Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 161744
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1144 AM MDT Tue May 16 2017

.SHORT TERM...(Today through Thursday night) Issued at 112 AM MDT
Tue May 16 2017

Imagery shows developing complex upper level pattern with longwave
trof across the western 2/3s of the CONUS and embedded lows over the
the Desert SW and another more vigorous one just coming into the PAC
NW. Exiting tail end of jet moving out of nrn/cntrl/ern WY. SFC has
nearly stationary front draped across srn/cntrl/sern WY with
associated developing lee side low ovr ern CO. Isolated showers and
a couple of thunderstorms over the area...strongest across ern
Johnson County.

Today, Desert SW low lifts and opens, headed across the srn/cntrl
Rockies as one SFC low that is developing this morning across ern CO
moves into the cntrl Plains of NE. Morning showers a possibility
along/near the front associated with the low as it slowly pushes n
into cntrl/nern WY. Tonight, the Desert low continues into/across
CO/NM while stronger lee side cyclogenesis takes place once again
from the High Plains of ern CO into the cntrl Plains of KS/NE which
will limit moisture return into WY east of the Divide along with
precip chances...at least for a while as a "dry slot forms between
this exiting system and the next one on its way into the region from
the PAC NW. Later tonight, upper level diffluence increases west of
the Divide with the approach of said PAC NW trof/low and attending
increasing deep moisture while very strong SFC/mid lvl frontogenesis
takes place across ID/wrn MT. Lee troffing will begin just east of
the Divide...a precursor to further strong development expected
through Wednesday morning. Additionally, seasonally very cold air
along with the cold front and terrain will combine with the
increasing available moisture from the EPAC and increase precip
chances rapidly...especially after midnight tonight. Snow levels
will initially begin above 10k ft...but quickly drop to (near)
valley floor level before sun-up Wednesday morning. A quick half to
2 inches of snow may be found in portions of the Star/Jackson
valleys through Wednesday morning...with another inch or so through
the rest of the day. Western mountains should see anywhere 2 to
perhaps 8 inches by the end of day Wednesday...with heaviest amounts
likely over the Teton, and Gros Ventre mountains. The Bighorn
mountains and Absaroka mountains are within potentially the heaviest
snow target area as the moisture gets rammed into these ranges
around the developing front/cyclone. So, when potential Wednesday
night amounts are also added to this, Advisory to possible Warning
amounts will be likely for most mountain locations...especially the
Absaroka and Bighorn mountains. Have issued a Winter Storm Watch for
these two locations beginning Wednesday morning and continuing
through Thursday morning and will let the day crew will in areas
around these with appropriate highlights. Later Wednesday night
through the end of the forecast, temps across the lower elevations
east of the Divide will probably see some snow and may need
additional highlights of their own as the cold front plunges
southward across central and eastern Wyoming...with the strongest
overall forcing...with and behind the front showing up over eastern
Fremont and all of Natrona and Johnson counties...continuing through
Friday morning before finally exiting the FA. Winter just never
seems to end!

.LONG TERM...(Friday through Tuesday)
Issued at 112 AM MDT Tue May 16 2017

The major spring storm impacting the area during the short-term
will be pushing into the Plains Friday. Wrap around moisture from
this system will keep chances of precipitation across much of the
forecast area Friday with an emphasis over the far east.

In wake of this system, northwest flow will develop and remain
across the area through early next week. In this flow a few quick
moving shortwaves will impact the area. The first one looks to
impact the area Sunday, and another one Monday/Tuesday time frame
with an associated cold front. This will keep isolated to
scattered convection each day peaking each afternoon/early evening
Sunday through Tuesday.

Temperatures will be well below average Friday with warming to
seasonal to slightly below average for the weekend into early next
week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday)
Issued at 1143 AM MDT Tue May 16 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS Terminals

VFR conditions will generally prevail until 05Z Wed. Scattered
showers and isolated thunderstorms will occur through this evening.
Local MVFR conditions will occur with some storms along with wind
gusts to 30 knots and small hail. A cold front will move into the
far west around 05Z Wed and progress to the continental divide by
12Z Wed. Rain and mountain snow will occur in the west with rain
changing to snow in the valleys between 09Z Wed and 12Z Wed. MVFR to
IFR conditions will prevail in the far west after 09Z Wed through at
least Wednesday morning and possibly into Wednesday afternoon.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will prevail most terminal sites today and tonight.
Scattered showers and isolated thunderstorms will occur through this
evening. Local MVFR conditions will occur with some storms along
with small hail and wind gusts to 30 knots. Isolated rain showers
will linger after 06Z Wed through 12Z Wed. Showers with MVFR to lcl
IFR conds will be on the increase Wednesday across the far north,
especially from a KBYG to KCOD line.

&&

.FIRE WEATHER...
Issued AT 112 AM MDT Tue May 16 2017

Fire danger low across the forecast area through the week as all
fuels are currently in green-up and a late season winter storm roars
through the forecast area. Precip will be around the area east of
the Divide and across southern WY today...increasing west of the
Divide later tonight with snow levels reaching valley/basin floor
level Wednesday morning. Expect breezy west to southwest winds
across the mountains and southern WY today...becoming quite gusty on
Wednesday. Smoke dispersion good to excellent this afternoon...fair
to good afternoons through the rest of the week. Significant
precipitation expected across much of western, northern, central,
and eastern Wyoming through Friday morning.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Wednesday morning through Thursday
morning for WYZ002.

Winter Storm Watch from Wednesday morning through Thursday
afternoon for WYZ008-009.

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Ross
FIRE WEATHER...Braun



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