High Wind Watch 10, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1109 AM MDT SUN OCT 2 2016

.SHORT TERM...Sunday through Tuesday night

Quite an active period is coming up with still some uncertainty with
some of the details. All this is courtesy of a bowling ball known as
an upper level low now moving into the Pacific northwest. Right now
a bit of a jet max is moving through the flow and bring some showers
and embedded thunder to western portions of county warning area.
Most of this should move out of the area around mid morning although
some showers will continue in the west. Things may then get a bit
more interesting in the afternoon. With a front approaching from the
west, some decent upper level divergence and a decent amount of
shear, a few thunderstorms could get feisty this afternoon and
evening. The best chance of this would be in the west where the best
shear is located and a closer proximity to the front. Much of the
day East of the Divide could end up bring mainly dry, especially the
further east you head. We did raise high temperatures east of the
divide a few degrees with much of the day expected to be dry though.
With the front to the west, snow levels should remain remain fairly
high. There is a chance more general showers could spread east of
the divide tonight, but a lot depends on what model you believe. For
now, we will keep POPS at the scattered level. The NAM is showing
some dry slotting moving into portions of the area as well.

The upper level low will approach the area on Monday and pass over
western and northern Wyoming on Monday. With cooler air moving in
behind the cold front 700 millibar temperatures are expected to drop
down to as low as around minus 3 in the afternoon that would drop
snow levels down to around 7000 to 7500 feet. This snow would
continue into Monday night before tapering off late Monday night as
the upper level low moves away. At this point, it looks more like
advisory level amounts rather than warnings but definitely something
to watch. Meanwhile, East of the Divide the concern shifts to wind.
The latest GFS shows the critical 700 millibar winds increasing to
around 50 knots in some locations. With the upper level low and
front in the area there looks to be good mixing as well. Both the
NAM and GFS MOS show high wind criteria on Monday as well. As a
result, some highlights may be needed. The models have also trended
a bit drier with the downsloping flow so we trimmed POPS. The
exception is across northern areas where the upper level low will
track. High temperatures east of the divide will be tricky as well,
with downsloping flow possibly warning things a bit more than we
have in the forecast.

The low will be lifting northward out of the area towards
Saskatchewan by Tuesday. It will be a brisk and cool day through
with most areas stuck in the 40s and 50s. Most of the day will be
dry. However, enough moisture might wrap back into the area and
combined with weak cyclonic curvature a few showers may pop up,
especially with the heating of the day. Some of these could have
some graupel with them and maybe even a couple of wet snowflakes
in the lower elevations if the precipitation is heavy enough.

.LONG TERM...Wednesday through Sunday

Longwave trough will be established across the western/central
U.S. by Wednesday with the main upper low over south-central
Canada. Global models show a couple of disturbances moving through
the base of the trough Wednesday and Thursday with the stronger of
the two occurring on Wednesday. This energy especially with
daytime heating, and steep lapse rates will keep isolated to
scattered showers each day with an emphasis across the west and
south on Wednesday. Have gone with a snow level generally between
6000-7000 feet both days, but decided to keep at least a rain/snow
mention to the valley floor even across the east due to to
anticipated low wet-bulb zero temps despite the contradictory
highs in the 40s to possibly 50 degrees.

Trough axis shifts east into the Plains Friday with a more zonal
flow developing by next weekend. Models show a shortwave trough
pushing across the Northern Rockies next weekend, but have varied
on the intensity and timing. Looks like the northwest and north
would see the most impact, and that does not seem to be that
impactful. For now will only have slight chance PoPs across the
northwest mountains.

Temperatures will be below average Wednesday through Friday with a
return to seasonal temps for next weekend. Also, most if not all
areas will see their first freeze Wednesday thru Friday mornings.
Each day has the potential for freezing conditions, but cloud
cover Tuesday night and Wednesday night might keeps some areas
above freezing.

