Severe Thunderstorm Warning 1, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
307 AM MDT WED MAY 7 2008

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT
FLAT RIDGE OVER WYOMING THIS MORNING WITH SOME SCATTERED
CONVECTION OVER YELLOWSTONE AND ABSAROKAS...WITH ISOLATED CONVECTION
OVER THE NORTH AND CENTRAL. ALSO HAVE A NORTHEAST GRADIENT OVER
THE CENTRAL AND NORTH WITH SURFACE LOW OVER SOUTHEAST WYOMING AND
EASTERN UTAH. THIS IS HELPING TO SLOWLY DRAW IN HIGHER
DEWPOINTS. SHORTWAVE MOVING ACROSS WA/OR AND N CA THIS
MORNING...WHICH WILL SPREAD EAST AND MOVE ACROSS WYOMING TONIGHT
INTO THURSDAY. H7 LOW TO DEVELOP OVER SOUTHWEST MONTANA AROUND MID
MORNING TODAY AND MOVE ALONG THE MT/WY BORDER...SPREADING SHOWERS
ACROSS THE NORTH...ESPECIALLY IN THE NORTHWEST AND NORTHERN
BIGHORN MOUNTAINS. DAYTIME HEATING WILL ALLOW SHOWERS TO SPREAD
FURTHER SOUTH ACROSS THE CENTRAL AND SOUTH THIS AFTERNOON AND
EVENING. WILL HAVE DECENT ENOUGH INSTABILITY AND MOISTURE...BUT
CLOUD COVER MAY INHIBIT CONVECTION. WILL LEAVE MENTION OF THUNDER
IN THE CENTRAL...SOUTH AND WEST. THE NORTH WILL REMAIN SLIGHTLY
STABLE...SO DONT EXPECT THERE TO BE THUNDER THERE. THE MAIN TROUGH
MOVES INTO THE WEST AROUND MIDNIGHT...THEN MOVING OUT OF THE
EASTERN PORTION OF THE CWA AROUND 18Z. COLD FRONT MOVES ACROSS THE
WEST IN THE LATE AFTERNOON AND INTO THE CENTRAL AROUND 00Z...THEN
THE EAST BY 06Z. SNOW LEVELS TODAY WILL BE QUITE HIGH...BETWEEN 9
AND 10K FEET. A FLAT TRANSITORY RIDGE QUICKLY MOVES THROUGH DURING
THE DAY THURSDAY. LINGERING MOISTURE AND INSTABILITY WILL RESULT
IN A FEW SHOWERS. TEMPERATURES TO COOL A BIT FOR TODAY BY 5 TO
10F...THEN ANOTHER FEW DEGREES FOR THURSDAY BEHIND THE FRONT. FLOW
BACKS FRIDAY AHEAD OF THE NEXT PACIFIC SHORTWAVE. INSTABILITY
CONTINUES FRIDAY AS WITH INCREASING MOISTURE TO RESULT IN
ADDITIONAL SHOWERS. THE SHORTWAVE MOVES THROUGH FRIDAY NIGHT WITH A
SPIN UP OF AN H7 LOW OVER THE NORTHERN UTAH AND NORTHERN COLORADO
AREA...BRINGING THE POTENTIAL FOR DEEP UPSLOPE FLOW PATTERN EAST
OF THE DIVIDE. COULD SEE A QUICK SHOT OF DECENT MOISTURE IF IT
OCCURS. GFS BRINGS THIS THROUGH FRIDAY RATHER THAN FRIDAY NIGHT.
HAVE RELIED MOSTLY ON NAM AS IT HAS BEEN PERFORMING BETTER. HAVE
INCREASED POPS FOR FRIDAY NIGHT.

