Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200826
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
226 AM MDT Mon Mar 20 2017

.SHORT TERM...Monday through Wednesday night

We currently have another fast moving shortwave moving across the
area and bring some valley rain and higher elevations snow to the
west as well as some precipitation across the Big Horns. Everywhere
else still looks largely dry. This wave should move away by sunrise.
However, another wave will move into the west around noon and bring
another shot of valley rain and mountains snow across the north.
Again, the wave is moving fast enough so areas should not need any
highlights. However, with mild temperatures continuing and some
reports of minor flooding, we will continue the flood watch
through today and add Sublette County.

In the meantime, a cold front will sag southward across
the areas East of the Divide. The air behind it is not that cold,
and temperatures will again average above normal. Most areas will
start out dry, but coverage of showers will increase through the
afternoon with the presence of the front, although it will wash out
through the day. In addition, the right rear quadrant of a jet
streak will move over the area this afternoon and evening. This
should enhance upper level divergence and increase coverage of
showers and even a few thunderstorms. The models are keying in on an
area of heavier precipitation over Casper late today and this
evening so we bumped up POPS and QPF in this area. The area across
the Big Horns and Johnson County could also see a decent amount of
precipitation, likely snow in the mountains and possibly down to
Buffalo although again, with the quick movement of the system
amounts should fall short of advisory levels. This system should
move away by Tuesday morning.

Quieter weather will move in for Tuesday as some flat ridging moves
over the area. The ridging is rather weak though so there could be a
few showers around, especially in Natrona County and in the west.
However, most areas should be dry most of the time. And again, most
areas should see another day of above normal temperatures. Another
shortwave then moves into the west for Tuesday night but does not
have a lot of punch with it so any precipitation amounts will remain
on the light side.

Most of Wednesday should start of mainly dry as well. The next in
the parade of troughs will then move into the west later in the day
at night. The GFS looks interesting in developing a mid level
circulation that brings a decent amount of QPF to portions of the
Big Horn Basin as well around Buffalo. The NAM is flatter though
with an area of showers mainly south of a Buffalo to Riverton and
Evanston line. For now, we half stepped and increased POPS but not
as far as the GFS. This will have to be watched though.

.LONG TERM...(Thursday through Monday)

A spitting trough will push across the Rockies Thursday/Thursday
evening with the main 500mb low pushing across the Four Corners/Grand
Junctions area. The associated cold front is expected to push
across the forecast area Thursday with some model solutions
continue to show 700mb mid-level circulation/wave pushing
east/northeast along the baroclinic zone Thursday which will
enhance the precipitation especially along and north of the
frontal boundary. As the main 500mb low approaches and front
continues to shift south, this should shift the bulk of the
precipitation across central/southern parts of the area.
Precipitation looks to be mountain snow/valley rain. The rain in
the lower elevations west of the Divide will change to snow with
frontal passage, and even some low-elevations east of the Divide
could as well, as 700mb temps dip to around -5C. Although there
could be some lightning will keep thunder out of the forecast for
now as models show little or no surface instability.

Shortwave ridging with mainly dry conditions are expected late
Thursday night and Friday.

This again will be short-lived with another trough takes aim at
the Cowboy State Saturday and Sunday. Based on current timing, the
west would see increasing precipitation Friday night into
Saturday with some precip spilling east of the Divide Saturday
night and Sunday with the cold front.

Elevated fire weather behavior possible east of the Divide on
Saturday.

Low-elevations snowmelt over western valleys will continue localized
flooding through the extended.

.AVIATION...12Z Issuance

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

Rain and high mountain snow showers will be isolated this morning
in NW WY. Then rain and high mountain snow showers will
redevelop around 18Z in northwest WY and continue through 06Z Tue.
Local MVFR conditions will occur in NW WY along with mountain
obscuration at times. Scattered showers and isolated thunderstorms
will develop in southern WY after 20Z and continue into this
evening. Then showers will become isolated after 06Z Tue.

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

A cold front will become nearly stationary in central WY today.
North of the front areas of low clouds will occur producing some
MVFR conditions. In central WY, VFR conditions will today. This
afternoon scattered showers will develop east of the divide along
with isolated thunderstorms from near KRKS to KCPR. This activity
will continue into this evening. Areas of MVFR ceilings will occur
east of the divide after 01Z along with some mountain obscuration.
Showers will become isolated after 06Z Tue.

.FIRE WEATHER...

A couple of disturbances will move through the area and bring some
rain and higher elevation today. The areas with the best chance of
precipitation will be across the northern and western mountains as
well as Natrona County. Relative humidity and wind should remain
below critical levels. A cold will settle near the continental
divide before washing out. Mixing and smoke dispersal will range
from fair to poor in the north to good to excellent in the south.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for WYZ013-023-025>027.

&&

$$

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


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