Area Forecast Discussion
Issued by NWS Riverton, WY

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391
FXUS65 KRIW 160933
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
333 AM MDT SAT MAY 16 2015

.SHORT TERM...SATURDAY THROUGH MONDAY NIGHT

IT WAS ANOTHER VERY ACTIVE DAY YESTERDAY WITH FUNNEL
CLOUDS...HAILSTORMS...SOME FLOODING AND EVEN HIGHER ELEVATIONS
SNOW. IN OTHER WORDS...IT WAS MAY IN WESTERN AND CENTRAL WYOMING.
AS OF NOW...A DRY SLOT HAS WORKED ITS WAY ACROSS MOST OF THE AREA
WITH QUIET CONDITIONS WITH THE EXCEPTION OF THE NORTHWEST CORNER
WHERE SOME WRAP AROUND MOISTURE CONTINUES TO BRING SOME MAINLY
LIGHT PRECIPITATION AT THIS TIME. IN ADDITION...WITH THE RAIN AND
SOME CLEARING SKIES...PATCHY FOG HAS DEVELOPED ACROSS AREAS THAT
SAW THE HEAVIEST PRECIPITATION YESTERDAY. AS FOR TODAY...BOTH THE
NAM AND GFS ARE FAIRLY CONSISTENT IN KEEPING THE VAST MAJORITY OF
PRECIPITATION ACROSS NORTHERN AND WESTERN AREAS THROUGH MOST OF
TODAY. IN THE WEST...RAIN AND SNOW SHOWERS WILL CONTINUE. AS FOR
SNOW LEVELS...THE MODELS HAVE TRENDED A BIT WARMER WITH 700
MILLIBAR TEMPERATURES ONLY FALLING TO AROUND MINUS 1 TO MINUS 2 BY
THE END OF THE DAY. THIS WOULD HOLD SNOW LEVELS ABOVE 7500 FEET
AND A LOT OF THE SNOW WOULD NOT STICK WITH THE STRONG MAY SUN.
MEANWHILE...EAST OF THE DIVIDE WE HAVE KEPT A SLIGHT CHANCE IN FOR
LATE DAY SHOWERS AND THUNDERSTORMS WITH A DECENT AMOUNT OF
MOISTURE AT THE SURFACE AND SOME FORCING FROM A COLD FRONT THAT
WILL MOVE TOWARD THE AREA. LIKE YESTERDAY...ANY STORM COULD HAVE
SOME GUSTY WINDS...HAIL AND EVEN SOME HEAVY RAINS. HOWEVER...MANY
AREAS WILL BE DRY A VAST MAJORITY OF THE DAY...ESPECIALLY THE
FURTHER SOUTH AND EAST YOU GO.

FOR TONIGHT...THE FRONT WILL MOVE SOUTHEASTWARD ACROSS THE STATE AND
BRING IN SOME COOLER AIR. AGAIN...THE MODELS HAVE TRENDED A BIT
WARMER WITH 700 MILLIBAR TEMPERATURES A BIT HIGHER THAN YESTERDAY.
SO...SNOW LOOKS TO HOLD ABOVE 7000 FEET WITH ANY DECENT ACCUMULATION
ABOVE 9000 FEET. THERE IS A CHANCE SOME AREAS WOULD COME IN WITH
ADVISORY LEVEL AMOUNTS...BUT SINCE IMPACTS WOULD BE MINIMAL...WE
WILL NOT ISSUE ANY. OTHERWISE...EXPECT MOSTLY CLOUDY AND UNSETTLED
CONDITIONS WITH SHOWERS POSSIBLE ACROSS MOST AREAS WITH A GUSTY WIND
DEVELOPING ACROSS AREAS FAVORED WITH NORTH TO NORTHWESTERLY FLOW AND
STRONG COLD ADVECTION SUCH AS THE NORTHERN BIG HORN BASIN AND
NORTHERN JOHNSON COUNTY. SUNDAY ALSO LOOKS LIKE A MOSTLY CLOUDY AND
COLD DAY WITH A CONTINUING CHANCE OF SHOWERS ACROSS MOST OF THE AREA
ALONG WITH CONTINUED GUSTY WINDS ACROSS THE AFOREMENTIONED AREAS.
CONTINUITY LOOKED REASONABLE FOR THIS DAY SO WE MADE FEW CHANGES.

SHORTWAVE RIDGING THEN MOVES ACROSS THE STATE FOR SUNDAY NIGHT TO
HELP SHUT OFF THE SHOWERS. HOWEVER...THE DRY PERIOD LOOKS TO BE
SHORT LIVED AS ANOTHER SURGE OF MOISTURE MOVES IN FROM THE SOUTHWEST
FOR MONDAY. AT THIS TIME...BOTH THE NAM AND GFS HOLD MOST OF THE
MOISTURE WEST OF THE DIVIDE FOR MONDAY. AS A RESULT WE DID TRIM POPS
BACK A BIT FOR MONDAY DURING THE DAY EAST OF THE DIVIDE. WE ALSO
DECREASED SKY COVER A BIT AND RAISED TEMPERATURES A COUPLE OF
DEGREES.


