Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190813
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
213 AM MDT Thu Oct 19 2017

.SHORT TERM...Thursday through Saturday night

There will be one more rain free day across Wyoming today with
above normal temperatures. However, fire concerns will ramp up
today. As a Pacific Cold front approaches from the west, the
pressure gradient will tighten across the southwestern Wind
Corridor from east of Rock Springs through Casper. In addition,
relative humidity will fall down to 10 percent. As a result,
critical fire weather will be likely so we upgraded the fire
weather watch to a Red Flag Warning. Natrona County should verify
no problem. Was a bit more on the fence about Johnson County
though in regards to wind. The best place for verification here
would be in southern Johnson County. Otherwise, expect a warm day
similar to yesterday. Some high level moisture will move in from
the south though and bring some high level clouds at times,
especially in the afternoon.

The aforementioned cold front will move into the west in the on
Friday. Most areas should start the day dry with the heaviest
precipitation holding off into the afternoon when the trailing
trough and jet dynamics catch up to the moisture. Snow levels will
start fairly high. However, with 700 millibar temperatures expected
crash around zero to minus 7, snow levels will fall to the valley
floors by late afternoon. And the problem here is that this will be
the time of the heaviest precipitation. It has been fairly warm
lately, so some snow may melt initially on the roads though and keep
accumulations down just a bit. Nevertheless, it could get hazardous
for a time late tomorrow afternoon and evening so future shifts may
want to consider an advisory, especially for the northwest.

Meanwhile, the concern East of the Divide will be wind. Relative
humidity should be higher, so critical fire weather is not expected.
It will be quite blustery though. The latest NAM is showing some 50
knot wind barbs at 700 millibars and sometimes that can be the
threshold for high winds. The NAM MOS also shows 34 knots
sustained at Casper and with the area in the right front quadrant
of the jet to promote downward forcing, there could be some rather
high wind gusts. The GFS is not as bullish however. Something to
watch though. High temperatures will be tricky depending on when
the front moves through. This could be a situation where Riverton
has a high around 60 and Casper rises into the mid 70s. As for
precipitation, other than around the mountains, strong downsloping
flow should keep things rain free.

Saturday at this point looks like a much cooler day. However, with
the Pacific origin of the air mass, it will not be frigid. Most of
the daylight hours look to be mainly dry with just a few mountain
showers in the west. Another wave will then approach the area late
Saturday night and bring another chance of snow to the northwestern
mountains. In addition, 700 millibar winds once again increase
across the Absarokas and with some downward forcing from a jet
streak, we could be looking at more high winds for the Absarokas and
Cody Foothills beginning on Saturday night, especially late.


.LONG TERM...Sunday through Thursday

Overview...Breezy to windy and milder conditions will prevail across
the area Sunday with some rain and snow showers lingering in the
northwest mountains.  High winds may develop along the Cody
Foothills and Absaroka Range into the Dubois area Sunday into Sunday
night.  A cold front will drop south along and east of the
Continental Divide on Monday bringing chances of light snow across
the northern mountains, with some rain and snow showers mainly along
and north of Interstate 90.  Gusty west to northwest winds will
prevail across much of the area.  Dry weather will prevail under a
building ridge of high pressure Tuesday through Thursday with above
normal temperatures expected to return mid-week.

Discussion...Upper air pattern on Sunday morning features Polar Jet
extending from the north central Pacific into the northern Plains
with shortwave making landfall along the NW U.S. coast.  At the
surface, cold front will extend WSW across northern ID from surface
low over southern Alberta/Saskatchewan, with a lee side trough across
eastern Wyoming.  Backing flow will bring drier air, decreasing
precipitation across western Wyoming through the day.

Strong mountain top winds Sunday and Sunday night will likely mix
down into the Cody foothills, Dubois area through Sunday morning.
The cold front will push across Montana into northern Wyoming Sunday
night, with GFS showing some 70KT H7 wind barbs along the east
slopes of the Absaroka Mountains, affecting mainly the higher
terrain above 8KFT as lower airmass becomes more stable behind cold
front.  The focus for strong winds will shift to NW prone areas
behind the cold front Monday morning, especially the northern
Bighorn Basin and NE Johnson County where winds may approach high
wind criteria.  Precipitation amounts across the north with the
frontal passage should be on the light side overall (0.10" or less)
on Monday. Upper air pattern amplifies on Tuesday with building
ridge over the western U.S., trough across the eastern U.S..  ECMWF
is quicker to build heights across the area Tuesday than GFS, so
EC`s surface temperatures are running about 8-10F warmer than GFS.
Forecast takes the middle road approach here, with weight given
toward the warmer side of guidance on Wednesday.

&&

.AVIATION...12Z Issuance

VFR conditions will occur through 12Z Friday. A gusty southwest wind
will occur today from 50NE KRKS to KCPR through about 03Z Friday.

&&

.FIRE WEATHER...

...RED FLAG WARNING FOR NATRONA AND JOHNSON COUNTIES...

A combination of relative humidity falling below 10 percent and a
gusty southwest wind will bring critical fire weather conditions
this afternoon across Johnson and Natrona Counties. Elsewhere, there
will be a bit of breeze, especially across the west and in eastern
Sweetwater County. Many areas East of Divide will see humidity fall
into the teens. Mixing and smoke dispersal will range from fair in
the basins to very good from eastern Sweetwater through Johnson
County.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM this morning to 7 PM MDT this
evening for WYZ280-281.

&&

$$

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



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