Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1130 AM MDT Thu May 24 2018

.SHORT TERM...Thursday through Saturday night

Other than a couple of stray showers, most of the convection has
come to an end early this morning. For today, we will continue the
trend of higher temperatures and lower chances of convection. A weak
upper level disturbance will move into western Wyoming later this
morning into this afternoon and bring another chance of
thunderstorms. However, with less moisture to work with we kept POPS
in the scattered category and this is mainly in the afternoon. At
this point, the best chance for storms to move off the mountains
into the lower elevations would be in northern Wyoming with slightly
lower heights and somewhat stronger steering winds. But coverage
would be isolated at best in the valleys. Highs will be the warmest
so far this week with 80s in some of the favored warmer locations.
Most convection should end after midnight.

Ridging should reach it`s greatest strength across the area on
Friday along with the the lowest levels of precipitable water. The
result will be the least active day for convection. Any
thunderstorms that do develop would be at most isolated and likely
restricted to the mountains and adjacent foothills with steering
flow looking very weak. Temperatures will continue to be on the rise
and it will definitely feel like summer as we approach it`s
unofficial start on Memorial Day weekend. Many of the lower
elevations will be well into the 80s with a couple of the warmer
spots like Greybull or Basin possibly seeing the first 90 degree
temperature of the year.

Another shortwave will begin to approach the area on Saturday and
again increase the chance of thunderstorms. The models begin to
diverge on timing of the shortwave though. The usual bias with the
models is too rush these systems in too fast though. So for now we
kept POPS mainly across the western mountains during the day.
Meanwhile, East of the Divide it will likely be a very warm day with
90s a decent possibility for many areas in the Big Horn Basin. With
a southwest wind developing ahead of the wave, there could be enough
downsloping to even warm a place like Casper to around 90.


.LONG TERM...Sunday through Thursday

A wetter and cooler pattern will prevail on Sunday and Memorial
Day. The culprit will be an upper low pressure system that is
expected to become nearly stationary over the northern Great Basin
on Sunday. This system is expected to finally lift out of the area
into the northern High Plains on Tuesday. Similar to the last few
of these systems, this system will be somewhat fragmented, with
both ECMWF and GFS showing at least one potent shortwave ejecting
north across the area on Sunday, encountering modest instability
(SB CAPES ~1000 J/kg) across central Wyoming. So isolated strong
to severe thunderstorms will be possible, but flooding will be the
main concern as rivers and creeks will already be running high
from snowmelt after warm mountain temperatures Friday and
Saturday, with the potential for another 1"-2" of rain.

Medium-range models show heavier band of precipitation moving
north into Montana and NW Wyoming Sunday night. Another round of
showers and thunderstorms is expected to blossom over the area
Monday afternoon and evening, possibly continuing well into Monday
night as the main upper low over the Great Basin begins to lift
out across the area. Heavier, more organized precipitation should
be mostly east of the area over the northern and central plains on
Tuesday with chances of showers and thunderstorms lingering
mostly over the west and north.

An active weather pattern looks to close out the month of May, as
next Pacific storm system will move onto the northwest coast on
Wednesday, and into the Intermountain West/northern Rockies on
Thursday. Accounting for models typical fast bias, most areas east
of the Divide should see a break in precipitation on Wednesday,
with chances of showers and thunderstorms returning on Thursday.
The western mountains will continue to see chances of showers and
thunderstorms through the extended period.

&&

.AVIATION...For the 18Z TAFS through 18Z Friday

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

Isolated showers and storms will form this afternoon and continue
through 03Z Fri. Local MVFR conditions will occur with thunderstorms
along with wind gusts up to 30 knots. Mountains will be obscured at
times.

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

Isolated shower or storms will occur over the north after 19Z and
continue through 03Z Fri. Local MVFR conditions will occur with
thunderstorms along with wind gusts up to 30 knots. Mountains will
be obscured at times.

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

&&

.FIRE WEATHER...

A weak weather system will move into western Wyoming today and bring
isolated to widely scattered showers and thunderstorms mainly to
western and central Wyoming with lessening chances further to the
south and east. Winds will be generally light to moderate, although
a gusty southwesterly breeze will be possible at times from the Red
Desert through Casper. Humidity should remain above critical levels,
although it could fall into the teens at times in Sweetwater County.
Mixing and smoke dispersal will generally range from good to
very good.

&&

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

$$

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



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