139  
FXUS65 KPSR 201309  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
609 AM MST MON AUG 20 2018  
   
UPDATE
 
 
UPDATED 12Z AVIATION AND FIRE WEATHER DISCUSSIONS.  
 

 
   
SYNOPSIS
 
 
ABOVE NORMAL TEMPERATURES ARE EXPECTED AGAIN TODAY ACROSS THE  
REGION. HIGHS OVER THE LOWER DESERTS WILL RANGE BETWEEN 105 AND  
110 DEGREES WITH THE WARMEST READINGS OVER SOUTHEAST CALIFORNIA.  
SUBSTANTIAL MOISTURE RETURNS TO THE REGION STARTING TONIGHT WITH  
SHOWER AND THUNDERSTORM CHANCES RETURNING ON TUESDAY. STORM  
CHANCES REMAIN ACROSS SOUTH CENTRAL ARIZONA THROUGH THE REST OF  
THE WEEK, BUT WILL MOSTLY BE FOCUSED ACROSS THE HIGHER TERRAIN  
NORTH AND EAST OF PHOENIX.  
 

 
   
DISCUSSION
 
 
THE UPPER LEVEL HIGH CENTER IS CURRENTLY SITUATED OVER CENTRAL  
ARIZONA, BUT WILL DRIFT SOUTHEAST OVER THE NEXT DAY OR TWO.  
LAST EVENING, SEVERAL THUNDERSTORM COMPLEXES DEVELOPED ACROSS  
NORTHERN MEXICO JUST SOUTH OF THE HIGH. AS THE HIGH SHIFTS  
SOUTHEAST, OUR FLOW WILL QUICKLY TURN SOUTHEASTERLY WITH MOISTURE  
LEVELS BEGINNING TO INCREASE FROM THE CURRENT 1.00-1.40" PWATS.  
MODELS CONTINUE TO SHOW THIS AFTERNOON AND EVENING WILL BE VERY  
ACTIVE ACROSS NORTHERN MEXICO AND POSSIBLY JUST SOUTH OF TUCSON  
THIS EVENING. AN INVERTED TROUGH IS ALSO SHOWN MOVING NORTHWARD  
OUT OF MEXICO INTO SOUTHERN ARIZONA SOMETIME AFTER MIDNIGHT. A  
STRONG MOISTURE SURGE WILL ACCOMPANY THIS INVERTED TROUGH, PARTLY  
COMING FROM THE CONVECTION OVER NORTHERN MEXICO AND ALSO FROM A  
GULF SURGE.  
 
THE FORECAST SCENARIO FOR TONIGHT AND TUESDAY IS RATHER COMPLEX  
WITH MULTIPLE FACTORS COMING INTO PLAY. CURRENT THINKING IS WE  
MAY SEE SHOWERS AND THUNDERSTORMS OVERNIGHT TONIGHT ACROSS SOUTH-  
CENTRAL ARIZONA FORMING ALONG THE NORTHERN EDGE OF THE INVERTED  
TROUGH AND/OR FROM GRAVITY WAVES PRODUCED FROM CONVECTION TO THE  
SOUTH. MODELS ARE STARTING TO SHOW THIS POSSIBILITY MORE IN THE  
LATEST RUNS, BUT THERE IS STILL QUITE A BIT OF DISCREPANCY AMONG  
MODELS. EVEN IF STORMS DON'T DEVELOP TONIGHT OVER SOUTHERN  
ARIZONA, THE MOISTURE SURGE WILL BE SIGNIFICANT WITH PWATS RISING  
TO BETWEEN 1.75-2.00" BY MID MORNING TUESDAY. POPS HAVE BEEN  
INCREASED SOME FOR TONIGHT, BUT MAY END UP HAVING TO BE RAISED  
EVEN MORE IF THE 12Z RUNS SHOW MORE MODEL CONSENSUS.  
 
