Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 140802
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
202 AM MDT Tue Aug 14 2018

.SHORT TERM...Today through Thursday night

We have two concerns today. The first is for the potential for
strong to possibly severe thunderstorms this afternoon. We have a
couple of thunderstorms out there this morning but these are likely
only producing small hail and some rain. After a break during the
midday, an approaching shortwave will fire off another round of
showers and thunderstorms this afternoon and evening. With the
shortwave, there will be some upper level forcing and the model
soundings are showing some decent shear as well. The limiting
factor, as it usually is in Wyoming, will be moisture. At this
point, the main threat will likely be hail given some fairly steep
lapse rates although strong wind gusts will also be a threat. A few
storms could linger late at night as well although all storms will
be over by Tuesday morning.

The next question is for fire conditions. At this point, we will
hold off on any highlights. Relative humidity will fall into the
teens and a gusty wind will develop this afternoon. However,
conditions do look borderline for any warnings. In addition,
Wednesday may be more of a threat with some dew points falling into
the teens and the gusty winds continuing. For now, we will issue a
special weather statement for near critical fire conditions. A few
spots could see critical fire conditions but at this point it looks
to be isolated.

The only concern on Wednesday looks to be for fire weather. Fire
weather highlights look to be a good possibility for this day
across southern Wyoming with the aforementioned low relative
humidity and gusty wind wind continuing. Both the NAM and GFS have
really backed off on any chance of precipitation with some higher
heights over the area. As a result, we have removed all POPS from
the area. The only chance at this point would be across far
southwestern Wyoming but even here the chance is so small we left it
out of the forecast.

On Thursday, some moisture will return to the west and could bring a
few showers and thunderstorms. With the lack of a trigger though,
coverage looks rather sparse so we kept the POPS in the slight
chance category and kept most of the activity across the west and
the adjacent foothills.

.LONG TERM...Friday through Tuesday

Medium range models are in decent agreement with A low-amplitude
trough over the Pacific Northwest/N. California at 12Z Friday.
This system is expected to push into the Intermountain West Friday
night, and then across the Northern Rockies Saturday/Saturday
night. The associated cold front is expected to push across the
area in the Saturday/Saturday night time frame. Synoptic forcing
from this system will begin to overspread the far west late
Friday, and then spread across the entire area Friday night and
Saturday. Chances of convection will be mainly over the west
Friday spreading east to include all of the area late Friday
night/Saturday morning. The best chances of convection will be
Saturday afternoon into Saturday night east of the Divide as the
cold front begins to slide across the area. Despite the main
forcing exiting the area Saturday night, decent chances of
precipitation will continue east of the Divide through Saturday
night into Sunday especially over the far east from a potential
combination of low-level northerly upslope, wrap around moisture
and/or a trailing shortwave. There could be enough cold air for
the northern mountains (Absaroka/Bighorn) to see rain changing to
snow above 10.5/11 KFT MSL Saturday night/Sunday.

For Monday and Tuesday there is quite a bit of spread between
models and their ensembles. They are showing a potential
shortwave in the northwest flow or a transitory ridge building
into the area, and how they handle a disturbance in the Pacific
that could impact the area Tuesday. For now will go for a warming
trend with a mainly dry forecast.

&&

.AVIATION...12Z Issuance

Lingering isolated showers and thunderstorms along and east of the
Continental Divide should mostly dissipate or move out of the area
by 12z, with the exception of vicinity KCPR where showers will
linger through the morning.

The west (KJAC, KRKS, KBPI, KPNA) will remain dry with possibly some
restrictions in visibility in FU from vcnty KJAC and north 12z
Tuesday through 12z Wednesday.

East of the Continental Divide, scattered thunderstorms are expected
to develop along and east of the Bighorn Range and south to vicinity
KCPR 20z Tuesday-02z Wednesday.  Isolated thunderstorms could be
strong to severe with large hail and wind gusts over 50kts.  Showers
and storms will be more isolated further west, vicinity KCOD, KWRL,
KRIW and KLND Tuesday afternoon and early evening.  Some lower MVFR
ceilings may push into KCPR after 06z Wednesday, but confidence is
too low at this time to go beyond SCT015.

&&

.FIRE WEATHER...

...NEAR CRITICAL FIRE CONDITIONS LIKELY ACROSS SOUTHERN AND PORTIONS
OF WESTERN WYOMING...

A weather system will move across the area today. Across the south,
a gusty west wind and relative humidity falling close to 15 percent
will bring near critical fire conditions this afternoon. Across many
areas East of the Divide, isolated to widely scattered showers and
thunderstorms are possible, some could be strong especially east of
a Greybull to Riverton line with strong wind gusts and hail. Mixing
and smoke dispersal will range from fair to poor in some of Basins
to very good to excellent in the west and south. Critical fire
weather is possible across southern Wyoming on Wednesday
afternoon.

&&

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

&&

$$

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


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