Tornado and Flash Flooding in California, August 3 to 4, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
845 AM PDT SUN AUG 3 2008

.SYNOPSIS...MOISTURE MOVING INTO THE MOJAVE DESERT AND SOUTHERN
GREAT BASIN WILL BRING AN INCREASING CHANCE OF THUNDERSTORMS TO THE
AREA THROUGH THE MIDDLE OF THE WEEK. TEMPERATURES WILL LOWER A FEW
DEGREES TODAY THEN DROP TO NEAR NORMAL MONDAY AND TUESDAY ONLY TO BE
OFFSET BY INCREASED HUMIDITY.
&&

.UPDATE...OVERALL TREND OF THE SHORT TERM LOOKS TO BE ON TRACK...
HOWEVER THIS MORNINGS WORDING OF PARTLY CLOUDY ACROSS THE SOUTHERN
ZONES OVERDONE. HAVE UPDATED ZONES TO REFLECT MOSTLY SUNNY
CONDITIONS INITIALLY WITH AFTERNOON CLOUDS THEN FORMING. 12Z NAM
WOULD TREND THAT MOST AFTERNOON CONVECTION TO BE MAINLY ACROSS
SOUTHEAST SAN BERNARDINO/SOUTHERN MOHAVE COUNTIES THIS AFTERNOON
WITH LITTLE TO NO ACTIVITY ACROSS CENTRAL CLARK NORTH INTO LINCOLN
COUNTY. FOR NOW WILL CONTINUE TO KEEP MENTION OF ISOLATED...LESS
THAN 20 PERCENT CHANCE OF THUNDERSTORMS FOR THIS AREA AND ADJUST IF
NEED BE BY EARLY AFTERNOON.
&&

.AVIATION...FOR MCCARRAN...FEW-SCT CLOUDS AOA 14K FEET TO START
OFF TODAY...HOWEVER AS DEEPER MOISTURE CONTINUES TO WORK INTO THE
AREA AS THE DAY PROGRESSES...AND THE ATMOSPHERE ALSO BECOMES
SLIGHTLY MORE UNSTABLE...CLOUD COVER LOOKS TO INCREASE INTO THE
SCT-BKN RANGE BY THIS EVENING AND REMAIN THAT WAY TONIGHT. NOT
REALLY IMPRESSED FOR A SHOWER OR TSTM IN THE LAS VEGAS VALLEY OR
EVEN IN THE SURROUNDING MOUNTAINS FOR THAT MATTER TODAY...BUT WILL
KEEP IN A CB WITH SKY COVER FOR THIS AFTERNOON/EVENING. THE BETTER
CHANCES FOR TSTMS THEN WILL BE ACROSS THE DAGGETT AND PEACH SPRINGS
APPROACH CORRIDORS WHERE A FEW TSTMS COULD DEVELOP. WINDS SHOULD
PICK UP THIS AFTERNOON/EVENING AND MAY GUST TO AROUND 20KTS...BEFORE
LESSENING AGAIN LATER IN THE EVENING. AN INCREASE IN MOISTURE AND
INSTABILITY WILL ALLOW FOR INCREASING CHANCES FOR A TSTM TO AFFECT
THE AIRPORT MONDAY BUT MORE SO ON TUESDAY AND WEDNESDAY.

FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST
CALIFORNIA...SCATTERED TSTMS LOOK POSSIBLE THIS AFTERNOON/EVENING
ACROSS MOHAVE COUNTY...WITH ISOLATED TSTMS POSSIBLE THEN ACROSS
CLARK AND EASTERN SAN BERNARDINO COUNTIES. OTHERWISE SCT-BKN CLOUDS
AOA 12K FEET IN THESE AREAS THIS AFTERNOON/EVENING. ELSEWHERE THIS
AFTERNOON/EVENING EXPECT FEW-SCT CLOUDS AOA 16K FEET...WITH
WIND GUSTS TO AROUND 20KTS MAINLY NEAR KDRA AND KDAG. TONIGHT...AN
INCREASE IN MOISTURE SHOULD RESULT IN SCT-BKN CLOUDS AOA 12K FEET IN
ALL AREAS WITH MAINLY DIURNAL WINDS EXPECTED WITH AN ISOLATED TSTM
EARLY IN NORTHWEST ARIZONA. TSTM CHANCES WILL INCREASE SLIGHTLY
ON MONDAY...BUT MORE SO TUESDAY AND WEDNESDAY ACROSS THE AREA.
&&

