Severe Thunderstorm Warning 7, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
155 PM MDT SUN JUN 20 2010

SHORT TERM...TONIGHT THROUGH WEDNESDAY
AS NOTED IN THE MORNING FORECAST UPDATE...IT LOOKS LIKE AN ACTIVE
AFTERNOON AND EARLY EVENING. OVERNIGHT OUTFLOW BOUNDARY HAS JUICED
UP THE LAYER BELOW H7 WITH DEW POINTS HOLDING AROUND 50. THE 18Z
SOUNDING INDICATES A VERY UNSTABLE...LI -7 AND ALMOST 2000
J/KG...ATMOSPHERE WITH A SMALL AMOUNT OF CIN AROUND H7. FOOTHILLS
ARE TRYING TO MIX SOME BUT MAINLY HIGHER UP OR WEST OF THE DIVIDE.
A QUICK MODIFICATION OF THE 18Z SOUNDINGS INDICATE THAT A 74 OVER
A 49 DEW POINT WOULD JUST ABOUT BREAK THE CAPE AND GIVE US 2200
J/KG TO WORK WITH ALONG WITH A -8 LI. ERN NATRONA HAS EVEN JUICER
AIR TO WORK WITH ALONG WITH SERN JOHNSON. THE ENTIRE WATCH AREA
HAS DECENT TO SIGNIFICANT CAPE. THERE IS A WEAK DISTURBANCE
PASSING THROUGH ON WATER VAPOR AND THERE MAY BE A LITTLE JET
SUPPORT LATER OR POSSIBLY ANOTHER WAVE COMING UP FROM UTAH BUT THE
THERMODYNAMICS AND OUTFLOWS SHOULD BE MORE THAN ENOUGH TO FIRE
SOME SEVERE STORMS INTO THE EARLY EVENING.

ON MONDAY...THE OREGON UPPER LOW OPENS UP AND MOVES INTO NRN MT
WITH A SHORTWAVE HITTING NW WYO ALONG WITH THE POSSIBILITY OF A FEW
STORMS. THE SERN ZONES SHOULD BE DRY. ON TUESDAY...IT LOOKS LIKE
ANOTHER STRONGER DISTURBANCE WILL FORM OVER THE GREAT BASIN WITH A
WEAK JET STREAK CUTTING ACROSS OUR SERN ZONES. AT THE SAME TIME...A
WEAK FRONTAL BOUNDARY WILL SAG SOUTH INTO OUR FAR ERN ZONES. HAVE
INTRODUCED AT LEAST ISOLATED STORMS TO MUCH OF THE AREA BASED ON
THIS SHORTWAVE AND MID LEVEL COOLING IN THE SW AND FRONTAL
BOUNDARY ACROSS THE NORTH. ON WEDNESDAY...WE START TO SEE SOME
WEAK RIDGING AND MID LEVEL WARMING WHICH SHOULD CUTOFF THE
CONVECTION IN MOST AREAS EXCEPT PERHAPS THE NW WHERE A WEAK THERMAL
TROUGH MOVES THROUGH.

LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY ABOVE NORMAL
TEMPERATURES EXPECTED THROUGH THE PERIOD WITH SLIGHT CHANCES OF
LATE DAY THUNDERSTORMS ACROSS NORTHERN WYOMING.

MEDIUM RANGE GLOBAL MODELS HAVE GENERALLY BACKED OFF FORCED CHANGES
TO PERSISTENT LARGE-SCALE PATTERN...SUB-TROPICAL HIGH REMAINING OVER
THE SOUTHERN PLAINS/SOUTHEAST U.S. WITH MEAN TROUGH ALONG THE WEST
COAST.  IN ECMWF CASE IT IS SHOWING LESS RETROGRESSION OF
RIDGE...WHILE GFS NOT QUITE AS AGGRESSIVE BRINGING IN WEST COAST
TROUGH. THUS...OVERALL THERE IS LESS SPREAD THROUGH FRIDAY WITH
INCREASING SPREAD OVER THE WEEKEND.

UPPER RIDGE BUILDS ALONG THE DIVIDE WEDNESDAY BETWEEN EXITING TROUGH
OVER THE NORTHERN PLAINS AND PERSISTENT TROUGH ALONG THE WEST COAST.
SURFACE HIGH OVER THE NORTHERN HIGH PLAINS WEDNESDAY WILL RESULT IN
LOW LEVEL UPSLOPE FLOW EAST OF THE DIVIDE WITH MARGINAL
INSTABILITY.  BASINS LIKELY TO BE CAPPED...HOWEVER A FEW
THUNDERSTORMS MAY FIRE OFF ALONG MOUNTAIN/VALLEY BOUNDARY ALONG EAST
SLOPES WITH ELEVATED CONVECTION MOVING EAST INTO BASINS.

RIDGE SHIFTS EASTWARD THURSDAY WITH SOUTHWEST FLOW ALOFT PREVAILING
OVER AREA ONCE AGAIN...WEAK SHORTWAVES IN THE FLOW...A PATTERN VERY
SIMILAR TO THE LAST COUPLE OF DAYS WITH STATIONARY FRONTAL
BOUNDARY ACROSS THE NORTH WITH THE BETTER MOISTURE AND INSTABILITY
ALONG AND NORTH OF THE FRONT. BOTH GFS AND ECMWF BRING DISTINCT
SHORTWAVE ACROSS MONTANA THURSDAY NIGHT WITH FURTHER SOUTHWARD
PUSH OF FRONT AND A BIT COOLER TEMPERATURES ON FRIDAY.

