712  
FXUS65 KPSR 211136  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
436 AM MST THU SEP 21 2017  
   
UPDATE...TO AVIATION AND FIRE WEATHER DISCUSSIONS
 
 

 
   
SYNOPSIS
 
 
AN AREA OF LOW PRESSURE AND ASSOCIATED COLD FRONT WILL SWEEP THROUGH  
THE REGION OVER THE NEXT TWO DAYS. THE END RESULT WILL BE  
SUBSTANTIALLY COOLER TEMPERATURES ALONG WITH OCCASIONALLY BREEZY  
CONDITIONS. PRECIPITATION CHANCES WILL BE NEGLIBLE AND LIMITED TO  
ONLY SOME SPRINKLES OVER THE HIGHER TERRAIN LOCATIONS. TEMPERATURES  
WILL ONLY SLOWLY WARM BACK TO NEAR AVERAGE NEXT WEEK.  
 

 
   
DISCUSSION
 
 
A POSITIVELY TILTED, YET SEASONALLY DEEP NEGATIVE HEIGHT ANOMALY  
THROUGH THE NW QUARTER OF THE CONUS WILL NEARLY STALL OVER THE GREAT  
BASIN AND SOUTHWEST THROUGH EARLY NEXT WEEK AS DOWNSTREAM HIGH  
AMPLITUDE RIDGING TEMPORARILY BLOCKS THE PROGRESSION OF THE WESTERLY  
FLOW PATTERN. H5 HEIGHTS BELOW 570DM WILL HOVER OVER THE NORTHERN  
HALF OF ARIZONA THROUGH THE WEEKEND EASILY PROVIDING THE COOLEST  
WEATHER OF THIS BURGEONING AUTUMN SEASON. THE BLOCKED FLOW REGIME  
WILL BE STUBBORN TO RELINQUISH ITS HOLD ON THE COUNTRY EVEN INTO THE  
MIDDLE OF NEXT WEEK WITH SOMEWHAT LOWER HEIGHTS LIKELY LINGERING  
OVER THE FORECAST AREA TOWARDS THE END OF THE MONTH. DESPITE THIS  
BLOCKING PATTERN AND LARGER ANOMALY FRAMEWORK, ENSEMBLE OUTPUT  
DISPLAYS REMARKABLE AGREEMENT THROUGH EARLY NEXT WEEK YIELDING  
BETTER THAN EXPECTED FORECAST CONFIDENCE.  
 
THOUGH THE FIRST VESTIGES OF HEIGHT FALLS AND COLD ADVECTION ALOFT  
ARE ALREADY IMPINGING ON SE CALIFORNIA EARLY THIS MORNING, THE MORE  
PRONOUNCED IMPACTS WILL BE EXPERIENCED LATE THIS AFTERNOON AND  
EVENING WITH A COLD FRONT AND MUCH LOWER THETA-E AIRMASS CROSSING  
THE SAN DIEGO MOUNTAINS. FROPA TIMING LOOKS QUITE FAVORABLE FOR  
TAPPING AND TRANSFERRING MOMENTUM IN THE H8-H7 LAYER (40 KT ABOVE  
THE SHALLOW WINDWARD MARINE LAYER) AND ENHANCEMENT THROUGH THE  
FORMATION OF STRONGER MOUNTAIN ROTORS ACROSS THE WESTERN HALF OF  
IMPERIAL COUNTY. THE WIND ADVISORY WAS EXPANDED SLIGHTLY EAST  
TOWARDS TERRAIN FEATURES INTO CENTRAL IMPERIAL COUNTY THOUGH IMPACTS  
WILL BE MOST EVIDENT ALONG THE I-8 CORRIDOR FROM EL CENTRO WESTWARD.  
OTHERWISE FURTHER EAST INTO CENTRAL ARIZONA, THERE IS MODEL EVIDENCE  
OF ENHANCED SFC PRESSURE PACKING ALONG A PRE-FRONTAL TROUGH SUCH  
THAT SOME LOCALIZED BLOWING DUST COINCIDENT WITH 30 MPH GUSTS ALONG  
I-8 FROM GILA BEND TO CASA GRANDE IS ALSO POSSIBLE.  
 
THE AFOREMENTIONED COLD FRONT WILL FORGE THROUGH THE STATE FRIDAY,  
FINALLY CLEARING INTO THE NEW MEXICO ON SATURDAY. MOISTURE  
AVAILABILITY ALONG THIS BOUNDARY WILL BE TENUOUS, AT BEST WITH ONLY  
A FEW MODEL ITERATIONS SUGGESTING A NARROW MOIST TONGUE PRESENT  
WITHIN A FRONTAL BASED OROGRAPHIC UPSLOPE REGIME (ALBEIT BEFORE MORE  
SUPPORTIVE COOLER TEMPERATURES ALOFT ADVECT EASTWARD). BASED ON  
FORECAST SOUNDINGS, LITTLE MORE THAN A BRIEF SHOWER/SPRINKLE LOOKS  
POSSIBLE ALONG THIS FRONT. WINDS WILL REMAIN OCCASIONALLY GUSTY  
FRIDAY AFTERNOON THOUGH BEGIN TO WEAKEN AFTER FROPA AND SUNSET  
FRIDAY EVENING.  
 
