Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220918
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
318 AM MDT THU SEP 22 2016

.SHORT TERM...Thursday through Saturday night

Well we can say goodbye to summer for another year as autumn arrives
at 8:21 am this morning. And we will be in a cool weather pattern
courtesy of a meteorological bowling ball, AKA an upper level low
now moving into the Great Basin that will drift east and keep the
weather rather unsettled. A front has settled across the area with
some cool temperatures across much of the area. We do have some
questions with the forecast today. The first is how much rain will
fall. With a deep moisture plume with an assist with moisture from
the remnants of Paine moving into the area, rain is ongoing as we I
write this. The heaviest so far has been across Idaho but it is
shifting eastward. It has been fairly dry, so many areas should be
able to absorb the rain. The exception are the numerous areas that
have seen wildfires over the summer where burn scars are present.
With some of these some mud flows are possible so we will keep the
Flash Flood Watch going. Some areas will see over an inch of rain in
the west. However, with the convective nature of it, pinpointing the
exact location is not possible. There could even be a few stronger
thunderstorms in the southwest where the left front quadrant of a
jet streak will enhance upper level divergence.

Meanwhile, East of the Divide many areas will start the day dry.
However, the aforementioned jet max will move into the area and
bring an increase in showers and thunderstorms as well. At this
point, the models agree that the wettest period will be from this
afternoon until around midnight. The general rule is the further
west you go, the more rain you will get. However, like West of the
Divide, the models disagree on the exact position of the heaviest
rain since it will be convective. Temperatures will also be tricky
as the cold front over the area will try to lift northward. A place
like Casper could mix out and get well into the 70s while Riverton
or Lander could be stuck in the lower 60s. Continuity had a fairly
good handle on this so we made few changes. The heaviest rain should
shift out of the area late tonight but there will still be showers
around. As for the chance of snow, snow levels will lower tonight
but for the most part should remain above pass level into early
Friday.

As for Friday, the precipitation at this point looks to be more
showery as the upper level low approaches. It will also be cooler
following the passage of a cold front. However, with the proximity
of the low there will still be showers around along with a few
thunderstorms. With the upper level low close and low wet bulb zero
levels, the main threat from the storms will shift to small hail.
The other threat will be snow. Snow levels will be lowering as the
upper level low approaches with the GFS showing 700 millibar
temperatures getting down to minus 3 that would put snow levels at
7500 feet. This may be a bit too low. However, it will be low enough
for snow levels to likely drop to pass level on Friday. The lowest
levels should be in southwest Wyoming which will be closest to the
upper level low. A place like Togwotee pass could get slick on
Friday at times if the rate of snowfall is heavy enough. Meanwhile,
East of the Divide some areas may be in a dry punch with many dry
periods. There will be some showers around through the day however.
Precipitation will continue into Friday night although the heaviest
may shift to northern Wyoming as the upper level low crosses the
area.

Saturday at this point looks like the driest of the three days as
the upper level low moves into Montana. It will be blustery and cool
day however and with unstable northwest flow continuing over the
area, so will the threat of showers across much of the area,
although more scattered in nature than the previous couple of days.
One area that will have to be watched are the western slopes of the
Big Horns where northwest flow can really dump snow in some areas.
Most of the area should begin to dry out Saturday night as the upper
level low moves away into the Dakotas.

.LONG TERM...Sunday through Thursday

Still plenty of uncertainty where the storm system ends up on
Sunday afternoon, and just what will be left of it. All four
models looked at offer something different. It seems a north to
northwest flow aloft will occur Sunday with isolated showers
lingering east of the divide. Brisk north to northwest wind in
places east of the divide. Sunday high temperatures on the cool
side. The weather pattern looks to settle down Monday through
Thursday offering up those pleasant days of fall. Dry weather
through the period per the GFS model. The ECMWF model wants to
bring in a few showers/storms on Wednesday and Thursday afternoon.
Have went with the dry forecast as have most of the neighboring
offices. Plenty of time to watch it and alter forecast as needed.
High temperatures will gradually warm Mon through Thurs.

&&

.AVIATION.../12Z 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
early this morning is expected to lift northward as a warm front
with the surface flow becoming east, southeast, or south in most
areas. Any low ceilings ongoing early this morning 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, and will only have VCTS for now.  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 far west is
expected to decrease in coverage this 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 WEATHER...

Fire concerns should remain low for the next few days. A cold front
and will be in the area today and low pressure approaching from the
west will spread some showers and thunderstorms across the area with
coverage and intensity increasing into the afternoon and evening.
The heaviest rain will fall in the western half of the area with a
good chance of a wetting rain. Thunderstorms will be possible with
erratic winds and small hail. Smoke dispersal will be generally poor
in the west and north and good in southeastern areas. Chances of
rain and higher elevation snow will linger into Friday and Saturday
as well.

&&

.RIW Watches/Warnings/Advisories...

Flash Flood Watch from noon MDT today through late tonight for
WYZ001-002-012-014-015-024.

&&

$$

SHORT TERM...Hattings
LONG TERM...AR
AVIATION...WM
FIRE WEATHER...Hattings



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