Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231744
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1144 AM MDT MON MAY 23 2016

.Short Term...Monday through Wednesday night

Active weather looks to continue through the period. However, for
today we will at least dial it back a bit. Flow across the area
today looks flat or even a little bit ridgy; I wonder if that is a
word. Anyway, both the NAM and GFS are rather dry. The GFS is the
driest and has almost everywhere east of the divide dry. The NAM is
a bit wetter bit even it has little coverage. Precipitable water is
very low with all areas under 0.50 inches and some areas under 0.25
inches. For today, we kept areas east of the divide dry for the most
part. Although we can not rule out a stray shower or thundershower,
almost all areas will be dry. There will be some west of the divide
this afternoon and evening, but even here most areas will be dry
most of the time. Temperatures should warm today...averaging about 5
degrees warmer than on Sunday.

Things will get more active for Tuesday as another shortwave swings
around the long wave trough and increases the chance for convection.
With the added forcing the chance of thunderstorms will increase. In
addition, the model sounding are showing increased directional shear
across the area with portions of the area in the left front quadrant
of a jet streak that could increase upper level divergence somewhat
across eastern portions of the county warning area. The Storm
Prediction Center has put Natrona and Johnson County in a marginal
risk for the chance of strong to possibly severe thunderstorms. The
limiting factor may be moisture as dew points still look rather low
and precipitable water values barely creep above 0.50 inches. There
could be some tilted updrafts that could sustain the storms given
the shear. For now we went with small hail and gusty winds but
eastern area wording may have to beefed up later. Temperatures on
Tuesday should be fairly similar to Monday. Another wave will then
approach for Wednesday with another chance of mainly afternoon and
evening showers and thunderstorms. Temperatures will remain below
normal through the period. As for the chance of mountain snow, with
the convective nature of the precipitation it will be difficult to
pinpoint and given the high late May sun angle anything that falls
during the day would have a hard time sticking.

.LONG TERM...TONIGHT THROUGH MONDAY

Consistent forecast for Thurs and Fri with a chance of showers and
thunderstorms in the afternoon and evening with lingering showers
late in the night. A new weather system settles into the Oregon
area on Saturday through Saturday night. This brings a south
southwest flow aloft into the area. The ECMWF model is a little
farther east with the circulation, in the vicinity of Western
Idaho/Eastern Oregon. It appears that the best coverage of showers
and storms would be in the west and north with lesser activity in
the central and south zones. Breezy and warmer temps as well
Saturday.

The models differ on where this system moves on Sunday and
Monday. The ECMWF model spins it into Northern Idaho/Western
Montana Sunday and then takes in across Montana Monday and
weakens it. Meanwhile, the GFS drifts the circulation toward the
Nevada/Idaho/Oregon triple point by 18z Monday. The Canadian
model is more like the ECMWF model Sun/Mon with its track of the
storm system. Confidence low on just how this plays out Memorial
Day weekend. The rule of thumb in WY is for unsettled weather over
this holiday weekend versus dry and warm. For now will go with
isolated storms central and south with a little better coverage in
the north and northwest Sunday and Monday. Will keep high temps on
the mild side Sun/Mon versus the cooler temps that the ECMWF model
suggests.

&&

.Aviation.../18Z issuance/

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

An upper level low pressure system over south central Canada will
continue to produce isolated to scattered rain and snow showers and
higher mountain obscurations over the northwest mountains,
vicinity of a KJAC-KDUB line and north through the period. To the
south, an upper level disturbance will spread scattered showers
and possible a few thunderstorms into the vicinity of KRKS-KPNA-
KBPI until 01z this evening along with occasional mountain
obscurations. This activity will move out of the area or dissipate
with sunset.

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

Gusty southwest wind of 15 to 30 mph will continue in many areas
east of the divide into this evening with stronger gusts near any
isolated showers or thunderstorms. The two main culprits of
showers and isolated through early this evening will be an upper
level low pressure center over south central Canada. This system
will produce isolated showers in the vicinity of KCOD with
mountains to the west occasionally obscured. The second system
will move across the south producing mainly isolated showers and
possibly a few thunderstorms mainly and south of a KLND- KCPR line
along with occasional mountain obscurations. Most of this activity
will dissipate with sunset this evening.

&&

.Fire Weather...

It will be a somewhat drier day today; especially east of the
divide. A few mainly higher elevation thunderstorms may develop
across the mountains of the west though. A gusty breeze will
continue across portions of the area, but high winds are not
anticipated. Relative humidity should remain above critical levels.
An upper level disturbance will approach on Tuesday and bring an
increased chance of afternoon showers and thunderstorms. Expect good
to excellent mixing and smoke dispersal across most of the area this
afternoon.

&&

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

&&

$$

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



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