Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 170736
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
136 AM MDT Tue Oct 17 2017

.SHORT TERM...Tuesday through Thursday night

Once again we will not have to worry about precipitation for the
next three days as a ridge controls the weather and brings mild, warm
and dry conditions. However, we do have concerns of the non
precipitation variety though.

The first is a concern that a popular unnamed 90s cartoon was banned
from saying, fire. With a shortwave approaching from the west, the
pressure gradient will tighten and 700 millibar winds will increase.
This will increase the wind across the southwestern wind corridor
later this morning and into this afternoon as mixing increases. In
addition, with very dry air in place relative humidity will fall to
around 10 percent. And as a result, red flag conditions look likely
for much of Natrona County. We had a few thoughts of issuing one for
Johnson County as well, but at this point the wind looks to remain
just below criteria. Otherwise, expect a mild to warm day today with
temperatures well above normal.

The other concern is for wind. With the shortwave passing by to the
north, 700 millibar winds will once again increase substantially
across the Absarokas and Cody Foothills, with both the NAM and GFS
showing up to 60 knot winds at that aforementioned level. There
seems to be a decent chance these winds will mix to the surface at
times tonight so we have hoisted a high wind watch for the Cody
Foothills, but mainly for the areas around what seems to be the
windiest place in the civilized world, Clark. These winds should
decrease by mid morning Wednesday.

At this point, Wednesday looks to have somewhat less breezy
conditions, so critical fire weather is not anticipated at this
point. Thursday is a different story though as winds increase
ahead of a stronger advancing cold front so more fire weather
highlights will be possible for Thursday afternoon in the zones
that are still critical.

.LONG TERM...Friday through Tuesday

Overview...A Pacific cold front will bring chances of rain and
snow to the west on Friday with generally mild and breezy to windy
conditions east of the Divide. Areas of light snow may spread
into central Wyoming Friday night behind the cold front. Cooler
and breezy/windy conditions will prevail Saturday with snow
showers lingering over the NW mountains. Another cold front will
push across the northern Rockies on Sunday spreading more snow
into the NW mountains Saturday night-Sunday while breezy to windy
and milder conditions prevail across central and southern Wyoming
on Sunday. Mild and dry conditions are expected Monday and
Tuesday.

Discussion...Upper air pattern on Friday will feature shortwave
trough translating the Intermountain West with progressive flow
upstream over the northern Pacific. Shortwave trough is expected
to split as it translates the Rockies into downstream ridge over
the eastern U.S. Friday night. GFS is faster than ECWMF with
trough and associated cold frontal passage across the area
Friday/Friday night, the GFS brining the cold front across most of
Wyoming Friday afternoon while the ECMWF keeps most of central
Wyoming in the warm sector Friday afternoon. Forecast was weighted
toward ECMWF here with shortwave running into strong downstream
ridge. GFS also on the fast end of guidance with trailing
system/cold front translating the northern Rockies on Sunday than
ECMWF. Both models show warm advection pattern with good moisture
fetch into the northwest mountains late in the weekend along with
favorable setup for high winds/mountain wave activity mainly along
the Cody Foothills Saturday night-Sunday morning. This system is
expected to carve out a longwave trough across the eastern U.S.
Monday and Tuesday with upper ridge amplifying over the western
U.S..

&&

.AVIATION...12Z Issuance

VFR conditions will occur over the region through tonight. A gusty
wind will occur at the KCPR airport today through about 00Z, then
decrease. A gusty wind at the KCOD airport will decrease toward
03Z Wed.

&&

.FIRE WEATHER...

...RED FLAG WARNING FOR NATRONA COUNTY TODAY...

A combination of a strengthening mid level winds and good mixing will
bring a gusty wind to the wind corridor from Rock Springs through
Casper late this morning and into the afternoon. With relative
humidity falling to around 10 percent, critical fire weather will
occur this afternoon in Natrona County. Johnson County will also
have elevated fire conditions, although critical fire weather is not
expected at this point. Mixing and smoke dispersal will range from
fair to poor in the basins and valleys to good across the normally
breezier locations.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT this evening for WYZ280.

High Wind Watch from this evening through Wednesday morning for
WYZ003.

&&

$$

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



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