000 FXUS65 KPSR 041819 AAA AFDPSR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PHOENIX AZ 1118 AM MST SAT JUL 4 2009 .UPDATE...UPDATED AVIATION DISCUSSION. && .SYNOPSIS... A GRADUAL DRYING TREND IS ANTICIPATED ACROSS SOUTH-CENTRAL ARIZONA INTO SOUTHEAST CALIFORNIA THE NEXT COUPLE OF DAYS. THIS WILL OCCUR AS HIGH PRESSURE SHIFTS FROM OUR EAST TO OUR WEST. THERE WILL STILL BE ENOUGH MOISTURE THROUGH THE HOLIDAY WEEKEND TO GIVE A SLIGHT CHANCE OF THUNDERSTORMS OVER MOST OF SOUTH-CENTRAL AND SOUTHWEST ARIZONA...BUT THIS WILL BECOME LIMITED TO THE HIGHER TERRAIN EAST OF PHOENIX BY MONDAY AS THE DRYING CONTINUES. MEANWHILE...DAYTIME TEMPERATURES ARE EXPECTED BE SEVERAL DEGREES ABOVE EARLY-JULY NORMALS THROUGH NEXT WEEK. && .DISCUSSION... WINDS OF CHANGE. THE FLOW PATTERN IS BEGINNING TO CHANGE TOWARD A MORE DRIER AND STABLE PATTERN AGAIN. 12Z PLOT DATA HOWEVER CONTINUED TO SHOW A RATHER HUMID AIRMASS IN PLACE...WITH THE MONSOON MOISTURE BOUNDARY ... I.E. 800/850 MB DEWPOINT ABOVE 8 DEGREES C ...LOCATED ALONG A LINE FROM YUMA...TO BETWEEN PHOENIX AND FLAGSTAFF...THEN NORTHEAST TOWARD SHOWLOW AND THE WHITE MOUNTAINS. IN OTHER WORDS SOUTHERN AZ WAS STILL IN THE SOUP. CONVECTIVE TRIGGER TEMPERATURES TODAY IN THIS MONSOON AIRMASS ARE NEAR THE FORECAST AFTERNOON HIGHS AT MANY LOCATIONS...EVEN THE MOUNTAINS. HOWEVER...THE UPPER LEVEL FLOW PATTERN - 300/250 MB - WILL NOT BE THE SAME AS FRIDAY EVENING. ITS BEGINNING TO CHANGE DIRECTIONS FROM RECENT DAYS...OUT OF THE SOUTH INCLUDING PERIODS OF UPPER DEFORMATION/DIVERGENCE TO SUPPORT LARGE THUNDERSTORM OUTBREAKS...TO A MORE SOUTHWESTERLY AND CONVERGENT FLOW TODAY ...SUNDAY...AND MONDAY...WHICH IS LESS FAVORABLE FOR TALLER AND WIDESPREAD THUNDERSTORMS. EVEN A DRIER AIRMASS FROM THE WEST WILL OVER SPREAD SOUTHWEST AND SOUTH CENTRAL AZ BEGINNING SUNDAY... PUSHING THE MONSOON MOISTURE BOUNDARY SOUTH TO THE MEXICAN BORDER BY MONDAY AFTERNOON. YESTERDAY FRIDAY THE AIRMASS WAS VERY HUMID OVER A LARGE PART OF SOUTHERN AZ. AND...UPPER LEVEL FLOW AT 300-250 MB WAS FAVORABLE FOR A LARGE THUNDERSTORM OUTBREAK IN SOUTHEAST AZ. THAT MEANT ANY THUNDERSTORM OUTBREAK IN SOUTH CENTRAL AZ...INCLUDING NEAR PHOENIX...WOULD HAD TO HAVE COME FROM GUSTY AND STRONG OUTFLOW BOUNDARIES...PARTICULARLY FROM THE TUCSON DIRECTION. TODAY... ALTHOUGH THE MONSOON HUMIDITY IS IN PLACE...BOTH THE GFS AND EUROPEAN MODELS FORECAST 300-250 MB VEERING WINDS...BECOMING MORE SOUTHWESTERLY AND CONVERGENT ALOFT. THIS MEANS SMALLER TOPPED T-STORMS AND NOT AS WIDESPREAD...PERHAPS EVEN IN SOUTHEAST AZ... BUT CERTAINLY IN OUR AREA FROM PHOENIX TO CASA GRANDE. UPDATES THIS MORNING WERE TO REMOVE THE MENTION OF THUNDERSTORM THREATS THIS MORNING...UNTIL LATE THIS AFTERNOON AND EVENING. PROBABILITIES OF PRECIP THIS EVENING IN THE GREATER PHOENIX AREA ARE SMALL DUE TO SHIFTING AND UNFAVORABLE UPPER WIND