AREA FORECAST DISCUSSION

 
 FXUS65 KPSR 140045
 AFDPSR
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE PHOENIX AZ
 545 PM MST THU AUG 13 2009
 
 .UPDATE...UPDATED AVIATION SECTION.
 
 &&
 
 .SYNOPSIS...
 A PACIFIC LOW PRESSURE SYSTEM WILL MOVE ACROSS ARIZONA TONIGHT AND 
 EARLY FRIDAY. A CHANCE OF SHOWERS AND THUNDERSTORMS WILL CONTINUE 
 OVER PARTS OF SOUTHWEST AND SOUTH CENTRAL ARIZONA TONIGHT. THE 
 SHOWER POTENTIAL IN SOUTH CENTRAL ARIZONA WILL DIMINISH FRIDAY. DRY 
 AND WARMER WEATHER IS FORECAST SATURDAY THROUGH THE MIDDLE OF NEXT 
 WEEK.
 
 &&
 
 .DISCUSSION...
 OUR MORNING STATEMENT WAS...A CONCERN FOR POSSIBLE FLASH FLOOD 
 POTENTIAL OVER PARTS EASTERN MARICOPA AND SOUTHERN GILA COUNTIES 
 THIS AFTERNOON DUE TO EXCESSIVE ATMOSPHERIC MOISTURE...WEATHER 
 DISTURBANCES MOVING THROUGH...AND A CONDITIONALLY UNSTABLE AIRMASS.  
 HOWEVER...AT 21Z CONVECTION WAS VERY SLOW TO START IF AT ALL. 
 AFTERNOON TEMPERATURES...ESPECIALLY OVER SOUTH CENTRAL AZ WERE STILL 
 RATHER COOL DUE TO CLOUD COVER AND HIGH BOUNDARY LAYER MOISTURE. 
 
 SHOWER POTENTIAL IS STILL MODERATE THE REMAINDER OF THE AFTERNOON 
 AND EARLY TONIGHT. THEREFORE A CHANCE OF SHOWERS AND THUNDERSTORMS 
 WILL CONTINUE OVER SOUTH CENTRAL AZ...WITH A SLIGHT CHANCE OVER 
 SOUTHWEST AZ.
 
 A DRIER AND INCREASINGLY STABLE AIRMASS WILL MOVE INTO THE AREA ON 
 FRIDAY...SO A SLIGHT CHANCE OF SHOWERS/THUNDERSTORMS IS CONFINED TO 
 SOUTH CENTRAL AZ.  
 
 UPPER AIR DATA AND VAPOR IMAGERY SHOWED SEVERAL DISTURBANCES 
 CONTINUED TO MOVE THROUGH AZ THIS AFTERNOON. THE LARGEST DISTURBANCE 
 WAS SLOWLY MOVING INTO AZ FROM SOUTHERN CA...AND BEST SEEN IN THE 
 300-200 MB STREAMLINE ANALYSIS AND VAPOR IMAGERY.  
 
 MOISTURE WISE...WOW! 12Z YUMA AND PHOENIX SOUNDINGS SHOWED 
 PRECIPITABLE WATER AMOUNTS NEARLY THE SAME AS GUAYMAS MEXICO...ABOUT 
 TWO INCHES. OUR CONCERN IS...800/700 MB DEWPOINTS AND PRECIP WATER 
 VALUES ACROSS SOUTH CENTRAL AZ WERE VERY HIGH SUGGESTING FLASH FLOOD 
 POTENTIAL...IF STORMS DEVELOP.  THE PROXIMITY OF A JET STREAK ACROSS 
 THE 4 CORNERS MAY CAUSE SHOWERS/THUNDERSTORMS TO DEVELOP LATER THIS 
 AFTERNOON AND EVENING. IF STORMS DEVELOP THEY WILL BE HEAVY RAIN 
 PRODUCERS.
 
