469  
FXUS65 KPSR 210401  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
900 PM MST THU JUL 20 2017  
   
SYNOPSIS
 
 
THE BEST THUNDERSTORM CHANCES WILL BE LIMITED PRIMARILY TO HIGHER  
TERRAIN COMMUNITIES OF CENTRAL AND EASTERN ARIZONA THROUGH THE  
WEEKEND BEFORE EXPANDING WESTWARD SOMEWHAT MONDAY. WHILE SOME  
THUNDERSTORMS COULD AFFECT A FEW LOWER ELEVATION AREAS, COVERAGE AND  
INTENSITY WILL BE LESS THAN THE PAST WEEK. AND DESPITE THE REDUCTION  
IN THUNDERSTORM ACTIVITY, TEMPERATURES SHOULD REMAIN NEAR THE  
SEASONAL AVERAGE INTO NEXT WEEK.  
 

 
   
DISCUSSION
 
 
IT'S BEEN ANOTHER RATHER INTERESTING DAY, CONVECTIVELY SPEAKING,  
OVER THE LOWER DESERTS EAST OF THE LOWER COLORADO RIVER VALLEY. HUGE  
UPPER HIGH STILL IN PLACE OVER MOST OF THE CONUS, CENTERED OVER NRN  
OK; WEAK SOUTHEAST FLOW ALOFT ON THE WESTERN PERIPHERY OF THE HIGH  
WAS PRESENT ACROSS ARIZONA. MOISTURE VALUES REMAIN ELEVATED WITH  
PWAT VALUES IN PHOENIX/TUCSON AROUND 1.6-1.7 INCHES. SURFACE  
DEWPOINTS HAVE NOT CHANGED MUCH, STILL RUNNING IN THE 60S ACROSS THE  
LOWER CENTRAL DESERTS. WEAK DISTURBANCES CONTINUE MOVE TO THE NORTH,  
ROTATING AROUND THE HIGH AND ADDING SUPPORT FOR THUNDERSTORMS.  
DESPITE THE PRESENCE OF THE EMBEDDED DISTURBANCES, ONE CHANGE OF  
NOTE IN THE FLOW HAS BEEN THE DECREASE IN THE STRENGTH OF THE  
STEERING FLOW - 700-300MB MEAN WINDS HAVE DROPPED BELOW 10KT AND IS  
NEAR CALM OVER MUCH OF SOUTH CENTRAL AZ. WITH VERY WEAK STEERING  
FLOW, STORMS WILL NOT MOVE VERY QUICKLY AND WILL TYPICALLY BE BOUND  
TO THE HIGHER TERRAIN WHERE THEY FORM. STORMS OVER THE DESERTS WILL  
PULSE UP AND THEN DISSIPATE WITH LITTLE OR NO STORM-RELATIVE FLOW TO  
MOVE THEM ALONG. THUS THE MAIN DRIVERS FOR CONVECTION THIS EVENING  
HAVE BEEN OUTFLOW BOUNDARY INTERACTION. WE SAW ISOLATED TO SCATTERED  
STORMS FORM NORTH OF THE CWA THIS AFTERNOON, AND OVER SE AZ THIS  
EVENING. BOUNDARIES HAVE SINCE MOVED INTO THE CENTRAL DESERTS FROM  
THE NORTH, THE NORTHWEST, THE SOUTH AND SOUTHEAST. NEW STORMS AND  
SHOWERS HAVE THEN FORMED WHERE BOUNDARIES INTERACT. DUE TO COOL  
AFTERNOON HIGHS (PHOENIX ONLY REACHED 103), INSTABILITY WAS LOWERED.  
VERY MOIST PROFILE IN THE LOW LAYERS (AS SEEN IN THE 00Z RAOBS)  
LIMITED POTENTIAL FOR STRONG OUTFLOWS DUE TO POOR EVAPORATIONAL  
COOLING). ALL THIS COMBINES TO LIMIT BOTH STRENGTH OF CONVECTION AND  
THEIR CORRESPONDING OUTFLOW.  
 
ALL THIS SAID, PREVIOUS FORECASTS UNDERPLAYED THE AREAL COVERAGE OF  
STORMS IN THE LOWER DESERTS THIS EVENING AND HAVE BEEN UPDATED TO  
SHOW 10-20 POPS OVER MUCH OF THE LOWER DESERT THRU MIDNIGHT. THE  
STORMS THAT HAVE DEVELOPED HAVE PROVEN TO BE GARDEN-VARIETY,  
TYPICALLY WELL BELOW SEVERE LEVELS. DUE TO THE HIGH MOISTURE,  
LOCALLY HEAVY RAINS HAVE OCCURRED BUT WE HAVE YET TO SEE AMOUNTS  
NEEDED FOR FLASH FLOODING. WE HAVE ISSUED A COUPLE OF SPS AND FLS  
PRODUCTS TO COVER THE EVENING CONVECTION AND DO NOT EXPECT FURTHER  
INTENSIFICATION OF CONVECTION OVERNIGHT. WILL CONTINUE TO MONITOR...  
   
