Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210450

Area Forecast Discussion
National Weather Service Riverton WY
1050 PM MDT Tue Mar 20 2018

.SHORT TERM...(This evening through Friday)
Issued at 136 PM MDT Tue Mar 20 2018

Imagery shows modest ridging across the western CONUS with a trough
over the east and the trough axis extending from south central
Canada through MN/IL and on down through the southeastern States
(severe wx FL today). Split flow within the ridging is occurring
across the western CONUS with the weaker northern branch moving
into/through BC/MT and merging with the strong southern branch of
the jet over the Central Plains. This jet does clip the southwest
corner of WY. Modest moisture currently continues to flow into
western WY from the PAC NW keeping low end chances for light terrain
forced precipitation over the western mountains. just upstream
within the the region just between the splits...a weak embedded
shortwave is moving out of OR and into/through central ID. The
surface has weak high pressure in place across much of the western
CONUS with also weak lee side troughing extending from eastern MT
through eastern WY and down the Front Range of CO. Other than a few
light showers across western WY/southern ID today, central/northern
California and western NV is where the nearest significant
precipitation resides.

Rest of today through Wednesday night: The weak embedded shortwave
mentioned above (OR->ID) moves into/across western WY this evening,
increasing precipitation chances slightly when added to the addition
forcing from the jet across southwest WY. This should equate to a
trace to a few tenths of an inch of snow in the valleys through
tonight...with a half inch to 2 inches possible over the western
mountains. Otherwise, ridging continues across the FA Wednesday and
Wednesday night with the next weak embedded shortwave moving through
the ridge and into western WY Wednesday afternoon...again increasing
precipitation/snowfall for a short period...even giving a few
locally moderate snow showers and another couple of inches of
snowfall to favored locations above 7500 feet.

By Thursday, a trough/ridge/trough pattern develops across the CONUS
with the slowly passing ridge axis just to the east of the forecast
area roughly along the eastern border of Wyoming. Upper level
heights rise significantly as flow itself transitions to southwest
across the FA with the nose of a secondary jet and associated
embedded shortwave moving into/over western/southwestern WY along
with what looks to be good Spring moisture from the EPAC by Thursday
afternoon/evening. The primary jet will have lifted into the PAC NW
and north of WY for the most part. The moisture trajectory will also
transition from the PAC NW southward out of the EPAC and across
central California, bumping into/over the Sierra Mountain range.
This will wring some of the available moisture out of the moisture
stream. Still, it will be raining/snowing (above 8500 feet) west of
the Divide in Wyoming by Thursday afternoon/evening, continuing into
Friday morning.

Friday, the main trough over the western CONUS pushes eastward
against the ridge...moving it over the Central Plains while a
relatively strong embedded shortwave associated with the secondary
jet moves across the southern half of WY along with an associated
surface front. This will slow precipitation production across the
west for a bit, moving the focus over southern WY where rain showers
and isolated thunderstorms will take over. The mountains too, on
Friday, will see increased chances for a few isolated (mainly)
afternoon thunderstorms.

Possible impacts during the Thursday afternoon/night through Friday
period includes: 1. Potential periods of moderate to occasionally
heavy rain/snowfall (stratiform/embedded convective) with possible
low end flooding concerns for portions of western Wyoming due to
increased/added snowmelt combining with increased liquid precip
through these periods. 2. Advisory to localized Warning level
snowfall across the western mountains above 8500 feet (9500 feet for
Warning amounts). 3. Brisk, locally high, winds across portions of
the Wind Corridor Thursday and Friday. 4. A few isolated
thunderstorms possible Friday...mainly across the southwest/southern
portion of Wyoming.

.LONG TERM...(Friday night through Tuesday)
Issued at 136 PM MDT Tue Mar 20 2018

Still lots of uncertainty for the long range. There is general
agreement with the pattern but the details are still quite
uncertain. The period should start off fairly dry but precipitation
chances then increase across the west as a Pacific trough and cold
front approach western Wyoming. At least at this point, the models
are in fairly good agreement across the west with some rain and snow
across the west. Meanwhile, mainly dry conditions will continue East
of the Divide. However, with a rather tight pressure gradient
developing it will turn breezy to windy in many areas. With some 50
knot wind barbs showing up at 700 millibars, there could be some
high winds in the favored areas. Too early to nail this down at this
point however. The front then sweeps across the area on Saturday
night and will usher in some cooler temperatures.

Now is when the models begin to diverge as an upper level low
effects the area. The GFS and European continue to have large
differences on the placement, intensity and timing of the expected
precipitation. Other than that, they are perfect (note sarcasm). The
European looks slower than the GFS at this point however. For now,
we kept POPS low until details become clearer. Even for Tuesday, the
models have slowed down on a possible clipper headed toward the area
so we removed the POPS from later Monday night into Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Thursday)
Issued at 1050 PM MDT Tue Mar 20 2018

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

VFR conditions will prevail at the terminal sites through the
period. Scattered rain and snow showers will occur in southwest WY
after 12Z Wed through 21Z with local MVFR conditions. Then a new
batch of rain and snow showers will occur over northwest WY from 00Z
Thurs through 12Z Thurs. MVFR to IFR conditions will occur in this
area with mountains frequently obscured.

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

VFR conditions will occur through the period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.


Issued AT 136 PM MDT Tue Mar 20 2018

Seasonal or below (west) to seasonal or slightly warmer (east)
temperatures expected across the the region today. A weak upper
level disturbance will move into/across western Wyoming this evening
offering increasing chances for precipitation (snow or rain/snow
mixed showers). Precipitation chances will then decrease across the
west after midnight. Snow accumulation today through this evening
along and west of the Divide over the mountains will range from 0.5
to 2 inches in the mountains with perhaps a few tenths of an inch in
the western valleys. Strongest winds (gusting 15 to 30 mph out of
the west/southwest) will be found across the Wind Corridor from
Sweetwater through southern Natrona Counties this afternoon.
Slightly warmer across the forecast area Wednesday, but
otherwise similar weather results to those of today.





LONG TERM...Hattings
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