Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 280507
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1107 PM MDT Mon Mar 27 2017

.SHORT TERM...(This evening through Thursday) Issued at 146 PM
MDT Mon Mar 27 2017

Imagery continues to show a very progressive 5 wave pattern with a
multitude of springtime highly baroclinic embedded waves moving
through a general longwave. Upstream trof just moving through ID
with lead portion of the wave pushing into ern ID/wrn WY. SFC has
elongated area of low P developing into/over the northern to
southern Rockies with with ongoing frontogenesis just to the west of
this area. Light precip just now moving into/through ern ID/nern UT.

Today, post frontal with weak ridging aloft moving east into the
Plains ahead of the next system/trof due to arrive this
afternoon/evening across western/southwestern WY. The intermountain
trof makes its way into/through wrn WY later this afternoon...while
the bottom portion of the trof digs southward and closes off near
the 4 corners area by Tuesday morning. The better energy with this
system will stay south of the FA...however, a decent return of
moisture still looks good up through eastern CO and into/across
eastern/central WY early Tuesday morning. With frontogenesis going
on west of and over southern WY tonight and early Tuesday morning,
temperatures will be border line over the lower elevations but will
be cold enough above 6500 feet east of the Divide to have rain mixed
with or a changeover to all wet snow at many locations around
midnight tonight.  These conditions will continue (at least) into
the late morning/early afternoon hours on Tuesday before changing
back over to rain or a rain/snow mix. Portions of southern to east
central WY, including portions of Sweetwater, Fremont and Natrona
counties could see a few inches of wet heavy snow through Tuesday
morning...with heaviest snow generally expected near/north of the
front in the slightly colder air. Winter Wx Advisory highlights have
now been issued for zones 8, 9, 19, 28, 29 and 30 which includes the
southeastern portions of Fremont county...the southwest portion of
Natrona county...and most of Sweetwater county. These Advisories are
partially impacted based for a few inches moderate wet snow mixing
with winds gusting 25+ mph at times, while above 6800 to 7000
feet...snow amounts may very well support Advisory amounts. Tricky
forecast with such borderline temps.

System exits Tuesday night into the CO high Plains and Central
Plains with high P and SW ridging and clearing skies returning to
the FA until Thursday morning. Then, the next in a series of
shortwaves heads inland from the EPAC with a new moisture feed ahead
of the trof making its way into wrn WY along with a weak embedded
lead wave. Through the boundary layer, upstream frontogenesis will
be proceeding through the day Thursday...arriving just west of WY
over eastern ID by the end of the forecast period. While there may
be a few late morning snow showers out west down to valley
levels...as the better moisture arrive in the afternoon...snow
levels will rise to near 7000 feet before dropping back down in the
evening.

.LONG TERM...(Thursday night through Monday) Issued at 146 PM MDT
Mon Mar 27 2017

Next storm system currently out near 48N/170W will be moving into
the Great Basin to start this period and then dig towards the 4-
corners region on Friday. 5-wave pattern shows mean ridge over the
Inter-mountain west which may keep this system from reaching its
full potential or shunting it slightly further south (like the last
one). Current GFS is not that impressive this far north (just the
far sern zones look good) and temps are borderline at best for
any significant snow in the lower elevations. ECMWF is in a
similar location but colder with more pcpn in Central Wyo. System
our near 38N/150E looks to be the system to help pump up the ridge
behind this system. Will just have to continue watching this
closely. Precip seems like a reasonable certainty for much of the
area but whether its significant and in the form of snow for the
lower elevations is tbd. Will continue idea of high pops and
cool/chilly temps on Friday with clearing Friday night into early
Saturday from north to south. The far north will see significant
improvement later Friday night with decreasing wind and not a bad
day Saturday. In the far south, ne winds increase Friday into
Friday night and if the rain changes to snow there could be
several hours of `blizzard` conditions late Friday afternoon or
evening depending on the eventual timing of the upper low. Brief
shortwave ridging Sunday ahead of our next system. The next in a
series of digging upper lows is likely the system out near
52N/150E and this one may be aided by the bigger picture mean
trough over the west. This upper low will begin impacting the
Monday into Monday night and then tracking east of the divide. New
Euro is now an open wave with this system while the GEM and GFS
remain a fairly significant digging and strengthening system.
Early on we lean towards a stronger system which fits the bigger
picture but it`s a week away yet so we`ll see how it develops.

&&

.AVIATION.../06Z ISSUANCE/

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

Complex forecast tonight with first batch of precipitation already
north of the region. Additional development occurring over northeast
Utah may reach KRKS late tonight, but its looking more and more like
precipitation will wane by 15Z/Tue. Four Corners low may end up
eating the precipitation and keeping it to our south and east by
late Tuesday morning. IFR or lower ceilings may persist at KRKS
until that time, but confidence in reduced visibility and prolonged
precipitation is waning. After 15Z/Tue, conditions should gradually
improve to low end VFR at KRKS with VCSH by 20Z/Tue. Northerly wind
will respond to tightening pressure gradient with 20-30kt winds for
a good portion of the day Tuesday at KRKS. Conditions will be
generally VFR at KBPI, KPNA, and KJAC, with the best chance of any
rain or snow coming prior to 14Z/Tue. Confidence is not high and
expect VFR at all three terminals through the period. Gusty
northwest wind will be favored at KBPI and KPNA from mid-morning
through about 00Z/Wed. Speeds of 12-25kts will be common at both
terminals. Any mountain obscurations will diminish from northwest-to-
southeast during the late morning and afternoon. There could be some
lingering mountain top obscurations until 00Z/Wed, primarily in the
vicinity or KRKS and along the Continental Divide.

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

Complex pattern evolving across the region late this evening. Wave
of mid-level moisture has spread north across the forecast area this
evening, but low-level upslope flow has been slow to develop. Thus,
most of this moisture has been little more than a few sprinkles from
a mid-cloud deck. Best chance for precipitation and MVFR conditions,
or perhaps briefly IFR, looks to be from about 10Z-16Z/Tue. There is
an increasing possibility that KCOD and KWRL will stay mainly VFR
once this first wave clears northern Wyoming after about 15Z/Tue.
Dynamics and moisture will begin to move to the eastern half of the
state after 18Z/Tue with gradually improving conditions from west-to-
east. This will leave KCPR with the best chance of precipitation,
mainly rain, during the day Tuesday. Light rain looks most likely
between 12Z-22Z/Tue, with mainly MVFR conditions. Favored northerly
wind at KCOD, KWRL, and KRIW will blow 12-25kts before diminishing
around 02Z/Wed. Mountain obscurations will be widespread until about
18Z/Tue, with mountain top obscurations persisting along and east of
the Bighorn Range through 02Z/Wed.

&&

.FIRE WEATHER...
Issued AT 146 PM MDT Mon Mar 27 2017

Fire danger will remain low through much of the week. A cool and
very progressive unstable pattern will overtake the forecast area
and will remain in place over the next week or so. Periods of
precipitation will move across WY every other day or so along with
several frontal passages. Temperatures will be seasonal to slightly
below seasonal for much of the time with gustiest winds accompanying
frontal passages. Smoke dispersion will be fair to good most
afternoons.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Tuesday for WYZ008-009-019-028>030.

&&

$$

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



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