933 FXUS65 KVEF 231757 AFDVEF AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LAS VEGAS NV 955 AM PST SUN NOV 23 2008 .SYNOPSIS...DRY AND MILD CONDITIONS WILL CONTINUE TODAY AND MONDAY BEFORE A DRAMATIC CHANGE IN THE WEATHER DEVELOPS TUESDAY THROUGH THANKSGIVING WEEKEND. THE FIRST IN A SERIES OF PACIFIC STORMS WILL REACH THE WEST COAST TUESDAY THEN MOVE INLAND TUESDAY NIGHT AND WEDNESDAY...BRINGING A GOOD CHANCE OF RAIN TO THE VALLEYS AND SNOW TO THE MOUNTAINS. A BRIEF BREAK BETWEEN STORMS IS EXPECTED THANKSGIVING DAY BEFORE A SECOND PACIFIC SYSTEM BRINGS ANOTHER CHANCE OF RAIN AND SNOW TO THE REGION NEXT WEEKEND. && .UPDATE...THE 12Z NAM MOS GUIDANCE IS GENERALLY 3-5 DEGREES HIGHER WITH TEMPS THIS AFTERNOON THAN THE 12Z GFS MOS. LACK OF MIXING MAY KEEP SOME VALLEY TEMPS FROM REALIZING THEIR POTENTIAL BUT FULL SUNSHINE SHOULD PROVIDE A BOOST. TEMPS SHOULD BE WITHIN A COUPLE DEGREES OF FORECAST HIGHS AND NO SIGNIFICANT CHANGES ARE NEEDED FOR TODAY. THE MAIN WEATHER PROBLEM WILL BE ADJUSTING FOR THE CHANGING TRACK AND TIMING OF THE INCOMING SYSTEM. THE GFS HAS BEEN TRENDING TOWARD THE SLOWER ECMWF. IT STILL LOOKS LIKE WEDNESDAY WILL BE A GOOD BET FOR PRECIPITATION AREA WIDE AND POPS WILL BE INCREASED. HOWEVER...THE LOW CENTER IS NOW FORECAST TO SLIDE DOWN THE SOUTHERN CALIFORNIA COAST WEDNESDAY THEN ACROSS NORTHERN BAJA TO NEAR YUMA ARIZONA BY THANKSGIVING MORNING. THIS WILL KEEP A WARMER AIR MASS OVER OUR REGION AND SNOW LEVELS WILL LIKELY NEED TO BE INCREASED 1-2K FEET FROM CURRENT LEVELS. THESE TRENDS WILL BE ADDRESSED IN THE NEXT ROUTINE PACKAGE. -ADAIR- && .PREV DISCUSSION.../335 AM PST SUN NOV 23 2008/ .SHORT TERM...SATELLITE SHOWS THAT CIRRUS HAS BEEN SUPPRESSED TO AREAS SOUTH OF INTERSTATE 15...WITH CLEAR SKIES FARTHER NORTH. THE CIRRUS KEPT TEMPERATURES DOWN MORE THAN EXPECTED SATURDAY...SO EXPECT A BIT OF A REBOUND TODAY WITH FULL SUNSHINE. A WEAK RIDGE OF HIGH PRESSURE WILL CONTROL OUR WEATHER THROUGH TONIGHT...THEN THE TROUGH OF LOW PRESSURE JUST WEST OF 140W THIS MORNING WILL COME INTO THE PICTURE. THERE IS STILL A LOT OF SPREAD AMONG THE MODELS WITH THE DETAILS OF HOW THIS STORM WILL EVOLVE...WITH THE NAM STILL SHALLOWEST AND FASTEST...THE ECMWF STILL SLOWEST...AND THE GFS STILL IN BETWEEN. FORECAST WILL AGAIN LEAN TOWARDS THE GFS...BUT IT MUST BE NOTED THAT THE SLOWER ECMWF SCENARIO REMAINS PLAUSIBLE /THE SAME CANNOT BE SAID FOR THE NAM/. THE GFS SPLITS THE TROUGH LATER TODAY /WHICH LOOKS VERY GOOD GIVEN CURRENT WATER VAPOR IMAGERY/ AND BRINGS THE SOUTHERN PORTION TO 34N 130W AS A CLOSED LOW MONDAY AFTERNOON. MEANWHILE...A FINGER OF MOISTURE FROM THE INTERTROPICAL CONVERGENCE ZONE LIFTS NORTH ALONG 120W DUE TO THE INFLUENCE OF THE DIGGING LOW. THE LOW PULLS THIS MOISTURE UP ITS EASTERN SIDE MONDAY NIGHT AS IT MOVES SLOWLY EAST...CAUSING PRECIP CHANCES TO BREAK OUT OVER CALIFORNIA. THE PRECIP CHANCES SPREAD OVER OUR ENTIRE CWFA TUESDAY AS THE LOW SLOWLY TRACKS EAST AND FINALLY REACHES THE SOUTHERN CALIFORNIA COAST TUESDAY NIGHT. DUE TO THE WARM TROPICAL TAP SNOW LEVELS WILL START OUT QUITE HIGH...BUT IF THE MOISTURE PLUME ORIENTS JUST RIGHT...THE HIGHER ELEVATIONS OF THE SIERRA COULD SEE SIGNIFICANT SNOW ACCUMULATIONS AS EARLY AS TUESDAY. CONFIDENCE IN THE