268  
FXUS65 KPSR 181134  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
434 AM MST THU OCT 18 2018  
   
UPDATE
 
UPDATED AVIATION DISCUSSION.  
 

 
   
SYNOPSIS
 
 
DRIER AIR AND WARMER TEMPERATURES WILL GRADUALLY SPREAD INTO THE  
REGION FOR THE REMAINDER OF THE WEEK AS A LOW PRESSURE SYSTEM LIFTS  
TO THE NORTH. TEMPERATURES WILL FINALLY WARM BACK NEAR NORMAL BY THE  
END OF THE WEEK AFTER THIS PROLONGED PERIOD OF COOL CONDITIONS.  
RAIN CHANCES WILL RETURN TO MUCH OF THE AREA THIS WEEKEND WHILE  
ALSO RESULTING IN PARTLY CLOUDY SKIES. TEMPERATURES THIS WEEKEND  
INTO NEXT WEEK LOOK TO STAY NEAR SEASONAL NORMALS.  
 

 
   
DISCUSSION
 
 
THE WARMING TREND WILL CONTINUE OVER THE NEXT FEW DAYS AS THE  
UPPER LEVEL LOW SHIFTS OUT OF THE REGION, BUT BROAD SCALE  
TROUGHING STILL REIGNS ACROSS THE SOUTHWESTERN U.S. HIGHS TODAY  
SHOULD TOP OUT AROUND 80 DEGREES IN THE PHOENIX AREA WITH THE  
MIDDLE 80S COMMONPLACE FOR THE DESERTS OF SOUTHEAST CALIFORNIA.  
THE BROAD TROUGHING OVER THE REGION WILL SHIFT A BIT WESTWARD  
FRIDAY AND SATURDAY AS THE LOW CLOSES OFF JUST OFF THE CALIFORNIA  
COAST. THIS LOW IS FORECAST TO DEEPEN SLIGHTLY INTO SATURDAY, BUT  
THE LACK OF A STRONG UPPER LEVEL JET WILL KEEP THE 500MB LOW  
CENTER ONLY AT AROUND 575DM.  
 
THE POSITIONING OF THE LOW TO OUR WEST FRIDAY NIGHT THROUGH  
SATURDAY WILL BRING MODEST SOUTHERLY UPPER LEVEL FLOW INTO  
ARIZONA WHILE THE LOW AND MID LEVELS TURN OUT OF THE SOUTHEAST.  
500MB HEIGHTS OVER ARIZONA RISE TO AROUND 582DM ON SATURDAY WHILE  
THE SOUTHERLY FLOW ADVECTS IN WARMER AIR MAINLY BELOW 700MB. HIGHS  
SATURDAY SHOULD APPROACH 90 DEGREES ACROSS THE LOWER DESERTS EVEN  
AS MOISTURE WILL BE ON THE INCREASE DUE TO A LOW LEVEL JET  
SETTING UP ACROSS SOUTHEAST AND SOUTH-CENTRAL ARIZONA. 850MB  
SOUTHEASTERLY WINDS INCREASE TO 20-30KTS ON SATURDAY WHILE DECENT  
MIXING SHOULD BRING BREEZY CONDITIONS AND SOME DOWNSLOPE WARMING.  
 
THE INCREASED SOUTHEASTERLY LOW LEVEL FLOW FRIDAY NIGHT INTO  
SATURDAY WILL ADVECT MOISTURE OUT OF NORTHERN MEXICO INTO ARIZONA  
ALLOWING FOR 1000-700 MIXING RATIOS TO INCREASE TO AROUND 8-9  
G/KG. FORECAST SOUNDINGS INDICATE THE MOISTURE WILL BE FAIRLY  
SHALLOW AND FOCUSED MAINLY IN A 800-700MB LAYER. GIVEN THE  
EXPECTED WARM DAY ON SATURDAY AND INCREASED LOW LEVEL MOISTURE,  
AIRMASS INSTABILITY WILL BE ON THE INCREASE. THE UPPER LEVEL COLD  
CORE TO OUR WEST WILL EXTEND FAR ENOUGH INTO THE DESERT SOUTHWEST  
TO ALLOW FOR GOOD TO EXCELLENT LAPSE RATES. CAPE WILL INCREASE  
DURING THE DAY SATURDAY, BUT FORECAST SOUNDINGS SHOW A PRONOUNCED  
STABLE LAYER CENTERED AT 600MB. THE COMBINATION OF WEAK VERTICAL  
ASCENT ASSOCIATED WITH THE UPPER LEVEL LOW TO THE WEST, UPSLOPE  
SOUTHERLY FLOW, AND MODEST LOW LEVEL INSTABILITY SHOULD BE ENOUGH  
FOR SCATTERED SHOWERS AND A FEW THUNDERSTORMS ON SATURDAY. BEST  
RAIN CHANCES SEEM TO BE FOCUSED ACROSS THE HIGH TERRAIN IN CENTRAL  
ARIZONA, BUT THERE SHOULD BE AT LEAST SOME ISOLATED ACTIVITY  
ACROSS SOUTHERN ARIZONA. SHEAR WILL BE FAIRLY STRONG TO SUPPORT  
SOME ORGANIZED STORMS, SO THERE COULD BE A STRAY STRONG  
THUNDERSTORM OR TWO WITH SMALL HAIL AND GUSTY WINDS. RAINFALL  
AMOUNTS SATURDAY WILL BE ON THE LOW SIDE WITH MOST AREAS SEEING  
LESS THAN A TENTH OF AN INCH.  
 