THE REST OF TUESDAY IS EVEN MORE UNCERTAIN AS WE COULD EITHER SEE  
A VERY ACTIVE STORM DAY WITH FLOODING CONCERNS ACROSS SOUTH-  
CENTRAL AND EASTERN ARIZONA, OR WE MAY END UP MOSTLY CLOUDY  
LACKING SUFFICIENT INSTABILITY FOR WIDESPREAD STORM ACTIVITY. AT  
THIS POINT MODELS ARE GENERALLY SHOWING THE LATTER WILL HAPPEN  
DUE TO TOO MUCH MOISTURE AND OVERALL FAIRLY CLOUDY CONDITIONS. IF  
TONIGHT ENDS UP BEING FAIRLY QUIET AND CLOUD COVER IS LESS DURING  
THE DAY TUESDAY, TUESDAY AFTERNOON AND EVENING SHOULD BE VERY  
ACTIVE WITH HEAVY RAINFALL PRODUCING STORMS MAINLY FROM MARICOPA  
COUNTY EASTWARD. POPS HAVE BEEN RAISED ON TUESDAY, BUT GIVEN THE  
UNCERTAINTY, CHANCES MOSTLY FALL BETWEEN 30-40%. WINDS ALOFT ARE  
INITIALLY STRONG TUESDAY MORNING WITH 20-30KTS OUT OF THE  
SOUTH/SOUTHEAST AT 500MB AND A 50KT UPPER LEVEL JET ACROSS  
WESTERN ARIZONA PROVIDING AMPLE DIVERGENCE ALOFT OVER CENTRAL  
ARIZONA. WE CAN'T RULE OUT A FEW STRONG WIND PRODUCING STORMS  
EARLY TUESDAY MORNING, BUT THIS IS NOT A FAVORABLE TIME OF DAY FOR  
STRONG TO SEVERE STORMS. WINDS ALOFT GRADUALLY WEAKEN DURING THE  
DAY TUESDAY WHILE THE STEERING FLOW VEERS OUT OF THE  
SOUTH/SOUTHWEST. THIS SHOULD LEAD TO MOSTLY HEAVY RAINFALL  
PRODUCING STORMS LATER IN THE DAY TUESDAY WITH LOCALIZED STRONG  
GUSTY WINDS STILL POSSIBLE WITHIN ANY MICROBURSTS. HIGH  
TEMPERATURES ON TUESDAY WILL DROP SEVERAL DEGREES FROM TODAY'S  
READINGS WITH TEMPERATURES TOPPING OUT AROUND 100 DEGREES IN THE  
PHOENIX AREA TO AROUND 105 ACROSS THE WESTERN DESERTS.  
 
THE HIGH CENTER WILL CONTINUE SHIFTING EASTWARD ON WEDNESDAY WITH  
OUR FLOW ALOFT TURNING SOLELY OUT OF THE SOUTHWEST. THIS WILL  
BRING DRIER AIR INTO THE REGION, BUT WE SHOULD STILL HAVE  
SUFFICIENT MOISTURE FOR AFTERNOON AND EVENING STORMS MOSTLY ACROSS  
THE HIGH TERRAIN NORTH AND EAST OF PHOENIX. SIMILAR CONDITIONS ARE  
SEEN FOR THURSDAY AND FRIDAY, BUT STORM CHANCES SHOULD DIMINISH  
EACH DAY WITH THE LOWER DESERTS LIKELY STAYING DRY. TEMPERATURES  
WILL REMAIN FAIRLY STEADY THROUGH THE REST OF THE WEEK AS 500MB  
HEIGHTS LOWER FROM 594DM ON WEDNESDAY TO 588DM ON SATURDAY. THESE  
LOWER HEIGHTS ARE DUE TO A DEVELOPING PACIFIC LOW WELL OFF THE  
CALIFORNIA COAST. THOUGH IT IS AROUND A WEEK AWAY, BOTH THE GFS  
AND EUROPEAN SHOW A MORE SIGNIFICANT PACIFIC TROUGH DIGGING  
SOUTHEASTWARD INTO THE PACIFIC NORTHWEST EARLY NEXT WEEK. THIS  
WILL DEFINITELY BE SOMETHING TO WATCH FOR THE MIDDLE PART OF NEXT  
WEEK.  
 