PREVIOUS DISCUSSION
 
 
NUMEROUS MCV'S CONTINUE TO FLOAT AROUND THE SW CONUS THIS AFTERNOON  
WITHIN A WEAKENING FLOW PATTERN CHARACTERIZED BY A PRONOUNCED  
EASTERLY WAVE LIFTING NORTH THROUGH NW NEW MEXICO AND A MORE SUBTLE  
ANTI-CYCLONE OVER SOUTHERN/SOUTHWEST AZ. OBJECTIVE ANALYSIS ALREADY  
SHOWS H2 JET WINDS TURNING WESTERLY ACROSS THE CWA IN RESPONSE TO  
THE AFOREMENTIONED EASTERLY WAVE EJECTING TO THE NORTHEAST AND  
PREVAILING STRONG JET CORE CUTTING THROUGH CENTRAL CA/NV. AS A  
RESULT, DIVERGENCE FIELDS ARE LIFTING INTO NORTHERN AZ, AS WELL AS  
DEVELOPING OVER FAR SE ARIZONA LEFT IN THE VOID OF THE DEPARTING  
EASTERLY WAVE.  
 
THE NET RESULT LATER THIS AFTERNOON WILL BE MORE CONFLUENT FLOW OVER  
THE FORECAST AREA LARGELY UNSUPPORTIVE FOR BETTER CONVECTIVE COMPLEX  
ORGANIZATION AND SUSTENANCE. ADDING TO THE REDUCED THUNDERSTORM  
ACTIVITY (AT LEAST COMPARED TO THE PAST WEEK) IS WEAKER OUTFLOW  
BOUNDARIES GIVEN THE MOIST ENVIRONMENT AND LOWER T/TD SPREADS. WHILE  
DISTINCT OUTFLOWS ARE LIKELY TO FORM WITH HIGHER TERRAIN STORMS AND  
FLOW DOWNHILL, COLD POOLS WILL LIKELY BE MORE SHALLOW AND PROBABLY  
INSUFFICIENT TO SPARK STRONG NEWER UPDRAFTS. ASIDE FROM SHOWERS  
ALREADY BUILDING ALONG THE TYPICAL TERRAIN FEATURES OF CENTRAL AZ,  
GILA COUNTY IS MOST UNDER THE GUN FOR STORMS AND LOCALLY HEAVY RAIN  
THROUGH EARLY EVENING.  
 
CONVECTIVE ACTIVITY MAY BE VERY MUTED FRIDAY AS THE COMBINATION OF  
CONFLUENCE ALOFT AND STRONGER ANTI-CYCLONIC SUBSIDENCE IN THE  
MIDLEVELS SPREADS THROUGH THE FORECAST AREA. NATURALLY, A FEW STORMS  
WILL LIKELY FORM OVER THE MOUNTAINS, YET STEERING FLOW WOULD DEFLECT  
ANY STORMS AWAY FROM THE BULK OF THE CWA. IT'S PROBABLY A BETTER BET  
THAT VERY LITTLE OF THE AREA EXPERIENCES RAIN FRIDAY, AND HAVE  
REALLY LIMITED MUCH IN THE WAY OF POPS TO TERRAIN AREAS EAST OF  
PHOENIX FRIDAY AFTERNOON/EVENING.  
 
THROUGH THE WEEKEND, MIDLEVEL FLOW RECONFIGURES SLIGHTLY WITH THE  
CENTER OF THE H5 ANTI-CYCLONE BECOMING ESTABLISHED OVER SOUTHERN  
UTAH RESULTING IN NORTHEAST FLOW INTO CENTRAL AND SOUTHERN AZ.  
HOWEVER AS THIS HAPPENS, THERE IS NO CHANGE IN THE MOSTLY  
UNSUPPORTIVE WESTERLY JET WINDS, SO THUNDERSTORM ACTIVITY MAY  
STRUGGLE SOMEWHAT TO MAINTAIN INTEGRITY TRYING TO PROPAGATE INTO  
LOWER ELEVATIONS (NOT TO MENTION OUTFLOW BOUNDARIES THAT MAY STILL  
BE WEAKER THAN EXPERIENCED EARLIER IN THE MONSOON SEASON). ENSEMBLE  
PROBABILITIES ARE NOT VERY OPTIMISTIC ABOUT ANY WIDESPREAD STORMS  
OVER THE WEEKEND, AND ITS LOOK MORE LIKE A TYPICAL AREAL AND  
TEMPORAL MONSOON STORM COVERAGE.  
 
