598  
FXUS65 KPSR 231923  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
1223 PM MST TUE APR 23 2019  
   
SYNOPSIS
 
 
A LOW PRESSURE SYSTEM HAS INCREASED SHOWER AND STORM CHANCES IN  
SOUTH-CENTRAL ARIZONA THROUGH THIS EVENING. AS HIGH PRESSURE BUILDS  
OVER THE REGION IN THE WAKE OF THE LOW, A WARMING TREND WILL BEGIN,  
STARTING OVER SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA TODAY AND  
THEN IN SOUTH-CENTRAL ARIZONA TOMORROW. THURSDAY THROUGH FRIDAY,  
HIGHS IN THE LOWER DESERTS WILL BE APPROACHING OR EXCEEDING 100  
DEGREES. ANTICIPATE COOLER TEMPERATURES BY LATE IN THE WEEKEND AND  
EARLY NEXT WEEK.  
 

 
   
DISCUSSION
 
 
THE UPPER LEVEL PATTERN SHOWS A PRONOUNCED UPPER LEVEL CLOSED LOW  
PRESSURE SYSTEM CENTERED OVER NW MX AND THE SSE AZ BORDER WITH A WSW-  
ENE ORIENTED RIDGE AXIS FROM N CA ACROSS THE INTERMOUNTAIN WEST AND  
CENTRAL ROCKIES. IR SATELLITE AND WV IMAGERY IS SHOWING AN INCREASE  
IN CLOUD COVER ACROSS THE REGION AS A PAIR OF VORT MAX DISTURBANCES  
PUSH SOUTHWEST ACROSS THE AREA ALONG THE NORTHERN PERIPHERY OF THE  
LOW. AS A RESULT, THE CHANCES FOR SHOWERS AND STORMS HAVE INCREASED  
THIS AFTERNOON AND EARLY EVENING FROM PHOENIX AND CASA GRANDE  
EASTWARD.  
 
HIGH-RES MODELS AND HREF MEMBERS ARE FAIRLY BULLISH ON DEVELOPING  
SCATTERED CONVECTIVE SHOWERS, STORMS AND OUTFLOW WINDS FOR GOOD  
PORTIONS OF S GILA, PINAL AND E MARICOPA COUNTIES BEGINNING EARLY TO  
MID AFTERNOON WITH THE ACTIVITY LINGERING INTO THE EVENING AND  
TAPERING OFF BY/AFTER 01-03Z. SURFACE HEATING WITH WARM DAYTIME  
TEMPERATURES COMBINED WITH COLD AIR ALOFT, NEAR -21C AT H5, WILL  
CONTRIBUTE TO INCREASING INSTABILITY THIS AFTERNOON. HREF MEAN CAPE  
BY MID AFTERNOON IS FORECAST IN THE LOW 100S J/KG WITH MAX CAPE  
VALUES FORECAST AT 250-750 J/KG. MOISTURE ALSO CONTINUES TO INCREASE  
THIS AFTERNOON WITH SPC MESOSCALE VALUES ALREADY ABOVE 0.5 IN. AS  
THEY HEAD TOWARDS VALUES NEAR 0.75 IN.  
 
THE MAIN IMPACTS FROM TODAY'S STORMS WILL BE GUSTY OUTFLOW AND  
CONVECTIVE DOWNDRAFT WINDS AND DRY LIGHTNING WHICH WILL POTENTIALLY  
IMPACT BOTH AVIATION AND FIRE WEATHER CONCERNS. THIS IS SUPPORTED BY  
RELATIVELY DRY INVERTED V BOUNDARY LAYER PROFILE COMBINED WITH FORECAST  
DCAPE VALUES NEAR 1400 J/KG BY 00Z. SUSTAINED EASTERLY COMPONENT  
OUTFLOW BOUNDARY WINDS NEAR 15-20KT WILL LIKELY EXPERIENCE LOCAL  
CONVECTIVE GUSTS TO 30-35KT CLOSER TO THE STORMS. OTHER NOTABLE  
IMPACTS WILL INCLUDE ABRUPT WINDSHIFTS AND ERRATIC WINDS, ESPECIALLY  
NEAR THE STORMS. DUE TO THE GUSTY WINDS, THERE IS ALSO A SLIGHT  
CHANCE OF LOCALLY LOFTED DUST. RAINFALL AMOUNTS WITH TODAY'S STORMS  
ARE ALSO FORECAST TO BE ISOLATED TO SPOTTY AND MOSTLY MODEST.  
 
ALSO FOR TODAY A WARM UP WILL BEGIN ACROSS SE CA AND SW AZ WITH  
TEMPERATURES CLIMBING INTO THE MID 90S IN THOSE AREAS. FOR WEDNESDAY  
THROUGH THURSDAY THE WARMING TREND SPREADS EAST AS THE LOW PRESSURE  
SYSTEM EXITS TO THE EAST AND HIGH PRESSURE EXPANDS AND BUILDS OVER  
AZ AND THE WESTERN US. BY LATE THU. H5 HEIGHTS REACH NEAR 583-584  
DAM. THE ASSOCIATED SUBSIDENCE AND WAA WILL ALLOW TEMPERATURES TO CLIMB INTO  
TO THE UPPER 90S TO LOW 100S LATE IN THE WEEK ACROSS THE LOWER DESERTS.  
LOCALIZED AREAS OF MODERATE HEAT RISK WILL SPREAD FROM SE CA ON  
WEDNESDAY EAST INTO THE LOWER DESERTS AND PHOENIX ON THURSDAY AND  
FRIDAY. THE HOTTEST DAY IN PHOENIX IS CURRENTLY EXPECTED ON FRIDAY  
WHEN HIGHS WILL REACH WELL ABOVE NORMAL IN THE LOW 100S.  
 
