Severe Thunderstorm Warning 5, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
400 PM MDT FRI JUN 5 2009

.SHORT TERM...TONIGHT THROUGH MONDAY
CLOSED LOW OVER CENTRAL CALIFORNIA EVER SO SLOWLY SHIFTING EAST-
NORTHEAST THIS AFTERNOON. STRONG JET STREAK ON EAST-SOUTHEAST SIDE
OF THE LOW CONTINUING TO DEVELOP CIRRUS AND ALTO-STRATUS OVER
CENTRAL AND EASTERN WYOMING. THE HIGH CLOUDS ARE LIMITING
CONVECTIVE DEVELOPMENT...KEEPING SURFACE TEMPS IN THE 50S AND 60S
OVER THE CENTRAL BASINS EARLY THIS AFTERNOON. GLOBAL MODEL RH
PROGS BETWEEN H5 AND H3 INDICATING THE POTENTIAL FOR A FEW
DRY/CLEAR AREAS DEVELOPING OVER CENTRAL WYOMING THIS EVENING...POSSIBLY
ALLOWING A FEW AREAS ENOUGH INCOMING SHORT-WAVE RADIATION TO
REACH CONVECTIVE TEMPERATURES...LOW TO MID 70S FOR MANY LOWER
ELEVATIONS. EXPECTING SHOWERS DEVELOPING ALONG AND WEST OF THE
DIVIDE WILL MOVE EASTWARD AND HELP INITIATE CONVECTION LATE THIS
AFTERNOON AND EARLY EVENING. STILL EXPECTING TO SEE AT LEAST
ISOLATED STRONG TO POSSIBLY SEVERE THUNDERSTORMS THIS EVENING FROM
THE CENTRAL BASINS EASTWARD...ESPECIALLY IN JOHNSON AND NATRONA
COUNTIES WHERE DIRECTIONAL WIND SHEAR REMAINS FAVORABLE. SURFACE
LOW PROGGED TO MOVE THROUGH CENTRAL WYOMING THIS EVENING WITH A
NORTHERLY FLOW DEVELOPING ON THE WEST SIDE OF THE LOW THIS
EVENING...WITH UPSLOPE CONDITIONS DEVELOPING EAST OF THE DIVIDE.
WIDESPREAD SHOWERS EXPECTED TO DIMINISH IN COVERAGE AFTER MIDNIGHT
TONIGHT BUT WITH STRONG VORTICITY LOBES AND -17C COLD POOL AT
H5...EXPECT ISOLATED TO SCATTERED SHOWERS WITH ISOLATED THUNDERSTORMS
TO CONTINUE GOING OVER THE NORTH THROUGH THE NIGHT AND INTO
SATURDAY MORNING.

GLOBAL MODELS CONTINUE TO CONSISTENTLY FORECAST THE QUASI-OMEGA
BLOCK TO CONTINUE OVER THE GULF OF ALASKA EXTENDING INTO CENTRAL
CANADA TO ALLOW A CLOSED LOW OVER CENTRAL ALBERTA TO MOVE SOUTH-
SOUTHWESTWARD SATURDAY. THE ASSOCIATED SURFACE LOW EXPECTED TO DEEPEN
OVER EAST-CENTRAL AND SOUTHEAST WYOMING THROUGH THE DAY. THE
NORTHERLY FLOW ON THE WEST SIDE OF THE LOW WILL BRING A STRONG
COLD FRONT THROUGH NORTHERN WYOMING DURING THE AFTERNOON. GFS AND
ECMWF IN GOOD AGREEMENT THAT THE MAIN CAA WILL OCCUR LATE SATURDAY
NIGHT/EARLY SUNDAY MORNING. WIDESPREAD RAIN AND MOUNTAIN SNOW IS
EXPECTED FROM THE BEARTOOTH AND ABSAROKA MOUNTAINS WESTWARD
THROUGH YELLOWSTONE NATIONAL PARK AND TETON NATIONAL PARK SUNDAY.
MODEL SOLUTIONS GENERATING ANYWHERE FROM 0.50 TO 1.25 INCHES OF
WATER LATE SATURDAY NIGHT INTO SUNDAY. THIS WILL HAVE BE WATCHED
CLOSELY AS H7 TEMPS PROGGED TO DROP THE SNOW LEVEL TO ABOUT 6500
FEET SUNDAY MORNING OVER THE NATIONAL PARKS AND POINTS EAST TO THE
BIG HORN MOUNTAINS. SIGNIFICANT SNOW IS POSSIBLE LATE SATURDAY
NIGHT THROUGH THE DAY SUNDAY...ESPECIALLY AT ELEVATIONS ABOVE 7K
FEET FROM THE TETONS NORTH INTO YELLOWSTONE...THEN EAST THROUGH
THE ABSAROKA AND BIG HORN MOUNTAINS. WIDESPREAD RAIN AND MOUNTAIN
SNOW IS EXPECTED FOR MOST AREAS SUNDAY WITH RAIN AND MOUNTAIN SNOW
CONTINUING SUNDAY EVENING. DRY NORTHWEST FLOW ALOFT PROGGED TO
DEVELOP LATE SUNDAY NIGHT THROUGH MONDAY FORENOON. THE
AFOREMENTIONED CLOSED LOW OVER ALBERTA CONTINUES TO SPIN ALONG THE
MONTANA-CANADA BORDER MONDAY...KEEPING SHOWERS POSSIBLE OVER THE
NORTH THROUGH THE DAY MONDAY.

