000 FXUS65 KPSR 161302 AAA AFDPSR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 600 AM MST FRI MAY 16 2008 SOUTH-CENTRAL AND SOUTHWEST ARIZONA...AND SOUTHEAST CALIFORNIA FORECAST DISCUSSION .UPDATE...UPDATED FIRE WEATHER DISCUSSION. && .SYNOPSIS... AN UPPER LEVEL LOW PRESSURE SYSTEM SOUTHEAST OF ARIZONA WILL GIVE A SLIGHT CHANCE FOR SHOWERS AND THUNDERSTORMS OVER THE HIGHER TERRAIN OF SOUTH-CENTRAL ARIZONA TODAY...OTHERWISE DRY AND LOCALLY BREEZY CONDITIONS WILL CONTINUE OVER SOUTHEAST CALIFORNIA AND THE SOUTHERN ARIZONA DESERTS. A STRONG RIDGE OF HIGH PRESSURE ALONG THE WEST COAST WILL GRADUALLY SHIFT EASTWARD AND OVER THE DESERT SOUTHWEST...WITH THE WARMEST TEMPERATURES OF THE SEASON EXPECTED LATE IN THE WEEKEND INTO EARLY NEXT WEEK. THE RIDGE WILL BREAK DOWN DURING THE MIDDLE OF NEXT WEEK...WITH COOLER TEMPERATURES EXPECTED ONCE AGAIN...AS ANOTHER WEST COAST TROUGH OF LOW PRESSURE MOVES THROUGH THE REGION. && .DISCUSSION... RATHER HIGH AMPLITUDE UPPER RIDGE REMAINS IN PLACE ALONG THE WEST COAST WITH THE CLOSED HI CENTER OVR NRN CA...AND MEANWHILE THE UPPER LOW OVER SERN AZ HAS MOVED SOUTH INTO NWRN MEXICO. ALL OF THIS IS CLEARLY SEEN IN THE MORNING IR/VAPOR IMAGERY. LOW CONTINUES TO CYCLE LOW/MID LEVEL MOISTURE INTO SERN AZ...BUT DVV FIELDS BEHIND THE LOW HAVE ACTED TO THIN MOST OF THE CLOUD COVER OVER ARIZONA DURING THE PAST SEVERAL HOURS. IR IMAGERY AT 2 AM SHOWED MOSTLY CLEAR SKIES ACROSS THE CWA...WITH THE MOST SIGNIFICANT CLOUD COVER CONFINED TO FAR SERN AZ. LATEST PROGS ARE IN GOOD AGREEMENT IN THE SHORT TERM...AND FORECAST THE UPPER LOW TO GRADUALLY SHIFT SOUTH AND EAST...WHILE THE UPPER RIDGE SLOWLY BUILDS INTO ARIZONA FROM THE NORTH AND WEST. AS THE LOW SHIFTS EAST IT WILL HAVE LESS INFLUENCE ON OUR WEATHER...AND WE WILL CALL FOR JUST A SLIGHT CHANCE FOR SHOWERS AND AFTERNOON THUNDERSTORMS TODAY OVER THE HIGHER TERRAIN EAST OF PHOENIX. BY SATURDAY THE THREAT FOR PRECIP SHOULD BE JUST ABOUT OVER...WITH ONLY SOME SINGLE DIGIT POPS LEFT IN THE FORECAST EAST OF GLOBE. AS THICKNESS VALUES RISE 90M OR SO OVER THE NEXT 2 DAYS...PLENTIFUL SUNSHINE AND MIXING WILL BRING THE WARMING ALOFT TO THE GROUND...AND MAX TEMPS WILL CLIMB SHARPLY. EXPECT HIGHS BY SATURDAY TO BE IN THE MID TO UPPER 90S OVER THE CENTRAL DESERTS AND INTO THE TRIPLE DIGITS OVER MUCH OF THE WESTERN DESERTS...WHICH ARE CLOSER TO THE RIDGE AXIS. LATEST 00Z GFS AND EUROPEAN AGREE NICELY THAT THE UPPER RIDGE WILL CONTINUE TO SHIFT EAST AND MOVE WELL INLAND OVER THE DESERT SOUTHWEST BY MONDAY...WITH TEMPS CONTINUING TO CLIMB EACH DAY. EXPECT PHOENIX TO REACH 102 SUNDAY...WHICH WILL BE THE FIRST 100 OF THE YEAR...AND THEN PEAK AT 104 MONDAY. MONDAY LIKELY WILL BE THE WARMEST DAY WITH THIS UPPER RIDGE BEFORE IT BEGINS TO BREAK DOWN AND SHIFT EAST. OF COURSE...THE AIRMASS WILL REMAIN QUITE DRY SO GENERALLY SUNNY OR MOSTLY SUNNY DAYS AND CLEAR NIGHTS WILL BE THE RULE EACH DAY. FOR THE LATTER PORTION OF THE EXTENDED PERIOD...TUESDAY THROUGH THURSDAY...THINGS START TO GET A BIT INTERESTING AS THE MEDIUM RANGE MODELS REALLY START TO DIVERGE. LARGE SCALE