Area Forecast Discussion
Issued by NWS Riverton, WY

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449
FXUS65 KRIW 191722
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1122 AM MDT THU MAY 19 2016

.SHORT TERM...Today through Saturday night

Imagery shows a similar pattern to the last few days, with a
longwave trough stretching across most of the CONUS and the axis
located from north of the eastern Great Lakes back to the southwest
and through southern Arizona. An open wave trof is embedded across
the central and southern Rockies with shortwave ridging just to the
west across the Great Basin and a large deep trof just now moving
into the PAC NW. Surface has weakening high p across WY with lee
side troughing just beginning over central WY. A few light showers
across southern WY with isolated showers and thunder moving through
the extreme northwest corner of the state.

Tonight/today, upper pattern finally becomes mobile...pushing the
trof into the high/central plains of Colorado and Kansas while the
large upstream trough moves into the PAC NW and northern Rockies.
Upper level flow begins to strengthen...increasing ascent across
western WY by this afternoon ahead of the trough while lee side
troughing begins to deepen over WY. Then, relatively weak low
pressure tries to spin up on the trough across central Wyoming ahead
of a quickly approaching front from the west. For a period of time
this afternoon and evening, in a thin "warm sector" just ahead of
the front, a few strong to marginally severe storms will be possible
across western/southwestern WY. Strong/severe wind gusts will be the
mostly likely form of severe with DCAPE values likely to approach
1000 J/KG while storms will already be moving at 30 to 35 kts from
the southwest. Some marginally large (.75 to 1 inch) hail will also
be possible - with VILs around 40 looking to produce near severe
based on storm tops of around 35k ft. Localized flash flooding may
also occur with a few of the stronger storms hitting high terrain
features. Better post frontal precipitation will follow this
activity into the night time period.

This evening, the front will push into/through western Wyoming
before fizzling out, with good low to mid level forcing combining
with the terrain and increasing PV aloft, squeezing out good precip
amounts from the moisture feeding into the area from the EPAC/PAC NW
with PWs increasing from around 0.45 to 0.65 inch. Snow levels,
however, will remain fairly high, at or above 10000 feet to start,
only falling to between 9000 to 9500 feet by early Friday
morning...with mixed precip perhaps down as low as 8500 feet. So,
while QPF will be decent, snowfall should be limited...well below
advisory levels for all elevations. By Friday morning, what`s left
of the the front should wash out across central and eastern WY as it
moves away from the upper level support of the main trough while the
trough itself slows down to a crawl as a weak blocking type pattern
develops. With the only good forcing west of the divide now coming
from the mid/upper levels around the trough base with the approach
of an associated jet, precipitation chances Friday afternoon/evening
will generally be confined to the mountains of the west with
isolated to widely scattered very fast moving showers and
thunderstorms storms cruising to the north at 40+ mph. These storms
will naturally produce quite a bit of wind, but much less in the way
of rainfall than the day before. There may be some severe gusts from
these storms just based on storm movement/speed itself. However,
shower/storm coverage should be isolated to widely scattered at best
...dying off through the evening period. Low pressure will try to
spin up once again over central/northern Wyoming Friday...producing
an unstable warm sector across eastern WY...with strong to severe
storms possible across southeastern WY. Depending on how close to
our eastern counties a local lee side trough is, an isolated strong
storm could possibly hit extreme eastern Natrona county. Not likely
however. Gusty south winds across southern and central WY will also
be a consequence of this set up.

Saturday and Saturday night, different day, similar results. The
trough does move east some while digging into the desert southwest
letting some stronger jet level flow spread over WY. Both lee side
troughing and frontogenesis across the west beginning, with chances
for showers/thunder now bookending the forecast area in the
afternoon. These storms will again be moving at a good clip from the
south or south southwest...all able to produce very gusty winds with
at least some potential for isolated severe gusts to occur.
Convective activity will decrease, but not completely disappear
overnight with continued upper level support keeping isolated
showers around for an extended period.

