636  
FXUS65 KPSR 150840  
AFDPSR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PHOENIX AZ  
140 AM MST FRI DEC 15 2017  
   
SYNOPSIS
 
 
QUIET WEATHER IS EXPECTED TODAY AND MOST OF SATURDAY BEFORE THE NEXT  
WEATHER SYSTEM MOVES IN. THIS ONE IS LOOKING A BIT STRONGER NOW,  
WITH CHANCES FOR RAIN AND MOUNTAIN SNOW NOW IN THE FORECAST FOR THE  
EASTERN HALF OF ARIZONA SUNDAY AND INTO EARLY MONDAY. TEMPERATURES  
WILL ALSO BE QUITE COOL WITH THIS SYSTEM, LIKELY BRINGING THE  
COOLEST TEMPERATURES OF THE WINTER SO FAR. TUESDAY AND BEYOND WILL  
SEE A RETURN OF DRY AND GRADUALLY WARMING WEATHER.  
 

 
   
DISCUSSION
 
 
EARLY THIS MORNING, THE UPPER RIDGE AXIS REMAINED NEAR THE WEST  
COAST, KEEPING A DRY NORTH/NORTHEAST FLOW ALOFT GOING INTO ARIZONA.  
PWAT VALUES FROM THE LATEST SOUNDINGS SHOWED JUST 0.10 AT FLAGSTAFF  
AND 0.30 AT TUCSON. SURFACE DEWPOINTS OVER THE CENTRAL DESERTS WERE  
MOSTLY IN THE TEENS OR LOW 20S. IR IMAGERY AT 1 AM INDICATED CLEAR  
SKIES AREA-WIDE. FOR TODAY WE CAN EXPECT CONTINUED SUNNY SKIES AS  
HIGH PRESSURE ALOFT CONTINUES TO DOMINATE OUR WEATHER PATTERN. HIGH  
TEMPERATURES OVER THE WARMER DESERTS WILL AGAIN BE WELL ABOVE  
NORMAL, CLIMBING INTO THE LOW TO MID 70S.  
 
OPERATIONAL AS WELL AS ENSEMBLE GUIDANCE HAS STARTED TO CONGEAL UPON  
A SOLUTION FOR THIS WEEKEND THAT INVOLVES BETTER CHANCES OF RAINFALL  
FOR SOUTH-CENTRAL ARIZONA (THO STILL NOT OVERLY HEAVY AMOUNTS OF  
RAIN). THERE ARE SOME MINOR DIFFERENCES IN THE DETAILS BUT THE  
OVERALL PICTURE INVOLVES THE UPPER RIDGE RETROGRADING A BIT OFF OF  
THE WEST COAST AND ALLOWING A RATHER VIGOROUS SHORT WAVE TO DIVE  
QUICKLY SOUTHEAST OUT OF THE PACIFIC NORTHWEST AND INTO ARIZONA. AS  
IT APPROACHES ON SATURDAY, CONSIDERABLE MID/HIGH CLOUDS SPREAD INTO  
THE LOWER DESERTS FOR PARTLY OR MOSTLY CLOUDY SKIES. HIGH TEMPS  
START TO FALL OFF AS HEIGHTS FALL AND CLOUDS INCREASE. BY LATE  
SATURDAY BOTH OPERATIONAL GFS AND ECMWF AS WELL AS MOST GEFS  
ENSEMBLE MEMBERS PROPOSE A LARGE UPPER TROF FORMING OVER THE DESERT  
SOUTHWEST, SITUATED SQUARELY OVER ARIZONA. GFS IS MORE OF AN OPEN  
WAVE WITH THE LATEST RUN (BUT THAT COULD EASILY CHANGE WITH THE  
NEXT RUN OR TWO) AND THE ECMWF HAS A LARGE CLOSED LOW CENTERED OVER  
THE FAR NORTHERN GULF OF CA. IN BOTH CASES THEY RAPIDLY DEVELOP HIGH  
MEAN RH IN THE COLUMN OVER SOUTHERN ARIZONA AND AREAS OF LIGHT RAIN  
OR SHOWERS WILL START TO BREAK OUT ACROSS SOUTH CENTRAL AZ BY EARLY  
SATURDAY MORNING. BEST RAIN CHANCES FOR SOUTH-CENTRAL ARIZONA WILL  
BE MID MORNING THROUGH LATE AFTERNOON; POPS HAVE BEEN RAISED AGAIN  
INTO THE LIKELY CATEGORY FOR THE HIGHER TERRAIN EAST OF PHOENIX, AND  
INTO THE GOOD CHANCE CATEGORY FOR THE PHOENIX METROPOLITAN AREA.  
 