RIDGE IS FORECAST TO RETROGRADE BACK OFF THE WEST COAST...ALLOWING SIGNIFICANT SHORT WAVE ENERGY TO DIG DOWN THE FRONT SIDE AND CARVE OUT A SIGNIFICANT TROF OVER THE WESTERN STATES. GFS OPERATIONAL RUN WANTS TO SEND MOST OF THE ENERGY NORTHEAST OF THE FOUR CORNERS AND DEVELOP THE MAIN TROF AXIS EAST OF ARIZONA...WHEREAS THE EUROPEAN DIGS THE TROF MUCH FURTHER WEST...AND HAS THE MAIN AXIS THRU CENTRAL AZ BY WED AFTERNOON. AT 144 HOURS...WED AFTERNOON...GFS H5 HEIGHTS ARE FORECAST TO BE OVER 200M HIGHER THAN THE EUROPEAN...200 METERS HIGHER. YOW. ENSEMBLE MEAN SHOWS THE TROF AXIS FURTHER WEST THEN OPERATIONAL GFS...AND MORE IN LINE WITH THE EUROPEAN...AND SPAGHETTI PLOTS SHOW HIGH DEGREE OF VARIABILITY AMONG THE MEMBERS WED THRU THURSDAY. SO...THERE IS STILL QUITE A BIT OF UNCERTAINTY REGARDING HOW DEEP...AND HOW FAR WEST THE TROF SETS UP DURING THE MIDDLE OF NEXT WEEK. AS FAR AS SENSIBLE WEATHER...IT IS MOST LIKELY THAT OUR CWA WILL REMAIN DRY NO MATTER WHAT HAPPENS...GIVEN THE TIME OF YEAR AND THE FACT THAT WE WILL LIKELY BE ON THE SRN PERIPHERY OF THE TROF AND ON THE ANTYCYC SHEAR SIDE OF THE JET. HOWEVER...MUCH MORE WIND AND COOLING WILL OCCUR FOR US SHOULD THE EUROPEAN RUN VERIFY. FOR NOW WE WILL LEAVE THE FORECAST DRY THRU THE END OF THE EXTENDED...AND SHOW A COOLING TREND EACH DAY...AND OUR HIGHS DAY WILL BE AT LEAST A FEW DEGREES BELOW MEX MOS VALUES. IF THE EUROPEAN ENDS UP THE CORRECT SOLUTION...WE WILL LIKELY HAVE TO LOWER OUR TEMPS QUITE A BIT MORE BEFORE IT IS ALL OVER. AS FUTURE MODEL RUNS COME IN AND CONFIDENCE INCREASES...WE WILL MAKE THE NECESSARY ADJUSTMENTS TO THE TEMP TRENDS...AND OTHER SENSIBLE WEATHER ELEMENTS. && .AVIATION... PHOENIX METRO AREA...SUFFICIENT MOISTURE REMAINS ACROSS THE EASTERN PORTION FOR FEW CLDS TO DEVELOP BTWN 10 AND 15 THSD FT. NO SIGNIFICANT UPPER AIR SYSTEMS EXPECTED TO AFFECT LOCAL AIRPORTS TODAY. AS HIGH PRESSURE SLOWLY BUILDS OVER THE REGION...CLEARING SKIES WILL SET IN TONIGHT AND SATURDAY. WINDS WILL NOT BE AN ISSUE LIKE YESTERDAY...ALTHOUGH SKY HARBOR CAN EXPECT EASTERLIES NEARLY ALL DAY...AND AN OCCASIONAL GUST TO NEAR 25 KTS. YUMA-BLYTHE-IMPERIAL AREA...MUCH DRIER AIRMASS HAS SPREAD INTO THE LOWER COLORADO RIVER VALLEY...AND CLEAR SKIES OR FEW CLOUDS ABOVE 20 THSD FEET. PREDOMINANT WIND DIRECTION LIKELY TO REMAIN NORTHEAST. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... FOR TODAY...PRIMARY CONCERN IS WITH THE LINGERING MOISTURE ACROSS ZONE 133. IN FACT...EARLY MORNING RH READINGS FROM GLOBE RAWS SITE WERE IN THE 50S AND 60S. CIRCULATION AND INSTABILITY NOT AS FAVORABLE AS YESTERDAY...AND WE ARE CALLING FOR ONLY A SLIGHT CHANCE MAINLY OVER HIGHER TERRAIN. WINDS WILL GUST UP TO ABOUT 25 MPH IN EASTERN ZONES...AND LESS OUT WEST. THUS...NO CRITICAL FIRE WEATHER THRESHOLDS EXPECTED TODAY. EVEN LESS WIND ON SATURDAY...ALONG WITH THE START OF THE WARMING TREND. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...NONE. CA...NONE. && $$ NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT WEATHER.GOV/PHOENIX DISCUSSION...CB AVIATION...SIPPLE FIRE WEATHER...SIPPLE