.PREV DISCUSSION...405 AM PDT SHORT TERM...THE OVERALL PATTERN IS
EVOLVING ABOUT AS EXPECTED. TODAY WILL STILL BE A TRANSITION DAY AS
DRY SOUTHWEST FLOW AT THE MID LEVELS REMAINS OVER INYO COUNTY
CALIFORNIA AND SOUTH CENTRAL NEVADA. SOUTHEAST TO SOUTHERLY 500 MB
FLOW OVER THE REST OF THE FORECAST AREA WILL PROVIDE SOME ADDITIONAL
MID LEVEL MOISTURE ADVECTION TODAY. ANOTHER CONVECTIVE COMPLEX OVER
NORTHWEST SONORA PUSHED ACROSS THE NORTHERN GULF OF CALIFORNIA LATE
SATURDAY EVENING. THIS APPEARED TO ENHANCE LOW LEVEL SOUTHERLY FLOW
UP THROUGH YUMA WITH A DEPTH AROUND 5K FEET AND SURFACE DEWPOINTS IN
THE LOW 70S. RECENT SURFACE ANALYSES SHOWED DEWPOINTS IN THE LOWER
60S AT NEEDLES AND IN THE MID 50S UP THE COLORADO RIVER VALLEY TO
LAKE MEAD. IT DOES APPEAR THIS LOW LEVEL MOISTURE IS QUITE SHALLOW
GIVEN THE 20 KNOT SOUTHWEST TO WEST FLOW AT 4-5K FEET EVIDENT ON
RADAR WIND PROFILES NEAR LAS VEGAS. HOWEVER...THE COMBINATION OF
ADDITIONAL LOW LEVEL AND MID LEVEL MOISTURE WORKING INTO SOUTHERN
NEVADA AND SURROUNDING AREAS WARRANTS A SLIGHT CHANCE OF AFTERNOON
THUNDERSTORMS AS FAR NORTH AS CENTRAL CLARK COUNTY TODAY. THE DOOR
CONTINUES TO OPEN FOR DEEPER MOISTURE TRANSPORT MONDAY AND
ESPECIALLY TUESDAY WITH THE SOUTHEAST FLOW AROUND THE FOUR CORNERS
HIGH. THE RECENT SOLUTIONS OF THE NAM...GFS AND ECMWF ARE ALL IN
GOOD AGREEMENT AT INDICATING THESE TRENDS. IT IS A LITTLE DIFFICULT
TO PINPOINT A SIGNIFICANT TRIGGER MECHANISM FROM THE LATEST MODEL
SOLUTIONS...ASIDE FROM DAYTIME HEATING AND OROGRAPHICS...THROUGH
TUESDAY. THE LATEST MOS GUIDANCE MAINTAINS THE COOLING TREND
CONSISTENT WITH THE MOISTURE INCREASE. LITTLE CHANGE WAS NEEDED TO
TEMPERATURE TRENDS.

.LONG TERM...THE MAIN ITEM OF INTEREST IN THE LONG RANGE IS AN UPPER
LEVEL VORTICITY MAXIMUM AND ASSOCIATED INVERTED TROUGH AXIS
CURRENTLY LOCATED ACROSS CHIHUAHUA. THIS FEATURE APPEARS SOMEWHAT
MORE DILUTED ON WATER VAPOR IMAGERY THIS MORNING AND BOTH THE ECMWF
AND OPERATIONAL GFS SHOW THIS FEATURE WEAKENING OVER THE NEXT 24
HOURS BEFORE BECOMING RE-ENERGIZED ONCE IT MOVES INTO SOUTHEASTERN
ARIZONA ON TUESDAY. THIS VORT MAX AND ASSOCIATED INVERTED TROUGH
AXIS ARE PROGGED TO MOVE NORTHWESTWARD TOWARDS AROUND FLAGSTAFF BY
WEDNESDAY MORNING. BY WEDNESDAY AFTERNOON...THE 0Z RUNS OF THE ECMWF
AND OPERATIONAL GFS DIVERGE...WITH THE ECMWF TAKING THIS FEATURE
INTO SOUTHWEST UTAH WHILE THE GFS TAKES IT INTO NEVADA AND ALSO
SPLITS UP SOME OF THE ENERGY. THE 6Z DGEX WAS MORE IN-LINE WITH THE
0Z GFS. THE APPROACH OF THIS FEATURE SHOULD PROVIDE AN ADDITIONAL
TRIGGER TO INCREASE THE POTENTIAL AND COVERAGE OF CONVECTION ACROSS
THE CWFA ON WEDNESDAY...ESPECIALLY EASTERN PORTIONS...WHEN FACTORED
IN WITH A JUICY ATMOSPHERE THAT FEATURE PWATS WELL OVER AN INCH
ACROSS THE CWFA. THE BIGGER ISSUE MAY BE WEDNESDAY NIGHT AS WITH
THIS VORTICITY FEATURE IN PLACE...CONVECTION MAY NOT BE SO QUICK TO
DIMINISH EVEN WITH THE LOSS OF DAYTIME HEATING. THE MOST LIKELY AREA
FOR STUFF TO KEEP KICKING THROUGH THE NIGHT LOOKS TO BE ACROSS
LINCOLN COUNTY AND DID TWEAK UP POPS HERE A LITTLE.

