Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180506

Area Forecast Discussion
National Weather Service Riverton WY
1106 PM MDT Tue Oct 17 2017

.SHORT TERM...(This evening through Friday)
Issued at 150 PM MDT Tue Oct 17 2017

Dry, mild period continues through Thursday with main concerns being
periods of gusty to strong (even high wind) and elevated to critical
fire weather conditions in Natrona and possibly southern Johnson
County. We will be upgrading the high wind watch for the Cody
Foothills with the afternoon package. The period of high wind
potential though looks quite short. We will go 9 pm to 3 am to cover
the entire possible period but the window is really confined to
maybe 3 hours or so roughly 4-7z before a weak cold front brings n-
ne winds to the area and shuts down any further high winds. Winds
should be less across Natrona County on Wednesday with surface high
behind the front producing higher pressure to the east of our area.
This will be short lived as sw winds increase again on Thursday
ahead of our fast approaching Friday system. Strong sw winds will
return to the wind corridor, and probably many e-ne facing foothill
and adjacent lower elevation areas. Could be another round of strong
winds along the Cody Foothills Thursday night. Strong cold front
will move into the west Friday morning and then sweep quickly east
to near our ern CWA by 00z Saturday. Strong to locally high wind
will accompany the front. Out west, we`ll get a quick shot of
forcing with the trough and strong front producing a period of light
to moderate mountain snowfall mainly during the day Friday. Amounts
look sub-advisory right now. One caveat to that is the potential
for a few heavier snow showers with thunder. The lower elevations
will see highs in the morning then steady to falling temperatures.
Timing will make a difference if the front is any earlier as pcpn
could start as snow and stay snow in the far west valleys if that
occurs. Otherwise, we get some rain showers with lowering snow
levels to the valley floor in the afternoon before tapering off
just beyond this period. The areas east of the divide will be warm
and windy to very windy with some showers possible with fropa,
especially I80 corridor and far north (and of course the
mountains). The I80 corridor could see enough cold air ushered in
with the front to change the rain showers over to snow showers
before ending near 00z Saturday.

.LONG TERM...(Friday night through Tuesday)
Issued at 150 PM MDT Tue Oct 17 2017

Beginning of fcst, progressive embedded upper shortwave trof axis
and associated cold front (just ahead of the trof axis) will be
moving through the wrn/nrn FA with light snow or rain/snow mix in
the valleys decreasing through the evening period leaving lingering
snow showers mainly across the FA`s nw mountains. Additionally, a
secondary area of precip (rain/snow mix or all rain) will be
progressing through srn WY along and behind the front...mainly
kicking through and out of the FA before midnight Friday. Gusty
west winds expected to accompany the front at all locations...with
gusts 35 to 45 kts possible. Mainly dry except for
sprinkles/flurries east of the Divide and may a few light snow
showers across the Big Horn Basin over the same time period.

Saturday will be post trof/frontal and cool with highs in the 40s
and 50s across the lower elevations...20s and 30s in the mountains
and a few lingering showers over the nwrn mountains. The upper
levels will become flat wnw with the nose of a rather strong jet
moving into/over cntrl/nrn WY...nearly orthogonal to the terrain. By
Saturday night, the jet drifts somewhat north and repositions mainly
over MT with the nrn/nwrn FA under the better upper level dynamic
forcing. Better EPAC moisture will begin to push back into nwrn/wrn
WY (door now "open" to the Pacific) while the terrain
orientation/jet and increased boundary layer flow (in response to an
ever growing P GRAD between high pressure over the Great Basin and
lee side troffing east of the Divide adding to increased flow aloft
and momentum transfer to lower levels under relatively strong
boundary instability) provide a good mechanism for getting precip to
the surface. Colder mid/upper temps during the day Saturday will be
replaced Saturday night by warmer temps and slowly rising snow
levels in a WAA regime. Snow levels may end up rising from the
valley floors (what it would have been during the day Saturday)
across wrn WY to 7.0 to 8.0kt ft by Sunday morning. Current POPs
across western/northwestern WY Saturday night through Sunday are too
low for obvious "FB" reasons...but will surely rise quite a bit as
EPAC moisture, QPF, incoming upper level forcing from an embedded
shortwave/jet, and agreement between MR mdls is fully realized. Did
raise winds across the Wind River and Absaroka Mountains/foothills
Saturday night through Sunday and Sunday night through Monday as a
"typical" near or high wind pattern develops with good upper level
wind speeds and possible mountain wave production in addition to
good cross barrier temp/pressure/omega gradients. Otherwise, precip
chances end from west to east mainly across nrn WY on Monday with
ridging, warming and drying returning to the FA as well as most of
the wrn CONUS...lasting through next week.


.AVIATION...(For the 06Z TAFS through 06Z Thursday)
Issued at 1030 PM MDT Tue Oct 17 2017

VFR conditions will occur through the forecast period. A gusty
southwest wind will occur at KCPR through 11Z before decreasing.
A gusty wind at KCOD through 10Z. Wind shear is likely at KCPR,
KCOD, and KLND through the early morning.


Issued AT 650 PM MDT Tue Oct 17 2017

Wednesday will see some breezy areas but overall, the wind looks a
little less than today. Elevated fire weather conditions though are
still expected. Winds are expected to increase again on Thursday
with possible critical fire weather conditions returning due to
those winds and low rh`s across Natrona County with elevated fire
concerns elsewhere across the lower elevations east of the divide.



High Wind Warning from 9 PM this evening to 3 AM MDT Wednesday
for WYZ003.



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