.LONG TERM...SUNDAY THROUGH THURSDAY

The surface cold front should be across the eastern portion of
the forecast area...or already through the entire area by Sunday
morning. Most of the upper level energy will lift into the high
plains Sunday. Despite this, enough moisture will be available
for isolated to scattered showers with possibly a few
thunderstorms across much of the area, with an emphasis across
the west. A longwave trough will remain across the west Monday
through Wednesday with a continued chance of convection each day.
Forcing remains weak Monday and Tuesday, so much of the activity
will be more than likely diurnally driven. By Wednesday, global
models indicate a distinct shortwave trough moving across the
Rockies. However the intensity and track remain uncertain with the
ECMWF the weakest and farther north across the northern Rockies
while the GFS shows a track across the Four Corners area which
would have a bit more impact on the forecast area, and even the
GFS does not show anything major at this time. Models then try to
build a transitory ridge into the area Thursday/Friday...ahead of
a decent trough pushing into the West Coast Friday.

Temperatures are expected to be below average to seasonal for the
extended with the coolest temps occurring Sunday and Monday.

&&

.AVIATION.../18Z Issuance/

West of the Continental Divide...KJAC/KRKS/KPNA/KBPI Routes

Southerly flow will increase Thursday afternoon ahead of the upper
low over the Pacific Northwest, located near the WA/BC border at
late morning. Jet streak seen in water vapor imagery over northern
California will round the upper low later Thursday. This will enhance
convection that forms over central and eastern Idaho Thursday
afternoon. Mid and upper-level difluence also expected to increase
during this timeframe across eastern Idaho and western Wyoming.
Strong convection, with gusty outflow wind being the main hazard,
looks to be most prevalent from about 21Z/Thursday to 03Z/Friday.
KJAC the most likely terminal to experience TSRA with associated
MVFR conditions during this 6-hour period. Have also included
TEMPO groups at KBPI and KPNA, but confidence is higher at KJAC.
Looks as though convection, albeit weaker, will continue through
06Z-09Z/Friday across northwest Wyoming. Expect mountain top
obscurations to prevail along and east of the Divide from
21Z/Thursday until 09Z/Friday. Other issue will be increasing
south-to-southwest wind this afternoon with the highest speeds
of about 15-25kts anticipated at KRKS. Southerly winds will also
pick-up Friday morning as pressure gradient tightens ahead of
upper low.


East of the Continental Divide...KCPR/KCOD/KRIW/KWRL/KLND Routes

South-to-southwest winds will increase east of the Divide
Thursday afternoon with the highest speeds of 15-25kts at KCPR.
KRIW and KLND will increase to 10-20kts after 21Z/Thursday as
full mixing occurs. Speeds will decrease overnight and then
pick-up again late Friday morning. Best chance for TSRA Thursday
will be at KCOD where convection will move off the Absaroka Range.
have also continued VCSH at KRIW, KLND, and KWRL, while KCPR looks
to remain dry with little chance of convection. Convection will
wane with the setting sun, but expect SSW-NNE oriented
precipitation axis to persists overnight as it moves east from
western Wyoming. This will necessitate VCSH in KCOD terminal
through the night.


&&

.FIRE WEATHER...

Fire danger low today as green-up and elevated humidity levels
continues across Wyoming. Strong showers and isolated severe
thunderstorms will be possible today, mainly west of the divide with
some storms producing localized heavy rainfall. More widespread
moderate to heavy rain possible tonight, especially across the west -
along and behind a front. Otherwise, warming and drying
continues...persisting east of the divide both Friday and Saturday.
Shower/thunder chances will then continue across the west and
extreme east through Saturday...with the central basins remaining
more on the dry side. Gusty south winds will be around both Friday
and Saturday across southern and central Wyoming. Cooler
weather across the entire state Sunday.


&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

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



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