Area Forecast Discussion
Issued by NWS Riverton, WY

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623
FXUS65 KRIW 170712
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
112 AM MDT Fri Aug 17 2018

.SHORT TERM...Today through Sunday night

We have a few blips on the radar this morning but many are not
reaching the ground given the still very dry atmosphere at the
surface. And for the most part, most areas will have a mainly dry
and warm day for the most part. Isolated showers and thunderstorms
will likely develop in the west, but almost all will be of the late
day and more than likely evening and night time variety. Areas East
of the Divide should have a dry day today. The models are showing a
little in the Big Horns but like it has been stated many times
before the models are notorious for overdoing the orographics there.
As a result, we kept things dry. Temperatures will average above
normal but not really hot.

Saturday at this point looks cooler and unsettled as a trough and
cold front work across the state. The result will be cooler
temperatures, more clouds and a greater coverage of showers and
thunderstorms. The next question is, how strong will the
thunderstorms be? There will be some decent shear with the trough
moving through the area. The best upper level forcing looks to stay
over Montana however. In addition, with more cloud cover, surface
instability may be limited somewhat. So for now, we have gone with
the gusty winds and small hail wording. The main threat may be for
somewhat larger hail, given the shear may allow for some tilted
updrafts and cooler air moving in may lower wet bulb zero levels. A
few storms could linger into the night as well. In addition, a gusty
wind will develop behind the front. It will not be high wind but
there could be some locally windy conditions, especially areas
favored by north to northwest flow with cold advection. This is
mainly the northern Bighorn Basin as well as Johnson County.

The models begin to disagree on Sunday. The NAM builds ridging
across the area with drying conditions from west to east. Meanwhile,
the GFS shows another weak shortwave moving through the flow that
could keep a few showers and thunderstorms in the forecast. Coverage
will be less than Saturday though. For now, we split the middle but
kept POPS somewhat low. AS for threats, with a cool day expected, it
would be hail although with less instability it could be of the small
variety. A gusty wind will continue in Buffalo as well although it
will decrease through the day as the pressure gradient gradually
relaxes.

.LONG TERM...Monday through Friday

A shortwave trough in the northwest flow and associated weak cold
front could push across the forecast area Monday into Monday
night. Global models are hinting at pseudo-monsoonal moisture
being pulled northward between a sub-tropical ridge over Desert
SW/Southern Rockies and an upper low in the Pacific Northwest/N.
California area. All of this will give the entire area a slight to
chance PoPs with an emphasis east of the Divide.

The upper low over the Pacific Northwest opens up and lifts
northeast Tuesday/Wednesday across the northern Rockies/SW Canada
as the subtropical ridge strengthens over the southern Rockies.
Disturbances on the northern periphery of this ridge could result
in isolated to locally scattered diurnal convection mainly on the
lee side of mountain ranges and adjacent foothills as well as SW
Wyoming Tuesday and Wednesday. The trend has been for more
activity across SW Wyoming with models showing some monsoonal
moisture lingering across the area.

By Thursday and Friday, models are showing a dry westerly flow
developing with much of the monsoonal moisture being suppressed
south of the area.

&&

.AVIATION...12Z Issuance

VFR conditions will continue through 12Z/Friday with only localized
MVFR visibility because of the western wildfire smoke. Lingering
isolated nocturnal showers will decrease by sunrise Friday. An upper
low over the Pacific Northwest will track east Friday and Friday
night. Light showers will increase late Friday evening and continue
overnight. The best coverage will be north of a KJAC-KBYG line.

&&

.FIRE WEATHER...

A few showers will linger through the early morning hours, then,
thunderstorms will develop this afternoon and evening, mainly over
the mountains. Precipitation is expected to decrease in the west
while shifting to the north and east later Saturday into Sunday.
Winds will be mostly light until Saturday afternoon when it will
become breezy across the area. Temperatures will be slightly
warmer than on Thursday and relative humidity values low, but not
critical. On Saturday, temperatures will be cooler and relative
humidities higher. Mixing heights will be favorable for very good
smoke dispersal in the mountains and west of the Divide and fair
in the basins east of the Divide.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Jones
FIRE WEATHER...Bourque



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