 FRIDAY...
 STRONG UPPER LEVEL SUBSIDENCE AND DRIER AIR WILL SPREAD INTO THE 
 FORECAST AREA BEHIND THE WESTERLY TROF ON FRIDAY. SHOWER POTENTIAL 
 THEREFORE WILL DIMINISH ...OR LIMIT THUNDERSTORMS TO SMALL 
 ONES...TOPS 30-35 THOUSAND FEET WHICH QUICKLY DISSIPATE AFTER DARK. 
 STORM POTENTIAL FOR FRIDAY IS MAINLY FROM PHOENIX EASTWARD.
 
 SATURDAY THROUGH THURSDAY...
 MUCH DRIER WESTERLY FLOW IS EXPECTED WITH NEAR NORMAL TEMPERATURES. 
 THERE ARE INDICATIONS OF MEXICAN MOISTURE RETURN ...WITH A CHANCE OF 
 SHOWERS AND THUNDERSTORMS BY THURSDAY.  A SLIGHT CHANCE OF SHOWERS 
 AND THUNDERSTORMS WILL BE INTRODUCED TO SOUTHERN GILA COUNTY FOR 
 THURSDAY.
 
 &&
 
 .AVIATION...
 FOR KPHX/KIWA AND SURROUNDING AIRFIELDS...LARGE AREA OF RAIN THAT 
 PREVIOUSLY FELL TODAY HAS STABLIZED THE ATMOSPHERE...THEREBY SERVING 
 TO LIMIT TSTM DEVELOPMENT THIS AFTN. MOISTURE STILL EXISTS...AND 
 PLENTY OF IT...AS DOES SOME DYNAMICAL SUPPORT...SO CANNOT RULE OUT 
 THE THREAT OF LATE EVENING OR NIGHTTIME STORMS. COVERAGE FOR ANY 
 ACTIVITY WOULD BE GENLY OF AN ISOLD NATURE...AND TIMING OF ANY 
 DVLPMT IS DIFFICULT AT BEST AT THIS HOUR. THEREFORE...CONFIDENCE IS
 TOO LOW TO INCLUDE MORE THAN CB CLOUDS IN 00Z TAFS. OF COURSE...IF 
 ANY STORMS DO FORM...GUSTY AND SHIFTING WINDS WILL BE POSSIBLE. 
 THREAT FOR SGFNT BLDU IS LOWER TONIGHT DUE TO PREVIOUS RAINFALL. 
 LESS ACTIVITY EXPECTED ON FRIDAY AS AIRMASS BEGINS TO DRY OUT. 
 
 FOR WRN TAF SITES...AIRMASS IS ALREADY DRYING OUT. A SLIGHT RISK OF 
 AN OUTFLOW BOUNDARY FROM ACTIVITY TO THE EAST...OTHERWISE LITTLE TO 
 NO THREAT OF STORMS TONIGHT...WITH MUCH DRIER AIR ON FRIDAY TOTALLY 
 ELIMINATING ANY CONVECTIVE THREATS. EXPECT WINDS TO BE GENLY SE/SWLY
 THRU FRIDAY WITH SPEEDS MOSTLY UNDER 15 KNOTS.  
  
 AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
 
 &&
 
 .FIRE WEATHER...
 SATURDAY THROUGH WEDNESDAY...
 DRY AIR IS EXPECTED TO SPREAD FROM WEST TO EAST ACROSS MOST OF THE 
 AREA SATURDAY. ONLY SOME LINGERING CLOUDS AND A SLIGHT CHANCE OF 
 SHOWERS ARE EXPECTED OVER ZONE 133 SATURDAY. THEREAFTER...LOOK FOR
 MOSTLY CLEAR SKIES. DRY AIR IS EXPECTED ON SUNDAY AND MONDAY ACROSS 
 THE DESERTS. THE MINIMUM RH VALUES THOSE DAYS MAY DROP BELOW 15 PER
 CENT.
 
 &&   
  
 .PSR WATCHES/WARNINGS/ADVISORIES...
 
 AZ...NONE.
 
 CA...NONE.
 
 &&
 
 $$
 
 NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
 
 WEATHER.GOV/PHOENIX
 
 DISCUSSION...VASQUEZ 
 AVIATION...ESTLE
 FIRE WEATHER...SIPPLE
 
 
 

Webmaster
US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
Phoenix Weather Forecast Office
P.O. Box 52025
Phoenix, AZ 85072

Tel: (602) 275-0073