Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180944
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
344 AM MDT MON MAY 18 2015

.SHORT TERM...MONDAY THROUGH WEDNESDAY NIGHT

RADAR CONTINUES TO SHOW COME LIGHT PRECIPITATION ACROSS THE CENTRAL
PORTIONS OF THE COUNTY WARNING AREA BUT NOTHING OF CONSEQUENCE. MOST
OF THIS SHOULD BE OVER BY AROUND SUNRISE. TODAY LOOKS LIKE A BREAK
FOR SOME AREAS BETWEEN THE ACTIVE WEATHER...MAINLY EAST OF THE
DIVIDE. THERE WILL NOT BE MUCH SUNSHINE BUT A BIT OF RIDGING BETWEEN
THE TWO TROUGHS SHOULD KEEP THE AREA DRY MOST OF THE DAY.
TEMPERATURES WILL LIKELY BE A FEW DEGREES WARMER THEN YESTERDAY BUT
STILL CHILLY FOR THIS TIME OF YEAR. HOWEVER...WEST OF THE DIVIDE
ANOTHER SURGE OF MOISTURE WILL MOVE IN AND KEEP THE CHANCE FOR
MAINLY AFTERNOON SHOWERS AND THUNDERSTORMS IN THE FORECAST. AND
AGAIN WITH THE LOW WET BULB ZERO TEMPERATURES...SMALL HAIL WILL BE
THE MAIN THREAT FROM THE STORMS. SOME MOISTURE WILL PRESS ACROSS THE
DIVIDE TONIGHT HOWEVER WITH A CHANCE OF SHOWERS FOR MOST AREAS.

THERE ARE SOME KNOWNS AND SOME UNKNOWNS FOR TUESDAY. ANOTHER UPPER
LEVEL LOW WILL MOVE THROUGH THE GREAT BASIN AND PASS SOUTH OF THE
COWBOY STATE. WE HAVE A RATHER MOIST FLOW MOVING INTO THE AREA SO
MUCH OF TUESDAY FOR MOST OF THE STATE WILL BE THE
3C`S...CLOUDY...COOL AND CRUDDY. WITH VERY LITTLE CAPE WE HAVE JUST
GONE WITH STRATIFORM PRECIPITATION EAST OF THE DIVIDE. WE ARE NOT
HAVING DOUBTS OF ANY THUNDER EVEN WEST OF THE DIVIDE WITH LITTLE
INSTABILITY BUT WE HAVE LEFT IT IN FOR NOW. THE NEXT QUESTION...AND
THIS ONE IS UP IN THE AIR...IS THE POTENTIAL FOR SOME LATE SEASON
SNOW FOR LATER TUESDAY INTO TUESDAY NIGHT. AND THERE ARE SUBSTANTIAL
DIFFERENCES IN THE MODELS...THE NAM IS COLDER WITH 700 MILLIBAR
TEMPERATURES FALLING TO AROUND MINUS 4 TO MINUS 5 WHICH WOULD DROP
SNOW LEVELS AS LOW AS 6000 FEET FOR TUESDAY EVENING. HOWEVER...IT
DOES MOVE PRECIPITATION OUT FAIRLY QUICKLY WITH ONLY SHOWERS LEFT
OVER BY TUESDAY NIGHT. MEANWHILE...THE GFS IS WETTER. HOWEVER...IT
IS WARMER WITH 700 MILLIBAR TEMPERATURES GENERALLY IN THE MINUS 2 TO
MINUS 3 RANGE WHICH WOULD HOLD SNOW LEVELS AROUND 7500 FEET. FOR
NOW...WE SPLIT THE MIDDLE. AS FOR ANY POTENTIAL HIGHLIGHTS...SINCE
IT IS STILL A WAYS OFF AND WITH THE UNCERTAINTY WE WILL PUNT FOR
NOW.

FOR WEDNESDAY...IT LOOKS TO BE A BIT LESS WET AS THE UPPER LEVEL LOW
MOVES AWAY TO THE EAST. AGAIN...THERE ARE DIFFERENCES IN THE MODELS.
THE NAM IS QUITE DRY WITH DRIER NORTHWEST FLOW AND ONLY SOME SHOWERS
OVER THE MOUNTAINS. MEANWHILE...THE GFS IS WETTER WITH SOME SHOWERS
HOLDING ON...ESPECIALLY ACROSS THE WEST. AGAIN FOR NOW...WE SPLIT
THE MIDDLE IN KEEPING THE WEST WETTER AND GRADUALLY DRYING OUT THE
EAST THROUGH THE AFTERNOON AND NIGHTTIME HOURS. TEMPERATURES SHOULD
MODERATE WEDNESDAY BUT REMAIN BELOW NORMAL FOR MOST AREAS.

.LONG TERM...THURSDAY THROUGH MEMORIAL DAY

SYNOPSIS...AFTER A LETUP IN PRECIPITATION ON WEDNESDAY...THE NEXT
PACIFIC STORM SYSTEM WILL BEGIN TO SPREAD SHOWERS AND THUNDERSTORMS
NORTH ACROSS THE AREA THURSDAY...WITH COOL AND SHOWERY WEATHER TO
FOLLOW ON FRIDAY AND SATURDAY.  COOL AND UNSETTLED CONDITIONS WILL
FOLLOW SUNDAY AND MEMORIAL DAY WITH THE BEST CHANCES FOR SHOWERS
ACROSS THE WESTERN MOUNTAINS.

