Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 150804

Area Forecast Discussion
National Weather Service Riverton WY
204 AM MDT Thu Mar 15 2018

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

Imagery continues to show stagnant amplified trough/ridge/trough
pattern across the CONUS with the large negatively tilted
trough/closed low still lying offshore the PAC NW but significantly
affecting areas from the PAC NW eastward into WY. The ridge axis
currently is set from Canada south and roughly along the MT/ND-WY/SD-
CO/KS borders. A modest WCB still lies across the southern CONUS
from Baja through NM and over TX. The surface has developing low
pressure across most of the Intermountain West including a couple of
long fronts - one stretching across portions of western WY. High
pressure follows these fronts along the West Coast. Most active
precipitation is falling under the areas of low pressure including
the western halves of WY and CO...west to central California.

The large offshore trough/closed low continues to tilt eastward and
move slowly inland. Wyoming will continue to be affected by this
trough/low from now till the end of the forecast period (even
longer). Still quite a conglomeration of kludgy model solutions with
regard to this evolving system.

West of the Divide today/tonight, light rain and/or snow showers
continue with a few embedded afternoon thunderstorms possible (small
hail gusty winds associated with the stronger
showers/storms...nothing but ordinary, however). Snow levels will be
near or just above valley floor levels today...then at valley floors
tonight. A half inch to 3 inches of additional snowfall possible
today through above 7500 feet. The next bout of
rain/snow showers comes late Friday night and continues into
Saturday night as the main upper trough/closed low opens and finally
makes its move toward the forecast area. Mostly sub-Advisory snow
amounts are in place for this period.

Strong cyclogenesis takes place across eastern CO today with
forecast strong moisture return taking place around the low by this
evening. Models have, for the most part, finally cut back a bit on
QPF and placed the most significant amounts of precipitation mainly
across far eastern WY/western NE. Far more reasonable amounts of
liquid precip are finally being forecast into the the eastern zones
(much less than the previous few days) as the developing cyclone
races eastward into the Central Plains tonight. Accumulating snow
east of the Divide will not really begin to fall until later tonight
...and then only on grassy areas to begin with as daytime high
temperatures keep roadway temperatures up well into the night. Best
areas for accumulating snowfall east of the Divide from tonight
through Friday will occur north of an intruding cold front -
including portions of the southeast Big Horn Basin, Cody Foothills,
Johnson County, and Natrona County in addition to the Bighorn and
Casper Mountains. Most all of these areas will see less than
Advisory level snowfall. Better amounts will be possible in the
northern Bighorn Mountains, southern Cody Foothills, southeast
Bighorn Basin and northern Johnson County. Still, only worthy of an
SPS at the moment. Friday night the system moves out with the next
chance for light precipitation not returning until later
Saturday/Saturday night.

.LONG TERM...(Sunday through Thursday)

Overview...Cool and unsettled weather will remain over the area
Sunday with scattered snow showers developing across the north and
west; snow showers will be less numerous across the south. A cool
northwest flow will remain over the area Monday with some
lingering snow showers mainly over and near the mountains. Dry
conditions with a warming trend will prevail on Tuesday, the first
day of Spring, and Wednesday. Spring officially begins at 1015 AM
MDT on Tuesday. The next Pacific system will spread chances of
rain and snow showers into the west on Thursday with breezy to
windy and unseasonably mild conditions expected east of the
Continental Divide.

Discussion...Upper air pattern on Sunday morning will feature
complex trough across the Rockies pushing into downstream ridge
over the Plains. ECMWF and GFS show better agreement this morning
depicting strong shortwave moving out of the southern/central
Rockies into the Oklahoma panhandle late Sunday, while weaker
orphan lows remain across the NW U.S. and northern Rockies. These
orphan lows are expected to get caught up in W-NW flow Sunday
night-Monday and should be mostly east of the area on Tuesday.
Best lift and moisture will likely occur Saturday night as
shortwave ejects NNE across western Wyoming before cutting off
over northern Montana on Sunday. Heavier precipitation is expected
across Colorado into the central and northern High Plains on
Sunday with west and central Wyoming while weak flow and slight
instability remain across west and central Wyoming. ECMWF and GFS
show differing scenarios ejecting out the remaining orphan lows
Monday/Monday night, but snow shower activity with these systems
is expected to be light and mostly isolated. Both ECMWF and GFS
have trended slower with next Pacific system coming onshore late
in the week, with ECMWF slower trend more pronounced. We have thus
backed off on bringing in higher precipitation chances into the
west until Thursday.


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

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

Scattered snow showers today and tonight with better coverage in the
mountains. Local MVFR and IFR conditions will occur near the
mountains. Mountains tops will be frequently obscured through the
period. VFR conditions will prevail at the terminal sites after 13Z.

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

Scattered rain showers will occur through 15Z. Then isolated showers
will occur the rest of today and tonight. VFR conditions will
prevail at the terminal sites through the period.


Issued AT 107 AM MDT Thu Mar 15 2018

Seasonal or below (west) to above seasonal (east) temperatures
expected across the the region today. The effects of the
spring/winter weather system continue on the lighter side today west
of the Continental Divide today...still with a little everything
possible - from rain and snow to an isolated thunderstorm. 1 to 3
inches of additional wet snowfall will be possible above 7000 feet
through today...with rain/snow and mixes and much less accumulation
(if any) below 7000 feet. Warm east of the Divide today, becoming
cool and showery again by this evening and continuing through
Friday. Anywhere from a few hundredths of an inch of precipitation
to as much as 2 to 4 tenths across eastern Johnson and Natrona
counties may be found by the end of the day on Friday. This will
equate to a trace to near 3 inches of snowfall on grassy areas
(highest northern Johnson County, southern Cody Foothills, and
southeast Big Horn Basin). Gustiest winds most probable across the
Wind Corridor from eastern Sweetwater through Natrona Counties both
this afternoon and Friday afternoon.




LONG TERM...Meunier
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