Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231810
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1210 PM MDT Thu Mar 23 2017

.SHORT TERM...(Today through Saturday night)
Issued at 205 AM MDT Thu Mar 23 2017

Imagery shows ridge axis over the cntrl CONUS with a large upstream
trof over the Great Basin with low centered across NV and a
baroclinic leaf from nern UT through wrn WY. Hints at the beginning
of cyclogenesis also starting to show.  A modest jet pushing
into/through the trof`s base and over AZ, sern UT and wrn CO. At the
SFC, weak but slowly deepening low P now across UT, wrn CO and srn
and cntrl WY. isolated to scattered areas of precip...snow above
7000 feet...now spreading across much of WY except for the far nw
portions of the state.

This morning, as the large upstream trof deepens moves across the
Great Basin, both a strong jet as well as increasing sub-tropical
moisture have started to make their way through the Great Basin and
into southwestern and central WY from the southwest. POP chances are
increasing as well as the probable QPF values with the southwest
quad of WY, including the Wind River and Green Mountains receiving
the better moisture so far...both liquid and frozen (mainly above
7000 feet ATTM. Up to a couple of (melting) inches of snow may be
possible by sunrise across...particularly over this region`s
Mountains.

Today and tonight: The deepening and closing trof/low swings
into/through the Great Basin and portions of the Desert SW with what
looks to be setting up as a classic Four Corners Upper Low and
strong cyclogenesis proceeding across ern/nern CO by this afternoon.
Very strong low to mid-level frontogenesis will also take place to
the north of the low over central WY roughly from southwest to
northeast. This frontal zone will focus and then push to the
southeast through this afternoon and overnight. Much of the FA will
have decent chance for precip, except for northwest WY...some
convective with thunder this afternoon. Then, more
stratiform/upslope precip is expected by later this evening and
overnight. Best moisture convergence through the cyclone`s
maturation will come together this afternoon where frontogenesis is
occurring through central WY...then over the southeastern portion of
the FA from eastern Fremont through Natrona counties. Snow levels
still tricky and borderline with the snow level now around 7k ft and
may have to wait for near sunrise this morning  and again with the
cold frontal genesis/passage late this afternoon and evening before
snow appears east of the Divide down to the Basin floor. In addition
to the mountain Winter Wx Advisories over the Wind River and Bighorn
ranges, have issued/added an Advisory for zone 19 as most of its
elevation is above 6500 feet and snow is expected down to this level
by this morning. A few locations across Sweetwater county may also
exceed 3 inches of snowfall by this evening, however as it is not
not widespread, have decided to cover this in an SPS. Otherwise,
flood potential will also rise appreciably across southwest WY where
the current Flood Watch resides with 48 hr QPF values ranging
between 0.3 and 1 inch (highest where terrain or frontally forced)
for much of the area...also locally higher where convective elements
increase precipitation. As both Crow Creek and the Bear River area
already nearly bankfull (and others on the way up), an Areal Flood
Warning has been issued earlier this evening for Lincoln County.
Precipitation values along these same lines will also occur over
portions of the Wind River Basin, the southern Big Horn Basin as
well as over the Bighorn Mountains.

Friday through Saturday night: Post frontal and drying with upper
ridging moving back over the region from the southwest and modest
high P building across the surface. Upstream, another trof will be
moving through the wrn CONUS with the lead portion of the wave and
moisture  arriving over wrn WY later Friday night with another round
of moderately unstable conditions producing periods of both rain and
snow. Accumulating snow levels across wrn WY will likely range near
or above 7000 feet through Friday night...perhaps dropping to the
valley floors by Saturday morning. Trof axis will cross the western
border Saturday...clearing from west to east. Jet position during
the day (Saturday) will also be favorable for a while from west
central to north central Wyoming. Current snow amounts look to
remain below Advisory criteria.