DISCUSSION...BLOCKING RIDGE OVER WESTERN CANADA FORECAST TO REMAIN
THROUGH MEMORIAL DAY WEEKEND AND BEYOND.  PACIFIC STORM SYSTEMS
MOVING INTO THE NORTHEAST PACIFIC WILL CONTINUE TO SPLIT AROUND THIS
RIDGE WITH THE SOUTHERN END OF THE SPLIT TAKING THE STORMS SOUTH OFF
THE WEST COAST BEFORE TURNING EAST INTO SOUTHERN CALIFORNIA...THEN
LIFTING NE ACROSS THE GREAT BASIN AND CENTRAL ROCKIES.

THE NEXT IN THIS SERIES OF STORMS MOVING ACROSS 150W EARLY THIS
MORNING WILL MOVE INTO SOUTHERN CALIFORNIA FRIDAY MORNING.  A
SOUTHERLY DIFLUENT FLOW WILL PRECEDE THIS SYSTEM ON THURSDAY WITH A
COUPLE OF SHORTWAVES IN THE FLOW.  SHOWERS AND THUNDERSTORMS ARE
EXPECTED TO SPREAD ACROSS THE AREA FROM THE SOUTH AND WEST THROUGH
THE DAY.  THE MAIN UPPER LOW WILL LIFT ACROSS THE FOUR CORNERS
REGION FRIDAY NIGHT WITH SOME SECONDARY CIRCULATIONS FURTHER NORTH
OVER WYOMING...WITH THE SYSTEM AS A WHOLE SLOWLY PROGRESSING
EASTWARD ACROSS THE CENTRAL ROCKIES ON SATURDAY...AND INTO THE HIGH
PLAINS SATURDAY NIGHT.  THIS SYSTEM WILL NOT BE NEARLY AS COLD AS
THE LAST FEW WITH SNOW LEVELS EXPECTED TO BE BETWEEN 8K-9KFT DURING
THE DURATION OF THE STORM.  SCT-NMRS SHOWERS AND ISOLATED
THUNDERSTORMS ARE EXPECTED OVER THE AREA FRIDAY THROUGH SATURDAY
WITH PRECIPITATION DECREASING ACROSS THE CENTRAL AND EAST THROUGH
SATURDAY NIGHT.

MEDIUM-RANGE MODELS CONTINUE TO ADVERTISE AN UPPER LOW THAT SPLITS
OFF ALONG THE WEST COAST OF BRITISH COLUMBIA THIS
WEDNESDAY...BECOMING TRAPPED UNDER THE WESTERN CANADIAN RIDGE FOR A
COUPLE OF DAYS BEFORE DROPPING SOUTH INTO THE NW U.S. OVER THE
WEEKEND.  THERE IS STILL NOT A GREAT DEAL OF CONFIDENCE IN WHERE
THIS LOW WILL EVENTUALLY WIND UP...BUT SOME MODEL CONSISTENCY WITH
THIS SYSTEM OVER THE LAST FEW RUNS WARRANTS KEEPING SOME POPS IN THE
FORECAST SUNDAY AND MEMORIAL DAY...ESPECIALLY ACROSS THE WEST.

TEMPERATURES THROUGH THE PERIOD WILL REMAIN BELOW SEASONAL NORMALS
WITH HIGHS ACROSS THE BASINS AND VALLEYS MOSTLY IN THE 55-65 RANGE
THROUGH MEMORIAL DAY WEEKEND...FRIDAY AND SATURDAY EXPECTED TO BE
THE COOLEST/WETTEST DAYS.  LOW TEMPERATURES WILL ALSO CONTINUE TO BE
MODERATED BY CLOUD COVER AND A MOIST BOUNDARY LAYER THROUGH THE
PERIOD.

&&

.AVIATION.../12Z ISSUANCE/

AREAS OF MVFR TO IFR CEILINGS THIS MORNING DUE TO LOW CLOUDS. POSSIBLE
FOG WEST OF THE DIVIDE UNTIL 17Z WITH LOCAL LIFR CONDITIONS. THE LOW
CEILINGS WILL GRADUALLY BREAK OR LIFT TO VFR CONDITIONS BY 18Z.

AFTER 19Z SHOWERS AND ISOLATED THUNDERSTORMS WILL DEVELOP ALONG AND
WEST OF THE DIVIDE AND OVER THE SOUTH. THIS ACTIVITY WILL CONTINUE
THROUGH 06Z TUESDAY. LOCAL MVFR CONDITIONS WILL OCCUR ALONG WITH
OCCASIONAL MOUNTAIN OBSCURATION. ELSEWHERE AFTER 19Z VFR CONDITIONS
WILL PREVAIL WITH VERY ISOLATED SHOWER ACTIVITY THROUGH 06Z TUE.
&&

.FIRE WEATHER...

FIRE DANGER RELATIVELY LOW INTO WEDNESDAY AS SEASONAL TO
COOL AND RELATIVELY MOIST CONDITIONS PERSIST. HOWEVER...IT WILL BE
UNSTABLE AGAIN TODAY IN THE SOUTH AND WEST...OFFERING UP CHANCES FOR
MORE THUNDERSTORMS AND ASSOCIATED LIGHTNING. EAST OF THE
DIVIDE...ANY LEFTOVER LIGHT RAIN SHOULD END EARLY WITH MAINLY DRY
CONDITIONS THROUGH THE DAYLIGHT HOURS. THERE IS A GOOD CHANCE FOR
WETTING RAIN TUESDAY WITH HIGH ELEVATION SNOW A GOOD POSSIBILITY FOR
TUESDAY AFTERNOON AND POSSIBLY TUESDAY NIGHT.
&&

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

$$

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







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