Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 152250
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
450 PM MDT Mon May 15 2017

.SHORT TERM...(This evening through Thursday) Issued at 354 PM
MDT Mon May 15 2017

Scattered thunderstorms have developed across the northern and
eastern areas of the CWA in response to a northeast bound vort
lobe along with the right rear quad of the jet ovhd with some of
the stronger storms expected to across Johnson County closer to
the tight cape gradient. In addition, that is where the best shear
and helicity will be for the rest of this afternoon and early
evening.

Then the long advertised well developed symmetrical low pressure
center over the Pacific Northwest will track toward Boise through
Tuesday night resulting in a west southwest flow Tuesday backing
to a more southerly increasingly difluent flow by Tuesday night.
Increasingly unstable conditions will occur out ahead of this
approaching system with yet another round of afternoon and evening
thunderstorms. A strong asct cold front will plow through the far
west Tuesday night ahead of the low which should be in the vcnty
of SLC Wednesday morning. H7 temps will drop 10c in just a few
hours with the fropa in the west, especially across the southwest
where the front will likely have the best catafront characteristics.
A quick shot of snow should occur at all elevations in the far
southwest, and eventually all of western Wyoming for that matter,
with good isentropic lift over the NW. Then on Wednesday the
snowfall will spread to the Northern Mountains east of the divide.
Like so many other potent weather systems this winter/spring
system, a weak mid level low will wobble around the Owl Creek
Mountains which means atleast some areas north of and in the vcnty
of the circulation will see alot of precip. Significant QG
forcing be found across the northern third of Wyoming Wednesday.
The left front quad of the jet will be ovhd Wednesday. In the
meantime, a dry slot is being advertised by the GFS/Canadian
models for Wednesday and Wednesday night over the central and
south CWA with lighter precip there. Then by Thursday, enough
cold air will filter down close to the basin floors east of the
divide along with deepening upslope around the backside of the
main low which should be over Vernal Utah by then. Then isentropic
lift will add to the deepening upslope Thursday. Models suggest a
broad area of one half to 1.5 inches of precip over many areas
with 1 to 2 inches over the Big Horn Mountains where there will be
a longer residence time with the precip. Have indicated up to 18
inches of snow in the grids over some of the northern mtns. Issued
an SPS addressing this precip event.

.LONG TERM...(Thursday night through Monday) Issued at 354 PM MDT
Mon May 15 2017

Large upper low is expected to be over northern CO to start this
period before beginning to spin down and lift/shear off to the ne
on Friday. Potential for significant pcpn will continue Thursday
night in much of the central zones before winding down by Friday
morning. GFS has trended towards the Euro idea from yesterday with
a slower evolution for Fremont and Natrona Counties and that is
what we leaned towards yesterday. Bigger question will be how
significant the band of cold rain and foothill/mountain snow will
be. Still looks like one good shot of heavier pcpn Thursday
afternoon and Thursday night where at least one half to one inch
could accumulate across the lower elevations with 1 to 1.5 in the
mountains with locally higher amounts. Euro may be too slow in
lifting this system out Friday night so leaning towards a GFS/GEM
type solution with improving conds by Friday morning. Still
unsettled but a less chance of significant pcpn continuing. Not
much break as several shortwaves start dropping down in the cool
nw flow Saturday night/Sunday and another one on Monday to produce
another good chance of showers across the lower elevations with
higher elevation snow. Much warmer than the upcoming system but
still cool enough for higher elevations snow for the nrn mtns
especially. Some significant foothill rain and higher mountain
snow will likely fall in the nrn mtns at times later Saturday
afternoon through at least Monday and probably longer in this very
unsettled nw flow.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 354 PM MDT Mon May 15 2017

VFR conditions will prevail for most areas through Tuesday. Areas
of low clouds with MVFR to LCL IFR ceilings may occur in Johnson
and Natrona counties again later tonight through 16z Tuesday,
possibly impacting KBYG and KCPR. Scattered showers and isolated
thunderstorms will move across the area at times through Tuesday.
Local MVFR conditions can be expected in the vicinity of the
storms. Outflow wind gusts to 35 knots and small hail will be
possible near storms. Please see terminal forecasts for more
details.|

&&

.FIRE WEATHER...
Issued AT 354 PM MDT Mon May 15 2017

Much of the week will be rather wet with chances for
precipitation through much of the week. Isolated to scattered
afternoon thunderstorms will continue across northern and eastern
portions of the area. Temperatures will be near to slightly above
normal. A gusty southwesterly breeze will develop at times,
especially from Rock Springs through Casper. Mixing and smoke
dispersal will range from fair in the far north to excellent in
the south. Temperatures will turn cooler through the week with a
good chance for a wetting rain Wednesday into Thursday with snow
possible in the mountains.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Lipson



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