Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 301749
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1048 AM MST Wed Nov 30 2016

.SHORT TERM...Wednesday through Friday night

Things are mainly quiet this morning with a ridge across the area.
And for the most part things should stay fairly quiet through the
next 24 hours. There will be some weak warm advection out west and
this will bring some light precipitation, emphasis on light. Amounts
will remain below 2 inches in the mountains and under an inch in the
valley. In this part of the state, it is called Wednesday. Mainly
dry conditions will prevail East of the Divide. There will be the
typical southwesterly breeze across the southwestern wind corridor,
but nothing unusual. Temperatures should be fairly close to normal.

A trough and cold front will approach the area for Thursday. Again,
there is not a lot of moisture associated with it. There will be
more light snow across the west but nothing they can`t handle. We
still expect a weak closed mid level low to form and bring a
brief period of upslope to the Lander Foothills. However, there is
not a lot of moisture to work with so we kept the continuity of
high POPS and low QPF. This trough will move away for Friday with
improving conditions as another shortwave ridge builds across the
area. Another trough will then move into western Wyoming later
Friday night with the next chance of snow that will last into
Saturday.

.LONG TERM...Saturday through Wednesday

Synopsis...A disturbance will spread light snow showers across the
northern mountains and far north on Saturday.  Strengthening westerly
flow will bring increasing snow and wind to the west Sunday while
windy and mild conditions prevail east of the Continental Divide. A
strong cold front will push across the area on Monday with a much
colder Arctic airmass sliding into the area Monday night through
Wednesday.

Discussion...Medium range models in better overall agreement with
timing of cold frontal passage through the area on Monday followed
by cold upper level trough and a cold dome of surface high pressure
building into area Monday night and Tuesday.

The main weather maker during this period will be upper trough
across the western Pacific towing some Siberian/Arctic air our way.
This trough will move onto the NW coast Sunday morning with strong
westerly difluent flow developing downstream.  This setup will
likely bring a period of moderate to heavy snow across the west late
Saturday night through Sunday afternoon.  Upper trough will dig into
the intermountain west Sunday night with associated cold front
pushing across Wyoming.  Snow should decrease and become more
showery across the west with the frontal passage with some snow
showers spreading east of the divide with the front.

ECMWF and GFS have their differences with development of upper
low over the northern High Plains on Monday, but both models keep
H7 flow westerly south of the low across Wyoming. So this should
have a moderating effect on temperatures with brunt of Arctic air
seeping in Monday night with N-NE low level flow. Surface high
pressure and modified Arctic airmass settle into area Tuesday
morning with H7 temps -18c to -20c. Low temperatures will be
mainly in the single digits, with sub-zero readings in the
mountains and west Monday night. Most high temperatures on Tuesday
are expected to be in the teens with single digits in the
mountains. The area should see the coldest temperatures Tuesday
night, expected to be -10F to -15F in the western valleys and 5
below to 5 above zero across the central basins. Upper ridge is
expected to build into the area Wednesday, with some stratus and
light snow wedged into Far West. Milder Pacific air may be
advecting in at mountain top level Wednesday, but shallow Arctic
air will likely remain entrenched across the lower elevations.

&&

.AVIATION.../18Z ISSUANCE/

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

VFR conditions are still expected for the next 24 hours east of the
divide.  Clouds will increase during the period as weak energy moves
through the area overnight.  Wind should be relatively light, expect
for KCPR with SW flow at 20 knots gusting to 30 knots this
afternoon.  Expect some areas of precipitation Thursday night.

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

Weak energy with moisture from the west continues to influence
western WY today and overnight.  Light snow expected to continued
for the next 24 hours along far western WY and around KJAC, with
potentially heavier snow later this afternoon.  KJAC expected to
waffle between VFR and MVFR conditions. Some snow showers also
expected at other terminal sites across western WY this afternoon as
weak wave moves from west to east. Potential for heavier snow again
late Thursday morning around KJAC as another wave moves into western
WY. Mountains definitely obscured through about 18Z/Fri.  Winds
generally light across the area except for SW to W flow over 15
knots across southwest WY.

&&

.FIRE WEATHER...

The west should be mainly cloudy today with some snow showers but
little accumulation is expected. Dry conditions should dominate East
of the Divide with a gusty southwest breeze from the Red Desert
through Natrona County. Mixing and smoke dispersal should be
generally fair to poor. Another upper level disturbance will
approach the west on Thursday with another chance of light snow.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Hattings
LONG TERM...AEM
AVIATION...McDonald
FIRE WEATHER...Hattings


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