Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200602 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1202 AM MDT FRI MAY 20 2016

.UPDATE...Added 06z Aviation Discussion.

&&

.AVIATION.../06Z ISSUANCE/

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

Widespread showers will continue to move into southwest Wyoming
through 9-10z with widespread mountain obscurations and MVFR
conditions vcnty KBPI-KPNA-KAFO.  This activity should gradually
weaken and lift northward, mainly east of KJACc.  Some patchy fog
may follow through 14z, partially obscuring terrain.  Scattered to
numerous showers and isolated thunderstorms will redevelop south to
north along and west of the divide Saturday afternoon with mountain
obscurations increasing with local MVFR conditions possible at KJAC,
KBPI and KPNA.  This activity will diminish in the late evening and
overnight Friday night.

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

Isolated to scattered showers across central Wyoming will gradually
dissipate 8-10z.  South winds will increase to 20-30 knots across
much of the area Friday afternoon.  A few strong thunderstorms with
large hail and damaging winds will be possible along and north of
Interstate 90 Friday afternoon and evening.  Otherwise, isolated
showers and thunderstorms will be high-based across most of central
Wyoming with the main hazard of strong erratic wind gusts over 40
knots.  Winds will diminish Friday evening and drier air will result
in showers dissipating and some clearing overnight.

&&

.PREV DISCUSSION... /issued 333 PM MDT THU MAY 19 2016/

SHORT TERM...Tonight Through Sunday

Moisture and instability associated with the area of low pressure
over the Pacific Northwest will continue shower and thunderstorm
activity this evening. Threats of gusty wind, frequent lightning,
and small hail will linger into the evening hours. This scenario
will remain strong enough to keep some isolated thunderstorm
activity through the overnight hours, with convection finally
weakening Friday morning. The break from thunderstorms will be
fairly brief, with a return of moisture and instability expected
Friday afternoon. Similar to today, activity will be generally
present over areas west of the Continental Divide, though some
activity may spill over into the Cody foothills through Friday
evening. Gusty wind looks to be the greatest threat with activity on
Friday afternoon, with diminished coverage compared to this
afternoon and evening.

The area of low pressure resulting in these showers and
thunderstorms will move into Canada by early Sunday morning.
However, a cold front will move into the area late Saturday and into
Sunday to bring slightly cooler temperatures, and showers across the
area. These showers will be fairly widespread by Sunday afternoon.

Hydrology concerns over the next few days do remain. Models indicate
a few days of decent warming, which will help to increase snowmelt
across the area. The majority of rivers, for now, are capable of
handling the amount of snowmelt expected, though some rivers across
central Wyoming will approach, if not exceed action levels. If the
melting is more efficient than currently expected, additional river
concerns may be possible. Stay tuned for future updates.

LONG TERM...Sunday night through Thursday...

By early Sunday evening the main trough axis will finally push
through the area for one last "hurrah" of convection, mainly across
the western and northern portions of Wyoming accompanied by
scattered thunderstorms along with some snow in the higher northern
mountains. The main trough axis will be exiting the area Monday but
will still be close enough for additional shower activity late
Sunday night with snow at the higher elevations in northern Wyoming
with some showers lingering through Monday morning and scattered
afternoon thunderstorms Monday, mainly in western Wyoming. Then
Tuesday and Wednesday will feature a continued chance of afternoon
thunderstorms, mainly in western wyoming, as another trough to the
west establishes itself and an unstable southwest flow gets
underway, but without sufficient lift from any particular passing
shortwave we will witness less coverage of the convective activity.
By Thursday, another closed low will form in the Pacific Northwest,
thus increase thunderstorm activity, again mainly in the west, as
the southwesterly flow increases and becomes increasingly difluent.
In fact, a closed low is progged to form just west of YNP Friday
which could mean sufficient precipiation in NW Wyoming by then. This
next upper level closed low could very well turn out to be be our
"semi annual Memorial Day weekend storm." Temperatures will
gradually warm through the week.

AVIATION.../00Z Issuance/

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

Ongoing convection over eastern Idaho will progress east later
Thursday afternoon aided by difluence aloft and jet streak tracking
across northern Nevada and mid-afternoon Thursday. 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 south-to-
southwest wind of 12-22kts Thursday afternoon. 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.
These winds will continue to increase through the afternoon with
widespread 15-25kt speeds...25-35kts at KRKS. Enough moisture and
instability to generate showers over the higher terrain of the west
Friday afternoon. Therefore, have included VCSH at all terminals
except KRKS where drier southwest flow should limit convection. Main
TS hazard would again be gusty outflow winds.

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

South-to-southwest winds will increase east of the Divide
late Thursday afternoon with the current speeds of 15-25kts
remaining at KCPR. Speeds will decrease overnight and then pick-up
again late Friday morning. Best chance for TSRA into Thursday
evening 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 persist overnight as it moves east
from western Wyoming. This will necessitate VCSH in KCOD terminal
through the night. KCOD and KLND will be the most likely locations
for VCSH Friday afternoon due to their foothill locations.
Otherwise, expect drier south-southwest flow to limit convection
Friday afternoon. Main concern will be increasing south-southwest
surface wind between about 17Z-19Z/Friday. Looks to be widespread
15-30kt speeds Friday afternoon.

FIRE WEATHER...

Showers and thunderstorms will remain possible through the overnight
hours. A reprieve from thunderstorm activity will be possible early
Friday morning, though another surge of moisture and instability
could result in more thunderstorms across portions of western
Wyoming. These thunderstorms are anticipated to produce gusty and
erratic winds that could be strong. A cold front is anticipated to
move through the area late Saturday into Sunday morning, causing a
more widespread wind shift, cooler temperatures, and more widespread
shower activity.

&&

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

$$

SHORT TERM...Branham
LONG TERM...Lipson
AVIATION...AEM
FIRE WEATHER...Branham



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