Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212041

Area Forecast Discussion
National Weather Service Riverton WY
241 PM MDT Wed Mar 21 2018

.SHORT TERM...(This evening through Saturday) Issued at 237 PM
MDT Wed Mar 21 2018

Imagery shows modest ridging across the western CONUS with a trough
over the east and the trough axis extending from south central
Canada/Lake Superior area through WI and on down into the south
Atlantic States. Modest, but increasing, moisture continues to flow
into western WY from the EPAC now keeping low end chances for light
mainly terrain forced precipitation over the western/southwestern
mountains. 110-130 kt jet now flowing over WY...but translating
northward as it does. The surface still has weak high pressure
across much of the western CONUS with also weak lee side troughing
extending from eastern WY down through the Front Range of CO. A weak
nearly stationary front/boundary is present over portions of central
WY. Only a few light showers are present across western
WY/southeastern ID/western CO today, with more significant
precipitation moving through California and western/northern NV.

Rest of today through tonight: Upper level ridging continues across
the FA today tonight with the next weak embedded shortwave moving
through the ridge across southern ID this afternoon. This shortwave
will arrive across western/southwestern WY this afternoon...again
increasing precipitation/snowfall into this evening...even giving a
few locally moderate snow showers and perhaps another couple of
inches of snowfall to favored locations above 7500 feet tonight
(only a trace to a half inch expected below this elevation).

By Thursday, a trough/ridge/trough pattern develops across the CONUS
with the slowly passing ridge axis over to just to the east of the
forecast area roughly along the eastern border of Wyoming by
Thursday evening. 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 well north through the PAC NW and into Canada.
The moisture trajectory will also transition from the PAC NW
southward out of the EPAC and across central California, bumping
into/over the Sierra and Wasatch Mountain ranges. The trend over the
last 24 hrs has been to sag a little further south with the
California and Utah mountains wringing some of the available
moisture out of the mid/upper level moisture stream. As heights
rise, so will snow levels...likely well above 8500 feet by Thursday
afternoon/evening. Still, decent moisture will make its way into
western/southwestern WY. However, have lowered QPF amounts slightly
in the western valleys, but increased them over the southern
Wyoming/Salt and Wind River Ranges compared to what they were
previously. Regardless, it will be raining and snowing (best
accumulating snow above 8000 feet) west of the Divide by Thursday
afternoon/evening, continuing into Friday morning. A Winter Weather
Advisory has been issued for the Tetons/Gros
Ventre/Salt/Wyoming/Wind River Mountains mainly for elevations above
8000 feet. 4 to 12 inches of heavy wet snow will be possible over
the western mountains with a few locations above 9500 feet seeing up
to 15 inches. Rainfall (or rain mixed with snow), moderate to
briefly heavy at times, will also fall across the lower elevations
with a half inch or more expected from early Thursday into Friday
morning. This will provide a period of elevated low end flood
potential for a few creeks, streams and rivers in the west central/
far southwestern valleys of Wyoming into Saturday morning.

Through the day 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 move a front through the area
east of the Divide and also give slight chances for some rain.
Meanwhile, precipitation will decrease west of the Divide for a bit,
moving the better focus for precipitation over southwestern and
southern WY where rain showers and isolated thunderstorms will take
over for a while.

Possible impacts during the Thursday morning through Friday period
includes: 1. Potential periods of moderate to occasionally heavy
rainfall/snowfall (stratiform/embedded convective) with some
increased low end flooding concerns for portions of
western/southwestern Wyoming due to increased/added snowmelt (rising
heights/temperatures) combining with increased liquid precip
("atmospheric river") through these periods. 2. Advisory to
localized Warning level snowfall across the western mountains above
8000 feet (9500 feet for localized higher amounts above 12 inches).
3. Brisk, locally high, winds across portions of the Wind Corridor
Thursday and Friday. 4. A few isolated thunderstorms possible
Friday...mainly across the southern portion of Wyoming.

Friday night through Saturday, southwest flow aloft continues, but
with the exiting embedded shortwave out of the area. This will allow
for a quiet period without much precipitation into Saturday morning.
Then, the next next embedded shortwave rushes into western WY out of
the main upper trough still to the west. This will effectively
increase precipitation once again - mainly snow this time across the
western zones, but without the atmosphere juiced as it was before.
Thunder will be possible on an isolated basis along with rain
showers becoming possible across southern Wyoming.

.LONG TERM...(Saturday night through Wednesday) Issued at 237 PM
MDT Wed Mar 21 2018

Things continue to look rather active through the long term. The GFS
shows an interesting situation for later Saturday night with an area
of precipitation breaking out along the right rear quadrant of a jet
streak as the cold front moves through the area. As of now, this
extends from Sweetwater into Natrona and Johnson Counties. The EURO
does not have it however. In addition, the GFS is probably overdoing
the QPF. Something to watch however.

In addition, confidence continues to grow for the potential of a
snow event East of the Divide Sunday night into Monday. However,
there continue to be disagreements in the details of the evolution
of it. These factors include placement of the heaviest precipitation
and exact timing of it. For now, we bumped up POPS East of the
Divide especially in the favored upslope locations. It is too early
to start talking about amounts however.

There also continue to be model differences for Monday night through
the rest of the period. The GFS lingers some snow across southern
areas into Tuesday morning while the European is a bit quicker in
moving things out. For now, we bumped up POPS a bit across the south
for Monday night but kept things dry for Tuesday. Temperatures will
be tricky for Tuesday as well since a lot will depend on where the
heavier snow falls. It does look chilly though. There are questions
for Wednesday as well. The clipper that showed for up for Wednesday
then magically disappeared has now reappeared with the 12Z model
runs today. The models tend to rush this stuff in too fast as well.
For now, we maintained a dry forecast until a couple of consecutive
runs show the clipper definitely arriving.


.AVIATION...(For the 00Z TAFS through 00Z Friday) Issued at 237
PM MDT Wed Mar 21 2018

West of the Continental Divide...KJAC, KPNA, KBPI and KRKS

Periods of snow showers will move through portions of the sites
through the period. This will be especially so in vicinity of KJAC,
where there will be frequent MVFR and local IFR conditions along
with frequent mountain obscurations. The steadiest precipitation is
expected before 03Z and after 16Z Thursday. Mainly VFR conditions
will occur in vicinity of KRKS with a gusty wind at times.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

VFR conditions are expected. Some showers may spread East of the
Divide Thursday evening but should hold off until after 00Z Friday.

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


Issued AT 237 PM MDT Wed Mar 21 2018

Seasonal or below (west) to seasonal or above (east) temperatures
expected across the the region today. Another weak upper level
disturbance will move into/across western Wyoming this
afternoon/evening offering increasing chances for precipitation
(snow or rain/snow mixed showers). Snow accumulation today through
this evening along and west of the Divide over the mountains above
7500 feet will range from 0.5 to 2 inches with perhaps a trace to 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. Warmer for most locations across the
forecast area Thursday, but with significantly increasing
rainfall/high elevation snowfall chances west of the Divide by
Thursday afternoon.


Winter Weather Advisory from 3 AM Thursday to 6 AM MDT Friday
for WYZ012-014-015-024.



LONG TERM...Hattings
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