Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211853

Area Forecast Discussion
National Weather Service Riverton WY
1253 PM MDT Mon May 21 2018

.SHORT TERM...Today through Wednesday night
Issued at 255 AM MDT Mon May 21 2018

Overview...Weak steering flow will continue over the region today,
with a closed low forming over central CA. Winds aloft will remain
south-southwesterly, with minor shortwaves moving northward over the
state. These troughs, along with warming temperatures and abundant
moisture, will aid in developing isolated thunderstorms this
afternoon and evening. The closed low will become cut off by
tonight, as it gradually moves over S NV. The nose of the channel
jet is expected to move over southern and central portions Tuesday
afternoon, with areas of U/L divergence over Sweetwater and Natrona
counties.  At the same time, the upper low will begin to fill and
slowly lift toward the NE. Minor shortwaves/perturbations will
continue to move over the state Wednesday, as the remnants of the
upper low move over northern portions of the Great Basin.

Discussion...Convective activity will once again be isolated today,
mainly over the northern and western mountains. PW`s will stay near
0.50" and dewpoints in the mid 30s to mid 40s. Locally heavy rain
will be the main threat from these storms. Drier air will make its
way over Sweetwater County, where dewpoints drop into the low to mid
20s. This will be temporary, as southerly winds return Monday night
into Tuesday. Dewpoints will range between the 40s and low 50s
across much of the forecast area Tuesday, with isolated convection
starting over the western mountains by early afternoon. Storms will
increase in coverage through the rest of the afternoon, with
portions of the far north staying dry. Some of these storms could
initially be strong, but the main threat will continue to heavy
rain. The focus will shift mainly east of the Divide Tuesday
evening, as these storms move northward through the overnight hours.
Heavy rain with localized flooding is likely with these storms
through midnight. By Wednesday morning, light showers could be
ongoing across the far north. The area will quickly reset, as
another shortwave moves over the forecast area during the day.
Scattered thunderstorms will occur over western portions, with
isolated thunderstorms east of the Divide. Stronger storms will be
possible over Johnson and Natrona counties. Most of the convection
should end/exit the area by midnight Thursday morning.

.LONG TERM...Thursday through Monday
Issued at 255 AM MDT Mon May 21 2018

Thursday will see scattered showers and thunderstorms over the
west. Some of this activity moves east later in the day into
Thursday evening impacting the north and central basins. Friday
will see only isolated shower and thunderstorm activity, mainly
over and near the mountains. Saturday looks like a dry day with
very isolated showers or storms over the mountains late in the day
or evening. The long range models continue to struggle with the
track of a weather system that moves into California Saturday. One
would think the weather system will weaken as it encounters the
warm ridge of high pressure over the Rockies on Sunday into Sunday
night. The 00Z model runs tonight are more aggressive with the
system smashing into the ridge with a track that takes the system
across NW WY Sunday night/Monday morning and then into Montana
Monday night. This would result in showers and storms over the
west and north Sunday afternoon and night with additional showers
and storms next Monday. Have added this to the forecast and can
refine it more in the coming days. High temperatures will be
warming up for Thurs with 70s for most places. Friday through
Sunday will see upper 70s to mid 80s across the basins with 70s in
the west and south. Next Monday will be cooler. High elevation
snow melt will occur with rising stream and river levels over the
Memorial Day weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1253 PM MDT Mon May 21 2018

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

VFR conditions to prevail through the forecast period. Scattered
showers and thunderstorms will continue over the west through 06Z.
Then isolated showers to linger through 12Z Tuesday. Local MVFR
conditions will occur with some thunderstorms. Mountains will be
obscured at times. Wind gusts to 30 knots and small hail will occur
with stronger storms. Showers and thunderstorms will be more
numerous Tuesday afternoon and evening.

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

VFR conditions to prevail through the forecast period. Scattered
showers and thunderstorms will continue over and near the mountains
through 06Z. Then isolated showers to linger through 12Z Tuesday.
Local MVFR conditions will occur with some thunderstorms. Mountains
will be obscured at times. KCOD airport looks to have the best
chance of seeing thunderstorm activity this afternoon and evening.
Showers and thunderstorms will be on the increase Tuesday afternoon
with widespread showers and thunderstorms Tuesday night. Some storms
will be quite strong to severe, especially eastern zones.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.


Issued at 255 AM MDT Mon May 21 2018

Isolated showers and thunderstorms possible across western and
northern portions this afternoon and evening. Relative humidity will
return to lower values across the lower elevations, with eastern
portions of Zone 279 reaching critical values. Mixing and smoke
dispersal will range from good to excellent. Showers and
thunderstorms will develop across southern portions and the higher
elevations by mid afternoon Tuesday, becoming widespread late in the
afternoon into the evening. Heavy rain with localized flooding will
be possible, as these storms move northward Tuesday night.





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