Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220503

Area Forecast Discussion
National Weather Service Riverton WY
1102 PM MDT WED SEP 21 2016

.SHORT TERM...Tonight Through Saturday

Significant storms system remains on track to impact the area
through much of the short term. The moisture from the remnants of
Hurricane Paine has juiced up the atmosphere over most of the area
to our ssw/sw with precipitable water values of 1.5 to over 2
inches in the southwest. PW values from .75 to 1 inch are expected
across the area Thursday with a ribbon across the middle of our
area that may reach even higher. Some of those values put us in
the 99th percentile. With all the recent fires and associated burn
scars and potential for heavy rain with some repeat/persistent
showers/tstms...we have issued a flash flood watch for the western
mountains for Thursday afternoon through Thursday night. Front
moving into the west Thursday while forcing begins to increase as
cyclonic nose of the upper jet moves into the southwest. The
amount of energy rotating nwd around the upper low increases with
time tonight into Thursday night with the threat for heavy rain
increasing significantly by Thursday afternoon and continuing
through Thursday night. Currently, the frontal boundary has sagged
into the far north with some low clouds and thus cooler
temperatures. This front will likely make a swd push tonight but
may wash out some Thursday and lift nwd into the northern
sections. Dynamics with the upper trough/jet and daytime heating
will increase the heavy rain threat Thursday afternoon and into
Thursday night with 1-2 inches of rain in the mountains and
foothills quite possible. Even the lower elevations of the far
west may see over an inch with local 2 inch amounts. Upper low
will likely bottom out over Northern Utah Thursday night before
lifting nne/ne across us on Friday. Very unsettled the path of the
upper low into Friday. Heaviest rain will be in the west with less
the farther east you go. Few active storms will likely be around
late Thursday into Thursday evening in the southwest where the
best forcing combines with some available cape. Added in isold
storms on Friday afternoon with upper low overhead. Hard to not
put a few in with this significant of an upper low moving through
even with limited instability. As the cold air works in later
Thursday night and Friday...snow levels will lower quite a
bit...especially in the southwest. Significant snowfall is quite
possible above 9 to 10k with some snowfall down to 8k or so. The
highest impact will be mainly for hunters, hikers and others who
might be in the higher mountains. Be prepared for significant rain
changing to significant snow if you`re going well up into the
mountains. Scattered showers will persist in the cool...cyclonic
northwest flow on Saturday.

.LONG TERM...Saturday Night Through Wednesday

Upper low will be slow to depart the region, although it will at
least head east from Wyoming. Medium-range models diverge early in
the week and then come back into better agreement mid-week. Overall,
temperatures will remain cooler than normal through Wednesday as
northerly flow persists. Brisk northwest winds are possible across
the far north Sunday as the low trudges east. Proximity of the low
will keep some gusty northerly wind across Johnson and Natrona
counties on Monday and Tuesday. Just where the low will be is the
big question and will have possible impacts on light precipitation
chances. GFS has the upper low slowly spinning over the Dakotas
Monday and Tuesday, while the ECMWF has an elongated trough just to
the east of the state. By Wednesday, the ECMWF has the energy within
this trough becoming concentrated over South Dakota/Nebraska. While
farther south, this is similar to the solution offered by the GFS.
The final answer looks plausible, but what happens in between is of
interest. Model blend does not produce precipitation chances across
our eastern zones Monday and Tuesday. Have continued this thinking
with current forecast of dry conditions. However, the concern is
that a pinwheeling shortwave will rotate south across the state and
produce low precip chances Tuesday. This is the solution offered by
the GFS, which seems within the realm of possibility. Regardless,
the western half of the state should be dry Sunday through
Wednesday. Beyond Wednesday, ridge slowly builds from the west and
the upper low will push east. This pattern will provide a quieter,
drier, and warmer late week pattern.


.AVIATION.../06Z Issuance/

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

Messy forecast with several waves of showers and thunderstorms with
periods of MVFR/IFR conditions. A cold front set up along the Divide
is expected to lift northward Thursday morning as a warm front with
the surface flow becoming east, southeast, or south in most areas.
Any low ceilings ongoing are expected to lift to VFR category by mid-
late morning. Models have been relatively consistent in developing a
strong band of showers and thunderstorms in the KRKS-KRIW-KWRL
corridor by around 18Z and keeping it going through the afternoon
into the early evening. Outside this area models continue to paint
scattered showers and thunderstorms. Confidence on exact location of
convective banding is low.  Scattered showers and even an isolated
thunderstorm will continue through the entire night. Frequent
mountain obscuration will occur.

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

Messy forecast with several waves of showers and thunderstorms with
periods of MVFR/IFR conditions. The first wave across the far west
is expected to decrease in coverage Thursday morning, but not
totally go away. Between 15Z and 18Z, a second wave of showers and
thunderstorms will begin to impact south/southwest WY lifting
northward with continued development in the afternoon. Between 03Z
and 06Z shower and thunderstorm coverage will decrease, but
scattered convection will continue through the night. Frequent
mountain obscuration will occur.



Fire concerns should remain low to moderate for the next few days as
a large upper level storm system moves into the area. Showers and
isolated thunderstorms will increase tonight in the west with a
chance east of the divide. The widespread showers and isolated
thunderstorms will continue in the west Thursday and spread east of
the divide by Thursday afternoon. The potential for heavy rain will
bring the threat of flash flooding in the western mountains,
especially around the recent burn scars. Widespread wetting rain
should impact much of the area Thursday into Thursday night.


.RIW Watches/Warnings/Advisories...
Flash Flood Watch from Thursday afternoon through late Thursday
night for WYZ001-002-012-014-015-024.



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