Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190601

Area Forecast Discussion
National Weather Service Riverton WY
1159 PM MDT Tue Apr 18 2017

.SHORT TERM...This evening through Friday

The next shortwave/vort lobe that has been producing
thunderstorms over Nevada will be tracking rapidly to Wyoming
tonight. As it amplifies, a well defined mid level circulation
will become established and track northeast from Sweetwater County
northeast to Natrona County tonight. The airmass over the CWA is
destabilizing with increasing cape and lowering lifted indices so
we should witness at least isold ltg through early this evening.
QG forcing asct with the low, and lift from a jet couplet, should
all add up to significant precip of around one half inch along
the track of the mid level low with less precip on either side of
the track. The Big Horn Mountains should be just close enough of
the swath of heavier precip to capture enough snowfall above 8500
feet to justify a low end winter weather advisory as elevations at
pass level where upwards to a half a foot of snow from midnight
tonight through 1 pm Wednesday will likely occur. Hence, an
advisory has been issued. Some rain and snow showers will linger
through the day Wednesday with nnw flow converging with wnw flow
encouraging an enhancement of shower activity along the Owl Creek
Mtns Wednesday. Natrona and Johnson Counties will be the last to
clear out Wednesday.

Wednesday night will be precip free.

Then on Thursday, the next well defined trough will arrive as a
splitting negatively tilted trough with much of the energy taking
aim at the Four Corners region of the country. This trough, as
many other splitting troughs have done in past weeks, will eject
another well defined mid level low northeast to eastern Fremont
County Thursday evening and then will just wobble around Natrona
County all day Friday with the GFS depicting as much as 4.5 inches
of precip falling on the Big Horn Mtns on the backside of the low.
In any event, it will be a soaker east of the divide with a
persistent narrow band of isentropic lift depicted along a
relatively narrow band extending from Johnson to Natrona County,
with the flow from the east. This system bears watching with the
mountain zones potentially receiving ample amounts of snow,
especially the Big Horn Mountains. Snow levels should generally
range from 8500 feet late Thursday to 6k feet by Friday. For now,
have indicated mainly snow amounts ranging from advisory amounts
in the western mountains to a foot and a half in the Big Horns
from the period Thursday through Friday.

.LONG TERM...Friday night through Tuesday

Synopsis...Rain and snow showers will continue Friday night/early
Saturday morning (mainly east of the Divide), as the low pressure
system continues its trek over the Central Plains. Conditions
will improve after sunrise Saturday morning, as a ridge of high
pressure builds over the region. An upper level disturbance will
impact the area by Sunday afternoon. Isolated to scattered
showers, with a few thunderstorms in the afternoon, will develop
across the west and spread across northern portions during the
overnight hours. Showers will linger across the northern and
western mountains on Monday, while the rest of the area remains
dry. The next Pacific storm will spread mountain snow and valley
rain across the forecast area on Tuesday.

Discussion...A period of heavier precipitation will occur over the
Bighorns and Johnson County Friday evening, as a mid-level
circulation trails behind the Central Plains storm. This circulation
will move southeastward from north-central WY to southeast WY
overnight Friday. This will be in addition to the rain and snow
showers that will be ongoing at the start of the period. Models may
be finally showing some consistency, as the ECMWF continues to prog
the next system to be flatter, while the GFS wants to dig this
trough somewhat as it traverses over the Northern Rockies Sunday
night/Monday morning. Kept isolated to scattered PoPs across the
west and north, along with isolated thunderstorms Sunday
afternoon/evening. The pattern remains a bit progressive for
Tuesday, with models spreading precipitation across the area after
12Z Tuesday. The best chances for remain across the west so far,
with isolated showers and PM thunderstorms east of the Divide.


.AVIATION...For the 06Z TAFS through 06Z Thursday


Ongoing mountain snow/valley rain should exit much of the area
between 09Z and 12Z Wednesday. In wake of the precipitation ending
there could be a few more hours of low ceilings or even fog
formation. However by 16Z, most locations are expected to VFR
conditions with sct to bkn stratocumulus. Breezy to Windy northwest
to westerly wind are expected in the Upper Green River Basin and
Sweetwater County. Wednesday night, the main concern will be the
possibility of fog formation, but increasing high cloudiness should
limit chances for fog.


The bulk of the ongoing mountain snow/valley rain early this morning
is expected to end from southwest to northeast between 09Z and 15Z.
MVFR ceilings or vsby can be expected with this activity and could
linger a few hours after the precip has ended. The associated weak
cold front is expected to push south across the area this morning,
shifting winds generally to the northwest and north as well as
becoming breezy. By 18Z, most terminals should be low VFR conditions
with bkn-ovc stratocumulus deck between 035-050 kft agl. There could
also be isolated convection in the afternoon, as there is some weak
instability, but not enough to include in area terminals for now.
The stratocu deck is expected to quickly dissipate by around dusk
with mainly SKC conditions expected. Also winds are expected to
weaken around 00Z Thursday.



Fire danger low across the forecast area through the work week.
All fuels currently in green-up. Periods of rain and high country
snow will occur periodically throughout the week. Isolated
Thunderstorms will also be possible for the rest of this
afternoon and evening in addition to Thursday afternoon and
evening. Heavy rain showers possible later today along and near a
front and developing surface low pressure system from Sweetwater
County through eastern Fremont County and into Natrona and
southern Johnson Counties this evening through Wednesday morning.


Winter Weather Advisory from midnight tonight to 1 PM MDT
Wednesday for Wyz008-009.



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