Area Forecast Discussion


690 
FXUS65 KPSR 130406
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
905 PM MST MON AUG 12 2013
.SYNOPSIS...
DRY WEATHER WILL CONTINUE FOR THE LOWER DESERTS FOR MOST...IF NOT 
ALL OF THE WEEK. AFTERNOON AND EVENING THUNDERSTORM CHANCES WILL 
REMAIN CONFINED TO THE EASTERN PORTIONS OF THE ARIZONA HIGH TERRAIN. 
HIGH PRESSURE WILL GRADUALLY BEGIN TO REBUILD INTO THE SOUTHWEST 
STATES...LEADING TO SLOWLY RISING TEMPERATURES AND CONTINUED STORM 
CHANCES OVER HIGHER TERRAIN...WITH LITTLE CHANCE FOR RAINFALL OVER 
LOWER DESERT COMMUNITIES.
&&
.DISCUSSION...
LITTLE CHANGE IN THE WEATHER PATTERN AGAIN TODAY...AS UPPER TROFFING 
TO OUR WEST HAS KEPT A DRY SOUTHWEST FLOW IN PLACE ACROSS SERN CA 
AND MOST OF ARIZONA. AS A RESULT VERY DRY AIR REMAINS ACROSS ALL OF 
OUR CWA WITH THE EXCEPTION OF ZONE 24 WHERE ISOLATED SHOWERS AND 
THUNDERSTORMS DEVELOPED AGAIN THIS AFTERNOON...MAINLY EAST OF GLOBE. 
LOW LEVEL MOISTURE BOUNDARY...OR A SORT OF DRYLINE...REMAINS OVER 
FAR EASTERN AZ...GRADUALLY PUSHING WESTWARD THEN RETREATING BACK TO 
THE EAST DURING THE EVENING HOURS AS DRIER WESTERLY FLOW SETS UP 
ACROSS THE DESERTS. EARLY THIS AFTERNOON SURFACE DEWPOINTS AT 
PHOENIX WERE IN THE MID 50S AS SURFACE WINDS GUSTED A BIT FROM THE 
SOUTHEAST. AT 9 PM DEWPOINTS FELL TO 29 AS WEST WINDS TO 15KT MIXED 
TO THE SURFACE AND PUSHED THE MOISTURE BACK TO THE EAST. AT 830 PM 
STORMS HAD MOSTLY ENDED ACROSS ZONE 24...AND ALL OF ARIZONA FOR THAT 
MATTER...WITH SKIES RAPIDLY CLEARING EAST OF PHOENIX. IR IMAGERY 
SHOWED CLEAR SKIES FROM PHOENIX WESTWARD. EXPECT GENLY CLEAR SKIES 
REST OF TONIGHT...EXCEPT FOR A FEW LINGERING MID LEVEL DEBRIS CLOUDS 
OVER THE HIGHER TERRAIN...AND MOSTLY LIGHT WINDS. FORECASTS IN FINE 
SHAPE AND NO UPDATES ARE NEEDED.
.PREVIOUS DISCUSSION...
DESPITE AN INCREASE IN PWATS PER 12Z SOUNDING DATA...BOTH SYNOPTIC 
AND MESOSCALE CONDITIONS APPEAR UNFAVORABLE FOR ORGANIZED AND 
PERSISTENT DEEP CONVECTION THIS AFTERNOON/EVENING (AND FOR THE NEXT 
COUPLE DAYS FOR THAT MATTER). MUCH OF THE INCREASED MOISTURE WAS 
SAMPLED IN A 600-700MB LAYER...AS WELL AS IN THE SHALLOW BOUNDARY 
LAYER AT LOWER ELEVATIONS WHERE THE MOISTURE GRADIENT/DRYLINE TYPE 
BOUNDARY HAS SLOSHED WESTWARD WITH THE OVERNIGHT DRAINAGE 
WIND/PRESSURE COMPONENT. HOWEVER...THIS NARROW CORRIDOR OF MOISTURE 
ALOFT ABOVE 700MB HAS BEEN JUST SUFFICIENT TO SUPPORT ISOLD PULSE 
TYPE CONVECTION OVER SOUTHERN GILA COUNTY IN AN ENVIRONMENT OF 
MLCAPE AOA 500 J/KG WITH AMPLE SHEAR...YET LACKING LAPSE RATES IN 
THE UNSTABLE LAYER. GIVEN UNSEASONABLY STRONG SWLY 850-300MB FLOW 
VECTORS AROUND 20KT AND AN INCREASINGLY HOSTILE ENVIRONMENT FOR 
SUSTAINED UPDRAFTS...MOST ACTIVITY SHOULD HAVE WANED OR PROPAGATED 
OUT OF THE FORECAST AREA BY THIS EVENING.  
RIDGING WILL RETROGRADE FROM THE SOUTHERN PLAINS AND AMPLIFY OVER 
THE SWRN CONUS FOR THE REMAINDER OF THE WEEK. DURING THIS TRANSITION 
PHASE DURING THE MIDWEEK TIME FRAME...WINDS IN THE H7-H5 LAYER WILL 
BACK TO A MORE PRONOUNCED S/SE DIRECTION ALLOWING MIXING RATIOS 
ALOFT TO INCREASE TOWARDS A 5-7 G/KG RANGE ACROSS THE ENTIRE REGION. 
