Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201922

Area Forecast Discussion
National Weather Service Riverton WY
122 PM MDT Fri Apr 20 2018

.SHORT TERM...This evening through Monday
Issued at 1226 PM MDT Fri Apr 20 2018

Models are having a better handle with this Four Corners system
today and verifying quite well. Showers have remained widely
scattered and very light so far today. Clouds are also beginning
to thin out over Natrona and Fremont counties, with the thickest
clouds streaming over Sweetwater County due to the proximity of
the upper low. Thus, temperatures remain well above freezing for
those areas from western Natrona to central Sweetwater counties.
The best chance for snow will now be later this evening toward
midnight, but most of the precipitation is progged to shift
further southeast out of the forecast area. Expect little to no
accumulation from this event now, with slushy roads and light
accumulation in grassy areas being the main impact. Otherwise,
windy conditions with north to east-northeast winds will continue
across much of the area east of the Divide before gradually
subsiding this evening. Gusty northeast winds will begin to
decrease after midnight over eastern portions of Sweetwater

Dry and mainly clear skies will be in place over the weekend. H7
temps will also warm during this time, ranging between +2C to +5C
on Saturday. By Sunday, these values will range between +3C to
+6C, with the cooler values over the north. This will help with
melting the currently high snow pack and could begin to cause
minor local runoff issues Sunday night into Monday morning.

A cold front will begin to impact the area Monday morning, quickly
moving through by the noon hour. Showers will develop along and
behind this front, with most of the precipitation falling as rain.
Isolated thunderstorms will also be possible across central and
southern portions. Much cold temperatures will begin to
infiltrate northern portions of the Bighorn Basin after 18Z
Monday, with rain changing to snow near Cody. This trend will
continue southward through the rest of the afternoon and into the
overnight hours. Precipitation is expected to end by 12Z Tuesday,
with snow possible Monday night for most areas east of the Divide.


.LONG TERM...Monday night through Friday night
Issued at 1226 PM MDT Fri Apr 20 2018

By Monday evening, the models all agree that the southern and
northern stream shortwaves will become congruent and track
southeast across the CWA around 00Z Tuesday. The rain and high
mountain snow showers along with isold thunder, mainly across the
northern half of Wyoming, will dissipate after 00Z Tuesday with
the troughpa. Showers will end during the course of Monday night.
Then on Tuesday, the Canadian/GFS have us under the northerly
steering flow with the Euro under a looser gradient. This brief
cool air advection from the north will knock temps down just a few
degrees for highs on Tuesday, along with dry weather. After a
ridge builds into the area Wednesday, along with high temps in the
upper 50s and 60s, the next upstream shortwave will approach from
the northwest during the middle of the week, with the
Euro/Canadian bringing it through around 00Z Thu, and the more
robust GFS around 12Z Thursday. The GFS still indicates some
shower activity with this feature, so, for now, we will keep the
idea of isold shower activity in the mtns mainly Wednesday night.
Expect slightly cooler and dry conditions in the wake of this
shortwave for Thursday, but with temps still rather seasonal.
Then, another amplified ridge builds in from the west resulting
in nicely warming temps for Friday with highs in the 70s at the
lower elevations east of the divide and 60s west.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1226 PM MDT Fri Apr 20 2018

An upper level low pressure system will move across southern
Colorado through this evening. Widespread MVFR conditions, local
IFR conditions in rain and snow will develop vicinity and south
and east of a KRKS-KLND-KRIW-KBYG line by 20z this afternoon with
snow levels around 6000 feet. Higher terrain above 8000 feet will
also become mostly obscured in this area; ceilings will likely
lower mostly into category after 00z Saturday. Precipitation will
decrease NW- SE 03z- 09z Saturday but areas of MVFR-IFR ceilings
and patchy fog are likely to linger through 12z.

Further north and west, VFR conditions will prevail through the
period with scattered MVFR shrasn pushing north to vicinity KBPI-
KWRL line by 20z this afternoon, followed by clearing 00z- 06z

Expect mainly VFR conditions after 15Z Saturday everywhere.


Issued AT 1226 PM MDT Fri Apr 20 2018

Widely scattered showers will continue over southeast portions of
the forecast area this afternoon. Expect some of this activity to
fill in this evening to include eastern portions of Fremont
County. Some rain or rain and snow mixture and gusty northeast to
east wind will extend as far north as Fremont and Natrona
Counties today into tonight with a brief period up into Johnson
County. The northern zones will be dry with a period of gusty
north wind as the storm moves by. Dry and warmer conditions over
the weekend, with the next chance for precipitation arriving
Monday and Monday night.




LONG TERM...Lipson
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