Flash Flood Warning 24, NWS Las Vegas, NV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
1035 AM PDT THU SEP 9 2004

.SYNOPSIS...MONSOONAL MOISTURE INCREASING OVER THE REGION WILL BRING
MORE WIDESPREAD THUNDERSTORM CHANCES THIS AFTERNOON AND EVENING.
THUNDERSTORMS OVER SOUTHERN MOHAVE...SOUTHERN CLARK...AND EASTERN
SAN BERNARDINO COUNTIES WILL HAVE THE POTENTIAL TO PRODUCE HEAVY
RAINFALL AND FLASH FLOODING THIS AFTERNOON AND EVENING. THE MOISTURE
WILL REMAIN OVER THE REGION THROUGH THE END OF THE WEEK BEFORE A
TROUGH BEGINS TO MOVE IT EASTWARD SUNDAY.
&&

.UPDATE...MOISTURE CONTINUES TO INCREASE ACROSS THE SOUTHERN REGIONS
OF THE CWA. MORNING MODELS SUPPORT PREVIOUS FORECASTER THOUGHTS ON
MORE WIDESPREAD HEAVY RAIN POTENTIAL ALONG THE SOUTHERN AND EASTERN
REGIONS OF SAN BERNARDINO COUNTY...SOUTHERN CLARK AND MOHAVE
COUNTIES. THUS...WILL CONTINUE THE FLOOD WATCH FOR FLASH FLOODING IN
THESE AREAS. IN THE LAS VEGAS VALLEY...TSTORMS SHOULD BE OF THE MORE
SCATTERED VARIETY BUT WITH THE MCCARRAN PW SENSOR CURRENTLY SHOWING
1.2 INCH PW LEVELS...EXPECT THAT ANY STORMS THAT DO FORM OVER THE
VALLEY WILL HAVE THE POTENTIAL FOR HEAVY RAIN. MOISTURE PARAMETERS
ARE FORECAST TO INCREASE THROUGH THE DAY AND THE NEARLY CLEAR
CONDITIONS WILL ALLOW QUICK TEMP RISE AND DESTABILIZATION OF THE
ATMOSPHERE. EARLIER...THUNDERSTORMS FORMED OVER RIVERSIDE COUNTY IN
CALIFORNIA AND MODEL MEAN WIND PROGS WOULD BRING THIS THUNDERSTORM
ACTIVITY INTO THE SOUTHERN SAN BERNARDINO COUNTY OVER THE NEXT
COUPLE OF HOURS AND THEN MOVE THEM TOWARD THE NORTHEAST. ONE GOOD
THING...THE STORMS SHOULD MOVE AT A FAIRLY GOOD PACE AS 850-300MB
WINDS OF 15KTS ARE FORECAST. SHOULD BEGIN TO SEE CELLS FORM OVER THE
HIGHER TERRAIN OVER MUCH OF THE SOUTHERN CWA BY NOON OR EARLY
AFTERNOON AND OVER THE DESERTS AND LOWLANDS DURING THE AFTERNOON.
THIS IS COVERED IN THE CURRENT FORECAST...SO NO NEED TO UPDATE.

