Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211725
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1125 AM MDT Wed Jun 21 2017

.SHORT TERM...Wednesday through Friday night

The showers and storms with the strong wind gusts have ended for the
most part and the balance of the night and morning should be rain
free. And all in all, most of the area should be dry with another
warm to hot day expected. However, with a front approaching from the
north it will tighten the pressure gradient and bring a bit more
wind than previous days. High wind is not expected though, just some
breezy conditions at times, especially in the afternoon after the
atmosphere becomes fully mixed. As for the chance of thunderstorms,
the best chance would be across the south and especially late today.
We also have some isolated activity in Johnson County with a bit of
jet energy and better moisture this evening. However, most areas
should be rain free.

The aforementioned cold front will then drop southward across the
area tonight and drop temperatures back toward seasonal levels for
Thursday. In addition, with a very dry airmass dropping into the
area, all areas should be rain free all day. There will be a gusty
breeze at times though and combined with low relative humidity
could bring elevated fire danger in some areas.

Friday at this point looks to be a mainly dry day. All of the models
have trended mainly dry for during the day on Friday. We left some
slight POPS up north late but even here things will be rain free
most of the time. Even Friday night had trended even drier. The NAM
and GFS are completely dry. Even the European, which was the wettest
model, had trended drier and keeps most of the QPF to the east of
the county warning area. For now, we kept some slight POPS in
northeastern portions of the area but all other areas dry. As for
temperatures, with more sunshine the highs have been trending warmer
so we raised high temperatures a few degrees.

.LONG TERM...Saturday through Wednesday

Overview...Cooler conditions will prevail over the weekend with some
showers and isolated thunderstorms across the north on Saturday.
Temperatures will warm up significantly Monday and Tuesday. Dry
conditions prevail on Sunday, and then slight chances of afternoon
and evening thunderstorms are forecast across the north Monday
through Wednesday.

Discussion...Synoptic pattern on Saturday features upper high
Arizona/New Mexico, with ridge extending NW along the west coast.
Northwest flow remains downstream across the northern Rockies and
northern Plains with shortwave forecast by both GFS and ECMWF to
dive southeast across northern-eastern Wyoming during the day.
Associated cold front is expected to push south along the east
slopes of the Divide, south of I-80 by 00z Sunday.  Expect mostly
cloudy conditions and some showers, possibly a couple of
thunderstorms to accompany the front; however, conditions will be
fairly stable overall with a strong frontal inversion expected
Saturday afternoon.  Highs are only expected in the 60s across most
of the central basins Saturday.  Heights gradually build on Sunday
as upper ridge shifts east into the Intermountain West with most
highs in the western valleys and central basins in the 70s.  The
ridge axis is expected to translate the area Tuesday and flatten as
Pacific shortwaves begin to to ride through the ridge.  These
shortwaves will bring chances of showers and thunderstorms mainly
across northern Wyoming.  H7 temperatures are expected to warm to 15-
18c Tuesday and Wednesday with high temperatures in the lower basin
returning to near 90F.

&&

.AVIATION...18Z Issuance

Conditions will remain VFR through 18Z/Thu. The best chance for
afternoon convection will be across the far south, possibly in the
vicinity of KRKS, with isolated convection near the higher terrain.
Main hazard again will be outflow wind gusts to 45kts. West-
northwest wind of 15-30kts will mix to the surface this afternoon,
especially at favored locations like KRKS, KBPI, KPNA, and KRIW.
Expect convection to wane with the setting sun and any lingering
convection will end by 04Z/Thu. Cold frontal boundary pushes south
across the Big Horn Basin late this evening and across central
sections east of the Continental Divide between 06Z-09Z/Thu. A north
wind around 10kts will be common behind the front with speeds
increasing to 14-24kts at KCOD between 14Z-18Z/Thu.

&&

.FIRE WEATHER...

A cold front will advance toward the area today and cross the state
tonight and tomorrow morning. As a result, a gusty breeze will
develop across many areas this afternoon and evening. Most areas
should remain rain free with any showers or thunderstorms confined
to the northern mountains and portions of Sweetwater County. Strong
and erratic winds will be the main threat from any storm.
Temperatures will remain warm to hot with relative humidity falling
into the teens in many lower elevations East of the Divide. Mixing
and smoke dispersal will range from good in the northern basins to
very good to excellent in the west and south.

&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...CNJ
FIRE WEATHER...Hattings



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