773
FXUS65 KPSR 071700
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
1000 AM MST SUN SEP 7 2008
SOUTH-CENTRAL AND SOUTHWEST ARIZONA...AND SOUTHEAST CALIFORNIA
FORECAST DISCUSSION
SYNOPSIS
MOISTURE LEVELS WILL BEGIN TO INCREASE TODAY ACROSS THE DESERT
SOUTHWEST...AND CONTINUE THROUGH MONDAY. THIS WILL LEAD TO A SLIGHT
CHANCE FOR THUNDERSTORMS ACROSS THE EASTERN ARIZONA MOUNTAINS TODAY
AND AGAIN MONDAY. THUNDERSTORM CHANCES INCREASE AND SPREAD TO ALL
AREAS TUESDAY AS A LOW PRESSURE SYSTEM MOVES ACROSS THE REGION.
SLIGHT CHANCES FOR PRECIPITATION WILL LINGER THROUGH THE END OF THE
WEEK...MAINLY OVER THE MOUNTAINS...AS THE FORECAST BECOMES
INCREASINGLY UNCERTAIN. TEMPERATURES WILL GENERALLY BE ON A COOLING
TREND THROUGHOUT THE WEEK.
DISCUSSION
MOISTURE...AT LEAST AT THE MID LEVELS...WAS CONTINUING TO INCREASE
ACROSS MOST OF THE PHX CWA THE PAST 24 HOURS...AS EVIDENCED BY THE
LATEST VIS SATELLITE LOOPS. LOCAL 88D RADARS AND LIGHTNING DETECTION
SYSTEMS EVEN SHOWING AN ISOLD TSTM OR TWO EMBEDDED FROM TIME TO TIME
IN THESE LIGHT SHOWERS. HAVE UPDATED ZONES AND GRIDDED DATABASE TO
ACCOUNT FOR THIS PRESENT WX.
LATEST 12Z MODEL DATA SHOWING VERY LITTLE CHANGE IS NEEDED TO OUR
CURRENT FORECAST THINKING NEXT FEW DAYS. GENERAL TROUGHINESS IS
EXPECTED TO DEVELOP NEAR THE WEST COAST...WITH OF COURSE DIFFERENCES
IN THE DETAILS AMONGST THE MODELS. BUT...THE GENERAL TREND WILL BE
TOWARDS CONTINUING AN INCREASE IN MOISTURE NEXT TWO TO THREE DAYS AS
AGEOSTROPHIC FLOW AT THE LOWER LEVELS BEGINS TO TAP AND IMPORT
MOISTURE SOURCE TO OUR S. AND...ATMOSPHERE SHOULD REMAIN UNSTABLE
ENOUGH TO SUPPORT AN INCREASE IN CONVECTION...ESPECIALLY BY TUESDAY
ACROSS MOST OF CWA. AS CLOUDS AND MOISTURE INCREASE...MAX TEMPS
SHOULD COME DOWN...AND MIN TEMPS SHOULD BEGIN TO WARM UP. ALL OF
THIS IS HANDLED WELL IN CURRENT FORECASTS...SO NO FURTHER UPDATES
NEEDED.
PREVIOUS DISCUSSION...
FEW SURPRISE SHWRS/TSTMS POPPING UP ACROSS OUR ERN CWA THIS MORNING.
THIS IS LIKELY DUE TO A RATHER WEAK VORT MAX MOVING ACROSS AZ PER
WATER VAPOR IMAGERY. IT IS ALSO INDICATIVE THAT MOISTURE LEVELS ARE
IN FACT INCREASING ACROSS AZ...WHICH HAS BEEN ANTICIPATED FOR
SEVERAL DAYS NOW. LITTLE CHANGE IN TODAY/S FORECAST FROM PREVIOUS
SHIFTS WITH SLIGHTLY COOLER MAX TEMPS FROM SAT AND LOW-END POPS OVER
SRN GILA COUNTY.
MONDAY...SLIGHTLY COOLER TEMPS AGAIN AND AN INCREASE IN POPS ACROSS
OUR CNTRL AND ERN MTNS AS MOISTURE CONTINUES TO INCREASE.
