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Area forecast discussion 
National Weather Service Riverton Wyoming
337 am MDT Wednesday Apr 1 2015

Short through Friday night

A warm Dry Ridge that has been over the area for the past couple of
days is being pushed eastward by a short wave trough plowing into
western Montana and Wyoming early this morning. It is pushing along
a cold front with it...a rather dry cold front as not much moisture
or upper level support is available to work with. It will however
push colder temperatures...increased cloud cover and a chance for
precipitation in the mountains with perhaps a few showers and even a
thunderstorm or two making it into the foothills along and east of
The Divide today. More importantly for fire weather...the front will
lead to a wind shift but higher relative humidities and lower
temperatures. The front will likely make it to southern Natrona
County and into the Wind River basin before the brisk westerlies
engage across the south. This is important because the front will
provide some protection from the drier...warmer and windier
conditions to the south...preventing extremely critical fire weather
conditions from returning to the areas across central Wyoming with
more critical fuels.

There will be a bit of a break tonight as the next trough digs to
our west. This system will have another cold front coming along with
it...that will be received into a more welcoming cool environment
seeded by today's system. It will also have some better support and
stronger qg forcing along with it. Previous runs tried to take most
of the better dynamics well to the south but it looks like they are
trending more northward today. This system also has colder air aloft
and more available moisture for precipitation...along with a longer
period of upslope flow along and east of The Divide in addition to
more abundant is not much stronger but definitely
more widespread than today's system. This will no doubt enhance the
rain and snowfall in some spots on Thursday with gradual improvement
expected through Thursday night.

Some temperature rebound on Friday with flow turning more westerly
by Friday night...which could lead to some strong wind along the
Cody foothills as we get into the downward forcing behind the
exiting trough.

Long term...Saturday through Wednesday

Synopsis...southwest flow will prevail over the weekend bringing
mild and windy conditions along with elevated fire weather
conditions across the central and south. Showers and possibly a few
thunderstorms will track across the northwest and far north.

A cold front will push across the area Sunday night and Monday
bringing cooler and unsettled weather through mid-week.

Discussion...upper low moving into the western Gulf of Alaska early
this morning will dig down off the northwest coast on Saturday.
Flow across the western U.S. Backs ahead of this system with Flat
Ridge translating The Divide into the High Plains. European model (ecmwf) ejects a
strong shortwave over the ridge across Montana on Saturday that
drives a cold front south across Wyoming Saturday night. The GFS
shears this shortwave out further north and keeps Wyoming in warm
sector. Ensemble means favor the GFS forecast keeps
low probability of precipitation confined to the northwest mountains on Saturday/Saturday night while
warm and breezy/windy conditions prevail east of The Divide. Upper
trough along the northwest coast will progress into the intermountain west
on Sunday with associated cold front expected to push into western
Wyoming during the afternoon. Showers will increase ahead of the
front across the west with possibly a few thunderstorms. GFS is on
the fast side of the front across the area Sunday
night...while the European model (ecmwf) does not bring the front east of The Divide
until Monday evening. Deterministic guidance begins to stray
further away from ensemble means by forecast confidence
in timing of individual shortwaves becomes much lower at this
point. The broader picture shows slow progression of upper trough
across the intermountain west/Great Basin Monday and Tuesday...into
The Rockies Tuesday night and Wednesday. Several shortwaves
ejecting out ahead of main trough will bring the best chances of
showers to the west Monday and Tuesday with more isolated activity
east of The Divide. Upper trough passage on Wednesday appears to
be split with the main upper low heading into The Four Corners
area...which would take most precipitation south of the area.


Aviation.../12z issuance/

VFR conditions will prevail at the terminal sites through 15z ahead
of another upper level disturbance and associated surface front
that will move through the forecast area today. This system will
produce scattered rain and snow showers and isolated thunderstorms
along and north of vicinity kjac - kwrl. Over the northern and
western mountains MVFR to occasional IFR ceilings and visibility
reduced in heavier snow showers will obscure terrain...especially
after 18z. While kcod...kjac...and kwrl may all experience MVFR
conditions in low ceilings...kcod will stand the best chance of seeing
MVFR to brief IFR conditions in reduced visible and ceilings due to possible
rain and snow showers after 02z Thursday.


Fire weather...

A mainly dry cold front moves across the area this morning...
affecting areas along and east of The Divide with increased relative
humidity and cooler well as increased cloud cover
and a chance for rain and snow...especially in the mountains. The
main hazard with this front will be the wind shift and some gusty
wind along and behind the front....especially in northern Johnson

The front will also bring an isolated thunderstorm or two in the
northwest this evening. The Wind River basin and Green River basin and
mainly the northern Bighorn Basin and northern Johnson County should
see steady west to northwest winds today at 15 to 25 miles per hour with some
gusts of 40 to 50 miles per hour in more favored areas...especially early this
morning. Excellent mixing can be expected in most areas today with
the frontal inversion limiting the mixing heights in the central
basins and across the north but most areas will have fair to good
smoke dispersal today.

Chances for wetting rain or snow increase dramatically on Thursday
as a stronger system moves through with drying expected on Friday.
Another front could approach late in the weekend with potentially
widespread elevated fire weather conditions ahead of the front on


Riw watches/warnings/advisories...


Short term...Allen
long term...aem
fire weather...Allen

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