DYNAMICS LOOK TO BE RATHER IMPRESSIVE WITH HEALTHY AMOUNTS OF PVA,  
MID LEVEL Q-CONVERGENCE AND LOW/MID LEVEL UVV. THERE ARE NO  
ATMOSPHERIC RIVERS FORECAST TO BRING LARGE AMOUNTS OF MOISTURE TO  
FAR SOUTHERN CA OR ARIZONA AND MEAN RH REALLY INCREASES QUICKLY  
DESPITE NO REAL MOISTURE TAP POTENTIAL (BASED ON CURRENT SATELLITE  
IMAGERY). IT IS LIKELY THAT MUCH OF THE RH INCREASE IS DUE TO RAPID  
COOLING OF THE AIRMASS MORESO THAN LARGE AMOUNTS OF MOISTURE  
INGESTED INTO THE SYSTEM. AS SUCH, QPF IS FORECAST TO BE RATHER LOW  
WITH THIS SYSTEM. BEST FORCING AND PRECIP POTENTIAL IS DURING THE  
DAY SUNDAY BEFORE ALL OF THE COLD AIR ARRIVES, SO WE ARE NOT LOOKING  
AT ANY SIGNIFICANT SNOWFALL EITHER. GIVEN THE SHARP LOWERING OF  
HEIGHTS AND THICKNESSES AND THE INCREASE IN CLOUDS AND SHOWERS, HIGH  
TEMPS WILL FALL WELL BELOW SEASONAL NORMALS FOR A CHANGE WITH  
CENTRAL DESERTS FALLING INTO THE UPPER 50S TO LOW 60S, AND ONLY  
SLIGHT WARMING IS EXPECTED MONDAY.  
 
MODELS AGREE THAT THE TROF/CLOSED LOW WILL BE QUICK TO PUSH OFF TO  
OUR EAST OVERNIGHT SUNDAY NIGHT INTO EARLY MONDAY AS STRONG DVV AND  
MID LEVEL DIVERGENCE OF Q MOVE INTO THE DESERTS; BY 12Z MONDAY RAIN  
CHANCES WILL BE DOWN INTO THE SINGLE DIGITS EAST OF PHOENIX AND  
SKIES FROM THE PHOENIX METRO WESTWARD WILL BE GENERALLY CLEAR.  
 
HIGH PRESSURE ALOFT WILL REBUILD OVER THE DESERT SOUTHWEST DURING  
THE EARLY TO MIDDLE PORTION OF NEXT WEEK ALLOWING HIGH TEMPERATURES  
TO GRADUALLY WARM BACK ABOVE SEASONAL NORMALS WITH WARMER DESERTS  
CLIMBING BACK INTO THE LOW TO MID 70S BY NEXT WEDNESDAY. ANOTHER  
SHORT WAVE IS FORECAST TO DROP THROUGH THE FOUR CORNERS THURSDAY BUT  
AS OF NOW IT LIKELY WILL BE DRY FOR OUR AREA AND POPS WILL STAY IN  
THE SINGLE DIGITS. IT WILL USHER IN SOME COOLER AIR AGAIN, DROPPING  
CENTRAL DESERT HIGHS BACK INTO THE UPPER 60S.  
 

