Flash Flood Warning 8, NWS Las Vegas, NV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAS VEGAS NV
1030 AM PST FRI MAR 4 2005

.SYNOPSIS...A PACIFIC LOW PRESSURE SYSTEM ALONG THE SOUTHERN
CALIFORNIA COAST WILL DROP DOWN ACROSS SAN DIEGO THIS AFTERNOON AND
HEAD TOWARD THE NORTHERN GULF OF CALIFORNIA OVERNIGHT. THIS WILL
PRODUCE SOME SCATTERED SHOWERS AND POSSIBLY ISOLATED THUNDERSTORMS
OVER PORTIONS OF THE SOUTHERN GREAT BASIN AND MOJAVE DESERT THROUGH
EARLY SATURDAY BUT WILL DIMINISH IN COVERAGE AS THE LOW MOVES EAST.
&&

.UPDATE...LIMITED MOISTURE WITH THE LOW CENTERED JUST OFF THE
SOUTHERN CALIFORNIA COAST CAN CLEARLY BE SEEN ON SATELLITE IMAGERY.
THE 12Z GFS HAS A BETTER 6 HOUR FORECAST OF THE LOW THAN THE NAM BUT
SATELLITE TRENDS INDICATE THE CENTER OF THE SYSTEM IS PROBABLY GOING
TO BE TO SAN DIEGO ABOUT 6 HOURS FASTER THAN THE GFS DEPICTS.
BELIEVE THE SYSTEM WILL REACH THE NORTHERN GULF OF CALIFORNIA BY 12Z
SATURDAY 6-9 HOURS FASTER THAN THE GFS INDICATES. NEED TO WATCH FOR
THE POSSIBILITY OF A MOISTURE TAP FROM THE PLUME OVER MEXICO BUT IT
DOES NOT LOOK TOO PROMISING FOR OUR AREA. IT APPEARS THE OVERALL
TREND IS FOR THE SYSTEM TO SLIDE UNDERNEATH OUR AREA AND LEAVE US
ONLY A BRIEF ROUND OF SCATTERED SHOWERS MAINLY THIS AFTERNOON AND
TONIGHT. CANCELED THE HEAVY SNOW WARNING FOR THE INYO COUNTY
MOUNTAINS AND THE WINTER STORM WATCH FOR THE SOUTHERN NEVADA
MOUNTAINS.
&&

.PREV DISCUSSION...332 AM PST...SATELLITE IMAGERY SHOWING UPPER LOW
JUST WEST OF POINT CONCEPTION WITH DECENT ENHANCEMENT OVER THE
NORTHEAST QUADRANT. HNX RADAR HAS BEEN SHOWING MODERATE TO
OCCASIONALLY HEAVY SHOWERS WEST OF INYO COUNTY...BUT SO FAR NOT MUCH
EASTWARD PROGRESSION. NEW 06Z NAM HAS TRENDED THIS LOW FASTER OVER
THE NEXT 24 HOURS AND PLACES THE LOW CENTER NEAR SAN DIEGO BY THIS
EVENING. HAVING A HARD TIME DETERMINING HOW MUCH PRECIPITATION WILL
FALL IN THE CWA OVER THE NEXT 36 HOURS. AS THE LOW CONTINUES TO
TRACK SOUTH...THE AREA OF ENHANCEMENT WILL ALSO SHIFT SOUTH AND MOST
OF THE WEST FACING SLOPES WILL SEE PRECIPITATION FIRST...HOWEVER AS
THE LOWER LEVEL WINDS SWITCH MORE TO THE SOUTH AND EAST THE UPSLOPE
COMPONENT WILL KICK IN. THIS WILL BRING THE BEST CHANCE OF PRECIP TO
THE EAST SLOPES OF THE SIERRA STARTING AROUND 18Z AND CONTINUING FOR
SEVERAL HOURS. FEEL THAT THE START TIME OF THE HEAVY SNOW WARNING IS
A LITTLE EARLY...BUT WITH GOOD UPSLOPE SNOW AMOUNTS SHOULD REACH
WARNING CRITERIA. ANOTHER CONCERN IS THE SOUTHERN NEVADA MOUNTAINS
WHERE A WINTER STORM WATCH IS IN EFFECT. THE MODELS ARE NOT SHOWING
MUCH QPF FOR THIS AREA WITH THE GFS STILL THE MOST AGGRESSIVE. EVEN
SO...IT GENERATES ABOUT A HALF AN INCH OF TOTAL QPF. MOST LIKELY I
BELIEVE A SNOW ADVISORY WILL BE ISSUED FOR LATER THIS AFTERNOON AS
SNOW AMOUNTS SHOULD BE LOWER THAN THE CURRENT 6 TO 12
INCHES...ALTHOUGH SOME LOCALIZED AREAS MAY SEE A BIT MORE. UPPER LOW
IS EXPECTED TO REMAIN OVER NORTHERN BAJA WITH ENOUGH LOW LEVEL
MOISTURE AND INSTABILITY TO KEEP THE THREAT OF SHOWERS AND AFTERNOON
THUNDERSTORMS FOR THE SOUTHERN CWA THROUGH SATURDAY. INSTEAD OF THE
LOW REMAINING IN PLACE WITH A REX BLOCK SETTING UP OVER THE WEST
COAST...THE MODELS NOW BUILD A STRONGER HIGH OFF THE COAST WHICH
PUSHES THE LOW EAST THROUGH NORTHERN BAJA. THIS WOULD BRING AN END
TO THE PRECIP CHANCES OVER THE SOUTHERN CWA BY SUNDAY AND HAVE
ADJUSTED THE FORECAST ACCORDINGLY.
&&

.LONG TERM...AS MENTIONED AT THE END OF THE SHORT TERM PERIOD...THE
MODELS HAVE CHANGED THEIR TUNE REGARDING THE UPPER LOW. THE
CONSENSUS IS NOW FOR A MORE PROGRESSIVE SOLUTION...WHICH WOULD
SUGGEST A DRY AND MUCH WARMER FORECAST FOR THE EXTENDED PERIOD AS A
RIDGE BUILDS IN. THE ONLY DOUBTER IS THE 12Z/03 ECMWF. IT LATCHES ON
TO A SHORTWAVE TROUGH RIDING OVER THE TOP OF THE RIDGE ON
WEDNESDAY...AMPLIFIES IT AND DIGS IT SOUTH OVER UTAH BY THURSDAY
MORNING...ERODING THE EASTERN FLANK OF THE RIDGE. THE 00Z/04 RUN HAS
A MUCH WEAKER DEPICTION OF THIS SHORTWAVE...WHICH AGREES BETTER WITH
THE GFS AND IS ACCEPTED. MAIN CHANGES FOR THE EXTENDED PERIOD WERE
HIGHER TEMPS AND LOWER HUMIDITIES.
&&

.AVIATION...MID LEVEL CLOUDS HAVE BEEN FILLING IN OVER CLARK COUNTY
DURING THE MID TO LATE MORNING WITH ENHANCEMENT OF LOWER LEVEL
CUMULUS EVIDENT ON SATELLITE PRODUCT. SURFACE OBSERVATIONS SHOW
CUMULUS CLOUD DECK CURRENTLY NEAR 7000 FEET AGL AND MODEL FORECAST
SOUNDING DATA SUGGEST THIS WILL CONTINUE THROUGH THE AFTERNOON.
CEILINGS WILL LIKELY LOWER TO NEAR 5000 FEET AGL UNDER AREAS OF
SHOWER ACTIVITY BY LATE AFTERNOON AND THROUGH THE EVENING HOURS.
&

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

ADAIR/GORELOW/MORGAN