.LONG TERM...MONDAY NIGHT THROUGH FRIDAY
LAST SHORTWAVE ROTATES SWD FROM LARGE...COLD SRN CANADA UPPER LOW
INTO WYOMING LATER MONDAY INTO MONDAY NIGHT WITH ANOTHER ROUND
OF SHOWERS AND THUNDERSTORMS. MODELS DIVERGENCE SIGNIFICANTLY AGAIN
FOR THE REST OF THE PERIOD BUT JUST ABOUT ANY SOLUTION PRODUCED AT
LEAST SCATTERED SHOWERS AND THUNDERSTORMS EACH DAY. THE GFS HAS A
SHORTWAVE/UPPER LOW FORMING OVER THE PACNW AND RACING ACROSS THE
AREA LATE TUESDAY INTO TUESDAY NIGHT WITH WIDESPREAD SHOWERS AND
SOME THUNDERSTORMS ACROSS THE WEST AND NORTH AND LESS FURTHER SOUTH.
THE ECMWF IS SIMILAR TO OUR CURRENT PATTERN WITH A TROUGH OVER THE
SW AND SHORTWAVE ENERGY ACROSS MONTANA IN THE CURRENT NW FLOW.
GENERALLY HITTING THE ENTIRE AREA WITH CHANCE POPS FOR NOW. THE
ECMWF IS MORE AGGRESSIVE WITH A NEW LOW COMING ON THE OREGON COAST
WEDNESDAY WITH INCREASING QG FORCING LATE IN THE DAY AND WIDESPREAD
PCPN IN THE WEST AND SOUTH. THE GFS HAS ANOTHER SHORTWAVE MOVING
SOUTH THRU MT AND WEAKER TROUGH IN THE FAR WEST WITH PCPN GENERALLY
CONFINED TO THE NW AND FAR SOUTH. THE LARGE DIFFERENCES CONTINUE
THURSDAY WHEN A SIGNIFICANT TROUGH HEADS INTO ERN OR AND WRN ID PER
THE ECMWF WHICH SHOWS WIDESPREAD QPF ACROSS THE ENTIRE AREA. THE GFS
SHOWS A WEAKER TROUGH ALONG THE COAST AND ANOTHER SHORTWAVE PASSING
SE ACROSS ERN MT. WILL KEEP IDEA OF CHANCE POPS EVERYWHERE AS EITHER
PATTERN FAVORS AT LEAST SCT DIURNAL CONVECTION IF NOT MORE
SIGNIFICANT PCPN POTENTIAL. ON FRIDAY...THE EURO TAKES THE UPPER LOW
AND WEAKENS IT SOME AS IT MOVES INTO WRN MT AND WRN WYO WITH ANOTHER
ROUND OF WIDESPREAD PCPN. THE GFS ALSO HAS WIDESPREAD PCPN BUT WITH
A DIFFERING PATTERN OF SW FLOW AND FRONTAL INTERACTION AS A NRN HIGH
PLAINS SHORTWAVE MOVES THROUGH THE DAKOTAS. THE PATTERN SHOULD
CONTINUE VERY ACTIVE THROUGH THE MEDIUM RANGE PERIOD BUT THE OVERALL
EVOLUTION AND EXACT DETAILS FROM DAY TO DAY ARE STILL UNCERTAIN.
FIVE DAY PERIOD SHOULD PRODUCE A MODERATE TO HIGH POTENTIAL OF MORE
MODERATE TO LOCALLY HEAVY PCPN ACROSS THE AREA. TEMPERATURES WILL
LIKELY REMAIN BELOW NORMAL AS A COLD FRONT NEVER SEEMS TO COMPLETELY
LEAVE THE AREA EAST OF THE DIVIDE...FED BY CONSTANT SHORTWAVES
DROPPING THROUGH MT OR THE NRN HIGH PLAINS...TROUGH TO THE WEST...
CLOUD COVER AND LATE DAY SHOWERS/THUNDERSTORMS KEEPING CONSIDERABLE
CLOUD COVER.

&&

.AVIATION...
UNSTABLE AIRMASS ACROSS WESTERN AND CENTRAL WYOMING THIS EVENING
WILL ALLOW ISOLATED SHOWERS AND THUNDERSTORMS TO CONTINUE THROUGH
06Z. OTHERWISE...MID LEVEL CLOUD DECKS BETWEEN 5000 AND 8000 FEET
AND VFR CONDITIONS ARE EXPECTED OVERNIGHT. SURFACE LOW PRESSURE WILL
DEVELOP IN EASTERN WYOMING SATURDAY...AND WEST TO SOUTHWEST WINDS AT
10 TO 20 KT WITH GUSTS OF 20 TO 30 KT WILL DEVELOP. STRONGER
PRESSURE GRADIENT IN THE WIND CORRIDOR WILL GENERATE GUSTS UP TO 40
KT AT KRKS AND KCPR. VFR CONDITIONS WILL CONTINUE ACROSS THE REGION
SATURDAY WITH WEAK HIGH PRESSURE AND DRIER AIR MOVING INTO WYOMING
FROM THE WEST.