ORIENTATION AND TIMING ARE NOT HIGH ENOUGH TO JUSTIFY A WATCH YET...BUT WILL BEEF UP WORDING IN THE SPECIAL WEATHER STATEMENT. ASSUMING THE GFS TIMING IS RIGHT...TEMPERATURES WILL START TO DROP OFF ON TUESDAY...BUT THIS WOULD BE POSTPONED UNTIL WEDNESDAY IF THE SLOWER ECMWF IS RIGHT. .LONG TERM...AGAIN MODELS START OUT THE EXTENDED...WEDNESDAY...IN GOOD AGREEMENT CONCERNING THE FIRST SYSTEM TO TRACK TROUGH HE AREA WEDNESDAY AND THURSDAY. IT APPEARS THIS SYSTEM WILL GRAB A DECENT AMOUNT OF MOISTURE AND BRING WIDESPREAD RAIN AND SNOW. LATEST MODEL RUNS HAVE THE TRACK OF THE LOW FURTHER SOUTH THAN 24 HOURS AGO. HOWEVER...WITH THE GOOD MOISTURE SOURCE AND GENERAL UPLIFT / INSTABILITY THINK THE TRACK VARIATIONS WILL NOT MAKE TOO BIG A DIFFERENCE. SNOW LEVELS STILL STAY UP THERE AROUND 6000 FEET OR HIGHER. SIGNIFICANT ACCUMULATIONS ARE LIKELY IN THE SIERRA AND SPRING RANGES AND HIGHER ELEVATIONS IN LINCOLN COUNTY. THIS WILL BE ADDRESSED IN THIS MORNINGS HWO AND SPS. I DID ADD IN A SLIGHT CHANCE FOR THUNDERSTORMS IN OUR FAR SOUTH AREAS FOR WEDNESDAY AND EARLY THURSDAY. THE LOW WILL TRACK A COLD POOL OVER THAT AREA AND MODEL STABILITY PROGS AND EXPERIENCE SUGGEST THIS SET UP CAN TRIGGER SOME CB / THUNDERSTORM DEVELOPMENT THEN. THE SECOND SYSTEM FOLLOWING THURSDAY IS STILL FORECAST IN VERY DIFFERENT WAYS BY THE ECMWF AND THE GFS. THE GFS HAS BEEN CONSISTENT IN FORECASTING ANOTHER LOW TO DROP ALONG THE WEST COAST FRIDAY AND PARK ITSELF OFF THE COAST FOR THE WEEKEND. IT IS EXPECTED TO ALSO PULL UP MOISTURE AND BRING ANOTHER VERY GOOD CHANCE AT PRECIPITATION OVER THE WEEKEND. THE ECMWF IS ALSO CONSISTENT IN MAKING THE NEXT ONE AN INSIDE...WAY INSIDE...SLIDER THAT CLOSES OFF A LOW SATURDAY OVER THE NORTHERN GULF OF CALIFORNIA. I HAVE LEANED TOWARDS A COMPROMISE WHICH THE GEM WOULD DEPICT...HAVING THE LOW PINCH OFF OVER NEVADA. FOR NOW DO NOT HAVE THAT HIGH A CONFIDENCE IN WHAT WILL TRANSPIRE AND WILL LEAVE IN THE RELATIVELY HIGH POPS WE HAVE SINCE IN EITHER SOLUTION THIS LOOKS REASONABLE. && .AVIATION...FOR MCCARRAN...LIGHT SOUTHWEST WINDS WILL GIVE WAY TO LIGHT AND VARIABLE WINDS ALONG WITH HIGH LEVEL CIRRUS THIS MORNING AND BACK TO LIGHT SOUTHWEST TONIGHT. STILL ANTICIPATE A BIG CHANGE BEGINNING LATE MONDAY AND ONWARDS FOR THE REST OF THE WEEK. BY LATE TUESDAY THROUGH THURSDAY A SYSTEM WILL BRING WIDESPREAD RAIN TO THE AREA ALONG WITH MOUNTAIN SNOW. CEILINGS ARE LIKELY TO DROP NEAR OR BELOW 5000FT AGL DURING THIS TIME. ANOTHER SYSTEM IS EXPECTED LATE FRIDAY TO BRING ANOTHER ROUND OF WIDESPREAD PRECIPITATION TO THE AREA FOR THE WEEKEND. FOR THE REST OF SOUTHERN NEVADA...NORTHWEST ARIZONA...AND SOUTHEAST CALIFORNIA...LIGHT WINDS AND CIRRUS LEVEL CLOUDS EXPECTED TODAY AND INTO EARLY MONDAY. STILL ANTICIPATE A BIG CHANGE BEGINNING LATE MONDAY AND ONWARDS FOR THE REST OF THE WEEK. BY LATE TUESDAY THROUGH THURSDAY A SYSTEM WILL BRING WIDESPREAD RAIN TO THE AREA ALONG WITH MOUNTAIN SNOWS. CEILINGS ARE LIKELY TO DROP NEAR OR BELOW 5000FT AGL DURING THIS TIME. ANOTHER SYSTEM IS EXPECTED LATE FRIDAY TO BRING ANOTHER ROUND OF WIDESPREAD PRECIPITATION TO THE AREA FOR THE WEEKEND. && .VEF WATCHES/WARNINGS/ADVISORIES... NV...NONE. AZ...NONE. CA...NONE. && $$ MORGAN/JACQUES HTTP://WEATHER.GOV/LASVEGAS