Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240942
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
342 AM MDT TUE MAY 24 2016

.Short Term...Tuesday through Thursday night

The active weather pattern looks to continue through much of the
workweek. All is quiet right now on the weather front. However, this
will not last long as a front and associated shortwave move into the
area. Showers and thunderstorms will break out in the west during
the late morning hours and then progress eastward through the
afternoon and into the evening. Out west, only run of the mill type
storms are expected. Some snow is also possible at the higher
elevations, but likely nothing of consequence. Things do get a bit
more interesting East of the Divide though.

The thunderstorms will wait a bit longer areas east of the divide
today, allowing for more instability to build up. Soundings are also
showing a decent amount of shear of the speed and directional
variety that could support tilted updrafts that could sustain the
storms. In addition, the left front quadrant of a jet streak could
enhance upper divergence a bit. This could spell some stronger
storms and possible a few severe. But there are some limiting
factors. Temperatures are not expected to be terribly warm, only
getting to around 70 in most locations. Also, the atmosphere is
starting off rather dry. There is some indication of moisture
recovery however as weak surface low pressure may wrap moisture back
from Natrona County westward into the Big Horn basin.
Nevertheless, a lot of thunderstorms look to be higher based. With
inverted V soundings there could be some downburst wind gusts and
the shear is sufficient for decent sized hail. The Storm Prediction
Center does have the area eastward from around a Cody to Casper line
in a marginal risk. This looks reasonable at this time. For now, we
will stick to the small hail and gusty wind wording. Should be time
to beef up the wording since any stronger thunderstorms would be
late this afternoon or evening. Some storms could go later in the
night with a boundary lingering in the area, mainly across the
north. Snow will still be possible in the mountains as well.

Wednesday and Thursday still look active as well. With a boundary
remaining in the area and low heights still around. There will be a
chance for mainly afternoon and evening showers and thunderstorms
each day. Most of each day should be rain free, but all areas will
have at least a chance. With more cloud cover and less jet support
stronger storms should be less of a threat however. Temperatures
will remain below normal for most areas as well. Like previous days,
the most coverage of showers will be in the western and northern
portions of the county warning area.

.LONG TERM...Friday through Tuesday

GFS and ECMWF models bring in a storm system into the Pacific NW
states for Friday. Best chance for storms will be the NW and in
the north with lesser coverage central and south.

On Saturday the system gets additional energy dropping into it
from Western Canada. This energy creates issues with the original
circulation as the storm system begins to elongate and change
orientation from Saturday night through Monday. The overall
pattern will be a SW flow aloft over the area all weekend into
next week. The flow is diffluent at times which may lead to
increased convection at times.

The models drive a cold front into western WY late afternoon
Saturday which may be the focus for some strong storms. The ECMWF
model pushes the cold front through the entire region Saturday
night leading to a cooler Sunday with isolated showers and
storms. Then the ECMWF model brings the other piece of energy
toward NW around 00z Mon and rotates it into Montana Monday while
driving another cold front into the area for Monday. Thus a chance
for showers and storms.

The GFS model has its own scenario on how this weather pattern
plays out. It has the initial cold front Saturday afternoon and
evening with the better storm coverage over NW and northern WY.
The big difference is that GFS does not have the second cold
frontal push Sunday night and Monday and thus much warmer temps
for Mon. Plus the GFS has isolated to scattered showers and
storms across the region Sun and Mon.

Bottom line forecast for Memorial Day weekend outdoor activity
plans...the threat for thunderstorms over and near the mountains
each day with strong wind gusts, hail, periods of heavy rain, and
frequent lightning from the stronger storms. The storm coverage
over the lower elevations will be less but the above threats are
still possible at times.

Next Tue looks like the usual afternoon and evening thunderstorm
chance with a frontal boundary draped across the state.

&&

.Aviation.../12Z issuance/

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

VFR conditions to prevail through at least 09z. A strong
disturbance will move across the region today bringing scattered
showers and thunderstorms to most of the west this afternoon and
evening...with periods of brief MVFR conditions in lower ceilings in
and near heavier showers and storms. Stronger embedded storms will
produce gusty erratic winds to 35 knots with small hail and brief
heavy rain. Stronger gusts in excess of 45 knots will be possible
across Sweetwater county including KRKS. Most activity will
decrease by 04z Wed...lingering a bit longer near KJAC. Early
morning low stratus and MVFR ceilings or VCFG return to some
terminals after 09z.

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

VFR conditions to prevail through the forecast period. A strong
disturbance will move across the region today bringing scattered
showers and thunderstorms east of the divide later this afternoon
through the evening...with periods of brief MVFR conditions in lower
ceilings in and near heavier showers and storms late this
afternoon and evening. More numerous and stronger thunderstorms
are possible in both Natrona and Johnson counties later this
afternoon and evening with some storms producing winds gusts in
excess of 45 knots...hail from one half inch to one inch...and
brief heavy rain with frequent lightning. This activity will
decrease from south to north after 04z Wed. Early morning low
stratus and MVFR ceilings or VCFG returns to some terminals after
09z.

&&

.FIRE WEATHER...

An upper level disturbance and cold front will move towards the area
today and bring a chance of mainly afternoon and evening showers and
thunderstorms with the most numerous storms in the west. A few
thunderstorms from the Big Horn Basin eastward could become strong
to possibly severe with strong and erratic wind gusts as well as
hail. A gusty southwest wind will develop across portions of
Sweetwater County and the Green and Rattlesnake Ranges. Cooler then
normal temperatures and the chance of mainly afternoon showers and
thunderstorms will linger through Thursday.

&&

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

&&

$$

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



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