Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212002

Area Forecast Discussion
National Weather Service Riverton WY
202 PM MDT THU JUL 21 2016

.SHORT TERM...Tonight through Sunday

As of now, today looks fairly similar to Wednesday. One important
difference is that there is less wind than yesterday and as a result
fire weather concerns are reduced somewhat. A couple of
thunderstorms have already fired across southern Sweetwater County
with the northern edge of the monsoonal plume. The models are
showing some activity across the mountains this afternoon. However,
the models do tend to overdo the coverage of storms there.
Especially so in the Bighorn range where they seem to think we have
been transported to the Congo rain forest. We have some POPS there
but we have kept them in the slight range. There is some chance a
few could drift into the basins this evening but this should be few
and far between. Any convection should end shortly after midnight.

Things could get interesting tomorrow. A shortwave and associated
cold front will sweep across the area. The first concern is for
thunderstorms. There will not be a tremendous amount of moisture to
work with though. However, there will be enough forcing to bring
isolated convection to much of the area into the evening. The main
threat will once again by gusty winds with the large temperature/dew
point spreads that would lead to some erratic downburst winds. The
second concern is for temperatures. As the front approaches, a
southwest breeze will develop and increase downsloping. This should
bring hot temperatures. Triple digit highs look likely in the warmer
spots like Greybull and Worland with 90s fairly widespread across
the lower elevations. The third concern is for elevated fire danger.
The combination of gusty wind and low relative humidity could bring
critical fire weather conditions for portions of the west. As a
result, we will issue a fire weather watch for critical zones west
of the divide. One will be issued for Fremont County as well as
northwest post frontal winds could get strong. Further east, wind
should be lighter so we will issue a headline and not a red flag.
Convection should end by midnight Friday as the front moves away to
the east.

Somewhat quieter conditions should return for Saturday. High
temperatures should be around 5 to 10 degrees cooler behind the
front. With moisture shunted to the south and high pressure building
over the area all areas should be precipitation free. Relative
humidity will be quite low, but with wind expected to remain light to
moderate, critical fire weather is not expected at this time. Sunday
looks to be fairly similar. The NAM does show some higher elevation
convection over the west but the GFS has nothing. The models have
also backed off on convection in the east with moving shortwave flow
a bit further to the east. We maintained some slight POPS in the
Bighorn range but nothing more.

.LONG TERM...Sunday Night through Thursday

Models in fairly good agreement through early next week with
the overall synoptic pattern before differences begin to emerge
Wednesday and Thursday. Generally dry zonal flow aloft to begin
the work week. Thunderstorms will be off to our east Monday
afternoon/evening, and monsoonal flow will be suppressed to our
south. Temperatures look to be near seasonal averages. Tuesday
looks to largely be a repeat of Monday with temperatures rising
a couple of degrees. ECMWF does try to hint at some showers
clipping the northeast, but these would likely be confined to
the Bighorns. For Wednesday and Thursday, the ECMWF has a ridge
building farther to our west allowing for west-northwest flow
to bring energy and mid-level moisture across the north. The GFS
holds off this shift until Thursday when a shortwave dives into
the Intermountain West. For now, limited precipitation chances
for Wednesday but did increase them for Thursday. Again, the
northern-third of the forecast area will be the most likely area
as the ridge looks to keep things dry over the southwest nudging
into central Wyoming. Backdoor cold front through the northern
forecast area could set the stage for a more active afternoon
next Thursday. Of course, that is 7 days into the future, so
details are likely to change.


.AVIATION.../00Z Issuance/

All terminals will remain VFR through 00Z/Sat, with the only
issue coming in the form of early morning wildfire smoke settling
in the valleys/basins. Thinking here is that KJAC, KRIW, KLND,
KPNA, and KBPI will remain above MVFR, especially since fires
will not burn as much timber today. Ongoing convection across
southwest Wyoming could still produce some 30-40kt gusts through
about 01Z/Fri before convection wanes with the setting sun. Winds
will be less than 12kts overnight before increasing ahead of an
approaching cold front Friday. Expect south-southwest wind
10-20kts to pick-up at KCPR around 14Z/Fri and for southwest wind
8-14kts to increase at KBPI, KPNA, and KJAC between 18Z and 20Z/Fri.
Wind in the west will further increase after 20Z/Fri and gradually
shift to the west-northwest. This will likely bring more smoke to
KPNA late in the day into Friday evening. Expect front to swing
across KLND/KRIW around 23Z/Fri with gusty west-northwest wind in
its wake. Have generally bumped up speeds, especially at KRIW,
given this scenario as we can sometimes get surprised with
stronger winds from this direction. A similar scenario could
unfold post- frontal at both KBPI and KPNA.




Any afternoon and evening thunderstorms should end by around
midnight. A cold front will approach from the west on Friday and
bring gusty southwest winds and isolated thunderstorms ahead of its
passage. Critical fire weather is possible in the areas with
critical fuels West of the Divide tomorrow afternoon a Fire Weather
Watch has been issued. There could be a period of critical fire
weather across Fremont County as well following the frontal passage.
Mixing and smoke dispersal should be good to excellent. Fire
conditions should ease somewhat on Saturday with less wind and
cooler temperatures.


.RIW Watches/Warnings/Advisories...
Fire Weather Watch from Friday afternoon through Friday evening
for WYZ414>416.

Fire Weather Watch from Friday afternoon through Friday evening
for WYZ283.



SHORT TERM...Hattings
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