ECMWF SHOWS MORE AMPLIFICATION THAN GFS BY NEXT SUNDAY...DROPPING
UPPER LOW SOUTHEAST ACROSS THE GULF OF ALASKA TO NEAR VANCOUVER
ISLAND...AND AMPLIFYING DOWNSTREAM RIDGE OVER THE CENTRAL U.S.  GFS
MUCH FLATTER WITH FLOW ACROSS THE NORTHERN TIER OF THE U.S.
FORECAST CONTINUES TO BE WEIGHTED TOWARD PERSISTENT SOUTHWEST FLOW
WHICH IS ALSO A ROUGHLY 50/50 BLEND OF THESE TWO SOLUTIONS.

AVIATION...
SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED THIS EVENING OVER
THE NORTHERN TWO THIRDS OF THE FORECAST AREA.  FREQUENT
LIGHTNING...BRIEF HEAVY RAIN... GUSTY WINDS TO 45KTS AND SMALL HAIL
ARE ALL POSSIBLE.  THERE IS A POTENTIAL FOR SEVERE THUNDERSTORMS
OVER JOHNSON COUNTY...NATRONA COUNTY AND THE BIG HORN BASIN THROUGH
03Z.  LARGE HAIL AND GUSTY WIND TO 5`+KTS ARE POSSIBLE WITH THESE
SEVERE THUNDERSTORMS.  WITH THE HEAVY RAIN...MVFR VSBYS AND CIGS ARE
POSSIBLE AS WELL.  VFR CONDITIONS WILL PREVAIL OVER SOUTHERN THIRD
WITH SOME SOUTHWEST WINDS THERE.  FOR MONDAY...A FEW SHOWERS AND
THUNDERSTORMS ARE POSSIBLE OVER THE NORTH...BUT SHOULD BE LESS
NUMEROUS THAN TODAY.  OTHERWISE...AFTERNOON SOUTHWEST WINDS WILL BE
ON THE INCREASE OVER MUCH OF THE AREA WITH VFR CONDITIONS PREVAILING.

FIRE WEATHER...
STRONG TO POTENTIALLY SEVERE THUNDERSTORMS WILL OCCUR ACROSS MUCH
OF NRN AND CNTRL PART OF THE DISTRICT INTO THE EVENING WITH A
DRIER SW FLOW IN THE SWRN SECTIONS. ON MONDAY...ISOLATED LATE DAY
THUNDERSTORMS ARE POSSIBLE IN THE WEST AND NORTH WITH A LITTLE BETTER
COVERAGE ON TUESDAY...ESPECIALLY THE ERN SECTIONS. WEDNESDAY WILL BE
WARMER AND DRIER EXCEPT FOR ISOLATED STORMS IN THE NW SECTIONS.
MIN RH'S WILL BE IN THE TEENS ACROSS THE SOUTH WITH MAINLY 20S NORTH.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...SEVERE THUNDERSTORM WATCH UNTIL
7 PM FOR FREMONT...NATRONA...JOHNSON...HOT SPRINGS...WASHAKIE...BIG
HORN AND PARK COUNTIES.

&&

SHORT TERM...SKRBAC
LONG TERM...AEM
AVIATION...TWB
FIRE WEATHER...AEM

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RIVERTON WY
128 PM MDT SUN JUN 20 2010

WYZ001>020-022>030-211100-
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-
128 PM MDT SUN JUN 20 2010

...SEVERE THUNDERSTORM WATCH FOR MOST OF CENTRAL AND FAR NORTHWEST
WYOMING UNTIL 7 PM MDT...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING.

.DAY ONE...TODAY AND TONIGHT

SEVERE THUNDERSTORMS ARE POSSIBLE ACROSS THE CODY FOOTHILLS...BIGHORN
BASIN...JOHNSON COUNTY AND SOUTH TO FREMONT AND NATRONA COUNTIES THIS
AFTERNOON AND EVENING. THESE STORMS WILL BE CAPABLE OF PRODUCING
LARGE HAIL...DAMAGING WINDS AND FREQUENT DANGEROUS LIGHTNING.

THE MAJORITY OF STREAMS AND RIVERS WILL REMAIN FAIRLY HIGH AND SWIFT
ACROSS THE AREA AS SEASONAL SNOWMELT CONTINUES.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

THE MAJORITY OF STREAMS AND RIVERS WILL REMAIN FAIRLY HIGH AND SWIFT
ACROSS THE AREA AS SEASONAL SNOWMELT CONTINUES.

A SLIGHT CHANCE OF THUNDERSTORMS WILL BE FOUND MAINLY ACROSS
NORTHERN WYOMING THROUGH THIS PERIOD.  SOME OF THE THUNDERSTORMS IN
NORTHEAST JOHNSON COUNTY MAY BE CAPABLE OF PRODUCING LARGE HAIL AND
DAMAGING WINDS MONDAY AFTERNOON AND EVENING.

.SPOTTER INFORMATION STATEMENT...

SPOTTERS ARE ENCOURAGED TO REPORT ANY STREAMS OR RIVERS RISING
RAPIDLY...NEAR BANKFULL OR WHERE FLOODING IS OCCURRING. SPOTTERS
ARE ALSO ENCOURAGED TO REPORT HAIL SIZE AS WELL AS WIND SPEEDS
OVER 40 MPH. THANK YOU.

TEAM RIVERTON

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