MONDAY THROUGH WEDNESDAY...  
A TRANSITION MAY BEGIN THROUGH NEXT WEEK WITH A POTENTIAL TURN TO A  
MORE SOUTHWESTERLY COMPONENT TO THE FLOW AND A GRADUAL DRYING TREND.  
HOWEVER, THERE IS SIGNIFICANT UNCERTAINTY IN THIS TREND AS FLOW MAY  
HOLD AT A SOUTHERLY COMPONENT WITH LOWER LEVEL TRAJECTORIES STILL  
FROM THE SE AND IMPORTING MOISTURE. AT SOME POINT NEXT WEEK, THERE  
WILL LIKELY BE A SHORTWAVE PARTIALLY ABSORBED IN THE SOUTHERLY FLOW  
AND SUPPORTIVE FOR ORGANIZED STORMS. PINPOINTING SUCH A DAY THIS FAR  
IN ADVANCE IS FRUITLESS, AND HAVE CONTINUED WITH MODEST POPS FOR THE  
TIME BEING.  
 

 
   
AVIATION
 
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, AND KSDL:  
 
EARLIER WE WERE NOT EXPECTING CONVECTION INTO THE GREATER PHOENIX  
AREA DUE TO VERY WEAK STEERING FLOW CONFINING STORMS TO THE HIGHER  
TERRAIN. HOWEVER, OUTFLOW BOUNDARIES PROVED STRONGER AND MORE  
PERSISTENT THAN EXPECTED; THEY HAVE MOVED INTO THE AREA FROM THE  
NORTHWEST, NORTH, SOUTH AND SOUTHEAST AND HAVE ALLOWED ISOLATED  
SHOWERS/STORMS TO FORM AND AFFECT THE TAF SITES. VARIABLE WINDS UP  
TO 25 KT HAVE RESULTED DUE TO VARIOUS MERGING BOUNDARIES. HAVE  
UPDATED TAFS TO ADD VCTS THIS EVENING AND EVEN MENTIONED TSRA  
EARLIER AT KDSL. EXPECT ISOLATED CONVECTION AFFECTING THE TAFS THRU  
AROUND 05Z THIS EVENING BEFORE DISSIPATING AND LEAVING SCT-BKN MID  
TO HIGH DECKS WHICH WILL PERSIST INTO THE MORNING HOURS. EXPECT LESS  
CONVECTION IN THE CENTRAL DESERTS/GREATER PHOENIX AREA TOMORROW DUE  
TO CONTINUED WEAK AND SOMEWHAT UNFAVORABLE STEERING WINDS AND A  
SLIGHTLY MORE STABLE ENVIRONMENT.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
 
NO CONVECTION EXPECTED IN THE WESTERN DESERTS FOR THE NEXT 24 HOURS  
AS THE ATMOSPHERE REMAINS RELATIVELY STABLE WITH VERY WEAK WINDS  
ALOFT. WILL KEEP TAFS SHORT AS ANY CLOUD DECKS WILL TYPICALLY BE FEW-  
SCT WITH BASES MOSTLY AOA 12K FEET. WINDS CONTINUE TO FAVOR THE SE  
AT KIPL AND THE SOUTH AT KBLH WITH SPEEDS BELOW 15KT. NO REAL  
AVIATION CONCERNS OVER THE WESTERN DESERTS OR THE TAF SITES THRU  
FRIDAY EVENING.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
SUNDAY THROUGH THURSDAY:  
AFTERNOON AND EVENING THUNDERSTORMS WILL CONTINUE NEXT WEEK,  
ALTHOUGH THE MOST CONCENTRATED STORMS WILL BE CONFINED TO THE HIGHER  
ELEVATIONS NORTH AND EAST OF PHOENIX. LOWER ELEVATIONS OF ARIZONA  
COULD STILL SEE SOME STORMS, BUT LESS COVERAGE THAN THE PAST WEEK  
WHILE SE CA WILL STRUGGLE TO RECEIVE ANY RAINFALL. OTHERWISE,  
MINIMUM AFTERNOON HUMIDITY LEVELS WILL REMAIN GENERALLY AROUND 20-  
40% RANGE WITH GOOD TO EXCELLENT OVERNIGHT RECOVERY. TYPICAL  
SUMMERTIME BREEZES CAN BE EXPECTED EXCEPT NEAR THUNDERSTORMS WHERE  
ERRATIC GUSTY WINDS WILL BE LIKELY.  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTERS SHOULD FOLLOW STANDARD REPORTING PROCEDURES.  
 

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

 
 

 
 
VISIT US ON FACEBOOK, TWITTER, AND AT WEATHER.GOV/PHOENIX  
 
DISCUSSION...CB  
PREVIOUS DISCUSSION...MO/AJ  
AVIATION...CB  
FIRE WEATHER...MO/WILSON  
 
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