&&

.FIRE WEATHER...
COOL AND WET WEATHER IS EXPECTED THROUGHOUT THE DISTRICT TONIGHT
THROUGH THE WEEKEND. NUMEROUS SHOWERS AND THUNDERSTORMS WILL
TRANSITION TO RAIN AND MOUNTAIN SNOW LATE SATURDAY NIGHT AND
SUNDAY AS AN UPPER LOW FROM CANADA DROPS SOUTHWARD.

&&

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

SHORT TERM...CHURCH
LONG TERM...SKRBAC
AVIATION...SMITH
FIRE WEATHER...CHURCH

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RIVERTON WY
411 PM MDT FRI JUN 5 2009

WYZ001>020-022>030-061200-
ABSAROKA MOUNTAINS-BIGHORN MOUNTAINS SOUTHEAST-
BIGHORN MOUNTAINS WEST-CASPER MOUNTAIN-CODY FOOTHILLS-
EAST SWEETWATER COUNTY-FLAMING GORGE-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-JACKSON HOLE-LANDER FOOTHILLS-
NATRONA COUNTY LOWER ELEVATIONS-NORTH BIG HORN BASIN-
NORTHEAST JOHNSON COUNTY-OWL CREEK AND BRIDGER MOUNTAINS-
ROCK SPRINGS AND GREEN RIVER-SALT RIVER AND WYOMING RANGES-
SOUTH LINCOLN COUNTY-SOUTHEAST BIG HORN BASIN-
SOUTHEAST JOHNSON COUNTY-SOUTHWEST BIG HORN BASIN-STAR VALLEY-
TETON AND GROS VENTRE MOUNTAINS-UPPER GREEN RIVER BASIN-
UPPER GREEN RIVER BASIN FOOTHILLS-UPPER WIND RIVER BASIN-
WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST-WIND RIVER MOUNTAINS WEST-
YELLOWSTONE NATIONAL PARK-
411 PM MDT FRI JUN 5 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING.

.DAY ONE...TONIGHT

MORE NUMEROUS SHOWERS AND THUNDERSTORMS ARE EXPECTED THIS EVENING.
SOME OF THE STORMS COULD PRODUCE SMALL HAIL ALONG WITH WIND GUSTS OF
40 MPH...HEAVY RAINFALL AND FREQUENT LIGHTNING. THUNDERSTORMS
COULD BE STRONGER IN EASTERN NATRONA COUNTY AND SOUTHEASTERN JOHNSON
COUNTY EARLY THIS EVENING WHERE ISOLATED SEVERE STORMS PRODUCING HIGH
WINDS AND LARGE HAIL WILL BE POSSIBLE.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

ANOTHER STORM SYSTEM WILL MOVE IN OVER THE WEEKEND WITH
WIDESPREAD SHOWERS AND ISOLATED THUNDERSTORMS EXPECTED FOR MUCH
OF THE AREA SATURDAY AND SUNDAY ALONG WITH MUCH COOLER WEATHER. 3 TO
5 INCHES OF SNOWFALL IS EXPECTED OVER THE NORTHERN PORTION OF
YELLOWSTONE PARK AND THE NORTHERN ABSAROKA MOUNTAINS SATURDAY NIGHT
AND SUNDAY AS WELL AS OVER THE REST OF THE MOUNTAINS ABOVE
9000 FEET EAST OF THE DIVIDE. THE REST OF THE MOUNTAINS WEST OF
THE DIVIDE AS WELL AS AT ELEVATIONS BETWEEN 7500 AND 9000 FEET
EAST OF THE DIVIDE SHOULD SEE ONE TO TWO INCHES OF SNOW DURING
THIS TIME FRAME. POWDER RIVER AND GRANITE PASSES IN THE BIG HORN
MOUNTAINS...THE CHIEF JOSEPH HIGHWAY AS WELL AS TOGWOTEE PASS
COULD BE IMPACTED BY THIS SNOW...MAINLY SATURDAY NIGHT AND SUNDAY
MORNING.

AN UNSETTLED WEATHER PATTERN WILL CONTINUE TO BRING A CHANCE OF
SHOWERS AND THUNDERSTORMS MUCH OF NEXT WEEK...ESPECIALLY DURING THE
AFTERNOON AND EVENING HOURS.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS ARE ASKED TO REPORT WIND GUSTS OVER 40 MPH...ANY HAIL AND
HEAVY RAINFALL. SPOTTERS ARE ALSO ENCOURAGED TO REPORT ANY
POSSIBLE BANKFULL STREAMS OR RIVERS. THANK YOU.

TEAM RIVERTON

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