&&

.AVIATION.../12Z ISSUANCE/

West of the Continental Divide...KJAC/KBPI/KPNA/KRKS

VFR to occasional MVFR in lower CIGS and VIS restrictions associated
with heavier storms will prevail for most terminal sites today.
Later in the afternoon, particularly after 20Z, isolated to
scattered showers and embedded strong to marginally severe
thunderstorms will develop and moving quickly to the north northeast
across the area...especially over the western/northwestern
mountains/valleys affecting KJAC the most. The main threat from the
stronger storms will be brief heavy rain, gusty winds to 40+ mph,
hail between one half and one inch in size, and frequent lightning.
Low level wind shear will also be prevalent at most terminal sites
by this evening lasting overnight. Then, as the large upper level
low that brought today's weather continues to progress to the
northeast across Wyoming, increasing showers and embedded thunder
will begin to pick up once again over the entire region west of the
Divide after 12Z Monday. Occasional MVFR/to IFR
conditions...especially near KJAC where morning snow showers will be
possible...can be expected.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL

VFR conditions to prevail through the forecast period. Isolated
showers or an isolated thunderstorm will occur through 23Z
increasing in coverage after 00Z Monday and lasting through 06Z
Monday...dissipating from south to north across the region. A few
strong to marginally severe thunderstorms will be possible across
the area late afternoon and early evening. The main threat from the
stronger storms will be brief heavy rain, gusty winds to 40+ mph,
small hail, and frequent lightning. Low level wind shear will also
be prevalent at many terminal sites by this evening lasting
overnight. Then, as the large upper level low that brought today's
weather continues to progress to the northeast across Wyoming,
increasing showers and embedded thunder will begin to pick up once
again over the much region east of the Divide after 15Z Monday.
Occasional MVFR conditions can be expected.

&&

.FIRE WEATHER...

Fire concerns should remain low today. Showers will continue across
the west with a chance for a wetting rain and snow on the mountain
peaks. East of the divide, it will be mild to warm but relative
humidity will remain well above critical levels. Mixing with be
generally fair to good east of the Divide and good to very good west
of the Divide. Fire danger may increase in some areas on Monday as
winds may become strong and relative humidity falls.&&

.RIW Watches/Warnings/Advisories...None.

&&

SHORT TERM...Hattings
LONG TERM...WM
AVIATION...Braun
FIRE WEATHER...Hattings

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RIVERTON WY
245 AM MDT SUN OCT 2 2016

WYZ001>020-022>030-031200-
YELLOWSTONE NATIONAL PARK-ABSAROKA MOUNTAINS-CODY FOOTHILLS-
NORTH BIG HORN BASIN-SOUTHWEST BIG HORN BASIN-
SOUTHEAST BIG HORN BASIN-OWL CREEK AND BRIDGER MOUNTAINS-
BIGHORN MOUNTAINS WEST-BIGHORN MOUNTAINS SOUTHEAST-
NORTHEAST JOHNSON COUNTY-SOUTHEAST JOHNSON COUNTY-
TETON AND GROS VENTRE MOUNTAINS-JACKSON HOLE-
WIND RIVER MOUNTAINS WEST-WIND RIVER MOUNTAINS EAST-
UPPER WIND RIVER BASIN-WIND RIVER BASIN-LANDER FOOTHILLS-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-
NATRONA COUNTY LOWER ELEVATIONS-CASPER MOUNTAIN-STAR VALLEY-
SALT RIVER AND WYOMING RANGES-UPPER GREEN RIVER BASIN FOOTHILLS-
UPPER GREEN RIVER BASIN-SOUTH LINCOLN COUNTY-
ROCK SPRINGS AND GREEN RIVER-FLAMING GORGE-EAST SWEETWATER COUNTY-
245 AM MDT SUN OCT 2 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS...ESPECIALLY WEST OF THE DIVIDE THIS AFTERNOON AND
EVENING. A FEW STORMS MAY BECOME STRONG IN THE WEST...WITH STRONG
WINDS BEING THE MAIN HAZARD. THERE IS ALSO A RISK OF LOCALIZED
FLASH FLOODING OR MUD SLIDES...MAINLY IN AND AROUND RECENT
WILDFIRE BURN SCARS.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

MONDAY INTO MONDAY NIGHT...STRONG TO POTENTIALLY HIGH WINDS EAST
OF THE DIVIDE...ESPECIALLY FROM ROCK SPRINGS TO CASPER.
SIGNIFICANT SNOWFALL POSSIBLE ACROSS THE NORTHWEST AND NORTHERN
MOUNTAINS.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS SHOULD REPORT HAIL...WINDS OVER 45 MPH...AND SIGNIFICANT
HEAVY RAINFALL OR OBSERVED FLOODING. THANK YOU.

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