.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...COOL AND SHOWERY WEATHER WITH ISOLATED TO SCATTERED
AFTERNOON/EVENING THUNDERSTORMS WILL CONTINUE ACROSS THE AREA
THROUGH THIS PERIOD.

DISCUSSION...A SERIES OF PACIFIC STORM SYSTEMS WILL CONTINUE TO
UNDERCUT A BLOCKING RIDGE OVER WESTERN CANADA THROUGH THIS PERIOD.
THE STORM TRACK WILL REMAIN RELATIVELY UNCHANGED WITH UPPER LOWS
DIVING DOWN THE WEST COAST INTO THE SOUTHWEST U.S. BEFORE LIFTING NE
ACROSS THE GREAT BASIN/CENTRAL ROCKIES.  THE NEXT IN THIS SERIES OF
STORM SYSTEMS (IN THE NE PACIFIC CROSSING 140W EARLY THIS MORNING)
WILL BE CENTERED NEAR LAS VEGAS BY 12Z TUESDAY WITH SHORTWAVE
EJECTING OUT AHEAD THIS SYSTEM NE ACROSS COLORADO INTO
WYOMING/WESTERN NEBRASKA DURING THE DAY.  AIRMASS EXPECTED TO BE
ONLY SLIGHTLY UNSTABLE AHEAD OF THIS SYSTEM WITH SFC HIGH PRESSURE
OVER THE NORTHERN PLAINS...RESULTING IN NUMEROUS SHOWERS BUT
THUNDERSTORMS SHOULD REMAIN ISOLATED AND BELOW SEVERE THRESHOLDS.
THE MAIN UPPER LOW IS EXPECTED TO SHEAR OUT ACROSS THE AREA ON
WEDNESDAY KEEPING SCT SHOWERS OVER MOST OF THE AREA. HIGHS BOTH
TUESDAY AND WEDNESDAY WILL BE 10-15F BELOW NORMAL ACROSS MOST
AREAS...OR HIGHS MOSTLY IN THE 50S ACROSS THE LOWER ELEVATIONS.

A BRIEF LETUP IN SHOWER ACTIVITY IS EXPECTED WEDNESDAY NIGHT INTO
THURSDAY MORNING BETWEEN SYSTEMS...WITH NEXT PACIFIC SYSTEM MOVING
ONTO (GFS) OR NEAR THE SOUTHERN CALIFORNIA COAST (ECMWF) THURSDAY.
SOUTHERLY DIFLUENT FLOW AHEAD OF THIS SYSTEM AND SLIGHT INSTABILITY
WILL RESULT IN SCT SHOWERS AND THUNDERSTORMS ACROSS MOST AREAS
THURSDAY AFTN/EVE...POSSIBLY LESS ACTIVITY OVER THE NORTH AND EAST
WHERE A DRIER AND MORE STABLE AIRMASS IS EXPECTED.  THIS SYSTEM WILL
THEN SLOWLY PROGRESS INTO THE GREAT BASIN/CENTRAL ROCKIES FRIDAY AND
SATURDAY...SO AT LEAST THE START OF MEMORIAL DAY WEEKEND IS LOOKING
LIKE MOST MEMORIAL DAY WEEKENDS IN WYOMING...COOL AND WET.


&&

.AVIATION.../12Z ISSUANCE/

RAIN AND MOUNTAIN SNOW FOR NW WY TODAY. THE RAIN AND SNOW WILL
SLOWLY DECREASE AFTER 00Z. EXPECT MVFR TO IFR CONDITIONS OVER FAR
NORTH AND NORTHWEST WY THROUGH 06Z SUNDAY WITH FREQUENT MOUNTAIN
OBSCURATION. SHOWERS WILL FORM IN SOUTHWEST WY THIS AFTERNOON AND
CONTINUE INTO THROUGH THE EVENING. ISOLATED THUNDERSTORMS WILL BE
POSSIBLE IN SOUTHWEST WY AS WELL. ISOLATED TO SCATTERED SHOWERS WILL
DEVELOP OVER THE REST OF THE REGION AFTER 21Z AND THIS WILL CONTINUE
THROUGH 06Z SUNDAY. EXPECT AREAS OF MVFR CONDITIONS IN SW WY AFTER
00Z. AREAS OF MVFR CONDITIONS WILL DEVELOP EAST OF THE DIVIDE
AFTER 03Z AS LOW CLOUDS DEVELOP.

&&

.FIRE WEATHER...

FIRE DANGER RELATIVELY LOW THROUGH THE WEEKEND AS SEASONAL TO COOL
AND RELATIVELY MOIST CONDITIONS PERSIST. HOWEVER...IT WILL BE
UNSTABLE AGAIN TODAY...OFFERING UP CHANCES FOR MORE THUNDERSTORMS
AND ASSOCIATED LIGHTNING. SNOWS IS LIKELY TO RETURN TO THE HIGHER
ELEVATIONS AGAIN THIS WEEKEND WITH SNOW LEVELS LOWERING TO AROUND
7000 FEET AT TIMES. AFTER A SHORT BREAK SUNDAY NIGHT...MORE SHOWERS
ARE POSSIBLE WEST OF THE DIVIDE FOR MONDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...HATTINGS
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...HATTINGS








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