 
   
AVIATION
 
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, AND KSDL:  
 
LIGHT AND VARIABLE WINDS THIS MORNING WITH A BRIEF PERIOD OF  
EASTERLY WINDS THIS MORNING WILL BECOME WESTERLY AT 10 KNOTS BY  
THE AFTERNOON WITH GUSTS GENERALLY REMAINING BELOW 15 KNOTS.  
MOISTURE WILL INCREASE TODAY TO ALLOW AT LEAST ISOLATED TO  
SCATTERED THUNDERSTORMS TO DEVELOP OVER SOUTHERN ARIZONA THIS  
AFTERNOON. THESE STORMS MAY BE CAPABLE OF PUSHING SOUTHERLY  
OUTFLOWS (AND CROSSWINDS) INTO THE PHOENIX SITES LATER TONIGHT  
AFTER 03Z WITH THE LATEST HRRR RUNS EVEN SHOWING A LINE OF  
THUNDERSTORMS PUSHING THROUGH. HOWEVER, MOST HI-RES AND GLOBAL  
MODELS SHOW THIS CONVECTION DISSIPATING WELL SOUTH OF PHOENIX  
WHICH SEEMS MOST REASONABLE AT THIS TIME.  
 
THE FORECAST FOR THE OVERNIGHT HOURS IS RATHER UNCERTAIN, SO FOR  
NOW WE HAVE VICINITY SHOWERS AND SOUTHWEST WINDS WITH BROKEN TO  
OVERCAST CLOUDS ABOVE 10 KFT AFTER 06Z. HOWEVER, THERE ARE MANY  
POSSIBLE SCENARIOS OVERNIGHT DEPENDING ON HOW CONVECTION AND  
MOISTURE RETURN EVOLVES THIS AFTERNOON AND EVENING, INCLUDING  
PREVAILING SHOWERS AND THUNDERSTORMS FOR SEVERAL HOURS. WE DO HAVE  
BETTER CONFIDENCE THAT SHOWERS AND THUNDERSTORMS WILL OCCUR AT  
SOME POINT LATE IN THE OVERNIGHT HOURS OR JUST AFTER SUNRISE, SO  
WE HAVE ADDED IN A PROB30 GROUP AFTER 12Z. HOWEVER, THIS MAY NEED  
TO BE MOVED UP OR REMOVED DEPENDING ON HOW STORMS EVOLVE TONIGHT.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
 
SOUTHERLY AND SOUTHEASTERLY FLOW WILL PREDOMINATE AT KBLH AND  
KIPL, RESPECTIVELY. WINDS THIS AFTERNOON WILL BE SUSTAINED TO  
15-20 KNOTS WITH GUSTS TO 25 KNOTS POSSIBLE AT KBLH. A FEW HI-RES  
MODELS SHOW CONVECTION THAT DEVELOPS OVER NORTHERN MEXICO MAY  
MOVE INTO THE VICINITY OF KIPL, BUT THE VAST MAJORITY OF MODELS  
KEEP THIS ACTIVITY OUT SO THE FORECAST REMAINS THUNDERSTORM FREE  
FOR NOW. ASIDE FROM THAT, NO AVIATION WEATHER IMPACTS EXPECTED.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
 
WEDNESDAY THROUGH SUNDAY:  
 
AMPLE MOISTURE DURING THE MIDDLE OF THIS WEEK WILL KEEP CHANCES  
OF THUNDERSTORMS ELEVATED OVER THE REGION, IN ADDITION TO KEEPING  
MINIMUM RELATIVE HUMIDITY VALUES ABOVE 20 PERCENT OVER THE LOWER  
DESERTS OF SOUTH CENTRAL ARIZONA. SIGNIFICANT DRYING IS EXPECTED  
OVER SOUTHEAST CALIFORNIA AND POSSIBLY SOUTHWEST ARIZONA FOR  
THURSDAY AND BEYOND, WITH A REDUCTION IN CONVECTIVE ACTIVITY  
SPREADING FURTHER EAST LATER THIS WEEK INTO THE WEEKEND ASIDE  
FROM THE HIGHER TERRAIN NORTH AND EAST OF PHOENIX. WIND SPEEDS  
WILL BE TYPICAL FOR MID-SUMMER WITH THE USUAL AFTERNOON BREEZES  
ASIDE FROM STRONGER WINDS WITH THUNDERSTORM OUTFLOWS. TEMPERATURES  
WILL REMAIN NEAR TO SLIGHTLY BELOW NORMAL THROUGH THE PERIOD.  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTERS SHOULD FOLLOW STANDARD REPORTING PROCEDURES.  
 

 
   
PSR WATCHES/WARNINGS/ADVISORIES
 
 
AZ...NONE.  
CA...NONE.  

 
 

 
 
DISCUSSION...KUHLMAN  
AVIATION...HOPPER  
FIRE WEATHER...HOPPER/PERCHA  
 
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