Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000 FXUS65 KPSR 030535 AAA AFDPSR AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE PHOENIX AZ 1035 PM MST TUE SEP 2 2014 .UPDATE...UPDATED AVIATION DISCUSSION. && .SYNOPSIS... MOISTURE WILL GRADUALLY RETURN TO THE REGION BEGINNING THURSDAY...WITH A MUCH MORE HUMID AIRMASS OVERSPREADING THE AREA BY THE WEEKEND. THUNDERSTORM CHANCES WILL FIRST START OVER SOUTHEAST ARIZONA AND THE HIGHER TERRAIN TO THE NORTH AND EAST THURSDAY...BUT INCREASE SIGNIFICANTLY OVER THE REMAINDER OF THE AREA BY THE WEEKEND. WITH THE INCREASING MOISTURE...TEMPERATURE WILL EVENTUALLY FALL BACK OR SLIGHTLY BELOW AVERAGE BY EARLY NEXT WEEK. && .DISCUSSION... ANOTHER VERY WARM DAY IS NOW ENDING ACROSS OUR CWA...WITH BOTH PHOENIX SKY HARBOR AND YUMA REACHING 109 THIS AFTERNOON...WHICH WAS 6 DEGREES ABOVE THE NORMAL HIGH OF 103 FOR THE DATE...UNDER CLEAR SKIES. NOT MUCH CHANGE IN THE WEATHER IS EXPECTED ON WEDNESDAY AS HIGH PRESSURE REMAINS OVER THE REGION WITH A VERY DRY AIRMASS IN PLACE. HOWEVER...THERE ARE ALREADY SIGNS THAT A NOTICEABLE CHANGE TO COOLER AND MORE HUMID WEATHER IS ON THE WAY FOR LATER THIS WEEK...WITH THE LATEST SATELLITE IMAGERY NOW SHOWING TS ACTIVITY JUST SOUTH OF THE INTERNATIONAL BORDER AS A WEAK INVERTED TROF MOVES SLOWLY ACROSS NORTHERN MEXICO. ALSO...TROPICAL STORM NORBERT...WHICH IS NOW CENTERED ABOUT 320 MILES SE OF THE SOUTHERN TIP OF BAJA...CONTINUES TO SLOWLY MOVE NNW AND IT IS NOW EXPECTED TO BECOME A HURRICANE BY LATE THURSDAY AS IT PASSES TO THE SOUTH AND WEST OF CABO SAN LUCAS...WITH THE POTENTIAL THAT AT LEAST SOME OF THE MOISTURE FROM THIS TROPICAL SYSTEM COULD EVENTUALLY FIND ITS WAY INTO THE DESERT SOUTHWEST BY THIS WEEKEND. AS FAR AS THE FORECAST FOR THE REST OF THIS EVENING AND TONIGHT IS CONCERNED...OTHER THAN SOME MINOR ADJUSTMENTS TO THE TEMP AND DEWPOINT GRIDS TO BETTER REFLECT HOURLY TRENDS...INHERITED GRIDDED AND ZONE FORECASTS ARE LOOKING QUITE GOOD...AND NO UPDATES ARE PLANNED. && .PREVIOUS DISCUSSION... A SPRAWLING HIGH PRESSURE RIDGE WITH H5 HEIGHTS NEAR 592DM CONTINUES TO ENVELOP AN AREA FROM THE SOUTHERN PLAINS THROUGH SWRN CONUS THIS AFTERNOON (AS HAS BEEN THE CASE FOR THE PAST SEVERAL DAYS). SUBTROPICAL MOISTURE REMAINS TRAPPED THROUGH CNTRL/SRN SONORA/CHIHUAHUA WITH 12Z KTWC/KPSR SOUNDING DATA SAMPLING PWATS UNDER 0.75 INCHES AND SFC-H7 MIXING RATIOS ONLY IN THE 5-6 G/KG RANGE. MEANWHILE...A PROGRESSIVE WAVE ACROSS THE PACIFIC NORTHWEST AND NRN/CNTRL ROCKIES WILL ACTUALLY BECOME THE IMPETUS TO SHIFT THE CENTER OF THE AFOREMENTIONED SUBTROPICAL HIGH EASTWARD...ALLOWING A DEEPER SOUTHERLY MONSOON WIND COMPONENT AND MOISTURE SURGE BACK INTO THE SOUTHWEST LATE THIS WEEK INTO EARLY NEXT WEEK. THOUGH TOMORROW NIGHT...THERE WILL BE LITTLE CHANGE IN THE THERMODYNAMIC STRUCTURE OF THE ATMOSPHERE OVER THE ENTIRE FORECAST AREA...AND HAVE CONTINUED TO FOLLOW A NEAR PERSISTENCE FORECAST. THE FIRST INKLINGS OF MOISTURE RETURN WILL COMMENCE THURSDAY MORNING...PRIMARILY IN THE MIDLEVELS AND POTENTIALLY LOOSELY TIED TO DISTANT OUTFLOWS FROM NRN MEXICO/FAR SERN ARIZONA. HAVE ALMOST COMPLETELY DISCOUNTED THE RECENT OVERZEALOUS GFS QPF FIELDS FORECAST FOR THURSDAY MORNING GIVEN GOOD AGREEMENT OF SUBSTANTIAL DRY AIR STILL RESIDING IN THE SFC-H7 LAYER. THUS WHILE VIRGA AND SOME HIGHER ELEVATION SPRINKLES ARE POSSIBLE DURING THE MORNING HOURS...HAVE HEDGED CLOSER TO A SREF/ECMWF BLEND FOCUSING FAR BETTER CONVECTIVE INITIATION CHANCES DURING AFTERNOON PEAK HEATING OVER HIGHER TERRAIN FEATURES. LOWER ELEVATIONS MAY STILL BE A TAD TOO DRY THURSDAY