Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 202058
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
258 PM MDT FRI MAY 20 2016

.SHORT TERM...Tonight Through Saturday

Models continue to support the idea that today will be the warmest
for a while. Showers and thunderstorms, especially across western
Wyoming will remain present and some storms could become strong with
gusty wind. This threat for the potential for enhanced storms may
also be found over Johnson county, with some storms already moving
off the Bighorn Mountains towards Buffalo. Shower activity will
weaken with the loss of daylight this evening, though some shower
activity will persist across the western mountains and valleys.

Saturday will see the advancement of the cold front through much of
the area. The front will move into western Wyoming late Saturday
morning/early afternoon, and will move east through the state. Ahead
of the front, another day of strong wind will be possible across
central and southern Wyoming, especially across Sweetwater, Fremont,
and into Natrona counties. As the front progresses, the wind shift
will accompany it, along with the development of additional showers
and thunderstorms.

.LONG TERM...Saturday Night Through Friday

Previous forecast and overall expected weather pattern is still on
track from Saturday through Wednesday. Thursday and Friday next week
is quite uncertain at this time given the large discrepancies in
the model solutions of the latest GFS and ECMWF, which will be
discussed later. Overall pattern for Sat through Wed is for a
continued large scale trough over the western US with multiple
shortwave troughs moving through the central and northern Rockies.
Saturday night will see the upper level low center moving from
Oregon into western Montana. Big story overnight will be the
cooling that occurs as 700mb temps of -4C moving into western WY,
which will drop the snow levels to the valley floors by Sunday
midnight. The cold front will move eastward through the state and
push through the Casper area Sunday morning. Temps Sunday will be
noticeably colder, and possibly colder than guidance indicates if
precipitation and cloud cover are stronger than expected. Sunday
should be brisk as well with more gusty westerly flow. Most
precipitation Sunday afternoon likely to be orographic in nature
across western and possibly northern WY.

Secondary shortwave trough drops along the west coast Monday to
bring the flow back more to the southwest across WY with 700mb temps
warming slightly to 0C to 4C.  Monday looks to be lightest
precipitation days with slight chance of showers and isolated
thunderstorms across NW WY.  As the trough drops into the Great
Basin Tuesday, instability and moisture look to increase across the
western half of the state, increasing the chance of showers and
thunderstorms.  Both the GFS and ECMWF drop this shortwave trough to
the Las Vegas area on Wednesday, with a northern trough moving from
western Washington into SW MT.  Thus, Wednesday also looks to be
active for showers and thunderstorms in the afternoon and evening
hours.

Where the models then disagree is Thursday into Friday.  The GFS is
currently wanting to show another trough dropping from Vancouver
along the west coast into northern CA by Thu afternoon.  This
results in a weakening of the troughs through the Rockies and less
activity for WY.  The ECMWF however has no such west coast trough
and deepens the southern trough through the 4 corners area, thus
pulling cooler mid level air over WY and assumably keeping
convective activity going.  Friday is actually similarly active in
both models but for opposite reason; either chilly unstable NW flow
from the ECMWF or warm moist southerly flow from the GFS.

&&

.AVIATION.../00Z ISSUANCE/

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

Showers and thunderstorms will continue this evening, with gusty
and erratic wind possible. Thunderstorm activity will weaken
after 03z this evening, though light showers will continue through
the overnight hours. Low pressure moving closer to western Wyoming
will send another push of moisture across the area after 15z
Saturday. A cold front will begin to move through the area
Saturday afternoon, spreading another round of shower activity
across the terminals. The cold front will generally move east of
the area by Saturday evening. VCTS has been included at numerous
terminals after 18z Saturday. Mountain snows and ongoing shower
activity will leave the potential for obscured mountain tops
through the TAF period.

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

Isolated showers and thunderstorms may continue through the early
evening hours, especially around KBYG. Elsewhere, coverage is not
anticipated to be significant, though some storms may produce
gusty and erratic wind at terminal sites. Additionally, it remains
difficult to completely rule out shower activity at those
terminals adjacent to shower receiving mountains, including KLND.
A rogue shower may remain possible this afternoon. The central and
northern mountains will see the potential for showers and
thunderstorms, with lowered ceilings to create obscured mountain
tops. Showers at the terminals will weaken after 06z this evening,
with VFR conditions then anticipated. The cold front will move
through the area after 18z Saturday, and is generally anticipated
to continue to move east through the remainder of the TAF period.
VCTS has been included at many of the terminals after 18z, with
some lowering ceilings. Showers and related mountain obscurations
will remain possible through much of the TAF period.

&&

.FIRE WEATHER...

Showers and thunderstorms continue across western and central
Wyoming this afternoon, with the strongest storm activity currently
noted over Johnson county. Activity will continue through the
evening, with thunderstorm coverage weakening with loss of daylight.
A cold front will sweep through the state beginning Saturday
afternoon. Western Wyoming will see the initial effects of the front
as it passes through, leaving cooler temperatures and shower
activity in its awake. Ahead of the front, breezy conditions are
anticipated across southern and central Wyoming through Saturday
afternoon. The front is anticipated to approach central Wyoming late
Saturday afternoon and into the early evening hours introducing the
opportunity for shower activity. The front will complete its
movement through the forecast area by early Sunday morning, with
widespread cooler temperatures expected. Afternoon shower and
thunderstorm activity will be found across the area through early
next week, with the best opportunity for development noted over the
western and central mountains.

&&

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

$$

SHORT TERM...Branham
LONG TERM...McDonald
AVIATION...Branham
FIRE WEATHER...Branham



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