.LONG TERM...(Friday through Wednesday)
Issued at 205 AM MDT Thu Mar 23 2017

Synopsis...A storm system will lift out of the southern/central
Rockies into the central High Plains on Sunday with precipitation
tapering off across central and eastern Wyoming Sunday morning.  The
next Pacific storm system will dig into the SW U.S. on Monday,
spreading valley rain and mountain snow showers into the west, with
a cold front spreading rain and snow showers south across central
Wyoming Monday night and Tuesday.  Mainly dry conditions are
expected on Wednesday.  Highs will mostly be in the 50s across the
central basins with 40s in the western valleys through the period.
Lows will mostly be in the 20s to mid 30s.

Discussion...Synoptic pattern on Sunday morning features second in
this series of Pacific troughs split over the Rockies with upper low
closing off over SE Colorado.  Shortwave ridge follows over the
Intermountain West with next upstream trough (#3) near 135W. 23/00z
ECMWF has trended toward GFS with this next trough splitting along
the west coast Sunday night, closing off and digging upper low
further south into Arizona/New Mexico Monday night and Tuesday.
Thus, forecast trends currently favor GFS with this particular
system. SW low may briefly cutoff Tuesday and Wednesday before
kicker system approaches NW coast on Wednesday, kicking SW low out
into the southern/central high plains on Thursday.  Difluent SW flow
will setup across western Wyoming on Monday before system closes off
with showers and a few thunderstorms increasing across the west.
Most favorable setup for central Wyoming will be Monday night into
Tuesday morning under southerly difluent flow aloft - cold front
dropping south ushering in northerly flow at the surface.   Kicker
system will dive into NW U.S. on Thursday and spread precipitation
chances back into the west.

&&

.AVIATION.../18Z ISSUANCE/

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

Mid-level circulation over Sublette County will continue to plague
KPNA and KBPI with IFR/LIFR conditions through at least 21Z/Thu.
IR imagery at 1730Z/Thu showing cooling cloud tops and expect at
least 3-4 more hours of these IFR/LIFR conditions as snow persists.
Low-topped convection has developed to the southeast of this
circulation and KRKS could see some rain and rain/snow showers this
afternoon with possible MVFR conditions. Downsloping northwest flow
should wrap-around the backside of the mid-level circulation late
this afternoon with gradually improving conditions at KPNA and KBPI
between 22Z/Thu and 03Z/Fri. VFR conditions will gradually spread
northwest-to-southeast from 06Z-12Z/Fri at all terminals. Could see
some late night and early morning VCFG if enough cloud cover clears
and gusty west-northwest winds diminish. Mountain obscurations will
be widespread through about 03Z/Fri with mountain top obscurations
continuing until about 10Z/Fri.

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

MVFR/IFR conditions will prevail through the 02Z-05Z/Fri timeframe
at the central terminals of KRIW, KLND, and KCPR. Colder air will
filter into the central basins as a large-scale upper low forms over
southern Colorado. KLND is going back-and-forth with rain/snow and
this will continue into the mid-afternoon. By 00Z/Fri, snow is
likely to be the predominant precipitation type at KRIW and KLND,
with KCPR changing over around that time or a little later.
Precipitation will gradually shift south of these areas by 06Z/Fri.
There could be a brief period of MVFR at KCOD this afternoon, but
confidence is not too high. Conditions will gradually become VFR
after 06Z/Fri with KCPR the last to hold on to the clouds and
precipitation. Would not be surprised to see some sunrise fog with
high boundary layer moisture and clearing sky. Mountain obscurations
will be widespread until 06Z/Fri with mountain top obscurations
lingering until 12Z/Fri.

&&

.FIRE WEATHER...
Issued AT 205 AM MDT Thu Mar 23 2017

Fire danger will be low through the weekend. There will be a few
chances for embedded thunderstorms across southern Wyoming today,
however the day on the whole will have increasing moisture spreading
across WY along with good chances for measurable precipitation at
many locations...both liquid and frozen. Friday night through
Saturday night the next spring/winter system will move through the
region with rain and snow most likely west of the Divide...isolated
to widely scattered east. Smoke dispersion will be fair to good most
afternoons.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT tonight for WYZ019.

Flood Watch until 1 PM MDT this afternoon for WYZ013-025-026.

Winter Weather Advisory until midnight MDT tonight for WYZ008-
009-014-015.

&&

$$

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


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