HOWEVER...SFC DEWPOINTS LOOK TO ONLY SLOWLY RESPOND IN KIND AND 
WITHOUT AN EVIDENT WELL DEFINED FORCING MECHANISM (I.E. INVERTED 
TROUGH OR PERSISTENT UPPER LEVEL DIFLUENCE)...RAINFALL CHANCES LOOK 
NO MORE THAN A CLIMATOLOGICAL PROBABILITY. OPERATIONAL MODELS HAVE 
CONTINUED THE TREND OF PORTRAYING LITTLE TO NO QPF...WHILE ENSEMBLE 
MEMBERS ONLY TARGET THURSDAY AS APPROACHING CLIMATOLOGICAL ODDS.
FOR THE LATE WEEK TO EARLY WEEKEND...OPERATIONAL AND ENSEMBLE
FORECAST MEMBERS AGREE ON THE GENERAL IDEA OF THE 500MB RIDGE CENTER
SHIFTING WESTWARD ENOUGH TO BE NEARLY OVERHEAD BY THURSDAY. RIDGE
HEIGHTS START PUSHING 595DM PLUS WITH 1000-500MB THICKNESSES SLIGHTLY
INCREASING EACH AFTERNOON. WARM AND ABOVE NORMAL AFTERNOON
TEMPERATURES WILL PERSIST THROUGH THE WEEK...WITH SOME OF THE LOWER 
ELEVATION LOCALES CLOSER TOWARDS THE COLORADO RIVER POTENTIALLY 
SEEING 110 PLUS READINGS. AT THE SAME TIME...MORNING LOWS WILL BEGIN 
A GRADUAL WARMING TREND DUE TO THE WARMER AFTERNOONS AND RETURN OF 
ELEVATED DEWPOINT READINGS TRYING TO RETURN TO MORE MONSOON-LIKE 
READINGS. GFS/ECMWF/GEM BEGIN TO DIVERGE ON ACTUAL RIDGE POSITIONING 
AND SUBSEQUENT ORIENTATION OF THE FLOW OVER AND INTO THE REGION. 
EXTENDED POP FORECAST STILL DEPICTS A SLIGHTLY LOW-GRADE /AND BELOW 
CLIMO/ MONSOON PATTERN...WITH TYPICAL AFTERNOON/EVENING SLIGHT 
CHANCES FOR THE ARIZONA HIGH TERRAIN AND MOSTLY SINGLE DIGIT POPS 
ACROSS THE LOWER DESERTS. 
&&
.AVIATION... 
SOUTH-CENTRAL ARIZONA...INCLUDING KPHX...KIWA AND KSDL...AND  
SOUTHEAST CA AND SOUTHWEST AZ...INCLUDING KIPL AND KBLH...
NO CHANGE IN THE WEATHER PATTERN EXPECTED FOR THE NEXT 24 HOURS AT 
LEAST AS DRY SOUTHWEST/WEST FLOW ALOFT WILL KEEP DRY AIR IN 
PLACE...LEADING TO SUNNY DAYS AND CLEAR NIGHTS. MAY SEE A FEW HIGH 
BASED CU FROM PHOENIX EASTWARD TUESDAY AFTERNOON AND EARLY EVENING 
OTHERWISE EXPECT CLEAR SKIES. THUNDERSTORMS WILL STAY OVER HIGHER 
TERRAIN WELL TO THE EAST OF PHOENIX AND NO OUTFLOWS ARE EXPECTED TO 
REACH INTO THE LOWER DESERTS THRU TUE EVENING. WINDS WILL CONTINUE 
TO FOLLOW NORMAL DIURNAL SPEEDS AND DIRECTIONS AT THE TAF SITE AND 
MAY EXHIBIT SOME LOCAL GUSTINESS DURING THE AFTERNOON AND EVENING 
HOURS. HAVE PLAYED LARGELY A PERSISTENCE FORECAST WITH THE WINDS AT 
ALL TAF SITES. NO AVIATION CONCERNS THRU TUESDAY EVENING.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
&&
.FIRE WEATHER...
THURSDAY THROUGH MONDAY...
OVERALL...DRY AIR WILL DOMINATE ALL BUT FAR EASTERN ARIZONA THROUGH 
THE PERIOD WITH SLIGHTLY ABOVE NORMAL TEMPERATURES. MODEST MONSOON 
MOISTURE ACROSS SOUTHERN GILA COUNTY WILL ALLOW FOR A CONTINUED 
SLIGHT CHANCE OF AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS. 
MINIMUM HUMIDITIES WILL GENERALLY BE IN THE 10-15 PERCENT RANGE 
ACROSS THE LOWER DESERTS. WEAK SYNOPTIC GRADIENT FLOW WILL ALLOW FOR 
TYPICAL MOUNTAIN VALLEY DIURNAL WINDS TO PREVAIL THROUGH THE PERIOD. 
$$ 
 
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&
$$
VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX
DISCUSSION...CB
PREVIOUS DISCUSSION...MO/NOLTE
AVIATION...CB
FIRE WEATHER...KUHLMAN