Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230824
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
224 AM MDT Wed May 23 2018

.SHORT TERM...Today through Friday night

Almost all of the thunder and heavy rain has now shifted north of
Wyoming for the most part early this morning. Some light to moderate
rain is lingering however, mainly across northern and western areas.
With the rain still falling on top of the heavy rain of yesterday,
We will let the flood watch ride for now, although chances of
flooding are decreasing. In addition, with the rain and narrow
temperature / dew point depressions, another creature may come out
of hiding. I am talking about the dreaded fog monster. As a
result, we added patchy fog to much of the area for early this
morning.

The rest of the forecast is based on a mathematical principle called
an inverse relationship. Otherwise known as, the more you get of
one thing, the less you get of another. In this case, it is the
higher the heights, the lower the chance of a thunderstorm. And
this is what will happen heading into the end of the workweek. The
low that brought the rain is now lifting northward and much of
the southern two thirds of the state looks to mainly dry through
much of the day. However, with the low expected to drift further
west once it lifts into Montana the chance of showers will linger
in northern and western Wyoming. It still looks as though most of
the area will have a chance of a shower or thunderstorm this
afternoon although coverage will be much less with more dry
periods. With a tightening pressure gradient and increasing
southwest flow, areas from Rock Springs through Casper will become
quite breezy today and possible windy in spots as well.

Convection will continue to decrease toward the end of the week as
ridging builds toward the area, heights rise and the atmosphere
continues to dry out. By Thursday, any thunderstorms should be
confined to northern and western portions of the area. A couple may
drift into the adjacent lower elevations but most areas should
remain rain free. By Friday, any convection should be restricted to
the mountains and even here should be isolated. Temperatures will be
on an upward swing as well with highs by Friday rising well into the
80s in the lower elevations. There is even a chance at some of the
warmer spots like Greybull or Worland could see the first 90 degree
day of the year.

.LONG TERM...Saturday through Wednesday

The next Pacific storm system will move onto the west coast late
Friday and slowly drift east across the Great Basin Saturday and
Sunday. This system looks to impact the area over Memorial Day
weekend into early next week in similar fashion to the last few
systems, drifting into the northern Great Basin Sunday before
lifting northward across Wyoming Memorial Day into Tuesday. The
23/00z ECMWF shows this persistent storm track better than other
medium range models this morning. Saturday looks to be the `warm
before the storm` with southerly flow, H7 thermal ridge 13c-15c,
over central Wyoming, pushing most highs into the 80s with even
some low 90s possible across the lower Big Horn Basin. Chances of
late day thunderstorms will be mainly west of the Divide and along
and east of surface trough/dryline roughly along and east of
Interstate 25. The rest of Memorial day weekend may be a washout
as upper low moves into western Utah on Sunday, with difluent
southerly flow aloft over Wyoming, low level easterly flow
bringing more abundant moisture and moderate instability into
central an northern Wyoming. So much of the area should see
at least scattered thunderstorms developing Sunday afternoon with
isolated strong to severe thunderstorms across central areas along
with heavy rain. ECMWF shows the heavier precipitation moving
north and east of the area by Memorial Day with a shortwave
lifting into Montana Sunday night (quite similar to our current
scenario) while GFS is much slower in this regard. We do not have
a great deal of confidence in the specifics at this time, but
favor persistence (ECMWF) until trends show otherwise. We should
see some drying later Tuesday as the remnants of this system
exit into the northern Plains.




&&

.AVIATION...12Z Issuance

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

Expect MVFR to lower end VFR ceilings and visibilities to continue at
the TAF sites through the initial few hours of the forecast
period. Will keep in borderline MVFR/low end VFR ceilings/visibility
values at KJAC until around 08Z Thursday. VCTS will be possible at
KBPI until around 02Z Thursday and expect at least VCSH at KJAC,KPNA
and KRKS until around 06Z Thursday. Mountains will be obscured at
times. Wind gusts to 30 knots and small hail will occur with any
stronger storms.

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

Widespread showers and isolated TSRA is expected at KCOD until around
02z Thursday, and only during the initial portion of the forecast at
KCPR, KRIW, KLND and KWRL. MVFR ceilings to lower end VFR ceilings
are expected through be balance of the forecast period. VCTS is expected
at KWRL until around 02Z Thursday.

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

.FIRE WEATHER...

A weather system will move into Montana today. Northern and western
Wyoming will see more clouds and continued showers and
thunderstorms. Shower and thunderstorm chances will decrease further
south but all areas will still have a chance of a storm this
afternoon and evening. A gusty southwesterly breeze will develop
across southern and eastern portions of the area. Relative humidity
should remain above 15 percent. Mixing and smoke dispersal will
range from poor in portions of Yellowstone to excellent in the
southern and eastern zones.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch until noon MDT today for WYZ002-004-006-008-009-015-
016.

&&

$$

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


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