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Area forecast discussion 
National Weather Service Riverton Wyoming
305 am MDT Sat may 23 2015

Short term...today through Monday night

WV imagery showing large upper trough across the western Continental U.S. With closed
low over western Arizona...embedded SW within closed portion rotating
anti-clockwise through central Wyoming as we speak. Surface has general low p across
the west...high p to the east and weak trough/boundary stretching from
central NE into northestern Colorado.

The forecast for the Memorial Day weekend...upper low migrates from west
of the 4 corners region slowly to the north...centering across western Colorado
but spreading over top of Wyoming by day's end today. Middle to upper level
vorticity will increase with the approach of the upper low. The day
will be marked by sufficient buoyancy but low shear through much depth.
Disorganized showers with some embedded thunder by afternoon will be
relatively slow movers again...from S to north and 10 miles per hour +/- east of The
Divide...southeast to northwest/east to west west of The Divide. Ml cape should top out
across the County Warning Area at <=500 j/kg...but average 250 j/kg for most locations.
Precipitable waters look to be a bit less than today...but still at or above 0.60 east of The
Divide...at or below 0.5 west of The Divide...with highest values settling in
over the Bighorn Basin and the extreme eastern portions of Johnson and
Natrona counties. Expecting some middle level drying through the morning hours
with bits of sunshine possible across the southern...central and eastern forecast area.
This will increase ll lapse rates and instability over these
area...embedding a few thunderstorms amidst the amorphous mass of
showers expected by middle to late afternoon. Additionally...with a
modest surface low moving into western Colorado and southern Wyoming...Erly return flow and
small warm front sets up across eastern and central Wyoming...allowing for
terrain induced convergence to set up from the eastern Wind River
mountains...eastward along the Green Mountains and south across eastern
Sweetwater County. This area would likely see a couple of the
stronger storms of the day.

Tonight through Sunday...early morning fog will also be possible over
many areas throughout the forecast. Otherwise...surface cyclogenesis takes
place from eastern/northestern Colorado into southeastern Wyoming and western NE. Between low to middle
level moisture return around the surface low...and middle upper moisture from
accompanying the upper wave/closed low...relatively plentiful rainfall
will be had over the far eastern forecast area through the day Sunday. This system
continues to look comparatively warm...with 700 mb temperatures remaining above
-1 degree c through the entire forecast period...and really above zero for
most of that time. Generally...a trace to and inch or two will be
possible during the coldest portion of the nights above 9500
feet...with only the highest elevations receiving an isolated 3 or
so inches. Storm movement improves slightly...now reversing and
becoming north to S at 15 to 20 miles per hour. Severe weather will remain mostly east
to southeast of forecast area through the period with continued low shear
conditions only able to provide occasional isolated small
hail...especially west of The Divide.

Sunday night through Monday...system pushes eastward into the plains with
transitional ridging taking place across the western forecast area while the eastern forecast area
is forecast to be the recipient of a backside upper SW rotating around the
exiting low...with better shower/thunder activity/coverage across
the eastern County Warning Area. Otherwise...a bit warmer and more unstable with perhaps
an better sprinkling of stronger thunderstorms mixed in with
showers. Low shear conditions also persist however...keeping storm
organization at a minimum...while storm motion improves even
more...from the northwest to southeast at 20 to 25 miles per hour.

Long term...Tuesday through Saturday

GFS model continues its consistent forecast for Tuesday through
Friday. It has a weather system spinning over the Pacific northwest states
which creates SW flow aloft over Wyoming Tuesday and Wednesday. The system moves
into Idaho Thursday and into northwest Wyoming Friday...then across northern Wyoming
Friday night and into western South Dakota next Saturday afternoon.
A daily chance for showers and storms in the afternoon and evening
hours with lingering showers late night and morning hours Tuesday
through Friday. Next Saturday the showers and storms look to be
mainly over the north with lesser coverage central and south. The
European model (ecmwf) model is similar to the GFS model with the weather pattern
through Thursday night. It has a different track on Friday but the
trough is still moving through Friday into Friday night. Then the European model (ecmwf)
model is only showing isolated showers or storms. The forecast will
reflect a chance of showers and storms each afternoon and evening
with lesser coverage of showers late night and during the morning.
Next Saturday will be mostly dry in the southwest with isolated
coverage for the rest of the area. Will however keep a little chance
of precipitation in the northeast zones next Saturday. As for high temperatures...
there will be little change from day to day with 60s to lower 70s
for the lower elevations and 40s and 50s for the mountains. These
highs assume some sunshine each day and not a low overcast day(s)
like we have seen during the past week.

&&

Aviation.../12z issuance/

Confidence is low on the extent/timing of IFR/MVFR conditions from
low ceilings...fog and/or rain through 00z Sunday. A dry slot is
expected over much of southern and eastern sections this morning
limiting rain showers activity...but fog/low ceilings remain possible.
Another wave pushes over the region Saturday afternoon...with
scattered to numerous showers with isolated thunderstorms over the
entire area in the afternoon. A more widespread area of MVFR/IFR
ceilings with a more steady rain should develop east of The
Divide...and eastern Sweetwater County tonight. Mountains will be
obscured much of the time. Please see the aviation weather center
for the latest information on icing forecasts.

&&

Fire weather...

Fire danger low and will remain low through the Holiday weekend
as cool and relatively wet weather remains in place as yet another
vigorous upper level system rotates across the region. Widespread
precipitation with light mountain snow is expected each day...with
embedded thunderstorms possible each afternoon and early evening.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...Braun
long term...Arkansas
aviation...wm
fire weather...Braun

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