000
FXUS65 KPSR 201142 AAA
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
442 AM MST THU NOV 20 2008
.UPDATE...UPDATED AVIATION AND FIRE WEATHER DISCUSSIONS.
&&
.SYNOPSIS...
HIGH PRESSURE ALOFT WILL CONTINUE TO WEAKEN TODAY UNDER MOSTLY SUNNY
SKIES. BY FRIDAY AND SATURDAY...SOME HIGH LEVEL CLOUDS WILL DRIFT
INTO THE AREA...BUT TEMPERATURES SHOULD REMAIN ABOVE THE SEASONAL
NORMALS. BY THE MIDDLE OF NEXT WEEK...A MAJOR CHANGE IN THE PATTERN
WILL LEAD TO COOLER WEATHER...ALONG WITH A CHANCE FOR SHOWERS.
&&
.DISCUSSION...
STRONG UPPER LEVEL HIGH PRESSURE RIDGE...WHICH HAS PRODUCED SEVERAL
DAYS OF NEAR RECORD WARMTH OVER OUR LOWER DESERTS...FINALLY STARTING
TO WEAKEN. 00Z PLOT DATA SHOWED 20-30M HEIGHT FALLS...WITH HEIGHTS
DOWN TO AROUND 582DM. AIRMASS CONTINUES TO BE VERY DRY THIS MORNING
WITH PWAT VALUES BELOW 0.10 INCHES...AND 2 AM SURFACE DEWPOINTS OVER
THE LOWER DESERTS RANGED FROM UPPER TEENS TO MID 30S. IR IMAGERY
EARLY THIS MORNING SHOWED CLEAR SKIES ACROSS THE AREA...WITH AREAS
OF HIGH CLOUDS SPREADING INTO CENTRAL CA AHEAD OF A LOW PRESSURE
SYSTEM APCHG THE PAC NW COAST.
AS A SERIES OF SHORTWAVES MOVE ACROSS THE NORTHERN TIER OF
STATES...THE RIDGE OVER AZ WILL CONTINUE TO WEAKEN AND FLATTEN...AND
THIS WILL ALLOW INCREASING HIGH CLOUDS TO OVERSPREAD THE AREA. IN
ADDITION...THICKNESS VALUES ARE FORECAST TO SLOWLY LOWER ABOUT
10-20M EACH DAY...LEADING TO A STEADY FALLOFF IN MAX TEMPS TODAY
THROUGH THE WEEKEND. STILL...HIGHS WILL REMAIN WELL ABOVE SEASONAL
NORMALS. PROGS INDICATE A BAGGY TROF WILL DEVELOP OFF NRN BAJA
FRIDAY...THEN MOVE SLOWLY EASTWARD WITH TIME...BUT THIS FEATURE WILL
DO LITTLE MORE THAN HELP SPREAD HIGH CLOUDINESS AHEAD OF IT AND INTO
AZ. THE INCREASING HIGH CLOUDS WILL LEAD TO PARTLY CLOUDY SKIES FROM
TIME TO TIME...INSTEAD OF CLEAR SKIES THAT WE HAVE SEEN THE PAST FEW
DAYS.
FOR THE EXTENDED PERIOD...MONDAY THROUGH WEDNESDAY...THE GFS AND
EUROPEAN HAVE COME INTO SURPRISING AGREEMENT WITH THE FORECAST
SOLUTION AND AS SUCH CONFIDENCE IS GROWING THAT WE WILL BE SEEING A
SIGNIFICANT CHANGE IN THE WEATHER PATTERN BY THE MIDDLE OF NEXT
WEEK. BOTH PROGS FORECAST AN UPPER LOW TO DEVELOP OFF THE SRN CA
COAST BY MONDAY AFTERNOON...AND THIS LOW IS THEN FORECAST TO SLOWLY
PUSH EAST AND APPROACH THE CA/NRN BAJA COAST BY WEDNESDAY. NO
WEATHER IS EXPECTED WITH THIS SYSTEM MONDAY THRU EARLY TUESDAY...BUT
WE SHOULD SEE VARIABLE MID/HI CLOUDS SPREAD ACROSS THE AREA AHEAD OF
THE LOW. BY TUESDAY AFTN...GFS ADVERTISES A FRONTAL BAND TO MOVE
INTO SERN CA AHEAD OF THE MAIN LOW...WITH QPF SPREADING INTO OUR
WESTERN CWA. HAVE ADDED SLIGHT CHANCE POPS FROM THE CO RIVER WEST ON
TUESDAY...THEN SPREAD THE THREAT FOR SHOWERS INTO THE SWRN AZ
DESERTS OVERNIGHT TUE NIGHT. ALTHOUGH THE UPPER LOW IS FORECAST TO
REMAIN JUST OFFSHORE THRU WED AFTERNOON...IT LOOKS AS IF A
SUBSTANTIAL SUBTROPICAL MOISTURE TAP WILL DEVELOP AND SPREAD
CONSIDERABLE MOISTURE INTO OUR CWA AHEAD OF THE MAIN LOW ON
WEDNESDAY. AS SUCH HAVE RAISED POPS QUITE A BIT WED...INTO THE 20-30
PCT RANGE...FOR THE ENTIRE CWA. ALSO LOWERED MAX TEMPS SEVERAL
DEGREES TO NEAR OR A BIT BELOW SEASONAL NORMAL VALUES. IT APPEARS
THAT THE MOST SIGNIFICANT PRECIP AND COOLING WILL OCCUR BEYOND THE 7
DAY FORECAST PERIOD...WHEN THE MAIN LOW FINALLY MOVES INLAND ACROSS
ARIZONA.
&&
.AVIATION...
HIGH CIRRUS CLOUDS WILL BEGIN TO SPREAD OVER THE PHX CWA THROUGH 12Z
FRIDAY. WINDS WILL REMAIN LIGHT...AS THE CWA IS STILL UNDER THE
INFLUENCE OF HIGH PRESSURE.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
&&
.FIRE WEATHER...
SATURDAY THROUGH THURSDAY...
AS AN UPPER LEVEL LOW PRESSURE SYSTEM DEVELOPS OFF THE CA COAST...WE
WILL BEGIN TO SEE CHANGES IN THE WEATHER PATTERN. CLOUD COVER WILL
INCREASE...TEMPERATURES WILL BEGIN TO COOL INTO THE 70S AND THE
CHANCES OF PRECIPITATION WILL BEGIN TO INCREASE STARTING IN
SOUTHEASTERN CA THEN SPREADING EASTWARD INTO THE SOUTHWESTERN AZ
DESERTS BY WEDNESDAY MORNING. AS A RESULT...HUMIDITY LEVELS WILL
BEGIN TO TREND UPWARD. WINDS WILL REMAIN LIGHT FOR THE BEGINNING
PORTION OF THE WEEK.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&
$$
NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
WEATHER.GOV/PHOENIX
DISCUSSION...CB
AVIATION...PADDOCK
FIRE WEATHER...PADDOCK
