Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181721
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1121 AM MDT Tue Apr 18 2017

.SHORT TERM...(Today through Thursday night)

Imagery shows general longwave ridging across the CONUS with an
embedded sw trof moving over MT/WY and a large upstream trof still
hanging off the west coast (EPAC). Under the large offshore trof,
another stream of copious moisture is headed toward WY from out
of cntrl CA and NV with a jet from nrn CA through nwrn NV/ID and
into MT/nwrn WY. SFC has backdoor cold front through WY stretching
from the nern corner to the sw. No current precip falling within
WY`s boundaries...with MCS forming and rolling across SD.

Today: The morning starts out under ridging...transitioning to sw
flow aloft...early with an embedded disturbance moving toward WY`s
western border through srn ID. The SFc will have inverted low P
forming to the s/se over CO and building into ern WY through the
day. Precip chances will also increase west of the Divide offering
up some morning snow mainly above 7500 feet with perhaps 1 to 3
inches by noon...mainly rain or rain/snow mix below 7500 feet.
Frontogenesis is expected to be ongoing through the day east of the
Divide with chances for mainly afternoon showers and isolated
thunder (CAPE 200-500 j/kg) near and south of this area...much
lesser chances to the north. Through or into Wednesday morning the
front should remain in roughly the same place as a stationary
front...providing the boundary around which much of the
precipitation will fall. Potentially, areas of moderate to heavy
rainfall (QPF > 0.50 in in some locations?) may occur Wednesday
during the morning hours along/near this boundary as an embedded sw
and right exit region of the jet induce good upper level forcing
before finally pushing the front out of the FA and WY by mid-day.

Wednesday afternoon, sw ridging occurs with only terrain forced
showers around some higher mountain locations Wednesday
afternoon/eve. By Thursday morning, the EPAC trof makes its way
across the Great Basin becoming somewhat negatively tilted, headed
toward/into the wrn portion of the FA. Precip chances increase
across the entire CWA Thursday and Thursday night as the upper trof
induces SFC lee side troffing and eventually lee cyclogenesis
somewhere over ern/sern WY under cold air aloft and more convective
circumstances. The terrain above 6000 feet will have the chance to
see some snow with locations above 7500 feet seeing anywhere from 2
to 8 inches of new snowfall by sun-up Friday. Highlights possible
for mountain locations beginning Thursday night and continuing
through the day Friday.

.LONG TERM...(Friday through Tuesday)

Synopsis...A low pressure system over the central Rockies will
likely bring widespread precipitation with snow levels between 6000
and 7000 feet across central and southeast Wyoming, south of a Cody
to Buffalo line, through Friday morning. This system will move out
into the central and southern High Plains Friday afternoon, with
showers becoming more scattered across the lower elevations, more
numerous snow showers over the mountains.  A ridge of high pressure
will bring dry and milder conditions to most of the area on
Saturday.  An upper level disturbance will spread isolated to
scattered showers along with a few thunderstorms across the area
Sunday afternoon/evening.  Dry conditions are expected to prevail
across most areas on Monday.  The next Pacific storm system will
spread valley rain and mountain snow showers east across the area on
Tuesday.

Discussion...Synoptic pattern at the outset on 12z Friday shows
upper ridge along the West Coast flanked by upper lows over the
eastern Pacific and Central Rockies.  ECMWF and GFS show a few
embedded mid-level circulations within the larger trough over the
Central Rockies, placing one of these circulations somewhere in the
vicinity of Casper Friday morning.  Inverted surface trough is
forecast to extend from eastern Colorado northwest along the Bighorn
Range, with a band of heavier precipitation along this trough axis.
This storm system will become more organized over the
central/southern High Plains Friday afternoon, but one trailing mid-
level circulation lurking over the northern Rockies is expected to
pulled down along and east of the Wyoming Divide Friday night.  This
secondary system will bring another round of numerous mountain snow
showers with scattered rain/snow showers over the central basins,
though some more organized banding may develop downwind of NNW-SSE
oriented ridgelines.  Upper ridge along the west coast is expected
to translate the Rockies on Saturday, followed by eastern Pacific
trough splitting, with a portion moving across Wyoming late Sunday.
ECMWF is flatter/weaker than GFS with this system, but neither model
has shown any degree of consistency to trend forecast one way or
another.  For now, spread higher PoPs into the northwest Sunday
afternoon, with at least a slight chance of a shower or thunderstorm
other areas Sunday afternoon/evening.  By Tuesday, broad ridge
builds across the eastern Pacific with strong Pacific jet stream
expected to carve a trough across the Great Basin/Rockies through
the day Tuesday, bringing a period of cooler and unsettled weather
through the mid-week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1121 AM MDT Tue Apr 18 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Unsettled weather will continue over the area this afternoon and
tonight as rain and mountain snow occurs. Expect areas of MVFR
conditions and mountain obscuration through the period, especially
after 00Z.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail at the terminal sites through 00Z.
Scattered rain showers will develop after 18Z with isolated
thunderstorms possible through 03Z. The north will see isolated
showers this afternoon into this evening. Widespread rain will
develop over central and southern WY after 01Z and continue
through 12Z Wed. Expect MVFR conditions to prevail in these areas
tonight.

&&

.FIRE WEATHER...

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 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.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...LaVoie
FIRE WEATHER...Braun



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