CURRENTS. DITTO FOR SUNDAY AND MONDAY...ALSO UNDER A DRIER AIRMASS COMING FROM THE WEST. PREVIOUS DISCUSSION... WE SHOULD START OUT THE DAY WITH PRETTY FULL SUNSHINE...AND WITH LOTS OF LOW LEVEL MOISTURE AVAILABLE...CONVECTION SHOULD BEGIN FIRING ONCE AGAIN LATE TODAY. AS ONE WOULD EXPECT...FIRST STORMS WILL FIRE OVER AND NEAR THE MTNS BY AFTN...WITH A SLIGHT EWD DRIFT WITH A WEAK STEERING FLOW. MODEL SOUNDINGS INDICATE WE MAY SEE SOME DRYING TODAY...SO AMOUNT OF CONVECTION WILL PROBABLY BE LESS THAN WHAT HAPPENED FRIDAY. AND...NO IMPULSES EVIDENT THAT COULD SERVE TO TRIGGER CONVECTION...SO ACTION WILL NEED TO BE SUSTAINED BY OUTFLOW BOUNDARY COLLISIONS AS IS SO OFTEN OUR CASE. MEANWHILE...PROGS ARE STILL DEVELOPING UPPER RIDGE TO OUR WEST THIS WEEKEND...WITH MONSOON MOISTURE STILL EXPECTED TO BE SHUNTED MORE TO THE EAST. ALL SHORT RANGE MODELS ARE TRENDING IN THIS WAY. THEREFORE WE WILL CONTINUE TO INDICATE LESS TSTM COVERAGE IN GRIDS AS THE WEEKEND PROGRESSES...WITH ACTIVITY CONFINED TO GENERALLY ZONE 24 BY MONDAY. AS NEXT WEEK PROGRESSES WE SHOULD SEE THE RIDGE BUILD BACK TO OUR E WITH RISING HEIGHTS GIVING WARM TEMPS...LIKELY WELL ABOVE 110 F IN THE WARMER DESERTS. HARD TO TELL EXACTLY WHERE DEEPER MOISTURE WILL BE SITUATED AFTER ABOUT MIDWEEK...BUT MODELS ARE HINTING THAT EVEN THOUGH THE FLOW WILL TURN BACK TO A MORE SELY DIRECTION...IT MAY TAKE AWHILE TO GIN US BACK UP. THEREFORE...WE MAY BE IN FOR A HOT AND RATHER DRY WEEKEND...STAY TUNED. && .AVIATION... PHX/KIWA AND SURROUNDING AIRFIELDS... CLEAR SKIES OR FEW CLOUD BUILDUPS THRU 19Z...WITH ISOLD CBS FORMING OVER HIGHER TERRAIN AFT 20Z. WESTERLY FLOW PATTERN HAS TAKEN OVER THROUGHOUT THE METRO AREA...AND THIS DRIER AIRMASS WILL MEAN MUCH LESS TSTM COVERAGE/INTENSITY COMPARED TO FRIDAY EVENING. SFC TO 6K WINDS ALOFT WILL BE WEST AT 10-12 KTS WITH GOOD CONFIDENCE FOR THE TYPICAL DIURNAL WIND SHIFT AT SKY HARBOR BTWN 06-10Z. DRY WESTERLY PATTERN TO CONTINUE SUNDAY...WITH FEW 12-15K DEBRIS CLOUDS AROUND SUNRISE SUNDAY. KIPL/KBLH AND SURROUNDING AIRFIELDS... DRY CONDITIONS WITH TYPICAL DIURNAL WIND PATTERN THROUGH PERIOD. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... MONDAY THROUGH FRIDAY... MONSOON HIGH WILL BECOME SUPPRESSED THIS WEEK AS A TROUGH MOVES TOWARD THE WEST COAST. MAIN PLUME OF MONSOONAL MOISTURE WILL SHIFT EASTWARD RESULTING IN ONLY A SLIGHT CHANCE FOR THUNDERSTORMS OVER THE EASTERN ARIZONA MOUNTAINS THROUGH THE WEEK. WITH INCREASING TEMPERATURES AND THE DRYING TREND EXPECT RH LEVELS TO DROP. SOUTHWEST WINDS COULD BE GUSTY AT TIMES DURING THE AFTERNOON HOURS THROUGH THE WEEK BUT NO SIGNIFICANT WINDS EXPECTED. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...NONE. CA...NONE. && $$ NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT WEATHER.GOV/PHOENIX DISCUSSION...VASQUEZ/ESTLE AVIATION...SIPPLE FIRE WEATHER...INIGUEZ