BY LATE IN THE WEEKEND AND EARLY NEXT WEEK, EXPECT LOWER TEMPERATURES  
CLOSER TO SEASONAL NORMS AHEAD OF AND WITH A COMPACT PACIFIC LOW  
PRESSURE SYSTEM THAT IS EXPECTED TO MOVE THROUGH S CA AND INTO AZ  
AROUND MONDAY.  
 

 
   
AVIATION
 
UPDATED AT 1746 UTC.  
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, KSDL, AND KDVT:  
SHOWERS AND THUNDERSTORMS OVER EASTERN AND NORTHERN GATES ALONG WITH  
GUSTY OUTFLOW WINDS (20-30KT) AND ABRUPT WIND SHIFTS WILL BE A MAJOR  
CONCERN FOR ALL PHOENIX AREA TERMINALS FROM MID AFTERNOON THROUGH  
EARLY EVENING.  
 
EAST WINDS SHOULD PREVAIL THROUGH THE LATE MORNING AS SHOWER  
ACTIVITY REMAINS MOSTLY WELL REMOVED FROM AERODROME SITES (BOTH TO  
THE SOUTHEAST AND NORTHEAST). AFTER A SWITCH TO A WESTERLY WIND  
COMPONENT EARLY AFTERNOON, HIGH RESOLUTION MODELS CONTINUE TO BE IN  
EXCELLENT AGREEMENT IN RAPIDLY DEVELOPING THUNDERSTORMS OVER HIGHER  
TERRAIN EAST OF PHOENIX WITH STEERING FLOW BRINGING AGGRESSIVE  
OUTFLOW BOUNDARIES INTO LOWER ELEVATIONS. A FEW TS COULD HOLD  
TOGETHER AND DIRECTLY IMPACT AIRPORTS (MOST LIKELY AT KSDL AND  
KDVT), THOUGH EASTERN APPROACHES WILL LIKELY BE IMPACTED  
NONETHELESS. OUTFLOW BOUNDARIES COULD BRING ABRUPT WIND SHIFTS  
(SOMEWHERE BETWEEN AN EAST AND NORTH-NORTHEAST WIND) AS EARLY AS  
22Z, BUT COULD HOLD OFF UNTIL 00Z-01Z. THERE IS A POTENTIAL FOR  
BRIEF WIND GUSTS AS HIGH AS 35 KTS AS OUTFLOWS FROM THE STRONGER  
CELLS MIX STRONGER WINDS ALOFT DOWN TO THE SFC. AT THIS POINT,  
POTENTIAL FOR SIGNIFICANT BLOWING DUST LOOKS LOW, WITH THE  
SHOWERS/STORMS MOVING IN FROM A LESS FAVORABLE NORTHEASTERLY  
DIRECTION. ALL ACTIVITY SHOULD DISSIPATE AFTER ABOUT 04Z.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
LITTLE TO NO AVIATION CONCERNS THROUGH THE TAF PERIOD UNDER MOSTLY  
CLEAR SKIES. WINDS TO REMAIN MOSTLY FROM A NORTHERLY TO  
NORTHWESTERLY DIRECTION THROUGH EARLY EVENING BEFORE BECOMING MORE  
WESTERLY AND DIMINISHING LATER TONIGHT AND WEDNESDAY. A FEW STRONGER  
GUSTS IN EXCESS OF 20KT (PARTICULARLY AT KBLH) ARE LIKELY THROUGH  
MID AFTERNOON BEFORE WEAKENING BY LATER IN THE AFTERNOON AND  
EVENING.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
THURSDAY THROUGH MONDAY:  
HIGH PRESSURE WILL BUILD INTO THE REGION THURSDAY, BRINGING A  
SIGNIFICANT WARMING TREND WITH HIGHS ON THE LOW DESERTS WELL INTO  
THE UPPER 90S TO NEAR 100 THURSDAY THROUGH SATURDAY. THE WARMEST  
DAY LOOKS TO BE FRIDAY. ACCORDINGLY, HUMIDITIES DECLINE WITH  
MINIMUM READINGS ON THE LOWER DESERTS HOVERING AROUND 10% AND  
OVERNIGHT RECOVERY DECLINING TO FAIR. LATE IN THE WEEKEND AND  
EARLY NEXT WEEK, A LOW PRESSURE SYSTEM IS EXPECTED TO BRING COOLER  
TEMPERATURES. AS THE SYSTEM APPROACHES, ANTICIPATE MORE  
NOTICEABLE BREEZINESS DURING THE LATTER PART OF THE WEEK. AT THIS  
TIME, CRITICAL FIRE WEATHER THRESHOLDS ARE NOT ANTICIPATED.  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTER ACTIVATION IS NOT EXPECTED FOR THE NEXT 7 DAYS.  
 

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

 
 

 
 
DISCUSSION...SAWTELLE  
AVIATION...PERCHA/MO  
FIRE WEATHER...ROGERS/AJ  
 
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