. This will
virtually eliminate northwest flow by Monday night. Tuesday will be another sunny
day across most of the forecast area except for the southwestern zones which will have
increasing clouds through the day as the Baja California col is kicked this
direction. Additionally...through the day on Tuesday...surface p field will
become better suited for producing gusty SW wind...with additional
help in this case from aloft as upper ridge orients itself far enough
east to to allow nearly unidirectional SW flow through middle levels. This
together with warming ll temperatures...will allow moderately stronger
winds aloft to increase the surface winds adiabatically.

Tuesday night into Wednesday the col from Baja California heads this will quickly become an open wave but still be able to
bring some moisture with it...brushing the western to northwestern forecast area
through the overnight period. As this has been forecast for some time
now...and assuming the upstream kicker arrives within the critical
distance by Monday afternoon...pop chances will rise accordingly
through the overnight period Tuesday. However...the fleeting nature of this
disturbance currently looks to preclude too much quantitative precipitation forecast or snowfall
through the end of this forecast period...remaining below advisory criteria
for the most part.

Long term...Wednesday through Sunday

Synopsis...cooler conditions with some rain and snow showers will
spread east of The Divide Wednesday. Generally dry and seasonably
mild conditions will prevail across most areas Thursday through
Sunday. The one exception will be chances of rain and snow showers
pushing north into southern Wyoming on Friday before being
suppressed further south over the weekend.

Discussion...both GFS and European model (ecmwf) have shortwave trough...remnants of
Baja California upper low...pushing east out of southeast Wyoming into the
Central High plains 12z-18z Wednesday...with clipper in northern stream
diving southeast into the northern High Plains. The northern stream
clipper will drive a Canadian cold front south across the area
during the day Wednesday. GFS and European model (ecmwf) both show fetch of
sub-tropical moisture lingering over area behind southern stream
system to interact with Canadian cold front through the day.
Overall....rain and snow shower coverage should be spotty over the
lower elevations through the day but we could see more organized
banding across central Wyoming in unstable northwest flow...especially
downwind of The Owl creek mountains and southern Bighorn range.

GFS keeps broad trough from Baja California region into the eastern Great Basin
on Wednesday night and Thursday...continuing to feed sub-tropical
moisture into southern and central Wyoming. European model (ecmwf) shows sharper
deformation area keeping this moisture to our south as broad ridge
builds across the northern rockies with warm air advection across Wyoming.
Forecast is weighted toward ecmwf's drier scenario.

Both GFS and European model (ecmwf) show overall agreement bringing sub-tropical
moisture plume into southern Wyoming on Friday with broad trough
over the SW U.S. This may bring some rain and snow showers up to
around Interstate 80 on Friday before upper low closes off over Baja California
California on Saturday with moisture plume getting suppressed
further south of Wyoming over the weekend. Overall model trend
with northern stream Friday through the weekend has been toward
broader ridge over the northern rockies...thus keeping Arctic
intrusions further east across the northern plains and Great Lakes
region. Seasonably mild and dry conditions expected for most of the
area over superbowl weekend.


Aviation.../18z issuance/

VFR conditions will prevail over most of the today. In the far
west from Yellowstone Park to Kemmerer areas of MVFR and IFR
conditions will occur until 21z due to low clouds and patchy fog
on the slopes. Then VFR conditions will prevail over the far west
after 21z. Gusty wind will occur at krks...kcod and kcpr airports
through most of the day with the wind decreasing at krks by 02z
while continuing to be windy at kcpr.


Fire weather...

Fire danger remains low for most areas...with relative humidity values remaining
above 50 percent for locations below 7000 25 percent or
lower above timberline where rocks and snow abound. Mixing/inversion
heights will remain on the lower side...1000 to 1500 feet at most...
through at least Tuesday for most locations south of a Rock Springs
to Casper line. Poor smoke dispersion is also forecast through the same
period. Significant precipitation is not expected anywhere over the
forecast area until Tuesday night when increased moisture following
an upper level disturbance heads into the west and northwest.


Riw watches/warnings/advisories...


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

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