476  
FXUS65 KPSR 172315  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
415 PM MST SAT AUG 17 2019  
   
UPDATE
 
 
UPDATED AVIATION  
 

 
   
SYNOPSIS
 
 
TEMPERATURES WILL COOL EVER SO SLIGHTLY THIS WEEKEND BUT REMAIN  
SEVERAL DEGREES ABOVE NORMAL. MOISTURE AND THUNDERSTORM ACTIVITY  
WILL BE VERY LIMITED THROUGH THE MIDDLE OF NEXT WEEK AS RECORD HIGH  
TEMPERATURES AND ANOTHER EXCESSIVE HEAT EPISODE IS EXPECTED FOR  
TUESDAY AND WEDNESDAY ACROSS MOST OF SOUTHERN ARIZONA AND SOUTHEAST  
CALIFORNIA. MOISTURE SHOULD RETURN BY THURSDAY TO BEGIN A GRADUAL  
COOLING TREND AND BRING BACK AT LEAST A SLIGHT CHANCE OF  
THUNDERSTORMS BACK INTO THE FORECAST FOR SOUTH CENTRAL ARIZONA LATE  
NEXT WEEK.  
 

 
   
DISCUSSION
 
 
WEST-SOUTHWESTERLY FLOW IS ENGULFING THE DESERT SOUTHWEST AND  
PUSHING MOISTURE EAST AND OF THE FORECAST AREA. THIS MORNINGS  
PHOENIX SOUNDING SHOWED A PW OF 0.87 INCHES, WHICH IS JUST ABOVE THE  
10 PERCENT MOVING AVERAGE. CURRENT MESOSCALE ANALYSIS ALSO INDICATES  
A DRY ATMOSPHERE OVERHEAD WITH PWS BETWEEN 0.4 AND 0.8 INCHES IN OUR  
FORECAST AREA (THE GREATEST PWS ARE OVER LA PAZ, YUMA, AND MARICOPA  
COUNTIES). MID-LEVEL TEMPERATURES HAVE ALSO CONTINUED TO WARM SINCE  
YESTERDAY AND AS A CONSEQUENCE IT'S GOING TO BE EXTREMELY DIFFICULT  
TO GET ANY STORMS GOING TODAY, ESPECIALLY WITH VERY LIMITED  
INSTABILITY. CAPE IS PRETTY MUCH NON-EXISTENT TODAY, WITH THE  
GREATEST INSTABILITY IN FAR SOUTHEASTERN ARIZONA. THE HREF IS IN  
AGREEMENT, SHOWING THE BEST CHANCES FOR ISOLATED CONVECTION IN FAR  
SOUTHEAST COCHISE COUNTY. OTHERWISE, TEMPERATURES TODAY WILL RISE  
INTO THE 105-110 DEGREE RANGE WITH MOSTLY CLEAR SKIES IN PLACE.  
 
