Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230550
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1150 PM MDT Sun Apr 22 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 128 PM MDT Sun Apr 22 2018

High pressure will be replaced by lower pressure as a Pacific
low pressure system over the Pacific Northwest moves east
tonight. This will bring increased clouds with increasing
snow showers by midnight. Snow levels around 8800 feet will
lower to 8500 feet Monday morning. Snow showers will be rather
widespread through Monday into Monday evening spreading east of
the Continental divide as a Montana cold front dives south.
most areas will see rain in the lower elevations but may change
to snow late monday night and Tuesday morning as temperatures
fall across the area.  Showers will be decreasing Tuesday
as the low pressure trough shifts east of the area.  Tuesday
night will be cold with a few lingering mountain showers
with clearing on Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 128 PM MDT Sun Apr 22 2018

Beginning of forecast: Flattening ridge, due to the approach of a
rather large embedded shortwave trough, will be moving out of Canada
and into/through the Northern/High Plains. A large closed/cut-off
low (COL) continues to spin off the west coast over the EPAC.
Falling pressure across the northern portion of Wyoming will soon
give way to the passage of a cold front.

Overall, the MR models diverge from the beginning of the extended
forecast all because of the COL sitting over the EPAC. The EC is
still somewhat more robust with regard to the actions of the
embedded shortwave trough over WY moving into and through the ridge
at the beginning of the forecast. The EC displaces the shortwave
further southwest and brings light precip into/through WY late
Wednesday night and Thursday while the GFS keeps precip over the
Northern Plains. Not too significant either way. Ridging then
returns to the forecast area by Thursday afternoon with the best of
the models` similarities throughout this forecast period showing
over WY under the warm/dry skies through the day Friday. But that`s
over WY. To the west, problems/differences with the evolution of the
COL system continue over the same period. By the time the forecast
gets to Friday night and on through the weekend, the model
divergence really begins to show. All MR models are having a great
deal of trouble solving the cut-off low riddle over the EPAC and the
results of eventually being kicked inland. The two most widely
divergent solutions have: 1. A kicker shortwave arriving upstream of
the COL on Friday, reconnects the COL with the westerlies, opening
and moving it across the Great Basin/Intermountain west then finally
WY by Sunday (GFS). 2. The EC grows a blocking ridge to the west of
the COL over the same period of time and keeps the low cut-off,
holding it back over the NW CONUS. Of these tow scenarios, the GFS
would have a greater impact to the forecast in this forecast period,
particularly on Sunday. Some weight has been given to all solutions
at this point with a slight advantage being given to the more
progressive GFS.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1150 PM MDT Sun Apr 22 2018

A cold front with an accompanying band of precipitation will move
across areas west of the Divide from 09z-14z, and across north
central Wyoming from 14z-18z.  Snow levels will be between 7-8kft as
the band of precipitation moves across the west with prevailing MVFR
conditions at KJAC, brief MVFR conditions possible further south at
KBPI-KPNA.  More widespread IFR conditions are expected to develop
vicinity and north of a KCOD-KPOY line 14-18z with snow levels
lowering to near 5kft.  MVFR ceilings will spread further south to
KWRL with lower ceilings eventually spreading south into KCPR and
KLND Monday evening with areas of -SN.  Ceilings will gradually
improve into VFR category across NW Wyoming Monday
afternoon/evening.

&&

.FIRE WEATHER...
Issued AT 128 PM MDT Sun Apr 22 2018

High pressure will be replaced by lower pressure as a Pacific
low pressure system over the Pacific Northwest moves east
tonight. This will bring increased clouds with increasing
snow showers by midnight. Snow levels around 8800 feet will
lower to 8500 feet Monday morning. Snow showers will be rather
widespread through Monday into Monday evening spreading east of
the Continental divide as a Montana cold front dives south.
most areas will see rain in the lower elevations but may change
to snow late monday night and Tuesday morning as temperatures
fall across the area.  Showers will be decreasing Tuesday
as the low pressure trough shifts east of the area.  Tuesday
night will be cold with a few lingering mountain showers
with clearing on Wednesday.  Smoke dispersal will be good
or better in central and southern areas Monday before
becoming poor as cold air and low clouds move in behind the
cold front Monday afternoon and evening.

&&

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

$$

SHORT TERM...Baker
LONG TERM...Braun
AVIATION...Meunier
FIRE WEATHER...Baker



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