Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211724
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1124 AM MDT SAT MAY 21 2016

.Short Term...Saturday through Monday night

So much time to talk about the weather and so little to talk about.
Wait a minute, strike that, reverse that. Thank you Gene Wilder for
that quote. Anyway, it looks like the active pattern will continue
for a few days. With the difference in weather; we will divide the
short term discussion into West and East of the Divide.

West of the Divide...The thunderstorms of yesterday have waned down
to only a couple of showers and Saturday should start out dry.
However, it will not last as a cold front and associated cold front
move across the area around midday and into the afternoon. The
result will be another scattering of showers and thunderstorms
across the area. The threat for stronger thunderstorms should be a
bit less through with the best instability further east. Much cooler
air will move in behind the front and drop snow levels possibly down
to the valley floors. A that point there will not be a lot of
moisture and lift to work with some any accumulation would be none to
negligible.

Tomorrow looks to start out dry as well. However, another shortwave
moving through the area with another chance of showers and
thunderstorms. It will be a cooler as well so in the mountains the
precipitation may fall as snow. There is not a ton of moisture to
work with though so any accumulation would not be substantial. At
this point Monday looks to be the driest of the three days with some
flat ridging across the area. There will be enough instability for a
few showers, but most of the day should be precipitation free.
Temperatures should also moderate a bit.

East of the Divide...This looks to be the most active day as far as
stronger thunderstorms are concerned. There will continue to be the
warm temperatures of the past couple of days. High temperatures
could be a bit tricky however with the front moving in. This will
help to provide instability. In addition; with the front and
shortwave moving in there will be additional forcing to get lift
going. Model soundings are also showing decent shear of the
directional variety. 700 millibar winds also look fairly decent with
most models showing 40 to 45 knots. The limiting factor, as it
always is in Wyoming, will be moisture. This does support high
based storms though which could bring some downburst winds and
mixing the strong winds aloft to the surface. Models are showing the
best chance of stronger storms across eastern areas, especially
eastern Johnson County where a slight risk has been issued by the
Storm Prediction Center. A marginal risk runs roughly from Greybull
down to Casper. For now, we have stuck with the small hail and
gusty winds; but this will have to be looked at by the day shift.
All areas will see a south wind ahead of the front, with gusts to 40
and possibly 50 mph ahead of the frontal passage. Convection should
end after midnight as the front moves away to the east.

Sunday will be a cooler day with high temperatures averaging 10 to
15 degrees cooler than Saturday. Most of the day looks dry. However,
another shortwave rotating across the area will bring the threat of
more showers and thunderstorms. Instability will be more limited,
but with lower wet bulb zero levels there will still be the threat
of small hail. It will still be breezy, but the wind will be much
lighter than Saturday afternoon.

Most areas should have a break on Monday as a flat shortwave ridge
builds across the area. For now we have kept the forecast dry except
for some of the higher elevations. With more sunshine temperatures
should also begin to warm by a few degrees, but remain below normal
for late May.

.LONG TERM...Tuesday through Saturday

The long range models continue their trends of a longwave though
over the western U.S. through the period. Looks like a chance of
showers and afternoon and evening storms Tuesday through Friday.
At this time, Tue and Wed look to have more coverage of showers
and storms with lesser coverage Thurs and Fri. However, model
timing of weather disturbances in this flow may change by next
week and impact the best days for storm activity. By next
Saturday a new storm system may develop in the Western U.S. which
could bring showers and storms to the west with only isolated
activity east of the divide. This is the first set of model runs
to indicate this possibility, so will watch it over time.

As for temps, not a lot of fluctuation in daily highs and lows Tue
through Fri. Looking like 60s to lower 70s with 55 to 65 in the
western valleys. Lows temps in the upper 30s and 40s with 30s in
the west. Next Saturday looks to be a little warmer.

&&

.Aviation.../18Z issuance/

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

A cold front will push into the forecast area today with a line of
showers and thunderstorms accompanying the front along with areas of
MVFR/brief IFR conditions and increasing mountain obscurations.
Drier air will push in behind the cold front this evening with
ceilings lifting, but then followed by local IFR/LIFR ceilings and
-SHRASN developing early Sunday morning.

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

VFR conditions will prevail through much of the forecast with some
brief periods of MVFR later this afternoon and evening near
TSRA/SHRA. South winds will increase to 20-30 knots with higher
gusts across much of the area ahead of an approaching cold front.
This front will spread a line of showers and thunderstorms into
central Wyoming 21z - 02z Sunday with a few strong to severe
thunderstorms with large hail and damaging wind possible vicinity
and north and east of a KCOD-KBYG-KCPR line.  Drier air will push in
behind the cold front overnight tonight with prevailing VFR
conditions expected after 07z Sunday.

&&

.FIRE WEATHER...

After a break early this morning; a cold front will move into
western Wyoming with more showers and thunderstorms that will
spread east of the divide later this afternoon and evening. Some
storms could have strong downburst winds and hail, especially from
the Big Horn Mountains and eastward. A strong south wind will
precede the frontal passage this afternoon for many area in the
south and east of the Divide. Much cooler air will move in
behind the front with snow levels possibly falling to the valley
floors by morning. Mixing and smoke dispersal will be fair to good
West of the Divide and very good to excellent east of the divide.
Sunday will be a cooler day but still with a chance of showers and
thunderstorms; most numerous West of the Divide.

&&

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

&&

$$

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


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