SIMILAR CONDITIONS AND TEMPERATURES ARE EXPECTED ON SUNDAY AS H500  
HEIGHTS REMAIN AROUND 589-590 DM AND H850 TEMPERATURES REMAIN AROUND  
28-29 DEG C. MID-LEVEL HEIGHTS WILL ONCE AGAIN REBUILD AND INTENSIFY  
SUNDAY INTO MONDAY, FIRST ACROSS THE SOUTH CENTRAL TEXAS BEFORE  
SHIFTING NORTHWEST INTO BIG BEND. TEMPERATURES WILL BEGIN TO  
INCREASE ON MONDAY, BUT WE SHOULD REMAIN BELOW EXCESSIVE HEAT  
THRESHOLDS SINCE THE HEIGHT INCREASES ALOFT WILL NOT TRANSLATE TO  
WARMER BOUNDARY LAYER TEMPERATURES. HOWEVER, THE MID-TO-UPPER-LEVEL  
ANTICYCLONE IS EXPECTED TO BECOME POSITIONED NEAR THE FOUR CORNERS  
FOR TUESDAY INTO WEDNESDAY, WITH H500 HEIGHTS INCREASING TO AROUND  
594 DM UNDER MOSTLY CLEAR SKIES. FORECAST H850 TEMPERATURES EXCEED  
31 DEG C ON TUESDAY AND 32 DEG C ON WEDNESDAY, ABOUT A DEGREE WARMER  
THAN THAN THE PAST WEEK'S EXCESSIVE HEAT EPISODE, SO ANOTHER  
EXCESSIVE HEAT EPISODE APPEARS LIKELY WITH DAILY RECORDS LIKELY  
BEING SET ON WEDNESDAY AND POSSIBLY ON TUESDAY. THEREFORE, THE  
EXCESSIVE HEAT WARNING FOR TUESDAY AND WEDNESDAY OVER THE LOWER  
DESERTS OF ARIZONA HAS BEEN EXPANDED FOR SOME HIGHER ELEVATION  
LOCATIONS IN PINAL AND GILA COUNTY INCLUDING SUPERIOR, GLOBE, AND  
MIAMI THAT WERE LEFT OUT OF THIS PREVIOUS EVENT. IN ADDITION, THE  
LATEST MODEL GUIDANCE SHOWS TEMPERATURES INCREASING MORE RAPIDLY  
THAN PREVIOUSLY THOUGHT ACROSS SOUTHEAST CALIFORNIA ON TUESDAY, SO  
WE HAVE INCLUDED THEM IN THE WARNING FOR TUESDAY IN COORDINATION  
WITH NEIGHBORING OFFICES. WE HAVE EVEN ADDED IN WESTERN PORTIONS OF  
JOSHUA TREE NATIONAL PARK FOR WEDNESDAY THAT ARE USUALLY LEFT OUT OF  
THESE WARNINGS AS SOME OF THE MORE ACCESSIBLE PORTIONS OF THE PARK  
WILL EXPERIENCE EXCESSIVE HEAT.  
 
FORTUNATELY, THIS EXCESSIVE HEAT EPISODE SHOULD BE SHORTER THAN  
WHAT WE SAW THIS LAST WEEK AS MOISTURE RETURNS FROM THE SOUTH  
WEDNESDAY NIGHT INTO THURSDAY, THOUGH THIS MAY NOT RETURN FAST  
ENOUGH FOR SOUTHEAST CALIFORNIA WHERE AN EXTENSION MAY BE NEEDED.  
A FAST-MOVING EASTERLY TROPICAL WAVE CURRENTLY EAST OF THE LESSER  
ANTILLES SHOULD CROSS THE YUCATAN EARLY THIS WEEK AND PROGRESS  
INTO CENTRAL AMERICA AND MEXICO BY WEDNESDAY, TRIGGERING A  
SIGNIFICANT AMOUNT OF CONVECTION IN THE TROPICS. AT THE SAME TIME,  
A WEAK FRONTAL SYSTEM OVER THE CENTRAL PLAINS AND THUNDERSTORMS  
ON THE LEE SIDE OF THE ROCKIES SHOULD HELP BRING IN MOISTURE FROM  
THE NORTHEAST AS THE CENTER OF THE ANTICYCLONE SLIDES WESTWARD.  
BOTH OF THESE FEATURES SHOULD AT LEAST END THE EXCESSIVE HEAT,  
THOUGH THUNDERSTORMS MAY BE LIMITED ON THURSDAY TO THE HIGHER  
TERRAIN NORTH AND EAST OF PHOENIX AND OVER SOUTHEAST ARIZONA AS  
MIDLEVEL STABILITY LEFT BEHIND BY THE RIDGE ALOFT WILL STILL BE IN  
PLACE. HOWEVER, BETTER SHOWER AND THUNDERSTORM CHANCES FOR THE  
LOWER DESERTS INCLUDING PHOENIX ARE EXPECTED ON FRIDAY AS WE MAY  
HAVE BETTER NORTHEASTERLY STEERING FLOW ALONG WITH CONVECTION TO  
OUR SOUTHEAST TO HELP MULTIPLE OUTFLOW BOUNDARY COLLISIONS  
OVERCOME ANY LEFTOVER MIDLEVEL STABILITY. REGARDLESS, WE WILL  
FINALLY HAVE A RETURN OF A MONSOON PATTERN WHICH APPEARS TO HANG  
AROUND FOR A BIT WITH PERHAPS SOME INFLUENCE FROM A TROPICAL  
CYCLONE AS BOTH THE ECMWF AND GFS SHOW DEVELOPMENT ONCE THE  
EASTERLY WAVE CROSSES INTO THE EAST PACIFIC LATER NEXT WEEK.  
 