.LONG TERM...SATURDAY THROUGH WEDNESDAY
MEDIUM RANGE DETERMINISTIC MODELS...GEFS AND NCEP ENSEMBLE MEAN IN
FAIRLY GOOD AGREEMENT THIS MORNING. PROGRESSIVE PATTERN CONTINUES
INTO EXTENDED PERIOD WITH TRANSITORY RIDGE BUILDING INTO AREA BY
SATURDAY AFTERNOON CLEARING SHOWERS AND CLOUDS OUT FROM WEST TO
EAST. UPPER RIDGE SHIFTS ACROSS AREA SUNDAY AS NEXT TROUGH DIGS
INTO NW U.S. THIS TROUGH HAS SIMILAR THERMAL PROFILE TO THIS
FRIDAY NIGHTS TROUGH BUT GFS AND ECMWF SHOW MORE DIGGING WITH THIS
NEXT SYSTEM.

WARMER AND MAINLY DRY CONDITIONS WILL PREVAIL SUNDAY
UNDER UPPER RIDGE. VALLEY RAIN AND MOUNTAIN SNOW SHOWERS WILL
INCREASE ACROSS THE WEST SUNDAY NIGHT AND MONDAY AS ATMOSPHERE
BECOMES INCREASINGLY BAROCLINIC. FRONTOGENESIS EXPECTED ACROSS THE
NORTHERN HALF WYOMING MONDAY AS UPPER TROUGH DIGS INTO GREAT BASIN
WITH BEST CHANCES FOR SHOWERS ALONG AND NORTH OF THIS FRONT. THE
FRONT WILL PUSH SOUTH ACROSS CENTRAL AND SOUTHERN WYOMING MONDAY
NIGHT SPREADING RAIN AND SNOW SHOWERS SOUTHWARD AS WELL. UPPER
TROUGH IS EXPECTED TO EXIT OUT INTO THE PLAINS TUESDAY WITH UPPER
RIDGE BUILDING IN WEST TO EAST...SO TUESDAY LOOKS TO BE SORT OF A
REPEAT OF SATURDAY WITH SOME SNOW SHOWERS LINGERING MAINLY IN THE
EAST DURING THE MORNING WITH DECREASING CLOUDS AND SHOWERS WEST TO
EAST THROUGH THE DAY. ALL MODELS ARE STARTING TO GRASP IDEA
INITIALLY HINTED AT BY ECMWF AFTERWARDS WITH MEAN RIDGE POSITION
OVER THE EASTERN PACIFIC INTO THE WESTERN U.S. AND TROUGH OVER THE
EAST WHICH WOULD SPELL THE BEGINNING OF POSSIBLY AN EXTENDED DRY
PERIOD WITH ABOVE NORMAL TEMPERATURES FOR THE MIDDLE PART OF MAY.

&&

.AVIATION...
AN APPROACHING STORM SYSTEM WILL SPREAD SHOWERS SOUTHEAST ACROSS THE
AREA TODAY. RAIN SHOWERS WILL BE MOST PREVALENT IN AND NEAR THE
NORTHWEST MOUNTAINS AND NEAR THE MONTANA BORDER THIS
MORNING...SPREADING SOUTHEAST THROUGH THE AFTERNOON AND EARLY
EVENING. A FEW THUNDERSTORMS ARE EXPECTED ACROSS SWEETWATER COUNTY
AND PERHAPS UP INTO FREMONT AND NATRONA COUNTIES. GENERAL VFR
CONDITIONS ARE EXPECTED BUT SHOWERS COULD BRIEFLY PRODUCE MVFR
CONDITIONS AND MOUNTAIN OBSCURATIONS...ESPECIALLY IN AND NEAR THE
NORTHWEST MOUNTAINS.

&&

.FIRE WEATHER...
A STORM SYSTEM WILL APPROACH THE AREA TODAY...BRINGING A DECENT
SHOT OF MOISTURE TO THE WEST AND NORTH. SCATTERED SHOWERS WILL
SPREAD INTO THE CENTRAL AND SOUTH THIS AFTERNOON...POSSIBLY BY
LATE MORNING. SOME THUNDER IS POSSIBLE ACROSS MUCH OF THE
AREA...EXCEPT FOR THE LOWER ELEVATIONS OF THE NORTH. THE SYSTEM
WILL MOVE THROUGH TONIGHT. SHOWERS WILL LINGER THURSDAY BUT BE
MAINLY ISOLATED TO LOCALLY SCATTERED. ANOTHER SYSTEM WILL APPROACH
FRIDAY INCREASING THE CHANCE OF SHOWERS ONCE AGAIN...THEN POSSIBLY
BRING A QUICK SHOT OF SIGNIFICANT PRECIPITATION EAST OF THE DIVIDE
FRIDAY NIGHT.