.PREV DISCUSSION...315 AM PDT THU SEP 9...MODEL...SATELLITE...AND
SURFACE DATA ALL SUPPORT A LARGE INCREASE IN MOISTURE THIS AFTERNOON
OVER THE SOUTHERN 1/3 OF THE FORECAST AREA. ETA/GFS RUNS ALSO
SHOWING A VERY LARGE AMOUNT OF INSTABILITY OVER THESE SAME AREAS
WITH LI FORECASTS AS HIGH AS -6. ONLY REMAINING QUESTION WOULD BE A
KICKER TO BETTER ORGANIZE CONVECTION. ETA/GFS HAVE BEEN ADVERTISING
A JET STREAK COMING THROUGH THE SOUTHERN CWA THIS AFTERNOON FOR
SEVERAL RUNS NOW...AND ALTHOUGH IT IS HARD TO FIND IT ON WATER
VAPOR...THERE IS SOME INCREASE CLOUD COVER THIS MORNING OVER
CALIFORNIA THAT COULD BE ASSOCIATED WITH THIS FEATURE. THEY ALSO PUT
THE THETA-E RIDGE AXIS RIGHT ALONG THE COLORADO RIVER VALLEY AND
BRING A NICE CONVERGENCE/DIVERGENCE COUPLET ACROSS THE SOUTHERN CWA
THIS AFTERNOON. GIVEN ALL THIS AND HEAVY DOPPLER ESTIMATES
YESTERDAY...THERE WERE NO RAINGAGES UNDERNEATH TO VERIFY MOST OF
THIS...WILL ISSUE A FLOOD WATCH FOR SOUTHERN MOHAVE...EXTREME
SOUTHERN CLARK...AND EASTERN AND SOUTHERN SAN BERNARDINO COUNTIES
TODAY. CONVECTION SHOULD BE MORE WIDESPREAD AND SLIGHTLY BETTER
ORGANIZED TODAY WITH A SMALL AMOUNT OF SHEAR EXPECTED THIS
AFTERNOON. THEY WILL HOWEVER MOVE SLIGHTLY FASTER THAN YESTERDAY.
INCREASED POPS FOR FORECAST PACKAGE AS A RESULT OF ALL THE NEW
DATA...MOS THUNDERSTORM POTENTIAL SUPPORTS FAIRLY HIGH POPS OVER
SOUTHERN 1/3 OF CWA THIS AFTERNOON. MOISTURE SHOULD REMAIN THROUGH
SATURDAY AS LATEST MODEL RUNS SUGGEST THE TROUGH EXPECTED FOR THE
WEEKEND WILL COME THROUGH SLOWER AND WEAKER THAN PREVIOUSLY
EXPECTED. BIGGEST IMPACT IS WEAKER WINDS AND BETTER THUNDERSTORM
COVERAGE ON SATURDAY.
&&

.LONG TERM...MODELS IN PRETTY GOOD AGREEMENT SUNDAY THROUGH
WEDNESDAY. BROAD LOW AMPLITUDE TROUGH OVER THE PACIFIC NORTHWEST
SUNDAY WITH A WEAK SHORTWAVE TROUGH AND NEARLY WESTERLY FLOW OVER
THE SOUTHERN GREAT BASIN AND MOJAVE DESERT. MONSOON MOISTURE PRETTY
MUCH GETS SCOURED OUT TO THE EAST WITH MAYBE A LITTLE MID LEVEL
MOISTURE OVER NORTHEAST MOHAVE COUNTY. WENT WITH A DRY FORECAST AND
TEMPERATURES PRETTY CLOSE TO SATURDAY. DOES NOT LOOK BREEZY FOR
SUNDAY LIKE IT DID WITH PREVIOUS MODEL RUNS WITH THE MAIN TROUGH
BEING FARTHER NORTH AND LESS AMPLIFIED. BROAD TROUGH REMAINS OVER
THE FAR WESTERN U.S. THROUGH WEDNESDAY BUT DEEPENS WITH TIME.
ASSOCIATED MOISTURE LOOKS TO REMAIN NORTH OF THE FORECAST AREA BUT
EXPECT BREEZY CONDITIONS WITH A COOLING TREND.
&&

.AVIATION...FAIRLY CONFIDENT THAT THUNDERSTORMS WILL AFFECT THE
TERMINAL AREA THIS AFTERNOON. ONLY QUESTION IS WHEN AND WILL NOT BE
ABLE TO ANSWER THAT UNTIL THEY FORM.  AS OF THE TIME OF WRITING THIS
THINK THUNDERSTORMS COULD FORM IN THE LAS VEGAS VALLEY BY 22Z.  MAIN
THREAT FROM THESE STORMS WILL BE VERY HEAVY RAIN FALL IN ADDITION TO
THE STANDARD HAZARDS LIKE TURBULENCE AND LLWS.
&&

.VEF WATCHES/WARNINGS/ADVISORIES...
.NV...FLOOD WATCH FOR FLASH FLOODING UNTIL 10PM FOR NV ZONES 21 AND
22.
.AZ...FLOOD WATCH FOR FLASH FLOODING UNTIL 10PM FOR AZ ZONES 02...03
AND 36.
.CA...FLOOD WATCH FOR FLASH FLOODING TODAY 10AM UNTIL 10PM FOR CA
ZONES 28 AND 29.
&&

SALMEN/JACQUES