BY THURSDAY MODELS MOVE THIS FEATURE NORTHWARD AROUND THE CENTER OF
THE WESTERN RIDGE. DRIER AIR IS THEN SHOWN TO SLOWLY WORK IN FROM
THE WEST ACROSS THE AREA. EASTERN PARTS OF THE CWFA...ESPECIALLY
MOHAVE COUNTY...STILL LOOK TO HAVE A HARD TIME DRYING COMPLETELY
OUT...SO HAVE KEPT IN SLIGHT CHANCE POPS HERE INTO NEXT WEEKEND.
TEMPS SHOULD SLOWLY REBOUND UPWARDS AS THE AIR MASS DRIES OUT AND
CLOUD COVER AND CONVECTION BECOMES LESS PREVALENT.
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
NV...NONE.
AZ...NONE.
CA...NONE.
&&

JENSEN/ADAIR/STACHELSKI

HTTP://WEATHER.GOV/LASVEGAS

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE LAS VEGAS NV
624 AM PDT SUN AUG 3 2008

AZZ001>003-036-CAZ519>527-NVZ014>022-040700-
NORTHWEST PLATEAU-LOWER COLORADO RIVER VALLEY-NORTHWEST DESERTS-
LAKE MEAD NATIONAL RECREATION AREA-EASTERN SIERRA SLOPES-
OWENS VALLEY-WHITE MOUNTAINS OF INYO COUNTY-
DEATH VALLEY NATIONAL PARK-WESTERN MOJAVE DESERT-
EASTERN MOJAVE DESERT-MORONGO BASIN-CADIZ BASIN-
SAN BERNARDINO COUNTY-UPPER COLORADO RIVER VALLEY-
ESMERALDA AND CENTRAL NYE COUNTY-LINCOLN COUNTY-
NORTHEAST CLARK COUNTY-WESTERN CLARK AND SOUTHERN NYE COUNTY-
SHEEP RANGE-SPRING MOUNTAINS-LAS VEGAS VALLEY-
SOUTHERN CLARK COUNTY-
624 AM PDT SUN AUG 3 2008 /624 AM MST SUN AUG 3 2008/

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTHWEST
ARIZONA...SOUTHEAST CALIFORNIA AND SOUTHERN NEVADA.

.DAY ONE...TODAY AND TONIGHT

MOISTURE INCREASING FROM THE SOUTH AND SOUTHEAST WILL LEAD TO
ISOLATED THUNDERSTORMS THIS AFTERNOON AND EVENING...MAINLY OVER
THE AREA SOUTH AND EAST OF INTERSTATE 15. THE PROBABILITY OF
HAZARDOUS WEATHER IS LOW FOR THE REST OF THE REGION.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

DEEPER MONSOONAL MOISTURE WILL PUSH NORTH INTO THE ENTIRE REGION
MONDAY AND TUESDAY BRINGING INCREASING THUNDERSTORM COVERAGE WHICH
WILL PERSIST THROUGH THE MIDDLE OF THE WEEK. THE MAIN THREAT WITH
THESE STORMS WILL BE HEAVY RAINFALL AND THE POTENTIAL FOR ISOLATED
FLASH FLOODING...ESPECIALLY TUESDAY AND WEDNESDAY. DRIER SOUTHWEST
FLOW IS FORECAST TO DEVELOP OVER SOUTHEAST CALIFORNIA AND SOUTHERN
NEVADA TOWARD THE END OF THE WEEK.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.