Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 150932
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
232 AM MST Wed Feb 15 2017

.SHORT TERM...Today through Friday night

Imagery shows breaking down as main trof and opening COL heads east
if the MI and amplified ridging takes over the wrn CONUS with a very
large trof to the w of the ridge out over the EPAC. Embedded SW/jet
just now ejecting/moving into the w side of the ridge and into the
coastal states. SFC has deep high pressure across the wrn CONUS. No
precip is currently falling anywhere in WY.

Today through Thursday morning, upper level ridging and deep high
pressure at the surface will be in place across the Great Basin and
Intermountain West with lower pressure off to the north and east.
These conditions will slowly increase the P GRAD at the SFC between
the two pressure zones along the Continental Divide...inducing gusty
southwest winds across the usual areas (Cody Foothills and Wind
Corridor) beginning this morning and lasting through Thursday
morning up north in the Cody Foothills...but extended through the
end of the forecast period across the Wind Corridor. ATTM, it
appears that the wind will mostly stay below warning
criteria...except for spurious high gusts near Clark up north or
Outer Drive near Casper.

Thursday night, the upper ridge axis will have passed to the east
over the Plains States with the first hints of the oncoming upper
trof/SW/jet moving into western WY. Moisture will be somewhat
limited with this first system, however the western mountains will
be close to an entrance region of a jet. When all this is combined
with terrain lift, precipitation will result. Currently H7 temps
associated with this lead SW trof should represent snow to the
valley bottoms by the time precip starts...except for perhaps a few
minutes at very beginning of the event. Overall, 24 hr snowfall
accumulation west of the Divide, through the day Friday, should
range up to 4 inches at best in the mountains...an inch or two in
the valleys... an inch or less over the southern basins...with
perhaps a trace elsewhere. No official highlights expected at this
time.

Friday night, upper lvl SW ridging quickly follows the SW trof with
a lull in the action early. The next embedded SW will be racing
toward swrn WY out of the desert SW, arriving near midnight with
some renewed, but also modest, moisture. Precip in this case could
be all snow or a mix of rain and snow. In either case, snowfall
accumulation overnight will generally only amount to an inch or
less, west of the Divide, by sunrise Saturday.

.LONG TERM...Saturday Through Wednesday

Models in reasonable agreement until the end of the period when the
big picture diverges significantly between the GFS and Euro. The GFS
has backed away from a significant mean trough over the west the
middle of next week while the Euro is still holding onto this idea
(albeit not quite as strong as last night). Models are in pretty
good agreement up until then with splitting trough still passing
through over the weekend (this disturbed area is out near 160W
attm) with periods of snow mainly out west and in the sw mainly.
More active sw flow develops early next week with mean trough to
the west spitting out several shortwaves with a good chance of
snow at times out west, possible mixed with rain in the lowest
elevations at times. Decent wind event possible for the wind
corridor early next week (Mon/Tue) and in the Cody Foothills
around Monday night with lee trough and jet passing overhead. Main
challenge is what happens at the end of the period. The GFS with
its weaker mean trough (almost zonal) has just open wave passing
through Wednesday with a decent frontal passage but no significant
snow potential east of the divide. The Euro and GEM have a much
more significant closed low developing from the inter-mountain
west to near the nrn CA coast while a surface high over srn Canada
provides upslope east of the divide (especially in the Euro
solution). Both the GFS and Euro showing strong upper high/ridge
pushing nwd through the Gulf of Alaska which would favor troughing
over the western U.S. but need a little more consistency before
hitting any storm potential harder. Pattern would seem to favor
significant troughiness near or just inland of the west coast with
a nrn stream influence providing surface high (upslope
potential). This pattern can produce a longer duration storm
potential east of the divide as upslope holds east of the divide
and slowly developing upper low/trough holds to the west. Its just
too early to see if this will be the final answer yet. Will watch
it closely the next couple nights. Either way, they are both
showing some snow potential east of the divide next Wednesday and
we`ll just go with that for now. Overall period though will
be somewhat mild for February with 40s and some 50s east of the
divide until cooling begins Wednesday. The west will be mainly 30s
with some 20s in the higher mountains.

&&

.AVIATION.../12Z ISSUANCE/

Patchy morning fog in the valleys and basins. Boundary layer
moisture is on the dry side, so not expecting widespread fog. The
best chance for fog Wednesday morning will be across the Big Horn
Basin along the Big Horn River. Otherwise, VFR conditions with
mainly clear skies are expected today. Increasing mid to high level
cloudiness is expected late Wednesday night across the west. Winds
will be light in most areas, but breezy to windy west to southwest
wind is expected from the Red Desert/South Pass area to Casper,
Dubois area, and on lee side of mountain ranges. Low-level wind
shear will be an issue at KCOD through the period, and could become
an issue at KJAC Wednesday night.

&&

.FIRE WEATHER...

Fire Danger low for all of WY through the weekend as a good existing
snow pack will combine with seasonal to below seasonal temperatures.
Winds begin to pick-up east of the Divide out of the southwest while
remaining relatively light out west. Smoke dispersion poor to
briefly fair afternoons through Thursday...fair to good east of the
Divide Friday. Areas of light snow to return to the region Thursday
night....continuing periodically west of the Divide through the
weekend.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...WM
FIRE WEATHER...Braun



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