Area Forecast Discussion
Issued by NWS Riverton, WY

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734
FXUS65 KRIW 181032
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
332 AM MST Sat Feb 18 2017

.SHORT TERM...Today through Monday night

A WEAK SHORTWAVE MOVING NORTHEAST ACROSS WYOMING ATTM ON THE
BACKSIDE OF THE FLAT RIDGE AXIS HAS BEEN PRODUCING AREAS OF LIGHT
SNOW IN THE WEST AND SOUTHWEST AS WELL AS VERY SPOTTY RAIN/SNOW
SHOWERS EAST OF THE DIVIDE. MOST OF THE PRECIP EAST OF THE DIVIDE
AS PROGRESSED EAST TO NATRONA/JOHNSON COUNTIES AND WILL BE EXITING
THE AREA IN A FEW HOURS. MEANWHILE, THE AREAS OF SNOW IN THE WEST
HAS BEEN TRENDING TOWARD DIMINISHING. LIGHT SNOWFALL WILL
CONTINUE IN THE WEST TODAY AND TONIGHT WITH THE CONTINUED
SOUTHWESTERLY UPSLOPE COMPONENT. AS THE TROUGH NOW Aligned ALONG
THE PACIFIC COAST APPROACHES WYOMING, SNOWFALL WILL BECOME HEAVIER
IN THE WEST SUNDAY WITH SOME OF THE SNOW SPREADING EAST INTO
SWEETWATER COUNTY. MUCH OF THE SNOW WILL BE OF THE CONVECTIVE
NATURE AS COOLER AIR ALOFT MOVES IN WITH THE APPROACHING TROUGH
AXIS. BY SUNDAY EVENING, RAIN TURNING TO SNOW WILL SPREAD EAST OF
THE DIVIDE. RAIN AND SNOW BANDS WILL LIKELY DEVELOP OFF THE OWL
CREEK AND BRIDGER MOUNTAINS AND WILL TRAIN SOUTHEAST IN THE NW
FLOW BEHIND THE MID LEVEL CIRCULATION WHICH BY THEN SHOULD HAVE
DEVELOPED OVER NORTHEAST COLORADO. THIS IS THE SECOND NIGHT IN A
ROW THAT THE GFS HAS ADVERTISED UP TO A HALF INCH OF PRECIP OVER
OUR EASTERN ZONES SO INCREASED QPF AMOUNTS ACROSS NATRONA/JOHNSON
COUNTIES, THINKING THAT CONVECTIVE PRECIP BANDING COMING OFF THE
BIG HORN MTNS COULD BE INFLUENCING THE INCREASED MODEL QPF. HAVE
INDICATED 4 TO 5 INCHES OF SNOW FOR SUNDAY NIGHT ACROSS THE BIG
HORN MTNS AND LESS THAN AN INCH ELSEWHERE AS PRECIP WILL BEGIN AS
RAIN AT THE LOWER ELEVATIONS. CASPER MTN COULD SEE AN INCH.
MEANWHILE, OUT WEST, EXPECT 3 TO 6 INCHES OF SNOW ARE EXPECTED TO
ACCUMULATE IN THE WESTERN MOUNTAINS AND 2 TO 4 INCHES IN THE
WESTERN VALLEYS SUNDAY AND SUNDAY NIGHT.