ON SUNDAY THE CLOSED LOW TO OUR WEST WILL BEGIN SHIFTING TO THE  
NORTHEAST TURNING FLOW MORE OUT OF THE SOUTHWEST. RAIN CHANCES  
WILL REMAIN ACROSS SOUTH-CENTRAL AND EASTERN ARIZONA, BUT DRIER  
AIR WILL START SHIFTING EASTWARD INTO SOUTHWEST ARIZONA.  
INSTABILITY WILL LOWER SOMEWHAT THROUGHOUT THE DAY SUNDAY AS THE  
DRIER AIR SLOWLY WORKS EASTWARD, BUT THERE SHOULD STILL BE ENOUGH  
TO GIVE A SLIGHT CHANCE OF THUNDERSTORMS ALONG WITH THE SHOWER  
ACTIVITY. THIS ACTIVITY WILL SLOWLY SHIFT EAST OF PHOENIX SUNDAY  
NIGHT WITH LINGERING RAIN CHANCES ACROSS EASTERN ARIZONA ON  
MONDAY. TEMPERATURES SUNDAY INTO EARLY NEXT WEEK WILL DROP A BIT  
FROM THE WARM SATURDAY READINGS, BUT WILL STAY AT OR JUST BELOW  
SEASONAL NORMALS. WE SHOULD SEE ANOTHER WARMING TREND AT SOME  
POINT DURING THE LATTER PART OF THE NEXT WEEK AS A STRONG RIDGE TO  
OUR WEST SHIFTS INTO THE DESERT SOUTHWEST.  
 

 
   
AVIATION
 
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, AND KSDL:  
MODERATE LLWS CONCERNS WILL CONTINUE THROUGH MID-MORNING WITH  
EASTERLY WINDS ABOVE 20 KT JUST ABOVE THE SURFACE, BUT SHOULD  
WEAKEN AFTER SUNRISE. THE SURFACE PRESSURE GRADIENT WILL FAVOR  
EASTERLY FLOW PERSISTING LONGER THAN NORMAL TODAY, ALTHOUGH STILL  
EXPECTING A WIND OF WESTERLY WINDS DEVELOPING AT KPHX AND KIWA  
AFTER 23Z. MAY SEE ANOTHER ROUND OF LOW-END LLWS INTO EARLY FRIDAY  
MORNING, BUT WINDS ABOVE THE SURFACE WILL BE SLIGHTLY WEAKER  
COMPARED TO THIS MORNING, WITH LOWER CONFIDENCE IN SIGNIFICANT  
CONCERNS. SKIES WILL REMAIN MOSTLY CLEAR THROUGHOUT THE PERIOD.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
NO AVIATION CONCERNS UNDER CLEAR SKIES THROUGH THURSDAY EVENING. SFC  
WINDS WILL FAVOR A NORTHERLY DIRECTION THROUGH THE ENTIRE PERIOD  
WITH SPEEDS REMAINING BELOW 12KT.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
SATURDAY THROUGH WEDNESDAY:  
RAIN CHANCES AND A NOTICEABLE INCREASE IN HUMIDITIES WILL RETURN TO  
SOUTH-CENTRAL AZ DURING THE SAT-MON PERIOD AS LOW PRESSURE ALONG THE  
SOUTHERN-CENTRAL CA COAST PULLS MOISTURE BACK INTO THE REGION FROM  
THE SOUTH. THE BEST CHANCES FOR RAIN WILL BE OVER THE HIGH TERRAIN  
NORTH AND EAST OF PHOENIX. DRY CONDITIONS AND SLIGHTLY LOWER  
HUMIDITIES SHOULD RETURN DURING THE TUE-WED PERIOD AS WINDS ALOFT  
BECOME MORE WESTERLY. THERE WILL BE A NOTICEABLE INCREASE IN WINDS  
ON SATURDAY AS THE MOISTURE MOVES INTO THE AREA, ESPECIALLY IN THE  
MOUNTAINS EAST OF PHOENIX, BUT OTHERWISE WINDS SHOULD REMAIN AT 15-  
20MPH OR LESS AREA-WIDE. MINIMUM HUMIDITIES IN THE 25-50 PERCENT  
RANGE DURING THE SAT-MON PERIOD (LOWEST OVER SE CA) WILL FALL INTO  
THE 20-45 PERCENT RANGE (LOWEST OVER SE CA) BY TUESDAY.  
 

 
   
SPOTTER INFORMATION STATEMENT
 
 
SPOTTERS SHOULD FOLLOW STANDARD REPORTING PROCEDURES.  
 

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

 
 

 
 
DISCUSSION...KUHLMAN  
AVIATION...ROGERS/MO  
FIRE WEATHER...ROGERS/PERCHA  
 
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