SHWRS/TSTMS SHOULD BE AIDED A BIT BY A WEAK SHORTWAVE LIFTING NE
ACROSS OUR CWA. ONE THING THAT MAY TEMPER ACTIVITY A BIT IS THE FACT
WE/LL BE IN THE LEFT ENTRANCE REGION OF A SRN BRANCH JET STREAK.
TUESDAY...MADE A NOTICEABLE INCREASE TO POPS ON TUESDAY AS MODELS
ARE INDICATING A MORE SIGNIFICANT SHORTWAVE WILL MOVE FROM SW TO NE
ACROSS OUR CWA. SREF POPS PEG-OUT TUESDAY AFTERNOON FROM MARICOPA
COUNTY EAST...WHICH IS USUALLY A GOOD AND STRONG INDICATOR OF MTN
TSTMS. STORM MOTION WILL BE TO THE NE AT 10-20 KTS SO NOT VERY
FAVORABLE TO BRING TSTMS INTO THE LOWER DESERTS.
WEDNESDAY A POTENT SHORTWAVE IS PROGGED TO DROP OUT OF THE PAC NW
AND INTO THE GREAT BASIN. OUR CWA SHOULD BE IN THE WAKE OF THE
FIRST...WEAKER...SHORTWAVE SO POPS WILL BE A BIT LOWER WEDNESDAY.
TEMPS CONTINUE TO COOL.
THURSDAY THROUGH SATURDAY FORECAST CONFIDENCE DROP AT THIS POINT AS
MODELS BEGIN TO DIVERGE ON THE H5 PATTERN AND OFFER LITTLE
RUN-TO-RUN CONSISTENCY. THE LATEST ECMWF AND GFS ENS MEAN ARE
POINTING TOWARD CONTINUAL TROUGHING IN OUR AREA AS THE POTENT
SHORTWAVE GRAZES BY TO THE NORTH AND A CUT-OFF LOW REMAINS OFF THE
SRN CA COAST. GIVEN THE UNCERTAINTY SHOWED A GRADUAL DOWNWARD TREND
IN POPS THROUGH THE REMAINDER OF THE FORECAST WITH MINOR UPWARD
FLUCTUATIONS IN TEMPERATURES.
AVIATION
SCATTERED MID-LEVEL CLOUDS ARE DEVELOPING OVER CENTRAL ARIZONA AS
A MODEST MOISTURE INCREASE IS TAKING PLACE. EXPECT THESE CLOUDS TO
LINGER THROUGHOUT THE DAY...WHILE MOSTLY CLEAR SKIES WILL BE FOUND
IN SOUTHEAST CALIFORNIA. THERE WILL BE A SLIGHT CHANCE OF SOME
AFTERNOON THUNDERSTORMS...MAINLY OVER HIGHER TERRAIN AREAS TO THE
EAST AND SOUTHEAST OF THE GREATER PHOENIX AREA...BUT THESE ARE NOT
EXPECTED TO AFFECT ANY AIRFIELDS. WINDS WILL BE LIGHT WITH TYPICAL
DIURNAL PATTERNS THROUGH THE NEXT 24 HRS.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
FIRE WEATHER
TUESDAY THROUGH FRIDAY...AN AREA OF LOW PRESSURE WILL DEVELOP OFF
THE COAST OF SOUTHERN CALIFORNIA...BRINGING A SLIGHT CHANCE OF
SHOWERS AND THUNDERSTORMS TUESDAY FOR THE ENTIRE FORECAST
AREA...WITH A BETTER CHANCE IN THE HIGHER TERRAIN OF CENTRAL
ARIZONA. SLIGHT CHANCES OF THUNDERSTORMS WILL CONTINUE WEDNESDAY AND
THURSDAY IN THE HIGHER TERRAIN AREAS OF SOUTHERN GILA COUNTY.
TEMPERATURES WILL REMAIN NEAR SEASONAL NORMALS...WHILE AFTERNOON
HUMIDITY VALUES WILL RANGE FROM THE MID-TEENS IN SOUTHEAST
CALIFORNIA TO THE MID-TWENTIES IN SOUTH-CENTRAL ARIZONA.
PSR WATCHES/WARNINGS/ADVISORIES
AZ...NONE.
CA...NONE.
NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
WEATHER.GOV/PHOENIX
DISCUSSION...ESTLE/INIGUEZ
AVIATION...ROGERS
FIRE WEATHER...ROGERS
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