MORE NOTABLE THROUGH THE WEEKEND WILL BE THE UNUSUALLY COOL  
TEMPERATURES. ENSEMBLE MEANS DEPICT HEIGHTS AND TEMPERATURES ALOFT  
WITHIN THE TROUGH CORE SOME 3 NORMALIZED STANDARD DEVIATIONS BELOW  
NORMAL, OR BELOW THE 2ND PERCENTILE FOR THE END OF SEPTEMBER. CLOSER  
TO CENTRAL ARIZONA, ANOMALIES WILL BE CLOSER TO THE LOWEST 10TH  
PERCENTILE, THAT IS, HISTORICALLY EQUATING TO HIGHS ONLY IN THE  
MIDDLE/UPPER 80S. EVEN MORE AMAZING WILL BE OVERNIGHT LOWS IN THE  
50S AND LOWER 60S AND THESE READINGS SOME 10F-15F BELOW AVERAGE WILL  
PERSIST INTO EARLY NEXT WEEK.  
 
THE OVERALL TROUGHING STRUCTURE WILL BE MAINTAINED OVER THE SW CONUS  
INTO THE MIDDLE OF NEXT WEEK, HOWEVER HEIGHTS WILL SLOWLY RISE AS  
WEAKENING AND FILLING OF THE TROUGH BEGINS. DOWNSTREAM EASTERN CONUS  
RIDGING WILL ALSO WEAKEN ALLOWING FOR A PATTERN SHIFT/ADJUSTMENT BY  
THE END OF NEXT WEEK. AT THIS POINT, OPERATIONAL AND ENSEMBLE OUTPUT  
DIVERGE WITH RESPECT TO HOW MUCH VORTICITY/CIRCULATION REMAINS  
CENTERED OVER ARIZONA OR GETS DISPLACED BY HIGH PRESSURE BUILDING  
FROM THE EAST PACIFIC. REGARDLESS, TEMPERATURES WILL UNDOUBTEDLY  
MODERATE MUCH CLOSER TO THE SEASONAL NORMAL BEYOND THE MIDDLE OF THE  
WEEK.  
 

 
   
AVIATION
 
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, AND KSDL:  
TYPICAL EAST-SOUTHEASTERLY AM WINDS WILL TURN SOUTHERLY MIDDAY  
WHILE BEGINNING TO PICK UP IN SPEED. A CONTINUED SWING TO WEST-  
SOUTHWEST HEADINGS WILL OCCUR BETWEEN MIDDAY AND EARLY AFTERNOON  
WITH GUSTS 20-25KTS POSSIBLE. WESTERLY ORIENTED WINDS WILL STAY  
LONGER IN THE TERMINALS OVERNIGHT THAN USUAL, SWITCHING EARLY  
FRIDAY AM TO EASTERLY HEADINGS FOR A BRIEF PERIOD. CONDITIONS WILL  
REMAIN DRY WITH LIMITED THIN HIGH CLOUDS POSSIBLE EARLY FRIDAY AOA  
25KFT.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
STRONG COOL SEASON FRONTAL PASSAGE WILL GENERATE WINDY CONDITIONS  
FOR THE TERMINALS TODAY AND INTO THE OVERNIGHT. WIND GUSTS  
25-35KTS WILL POSSIBLE, MORE LIKELY AT KIPL THAN KBLH, FROM WEST-  
SOUTHWEST HEADINGS BY THE AFTERNOON. SKIES WILL REMAIN MOSTLY  
CLEAR AND WINDS WILL SUBSIDE INTO THE OVERNIGHT THURSDAY.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
SATURDAY THROUGH WEDNESDAY:  
POST-FRONTAL WEEKEND TEMPERATURES WILL FALL BELOW SEASONAL NORMALS  
FOR THE DISTRICTS WITH PERIODIC BREEZINESS POSSIBLE OVER AREA  
RIDGETOPS. CONDITIONS WILL LARGELY REMAIN DRY AND UNDER MOSTLY  
CLEAR SKIES AS TROUGHING HOLDS OVER THE REGION. SOME MOISTURE FROM  
THE EAST MAY BEGIN TO BE WRAPPED INTO TROUGH CIRCULATION MID NEXT  
WEEK, RAISING AREA DEWPOINTS AND INTRODUCING SOME VERY SLIGHT  
CHANCES FOR PRECIPITATION OVER SOUTHEAST AZ AND THE WHITE MTNS.  
OVERALL, HUMIDITIES WILL FALL MAINLY INTO THE 15-30 PERCENT RANGE  
WITH GOOD OVERNIGHT RECOVERIES.  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTERS SHOULD FOLLOW STANDARD REPORTING PROCEDURES.  
 

 
   
PSR WATCHES/WARNINGS/ADVISORIES
 
 
AZ...NONE.  
CA...WIND ADVISORY FROM 11 AM THIS MORNING TO 1 AM PDT FRIDAY FOR  
CAZ560>568.  
 

 
 

 
 
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DISCUSSION...MO  
AVIATION...NOLTE  
FIRE WEATHER...NOLTE  
 
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