 
   
AVIATION
 
UPDATED AT 2315 UTC.  
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, KSDL, AND KDVT:  
NO MAJOR AVIATION CONCERNS THROUGH SUNDAY EVENING WITH JUST  
INCREASING HIGH CIRRUS DECKS. WINDS COULD REMAIN SOMEWHAT VARIABLE  
INTO THE EVENING BEFORE SETTLING ON A WSW COMPONENT; AND CONFIDENCE  
IS ONLY MODERATE REGARDING THE TIMING OF THE SWITCH BACK TO EASTERLY  
OVERNIGHT. SIMILAR TO TODAY, THE WIND TRANSITION SUNDAY AFTERNOON  
WILL ALSO LIKELY INCORPORATE SEVERAL HOURS OF LIGHT AND VARIABLE  
CHARACTER.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
LITTLE TO NO AVIATION CONCERNS THROUGH SUNDAY EVENING UNDER  
INCREASING THICKER CIRRUS DECKS. WINDS WILL FAVOR A GENERAL  
PERSISTENT SOUTHERLY COMPONENT WITH THE TYPICAL DIURNAL VARIATION  
BETWEEN A ESE AND WSW. SOME OCCASIONAL GUSTS TO 20 KT ARE POSSIBLE.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
TUESDAY THROUGH SATURDAY:  
A WARMING TREND WILL TAKE PLACE THROUGH THE MIDDLE OF NEXT WEEK.  
HIGH TEMPERATURES WILL CLIMB WELL ABOVE NORMAL BY NEXT TUESDAY AND  
WEDNESDAY WITH MANY DESERT TEMPERATURES INCREASING INTO THE 110 TO  
115 DEGREE RANGE AND SOME ISOLATED SPOTS EXCEEDING 115 DEGREES IN  
SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA. MOISTURE SHOULD RETURN  
BY THURSDAY TO BEGIN A GRADUAL COOLING TREND AND BRING BACK AT  
LEAST A SLIGHT CHANCE OF THUNDERSTORMS INTO THE FORECAST FOR THE  
HIGHER TERRAIN OF GILA COUNTY AND PERHAPS THE LOWER DESERTS  
INCLUDING THE PHOENIX METRO FRIDAY AND SATURDAY. MINIMUM RELATIVE  
HUMIDITY VALUES THROUGH WEDNESDAY WILL DECREASE INTO THE SINGLE  
DIGIT RANGE BEFORE AN UPWARD TREND COMMENCES THURSDAY THROUGH  
SATURDAY TO KEEP ALL AREAS IN THE 15 TO 25 PERCENT RANGE. WINDS WILL  
FOLLOW TYPICAL DIURNAL PATTERNS WITH AFTERNOON AND EARLY EVENING  
GUSTINESS.  
 

 
   
CLIMATE
 
 
RECORD HIGH TEMPERATURES  
 
DATE PHOENIX YUMA  
---- ------- ----  
AUG 17 114 IN 2013 114 IN 1992  
AUG 18 112 IN 2011 116 IN 1960  
AUG 19 113 IN 1986 116 IN 1915  
AUG 20 112 IN 1986 114 IN 1982  
AUG 21 110 IN 2007 115 IN 1969  
AUG 22 113 IN 2011 115 IN 1969  
AUG 23 114 IN 2011 115 IN 2011  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTERS SHOULD FOLLOW STANDARD REPORTING PROCEDURES.  
 

 
   
PSR WATCHES/WARNINGS/ADVISORIES
 
 
AZ...EXCESSIVE HEAT WARNING FROM 11 AM TUESDAY TO 8 PM MST WEDNESDAY  
FOR AZZ530>546-548>556-559>562.  
 
CA...EXCESSIVE HEAT WARNING FROM 11 AM TUESDAY TO 8 PM PDT WEDNESDAY  
FOR CAZ561>570.  
 
EXCESSIVE HEAT WARNING FROM 11 AM TO 8 PM PDT WEDNESDAY FOR  
CAZ560.  
 

 
 

 
 
DISCUSSION...HERNANDEZ/HOPPER  
AVIATION...MO  
FIRE WEATHER...HERNANDEZ/HOPPER  
CLIMATE...ROGERS/KUHLMAN/MO  
 
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