Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 281925
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
125 PM MDT Wed Jun 28 2017

.SHORT TERM...(This evening through Thursday)
Issued at 126 PM MDT Wed Jun 28 2017

First line of convection moving into the nw early this afternoon
ahead of upper circulation moving into wcntrl MT with fairly strong
thermal trough extending swd in the mid levels. Also approaching
next jet streak moving into Oregon attm. Nose of this jet may add
some upper support later this afternoon into this evening as it
starts moving ewd later. Strong to possibly severe storms are
still expected the this afternoon and early evening across the nw.
As this first line gets farther east, it will hit some better low
level moisture and lower lcls so idea of some strong/svr storms
in the ern Bighorn Basin over to Johnson is still good. Other
areas across the west and north half could see strong to
marginally severe due to both hail and wind (spreads increasing to
40-45 already at noon) and with front pushing s/se across the
area. Storms will last well into the evening and even after
midnight, especially east of the divide. On Thursday, cold trough
axis moves across the area with much cooler air and considerable
showers and thunderstorms for the west and north half. Cold enough
for snow in the nw mtns and even down to the floor in Yellowstone
National Park during convective showers (snow pellets). Quite
chilly in the park with much of the day only in the 40s above 7K
feet. Even the lower elevations east of the divide will mostly see
60s behind the next front. Even though the instability is not as
great as today, the cool mid levels and decent sfc dp will still
create a favorable environment for more tstms in this nw flow
(and next cold front). Quite cool/chilly behind this trough
Thursday night with mainly 40s east of the divide and upper 20s to
30s west. Mostly dry Friday with the potential for some building
showers/isold storms off the nrn mtns. Shortwave ridge moves ovhd
Saturday for a warmer day with an approaching shortwave late in
the day or early evening. Enough heating to fire some isold storms
across the nw and north with gusty wind and maybe some small mtn
hail. Spreads increase so strong wind gust threat will be there
with any storm, especially across the lower elevations.

.LONG TERM...(Thursday night through Wednesday)
Issued at 126 PM MDT Wed Jun 28 2017

The middle of the weekend will see a departing weak ridge to the
east, while a shortwave trough moves into western WY from the Pacific
Northwest.  The disturbance should bring scattered showers and
isolated thunderstorms to western and northern WY from Saturday
night through Sunday night, as it moves slowly eastward.
Temperatures will warm slightly on Sunday, but moreso on Monday and
Tuesday as the upper level high pressure over the southwest US
strengthens.  Still forecasting some isolated thunderstorms mainly
over the mountains Monday and Tuesday, with a better chance across
northern WY Monday evening as a weak wave moves across MT.  Tuesday
evening on the Fourth of July may see more typical diurnal
thunderstorms for natural fireworks.  Wednesday looks to be a
continuation with less convective activity, although the NE areas
may see a weak boundary move south.  Overall, the strongest winds
during the period should occur from thunderstorm outflow as the
surface pressure gradient weakens from the weekend into the early
part of the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday afternoon)
Issued at 126 PM MDT Wed Jun 28 2017

Increasing instability with advancing shortwave trough and
daytime heating will aid convection into the evening hours.
Convection over the northwest part of the area will spread to
much of western and central Wyoming through the afternoon and
evening. Likely yesterday, wind gusts over 40 kts can be expected
along with small hail and brief heavy rain. VFR conditions
generally expected, with local MVFR/IFR possible under and around
storms and showers. Most areas will see a W to NW wind spreading
SE as outflow and pressure gradient combine, making for another
windy afternoon and evening across the area. Could see a couple of
waves of thunderstorms move west to east through the area through
this evening. Lingering shower activity expected after 06Z/Thu.
Another disturbance will move south out of Canada and MT early
Thursday, brining more showers to NW WY by 15Z/Thu. A northerly
push will cool temperatures and shift winds, primarily along and
east of the divide. The showers Thursday could also have isolated
thunderstorms. This activity is expected to last through 06Z/Fri.

&&

.FIRE WEATHER...
Issued AT 126 PM MDT Wed Jun 28 2017

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of the Divide over portions of southern
and central WY. These areas with elevated fire concerns will
generally coincide with low afternoon RH values and/or gusty
winds...through Friday. Winds west to northwest 15 to 25 mph into
this evening...north to northwest 10 to 20 on Thursday...and north
to west 5 to 15 on Friday...today and Thursday with higher gusts in
the afternoons and especially near stronger showers/storms. This
afternoon and evening, we will see isolated to scattered storms
producing brief heavy rainfall, hail to 1 inch and strong gusty
outflow winds of 40 to 55 mph...especially across portions of the
northern half of Wyoming. This unstable pattern will continue
through Thursday night. Smoke dispersion will be good to excellent
each afternoon and evening.

&&

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

&&

$$

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


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