EVENING WITH BOUNDARY LAYER MIXING RATIOS ONLY SNIFFING 10 G/KG...HOWEVER STRONGER OUTFLOWS WITH BLOWING DUST WILL STILL BE A THREAT GIVEN DEEP INVERTED-V SOUNDING PROFILES. THE FORECAST FOR FRIDAY IS A BIT TRICKIER...AS BETTER LOW LEVEL MOISTURE PROFILES SHOULD SPREAD NORTH BUT THE MAGNITUDE AND POSITIONING OF UPPER LEVEL SUPPORT REMAINS IN QUESTION. ONE LARGER WILD CARD IS THE CIRCULATION REMNANTS OF TROPICAL SYSTEM DOLLY FROM THE GULF OF MEXICO CROSSING WEST OVER THE MEXICAN PENINSULA. WHILE GLOBAL MODELS START LOSING ANY DEFINED CIRCULATION OVER THE MOUNTAINS...THERE IS SOME EVIDENCE OF A LINGERING INVERTED TROUGHING AND DIFLUENCE HEADING INTO CHIHUAHUA/SONORA LATE FRIDAY. MADE SOME MODEST INCREASES TO POPS FRIDAY EVENING...THOUGH HOLDING OFF THE MORE APPRECIABLE RAINFALL CHANCES UNTIL THE WEEKEND. SATURDAY INTO EARLY NEXT WEEK...SOME MEASURE OF TROPICAL INFLUENCE IS LOOKING MORE POSSIBLE AS THE COMBINATION OF STRONGER UPPER DIFLUENCE (SOME INPUT FROM TS DOLLY) AND DEEPER TROPICAL MOISTURE INFLUX FROM EAST PACIFIC TROPICAL SYSTEM NORBERT MAY MEET OVER THE FORECAST AREA. HOWEVER...THE LATEST 12Z AND 18Z HURRICANE CENTER CONSENSUS MODEL TRACKS KEEP THE CENTER OF NORBERT WELL TO THE SOUTHWEST OF THE BAJA PENINSULA...AND MOISTURE INFLUENCE MAY BE MORE LIMITED OR SECONDARY THAN PREVIOUS GLOBAL SCALE MODELS MAY HAVE PRESUMED. REGARDLESS...PRECIP CHANCES WERE FURTHER INCREASED GIVEN OVERWHELMING OPERATIONAL AND ENSEMBLE MODEL CONSENSUS ON MOISTURE AND CAPE/OMEGA FIELDS SUNDAY AND MONDAY. HOWEVER GIVEN THE TIME FRAME 5 TO 6 DAYS OUT...KEPT POPS CAPPED AROUND 50 PERCENT FEELING ONE OF THESE DAYS WILL BE VERY CONVECTIVE ACTIVE AND OTHER POTENTIALLY NOT SO ACTIVE. SOME DISCREPANCIES REMAIN IN MOISTURE PLACEMENT AND INFLUENCE OF DRY AIR INTRUDING FROM A TROUGH TO OUR NW BY THE MIDDLE OF NEXT WEEK. MANY 12Z OPERATIONAL MODELS DEPICT RATHER RAPID DRYING IN NWLY FLOW AS EARLY AS TUESDAY...THOUGH CERTAINLY NOT ALL ENSEMBLE MEMBERS SUPPORT SUCH AN IDEA OF STRONGER TROUGHING AND NORTHWEST FLOW. NEVERTHELESS...WITH THE UPTICK IN STORM ACTIVITY AND EXPECTATION OF MOSTLY TO CONSIDERABLY CLOUDY SKIES...A COOL DOWN OF WELCOME PROPORTIONS WILL RESULT FOR THE WEEKEND...WITH SEVERAL BEST PERFORMING AND CONSENSUS TEMPERATURE FORECASTS COOLING MUCH OF THE FORECAST AREA BELOW 100F BY MONDAY AND BEYOND. && .AVIATION... SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA...AND KSDL...AND SOUTHWEST ARIZONA AND SOUTHEAST CALIFORNIA INCLUDING KIPL AND KBLH... HIGH PRESSURE OVER THE REGION WILL KEEP SKIES MOSTLY CLEAR AND WINDS ON THE LIGHT SIDE THROUGH WEDNESDAY MORNING. INCREASING SOUTHERLY FLOW ALOFT IS THEN EXPECTED TO PUSH SOME HIGH CLOUDINESS INTO THE REGION ON WEDNESDAY AFTERNOON AND EVENING...WITH FEW-SCT CIRRUS CLOUD LAYERS PASSING OVER THE TERMINALS FROM TIME-TO-TIME. AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS. && .FIRE WEATHER... FRIDAY THROUGH TUESDAY... MONSOON MOISTURE WILL CONTINUE TO INCREASE AND SPREAD WESTWARD REACHING THE LOWER DESERTS OF SOUTHEAST CALIFORNIA BY SATURDAY. STORM CHANCES CONTINUE THROUGH MONDAY BEFORE TRENDING DOWNWARD TUESDAY. HUMIDITIES WILL FOLLOW A SIMILAR TREND. MEANWHILE...TEMPERATURES TREND DOWNWARD. WINDS WILL FOLLOW FAMILIAR SUMMERTIME SLOPE-VALLEY PATTERNS BUT WITH SIGNIFICANT DISRUPTIONS DUE TO THUNDERSTORM ACTIVITY. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...NONE. CA...NONE. && $$ VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX DISCUSSION...PERCHA/MO/NOLTE AVIATION...PERCHA FIRE WEATHER...AJ

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