Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Riverton Wyoming
230 PM MDT Wednesday Apr 23 2014

Short term...tonight through Thursday

Sat imagery shows trough cutting from north to S over central Wyoming...then off
to the Continental U.S. SW. Upstream...small fast moving SW that will affect
our western mountains tomorrow...located base of broad trough west-southwest of
Oregon near 140w. Surface shows north/S oriented trough/front to our west through
the Dakotas and western NE...anchored in east central Colorado. High p located west
across the Great Basin.

Through the period...upper trough current in place overhead exits stage
east west/ ridging proceeding from west to east. Generally...high p will
build in west/ the ridging west/ clearing skies overnight most areas and
decreasing precipitation chances western mountains. Lows dropping 5 to 10 degree over
those this morning...with highs warming some 10 +/- degree Thursday as 700 mb
temperatures also rise 4 to 6 degree c. The small fast SW mentioned above will
cruise into and through the building ridge...arriving in our western zones
by early/middle afternoon along with some modest moisture from the
epac. Terrain orographics will largely drive precipitation ode early...with
some help through convective Mode in the afternoon. No highlights with
this activity excepted as mostly low end quantitative precipitation forecast should only accumulate
an inch or two at best in preferred locations at high elevation.
Otherwise...while surface and middle level T and p pattern will maintain
gusty west to SW does not look like a warning type
situation...although will certainly be Worth monitoring the Cody
foothills for isolated severe gusts...especially tonight. Current
Special Weather Statement reflects this.

Long term...Thursday night through Wednesday

Flat Ridge moves over at the start of this period before the first
disturbance moves in from the southwest in advance of our
significant weekend trough. The first weak wave and some isentropic
upglide is worthy of some lower probability of precipitation in the far west...especially in
the evening. On Friday...the flow begins to back ahead of our trough
which will be nearing the West Coast. Another potential shortwave is
expected to impact the west...especially Friday afternoon with a
high likelihood of rain showers with snow above 8500 feet or so. A
few showers may make it east of The Divide as this wave lifts NE
across the area but not counting on much...especially later Friday
evening and overnight so lowered probability of precipitation east of The Divide. Even into
Saturday morning with the system digging into the desert SW...not
too excited about probability of precipitation east of The Divide. Out west...yes. As the
day wears on...however...we could see a shortwave lift northward and
combine with afternoon instability to break out more showers across
the south for sure and probably in and near the higher terrain east
of The Divide and possibly anywhere. The west will remain wet with
snow levels of 8000 feet or so depending on exact location of the
upper low. The new 12z European model (ecmwf) is slower and further south with the
system through the day Saturday and is thus warmer with less of a
chance of anything making it in central Wyo. One caveat to that
would be potential northern surface front sagging into the far north bringing
some lower clouds...light overrunning precipitation. Most will likely remain
north in Montana but Worth monitoring. Models are diverging more today as
to where this system will set up once it moves east of The Divide
and how slow it will move east or NE. 700 mb temperatures are in the -2 to -4c
range on Sunday on the GFS and even warmer on the European model (ecmwf) and Gem.
Most of the lower elevations will likely be rain with the foothills
and far northern portions above 5k feet or so having a better shot of at
least a mixture if not a changeover Sunday morning. Model solutions
for early next week have significant differences with the European model (ecmwf)
showing more retrogression and high over low pattern leading to a
slower movement and higher chances of precipitation lingering east of The
Divide into Wednesday. The GFS has the ridge nosing in late Tuesday
into Tuesday night already. So far this winter and Spring...a faster
progressive solution has been the way to go and thus the model blend
which is faster was generally followed. Will definitely need to
watch this sytem though as it should have high impact along its path.


Aviation.../00z issuance/

West of The Divide...kjac/kpna/kbpi/krks routes

VFR conditions for most locations through the 18z Thursday. In the
meantime...isolated snow showers will be possible over the western
mountains with some MVFR/IFR conditions until 05z...then again after
15z Thursday. Otherwise...increasing and lowering cloud
cover...along with rain/snow and gusty southwest winds beginning by
18z Thursday near kjac....with IFR conditions being replaced in the
afternoon by MVFR/IFR conditions. For kbpi kpna and krks...while
could will increase by Thursday afternoon...VFR conditions should
still be the rule rather than the exception.

East of The Divide...kcpr/kcod/klnd/kriw/kwrl routes

VFR conditions will dominate through 00z Friday. However..isolated
showers will occur near and north of kbyg...with some rain and snow
showers over the Bighorn Mountains through 03z tonight with local
MVFR conditions until as a cold front and upper level disturbance
move out of the area. Gusty west to southwest winds continue for
much of through 03z in/near most terminals.


Fire weather...

Thursday will be warmer breezy...and continue mostly dry east of The
Divide with min rh's in the teens and 20s across the lower
elevations...with 40 to 60s in the mountains. However...rain showers
will return to the west by Thursday afternoon as a quick moving
upper level disturbance moves through the region. Mainly warm and
dry again on Friday east of The Divide...with a little less wind.
Rain showers will continue a possibility across the west once
again...with a slight chance at lower elevations east of The Divide
by Friday evening. This weekend will cool back off quite a bit as
yet another Spring system moves through...with everyone getting some
chance for precipitation...along with increased winds.


Riw watches/warnings/advisories...


Short term...Braun
long term...skrbac
fire weather...Braun