THEN THE NEXT WEATHER FEATURE WILL QUICKLY MOVE IN BEHIND SUNDAYS
SYSTEM ON MONDAY IN THE FORM OF A STRONG MOIST WEST SOUTHWESTERLY
PACIFIC FLOW THAT WILL DRAW IN ENOUGH MOISTURE TO PRODUCE ANOTHER
SIGNIFICANT SNOWFALL ACCUMULATION IN THE WEST. FOR NOW, HAVE
INDICATED ANOTHER 8 TO 12 INCHES OF SNOWFALL IN THE TETONS/GROS
VENTRE RANGE, 6 TO 9 INCHES IN THE JACKSON VALLEY, 5 TO 8 INCHES IN
THE SALT AND WYOMING RANGES, 4 TO 6 INCHES IN THE STAR VALLEY, AND 2
TO 5 INCHES IN THE UPPER GREEN RIVER BASIN FOOTHILLS. FOR MONDAY AND
MONDAY NIGHT. ISENTROPIC LIFT WILL BE A STRONG COMPONENT WITH THIS
EVENT AS WELL AS THE WEST SOUTHWESTERLY UPSLOPE FLOW. SOUTHWEST
WINDS WILL INCREASE FROM RKS TO CPR AS WELL AS OVER THE HIGHER
TERRAIN AS THE WEST SOUTHWEST GRADIENT TIGHTENS ALONG WITH SURFACING
50 TO 60 KNOT 700MB WINDS SURFACING UNDERNEATH THE RIGHT FRONT QUAD
OF THE JET.

ASIDE FROM A SLIGHT COOL DOWN AFTER SUNDAY NIGHT BEHIND SUNDAYS
TROUGH AXIS, TEMPERATURES DURING THE NEXT FEW DAYS WILL BE MORE OR
LESS SEASONABLY COOL UNDER THE MODERATING FLOW OFF THE PACIFIC.

.LONG TERM...Tuesday through Saturday

A significant trough will push into the West Coast Tuesday into
Tuesday night. Ahead of this trough, a moist westerly flow and a
shortwave will likely result in widespread precipitation west of
the Divide, and mild and mainly dry conditions east. Significant
mountain snow is possible, but lower elevations could be mixed
with rain or all rain Tuesday morning before changing to snow as
a Pacific cold front moves across the area Tuesday/Tuesday
evening. This front could also result in some precip shifting east
of the Divide. However the main issue east of the Divide Tuesday
will be strong to potentially high wind.

The main trough moves into the Great Basin/Intermountain West
Wednesday with snow continuing out west. As this occurs, good
chances of snow remain across the west in the moist west/southwest
flow. East of the Divide will be much cooler, but precipitation
chances still look iffy. This will change Wednesday night and Thursday
as the trough pulls across the Rockies. Models in decent agreement
with a moist upslope pattern taking shape late Wednesday
night/early Thursday with possibly some isentropic lift ahead of
the 700mb low Wednesday night. This has the potential to bring
significant snowfall for areas east of the Divide across central
Wyoming Wednesday night through Thursday evening time frame if
models don`t change. Kept forecasted likely PoPs with no major
changes there.

Drier conditions are expected in wake of this system Friday into
Saturday, but waves in the flow still warrant slight to some
chance PoPs im most areas.

Above average temperatures Tuesday most areas, with more seasonal
temps Wednesday in wake of the cold front late Tuesday/Tuesday
night. Mainly below average temperatures are then expected
Thursday into the weekend, but nothing brutal considering it is
still February.

&&

.AVIATION.../12Z ISSUANCE/

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Areas of snow will occur through 18Z with MVFR to IFR conditions.
Mountains will be frequently obscured. After 18Z, snow will be
mainly confined to the mountains and foothills with MVFR conditions
in these locations. The terminal sites will improve to VFR after
18Z through the rest of the period.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail at the terminal sites through the
period. Isolated rain and snow showers will occur through 18Z.

&&

.FIRE WEATHER...

THE NEXT FEW DAYS WILL FEATURE MORE ACCUMULATING SNOWFALL IN THE
FAR WEST WITH SOME ACCUMULATING SNOWFALL ALONG AND EAST OF THE BIG
HORN MOUNTAINS SUNDAY NIGHT, MAINLY IN THE BIG HORN MOUNTAINS.
EXCEPT FOR A SLIGHT COOL DOWN SUNDAY NIGHT, TEMPERATURES WILL BE
SEASONABLY COOL FOR THE NEXT FEW DAYS. EXPECT WEST SOUTHWEST WINDS
TO INCREASED MONDAY AND MONDAY NIGHT.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Lipson
LONG TERM...WM
AVIATION...AR
FIRE WEATHER...Lipson



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