Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210852

Area Forecast Discussion
National Weather Service Riverton WY
252 AM MDT Sun May 21 2017

.SHORT TERM...Sunday through Tuesday night
Issued at 240 AM MDT Sun May 21 2017

We still have a few showers out there this morning and these will
likely continue into the morning as well. As for today, another
shortwave will drop southward across the area and bring another
chance of showers and thunderstorms. Like yesterday, activity is not
expected to be widespread and most areas will be dry most of the
time. The difference today is the best chance will shift further
south and east. There could also be some snow above around 8000 feet
in the Bighorn range. The models are overdoing the QPF given the
origin of the shortwave however. And, like yesterday, with low wet
bulb zero levels, the main threat from any thunderstorm would be
small hail. Temperatures should be fairly similar to yesterday. The
difference today will be a bit more wind across Johnson County, but
nothing extreme. There should be a bit of a break this evening.
However, the break will likely not last very long.

After midnight Sunday night, a cold front will drop southward out of
Montana and the Dakotas and into northern Wyoming and the drop
southward through the day. Like previous days, the models show the
bulk of the forcing missing the area to the east. However, it will
be enough to bring a chance of showers and thunderstorms to most
areas East of the Divide sometime on Monday, with later times the
further south you go. Again, with the origin of the system, the
models are likely overdoing the QPF, especially in the mountains.
But again, there could be some snow in the Bighorn range. The chance
of showers will be highest in Johnson and Natrona Counties with
gradually decreasing chances further west. In addition, places that
are favored for windy conditions with north to northwest cold
advection will see possibly strong winds, especially the northern
Bighorn Basin and Johnson County. At this time, 700 millibar winds
do not look strong enough for high winds, but it will become
blustery behind the frontal passage. Showers should end fairly
quickly Monday evening as the front moves away to the south and east.

Dry weather should return Tuesday as ridging builds across the area.
Temperatures should also be quite pleasant. We did raise high
temperatures given that most areas should see plenty of sunshine and
guidance usually under does high temperatures. Dry weather will last
into Tuesday night as well.

.LONG TERM...Wednesday through Sunday
Issued at 240 AM MDT Sun May 21 2017

Wednesday still looks like the warmest day of the period along
with breezy to windy conditions over most of the region. A cold
front will be nearing northwest WY around 00Z Thursday which may
induce a few showers or storms in NW WY toward Wed evening. As the
cold front moves south southeast Wed night shower and isolated
thunderstorm activity will occur in the west and around the
Bighorn mountains. The models are also hinting at some showers and
isolated storms in southern WY Wed night, so will include that
for now. The weather pattern Thursday through Sunday will feature
a trough over the Pacific NW eastward through the Northern Rockies
into the northern plains states. Numerous disturbances will be
moving through this trough impacting WY from time to time through
the holiday weekend. Scattered showers and some thunderstorms
will be in the forecast each day and night. Timing of the most
intense activity is difficult this far out and will be refined as
the week progresses. Temperatures will be mild to cool as each
disturbance will have some cooler air behind it. This will keep
the snowmelt in the higher elevations in check for the most part.
Plus with snow falling above 9500 feet at times, some mountain
passes will be impacted over the weekend. Overall, be prepared for
unsettled weather at times Memorial Day weekend and cool to mild


.AVIATION...(For the 12Z TAFS through 12Z Monday)
Issued at 240 AM MDT Sun May 21 2017


Isolated to widely scattered showers will linger mainly along and
south of a KBPI-KPNA line through Sunday afternoon as an upper level
low pressure system continues to dive south across the northern
Great Basin. Drier and more stable air will prevail across northwest
Wyoming, vicinity KJAC.  Showers across southwest Wyoming will
dissipate early Monday evening, leaving Few-sct FL110-130 across the
area overnight.


Low level upslope northeast flow may result in the development of
MVFR ceilings along the east slopes of the Wind River Range,
extending into vicinity KLND by 12z, possibly as far east as
vicinity KRIW.  An upper level disturbance will move south across
the area Sunday morning and early afternoon with isolated to widely
scattered showers developing ahead of this system.  Most of this
activity will blossom in the late morning and afternoon vicinity and
south of a KRIW-KCPR line.  Some drying and clearing will then occur
north to south late Sunday afternoon through Sunday evening.  A cold
front will plunge south into northeast Wyoming 09z-12z Monday, near
a vicinity KCOD-KWRL-40SM north KCPR line by 12z Monday.  Lower
MVFR/IFR clouds are expected mainly along and east of the Bighorn
Range through 12z, remaining north of KCPR until later Monday


Issued at 240 AM MDT Sun May 21 2017

Unsettled weather will continue for the next couple of days. An
upper level disturbance will drop southward across the area and
bring another chance of isolated showers and thunderstorms, with the
best coverage in southern and eastern areas. Relative humidity will
again remain above critical levels with cool temperatures
continuing. A gusty wind will develop across Johnson County. Mixing
and smoke dispersal will be generally fair to good. A cold front
will bring another chance of showers for Monday before dry weather
returns for Tuesday.





SHORT TERM...Hattings
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