Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211740

Area Forecast Discussion
National Weather Service Riverton WY
1140 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 18Z TAFS through 18Z Monday)
Issued at 1140 AM MDT Sun May 21 2017

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

All terminals will be VFR through the forecast period despite
another round of afternoon convection today. Convective cloud
elements already forming over the higher terrain of western Wyoming
late this morning. Elongated area of instability stretching from
northern Utah to central Wyoming will present the best chance for
more organized showers this afternoon. This will favor KBPI, KPNA,
and KRKS. This instability axis and associated moisture will push
south by this evening with clearing conditions in its wake. An
additional round of clouds will push south between 15Z-18Z/Mon ahead
of a more vigorous shortwave diving into eastern Wyoming Monday.
West-northwest wind will begin to respond late Monday morning as the
pressure gradient tightens ahead of this shortwave. Wind speeds of
10-15kts will be common. Occasional mountain top obscurations will
be noted through 01Z/Mon because of convection.

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

Series of weak shortwaves in northerly flow aloft has kept light
showers going in the central basins this morning with another batch
moving into northern Wyoming late this morning. The second shortwave
over the north will continue south and clear most terminals around
01Z/Mon. Despite the showers, VFR conditions will prevail through
the forecast period. Cloud cover in central areas will limit surface
heating and instability. Therefore, deeper convection may not occur
until much later this afternoon and just ahead of the drier, more
stable air moving south behind the second shortwave. Clouds will
decrease rapidly this evening before another push comes from the
north late tonight. Vortex over central Saskatchewan will dive south
tonight with mid-cloud decks increasing from north-to-south Monday
morning. The pressure gradient will increase prior to 18Z/Mon with a
cold front set to pass through KCOD, KWRL, and KCPR between 15Z-
18Z/Mon. Surface speeds of 15-25kts will be common following frontal
passage. Mountain top obscurations will be frequent along the east
slopes of the Wind River Range and over the higher terrain of
northern Wyoming through 01Z/Mon. Additional obscurations will
return across the north between 12Z-18Z/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.





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