Area Forecast Discussion
Issued by NWS Riverton, WY

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535
FXUS65 KRIW 201730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT Tue Jun 20 2017

.SHORT TERM...Tuesday through Thursday night

All is quiet right now across the area after a very nice late spring
day. And spring will come to an end today, with the summer
solstice at 10:24 pm this evening marking the beginning of summer.
And for the last day of Spring, it will certainly feel like
summer. With 700 millibar temperatures rising to over 15 degrees,
most of the lower elevations East of the Divide will see high
temperatures into the 90s. And in addition, we will have to deal
with some thunderstorms as a shortwave passes to the north of the
area. At this point, the best chance for any stronger storms will
be across northern and eastern areas where there will be better
moisture. In addition, with there is some directional shear so a
few storms could get stronger. The Storm Prediction Center also
has a marginal risk area across the northern border. The CAPE
isn`t the highest in the world though. For now, we will go with
the gusty winds and small hail wording but a storm or two could
reach severe limits. Further south, especially south of the Owl
Creek Range, this remains me of a B movie from long ago. But
instead of "Attack of the Killer Tomatoes", it is "Attack of the
Little Green Blobs." With less moisture in these areas, we could
have some 50 degree plus temperatures / dew point spreads. With
some inverted V soundings, this means the potential for some
strong wind gusts. A lot of times, these come out of innocent
looking showers. As a result, we kept the gusty wind wording
across the remainder of the area. Most of these thunderstorms
should end by midnight.

Largely zonal flow should take over for Wednesday. Temperatures
may be a couple of degrees cooler, but it will still be quite hot.
The main differences are two. One, with drier air over the area,
the coverage of thunderstorms will be much less. If any storm, it
would be in the Bighorn Range and eastward into Johnson County
where there will be somewhat better moisture. Also, with a tighter
pressure gradient and stronger 700 millibar winds, winds will be a
bit stronger. With the northwest flow, is will be especially so
across areas West of the Divide, portions of Sweetwater County as
well as the Wind River Basin. At this point, Thursday looks cooler
with temperatures close to seasonal averages. With even less
moisture around, we kept the forecast dry. And again, it will
become breezy to even windy in many areas, especially in the
afternoon as the atmosphere becomes fully mixed.

.LONG TERM...Friday Through Tuesday

Some significant model differences continue to start this period in
regards to how the energy with our upper low in the Gulf of Alaska
affects us Friday through Saturday. The problem is with the handling
of the low as it opens up and moves east towards us while at the
same time, we start to see rapidly building heights originating from
a upper high out near 40N/135W. The Euro keeps this energy/wave
across MT/Canadian border area while the GFS shows it dropping
more se into Montana and nrn Wyoming by Friday morning as it
responds to building upstream heights. The second piece of the
upper low are both shown coming through Friday night and Saturday
(Euro is stronger with the center). The challenge is which one
will be right with the first piece of the upper low Thursday night
and Friday. The GFS 5-wave chart (which has a mean ridge extending
from the Desert SW nw to off the PacNW coast) would at least
support a compromise with a chance of a stronger/further west wave
like the OP GFS. Will have to lean towards a middle of the road
for now but we need to watch it closely as it would be a lot
cooler behind it Friday with a better chance of showers/storms if
the OP GFS is more right. They are both quite cool Saturday with
H7 temps dropping to below 0C even into srn Fremont County. Either
way, quite cool for late June. Will also gradually play some
showers/slgt chance tstm spreading from the mainly the nrn
mtns/far north during the day Friday into most of the north half
Friday night. The 00z Euro is much more bullish with the precip
spreading south Friday night into Saturday with some right
entrance jet support and qg forcing as it`s strong vort moves se
across the area. Will need to watch this as we get closer but the
superblend was more like the OP GFS with low pops in the north and
east vs the quarter to half inch of pcpn the Euro shows in the
north and central zones between Friday night and Saturday. Since
the Euro has more with the second piece of the upper low, it`s
cooler weather lingers into Sunday with H7 temps still at or below
0C at noon on Sunday down to Fremont County. GFS has the ridging
building in more quickly behind it`s weaker second system.
Confidence low towards a rapid warmup with the strengthening ridge
near or off the coast so model blend (which is about half way in
between) is ok for now. Dry and warmer Monday. Euro continues that
warm-up into Tuesday with less influence from a Canadian
shortwave and associated cold front that the GFS shows. With the
mean ridge holding over the far ern Pacific up into wrn Canada,
will not warm temps up Tuesday. Both models are also showing a new
system approaching the west later Tuesday night or early
Wednesday with some weak ripples around the ridging to our sw
similar to today`s pattern. Will have some 20-30 pops in the
mountains spreading across the nearby lower elevations by next
Tuesday evening.

&&

.AVIATION...18Z Issuance

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals

Conditions will be VFR through the forecast period. Showers and
thunderstorms will develop over the higher terrain early Tuesday
afternoon and move east-southeast over the basins and valleys.
Gusty outflow wind of 30-40kts will be the main hazard. Best chance
for rain and potentially very small hail would be at KJAC.
Convection will push east and wane between 02Z-03Z/Wed. Surface
winds will increase at all terminals between 19Z-21Z/Tue with speeds
generally 15-25kts. These surface winds will decrease by 02Z/Wed.
Clouds will gradually dissipate during the overnight hours.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will prevail through the forecast period. Showers and
thunderstorms will develop over the higher terrain of the northwest
mountains by mid-afternoon Tuesday. This convection will move over
the basins and valleys in west-northwest flow aloft. Temperature-dew
point spreads will be sufficient to support some 45kt outflow wind
gusts with the convection. Best chance for rain or small hail will
be at KCOD through early evening. Surface winds will increase from
the west to northwest as well Tuesday afternoon with speeds commonly
14-24kts. onvection may eventually become more organized and make
its way across central Wyoming between 01Z-05Z/Wed. KCPR could see a
period of -TSRA during that time. After about 05Z/Wed, cloud cover
will dissipate and surface winds should be less than 10kts. Likely
to see a boundary push south across KCOD late in the forecast period
with north wind 10-15kts just prior to 18Z/Wed.

&&

.FIRE WEATHER...

An upper level disturbance will move across the area today. After
a largely dry morning, scattered showers and isolated
thunderstorms will develop this afternoon and evening. Some could
be strong across northern Wyoming with hail along with strong and
erratic winds. Although coverage of storms will be less further
south, any shower or thunderstorm will likely have erratic winds.
Relative humidity will fall into the teens across many of the
lower elevations East of the Divide. Some areas will see an
afternoon breeze develop. Mixing and smoke dispersal will range
from good to excellent for the most part.

&&

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

$$

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



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