Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160804

Area Forecast Discussion
National Weather Service Riverton WY
204 AM MDT Fri Mar 16 2018

.SHORT TERM...(Today through Sunday night)

Imagery continues to show slowly developing amplified
trough/ridge/trough pattern across the CONUS with the large
negatively tilted trough/closed low(s) system stretching from the
PAC NW to the western portion of the Central Plains. The embedded
developing closed lows reside over the west coast of OR and the CO
High Plains. The ridge axis currently is set from southern Canada
south through the eastern Dakotas. A bunched-up modest WCB still
lies across the southern CONUS from Baja through TX and into MS/AL.
The surface has a fully blown cyclone over eastern CO with an
inverted low pressure trough up into southeastern WY. Weak to
moderate low pressure lays across most of the rest of the western
CONUS. A nearly stationary front stretches from the cyclone in
eastern CO up through northeast WY and into central MT. Significant
precipitation in centered around the cyclone into southeastern WY
and then across central California and western NV up to northwestern
OR. Only a few patchy areas of light showers are currently
falling in this CWA.

The western CONUS trough is mashing into the ridge to the east a
pretty much destroying it while the developing strong cyclone over
eastern CO starts to move into the Central Plains. Return moisture
is not making it very far to the west and have used the current NAM
to dial back this evening`s as well as the overnight WPC/MR models
forecast QPF and subsequent snowfall (as has been done
routinely/continuously this week).

West of the Divide this morning through Saturday night: Only a few
light showers around this morning/today ahead of the main upper
trough/west coast closed low which will open and finally make its
move toward the forecast area late tonight through Saturday night.
Mostly sub-Advisory snow amounts continue through this period with
36 hour totals ending Sunday morning ranging from 2 to 6 inches over
the mountains and from 1 to 3 inches in the valleys. Sunday there
will be a reprieve from significant precipitation/snowfall for most
locations except for a few areas of scattered light snow showers
occurring over northwest and southwest/southern WY (former - terrain
forced...the latter near boundary convergence).

Strong cyclogenesis currently taking place across eastern CO will
moving into/through the Central Plains today taking the best
moisture along with it, only allowing rather modest moisture to
reach back into the portions of Central WY that include Natrona and
Johnson Counties. Most of the rain/snow showers will by out of this
portion of the CWA by early afternoon (Johnson County seeing the
last of the precipitation). Therefore, accumulating snowfall total
have been trimmed east of the Divide once again. Grassy areas over
the eastern zones will see the better accumulations of snow while
roadways become wet and slushy at times/places where snow does fall
later this morning. Best chances for accumulating snowfall east of
the Divide will occur mainly over the Bighorn and Casper mountains
(orographic convergence) in addition to portions of the lower
elevations of Johnson County, Natrona County, and southeast Big Horn
Basin (low level boundary convergence) this morning/early afternoon.
2 to 6 inches will be possible over the Bighorns...1 to 4 inches
over Casper Mountain... and anywhere from a trace to 2 inches
(locally 3 inches) across the previously mentioned lower elevations.
Tonight the system moves out with the next chance for light
precipitation not returning until later Saturday night through
Sunday night.

.LONG TERM...(Monday through Friday)

Overview...A cool and unsettled northwest flow will linger over the
area Monday and Tuesday with chances of snow showers mainly over the
mountains.  Warmer and dry conditions will prevail on Wednesday. The
next Pacific cold front will approach the region on Thursday,
spreading chances of rain and snow showers into the west with breezy
to windy and unseasonably mild conditions expected east of the
Continental Divide.  Chances of valley rain and mountain snow
showers will spread east of the Divide on Friday.

Discussion...Upper air pattern on Monday morning expected to feature
closed low moving across the southern and central Plains, leaving
with a few weakening, orphan lows lingering in weak flow from the NW
U.S. coast into south central Canada.  Upper ridge will be
amplifying along the west coast in response to approaching closed
low across the eastern Pacific.  Main forecast challenges involve
some of the orphan lows getting caught up in NW flow and moving
across the area Monday and Tuesday, and then further on down the
road, the shape the eastern Pacific trough will take as it kicks
across the west late in the week.

For Monday and Tuesday, ECMWF shows more separation between the
various orphan lows than GFS, kicking low out of the NW U.S.
across Wyoming on Tuesday. GFS shows a more phased trough (energy
alone NW coast combining with south central Canada system),
pushing across the area Monday night-Tuesday morning. We do not
have much confidence in the timing details of either model, but
favor ECMWF`s weaker more separated flow. Forecast continues to
keep PoPs confined mainly over the high terrain Monday and
Tuesday, and we may need to add some PoPs to lower elevations when
timing issues are resolved.

ECMWF has continued to trend slower over the last few runs with the
progression of next Pacific trough onto the west coast Thursday.
Eastern Pacific low is expected to open up and get kicked onto the
west coast, into the Four Corners region Friday morning.  Similar to
the last few systems in this split flow, the heavier precipitation
should stay to our south and east on Friday.


.AVIATION...(For the 12Z TAFS through 12Z Saturday)

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

Isolated snow showers will occur through 18Z. Local MVFR ceilings
will occur until 15Z. Otherwise, VFR conditions are forecast at the
terminal sites through the period. Isolated snow showers will return
to the western mountains after 06Z Saturday.

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

Rain and snow will change to all snow from Buffalo to Casper by 12Z.
Snow will end around 18Z in this area. MVFR to IFR conditions will
occur, becoming VFR after 18Z. This includes the KCPR terminal site.
Isolated rain showers will occur over the rest of the region through
15z. All other terminal sites will have VFR conditions forecasted
through the period.


Issued AT 116 AM MDT Fri Mar 16 2018

Seasonal or below (west) to seasonal (east) temperatures expected
across the the region today. The effects of the spring/winter
weather system continue with isolated light showers west and
isolated to scattered showers east. Anywhere from a few hundredths
of an inch of precipitation (most locations) to as much as 1 to 3
tenths of liquid can be expected (eastern Johnson and Natrona
counties) through the end of the day today. This will equate to a
trace to near 2 inches of snowfall (locally 3 inches and more over
the Bighorn and Casper Mountains) on grassy areas. Higher amounts
will be found across northern/eastern Johnson County, eastern
Natrona County, and the southeast portion of the Big Horn Basin.
Gustiest winds most probable across the Wind Corridor from eastern
Sweetwater through Natrona Counties this afternoon.




LONG TERM...Meunier
FIRE WEATHER...Braun is the U.S. government's official web portal to all federal, state and local government web resources and services.