&&

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

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

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
1252 PM MDT WED MAY 7 2008

WYZ019-020-071945-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-NATRONA COUNTY LOWER
ELEVATIONS-
1252 PM MDT WED MAY 7 2008

...SIGNIFICANT WEATHER ADVISORY FOR NATRONA COUNTY...

AT 1250 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
THAT THUNDERSTORMS WERE RAPIDLY DEVELOPING 9 MILES NORTHEAST OF
ERVAY...OR 46 MILES WEST OF CASPER...MOVING EAST AT 30 MPH. THESE
STORMS SHOULD AFFECT PORTIONS OF HIGH 26 BETWEEN WALTMAN AND POWDER
RIVER THROUGH 230 PM.

SOME SMALL HAIL IS POSSIBLE WITH THESE STORMS. DANGEROUS
CLOUD-TO-GROUND LIGHTNING IS EXPECTED WITH THESE STORMS.

REPORT SEVERE WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN
RIVERTON AT 1-800-211-1448.

LAT...LON 4317 10659 4279 10658 4284 10734 4302 10733
TIME...MOT...LOC 1852Z 271DEG 26KT 4293 10720

LIPSON

SPECIAL WEATHER STATEMENT...CORRECTED
NATIONAL WEATHER SERVICE RIVERTON WY
1252 PM MDT WED MAY 7 2008

WYZ019-020-071945-
GREEN MOUNTAINS AND RATTLESNAKE RANGE-NATRONA COUNTY LOWER
ELEVATIONS-
1252 PM MDT WED MAY 7 2008

...SIGNIFICANT WEATHER ADVISORY FOR NATRONA COUNTY...

AT 1250 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
THAT THUNDERSTORMS WERE RAPIDLY DEVELOPING 9 MILES NORTHEAST OF
ERVAY...OR 46 MILES WEST OF CASPER...MOVING EAST AT 30 MPH. THESE
STORMS SHOULD AFFECT PORTIONS OF HIGH 26 BETWEEN WALTMAN AND POWDER
RIVER THROUGH 145 PM.

SOME SMALL HAIL IS POSSIBLE WITH THESE STORMS. DANGEROUS
CLOUD-TO-GROUND LIGHTNING IS EXPECTED WITH THESE STORMS.

LAT...LON 4317 10659 4279 10658 4284 10734 4302 10733
TIME...MOT...LOC 1852Z 271DEG 26KT 4293 10720

LIPSON

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
146 PM MDT WED MAY 7 2008

WYZ020-022-072030-
CASPER MOUNTAIN-NATRONA COUNTY LOWER ELEVATIONS-
146 PM MDT WED MAY 7 2008

...SIGNIFICANT WEATHER ADVISORY FOR NATRONA COUNTY...

AT 142 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
STRONG THUNDERSTORM 11 MILES SOUTHEAST OF POWDER RIVER...OR 28 MILES
WEST OF CASPER...MOVING EAST AT 19 MPH.

PEA SIZE HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THIS STORM.

THIS STORM WILL BE...
  5 MILES SOUTH OF U.S. HWY 20/26 MILE MARKER 20 BY 220 PM MDT.
  NEAR EMIGRANT GAP BY 230 PM MDT.

REPORT SEVERE WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN
RIVERTON AT 1-800-211-1448.

LAT...LON 4298 10639 4274 10642 4281 10698 4299 10696
TIME...MOT...LOC 1946Z 275DEG 16KT 4289 10685

LIPSON