 
   
AVIATION
 
 
SOUTH-CENTRAL ARIZONA INCLUDING KPHX, KIWA, AND KSDL:  
A NORTH-NORTHEASTERLY FLOW HAS DEVELOPED ACROSS THE PHOENIX AREA  
IN THE WAKE OF THE DEPARTING TROUGH. WINDS WILL VEER OVERNIGHT TO  
THE EAST-NORTHEAST WHILE SUBSIDING SOMEWHAT. HOWEVER, LATEST MODEL  
GUIDANCE SUGGESTS STRONGER WINDS AROUND 30 KT JUST ABOVE THE  
SURFACE WILL PERSIST OVERNIGHT, WHICH WILL CREATE ISOLATED POCKETS  
OF LOW-LEVEL WIND SHEAR. FOR FRIDAY, WINDS WILL PREDOMINANTLY  
REMAIN OUT OF THE EAST LESS THAN 10 KT, EXCEPT FOR THE LATE  
AFTERNOON WHEN A PERIOD OF LIGHT AND VARIABLE WINDS IS POSSIBLE.  
THERE IS ALSO A SLIGHT CHANCE THAT LOCAL TOPOGRAPHIC INFLUENCES  
WILL YIELD A PERIOD OF LIGHT SOUTHWESTERLY WINDS AT KPHX EARLY  
FRIDAY MORNING.  
 
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:  
NO AVIATION WEATHER IMPACTS EXPECTED. THE WINDY CONDITIONS FROM  
EARLIER TODAY WILL CONTINUE TO SUBSIDE OVERNIGHT ACROSS  
SOUTHEASTERN CALIFORNIA. WINDS WILL REMAIN OUT OF THE NORTH AT  
BLYTHE AROUND 10 KT FRIDAY, WHILE LIGHT AND VARIABLE WINDS WILL  
PREVAIL AT KIPL.  
 
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.  
 

 
   
FIRE WEATHER
 
 
SATURDAY THROUGH WEDNESDAY...  
A FAST MOVING UPPER TROUGH WILL CUT SOUTHWARD THROUGH SOCAL AND  
DIVE OFF THE WESTERN BAJA COAST SATURDAY INTO SUNDAY, DEVELOPING  
ANOTHER ROUND OF ELEVATED NORTHERLY WINDS OVER SOUTHWEST AZ, THE  
LOWER COLORADO RIVER VALLEY AND PORTIONS OF SOUTHEAST CA. WITH  
COOLER HIGH TEMPERATURES IN THE FORECAST DAYTIME HUMIDITIES RANGE  
10-20 PERCENT OVER THE WEEKEND WITH MOSTLY FAIR OVERNIGHT  
RECOVERIES. CURRENT FIRE WEATHER FORECAST ELEMENTS SUPPORT VERY  
SPARSE AND SPOTTY RED FLAG CONDITIONS FOR SUNDAY WEST OF THE  
COLORADO RIVER FOR AN HOUR OR TWO, BUT NOTHING AT THIS TIME  
WARRANTING MORE THAN MENTION HERE. SOME RECENT CHANGES TO MODEL  
FORECASTS NOW HAVE SOME MOISTURE IN PLAY TO THE SOUTH ACROSS  
NORTHERN MEXICO. SOME INCREASED IN HUMIDITIES REGIONALLY AND EVEN  
REMOTE CHANCE FOR LIGHT PRECIP (MAINLY EASTERN-SOUTHEAST AZ) COULD  
DEVELOP. QUASI-ZONAL FLOW WILL SETTLE OVER THE REGION BY EARLY  
NEXT WEEK, KEEPING TEMPERATURES ON THE MORE MILD SIDE BUT STILL  
SLIGHTLY ABOVE SEASONAL NORMALS.  
 

 
   
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  
AVIATION...HIRSCH  
FIRE WEATHER...NOLTE  
 
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