FXUS65 KPSR 061649
AFDPSR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
945 AM MST MON OCT 6 2008

SOUTH-CENTRAL AND SOUTHWEST ARIZONA...AND SOUTHEAST CALIFORNIA
FORECAST DISCUSSION

.SYNOPSIS...
A WARMING TREND WILL PERSIST THROUGH WEDNESDAY AS HIGH PRESSURE
BUILDS BACK OVER THE REGION. TEMPERATURES WILL WARM TO WELL ABOVE
SEASONAL NORMS BY WEDNESDAY AFTERNOON ACROSS THE LOWER DESERTS. A
STRONG LOW PRESSURE SYSTEM WILL DEVELOP OVER THE WESTERN STATES
LATER THIS WEEK...WITH COOLER TEMPERATURES AND BREEZY CONDITIONS BY
THE WEEKEND.

&&

.DISCUSSION...
TEMPERATURES WERE VERY PLEASANT THIS MORNING ACROSS THE FORECAST
AREA...WITH MOST LOCATIONS REPORTING LOWS IN THE UPPER 50S TO LOWER
60S. WARMING TREND WILL TAKE PLACE OVER THE NEXT SEVERAL DAYS AS
RIDGE STRENGTHENS OVER THE PACIFIC...LEAVING THE FORECAST AREA IN A
DRY NORTHWESTERLY FLOW. TEMPERATURES WILL PEAK ON WEDNESDAY WITH
FORECAST HIGH TEMPERATURES NEAR 100 DEGREES ACROSS THE LOWER
DESERTS. AFTER WEDNESDAY...THE PATTERN BECOMES INTERESTING AS MODELS
ARE ADVERTISING AN UNSEASONABLY STRONG TROUGH TO MOVE INTO THE
SOUTHWEST...WHICH COULD BRING COOLER TEMPERATURES AND BREEZY TO
WINDY CONDITIONS TO OUR AREA FOR THE WEEKEND. THERE IS STILL A HIGH
DEGREE OF UNCERTAINTY FOR THIS TIME PERIOD. I WILL HAVE MORE
THOUGHTS ON THIS SYSTEM IN THE AFTERNOON UPDATE. FORECAST IS IN GOOD
SHAPE...SO NO UPDATES PLANNED.

PREVIOUS DISCUSSION...
MODELS STILL ON TRACK WITH WARM UP THE FIRST HALF OF THIS WEEK AS
THE EASTERN PACIFIC RIDGE EXPANDS INTO THE DESERT SOUTHWEST. 00Z
MAV/MEX GUIDANCE SLIGHTLY COOLER WITH PEAK TEMPERATURES WEDNESDAY
THAN PREVIOUS RUNS. OVERALL...NOT ENOUGH TO ALTER PREEXISTING
FORECAST. MIN TEMPS IN THE SHORT TERM COULD BE TRICKY AS AN EASTERLY
GRADIENT SETS UP WHICH COULD KEEP MIXING GOING FOR MORE MILD MIN
TEMPS. HOWEVER THIS WILL MOSTLY BE FELT AT RIDGETOP AND FOOTHILL
LOCATIONS MORE THAN VALLEY FLOORS.

IN THE EXTENDED...GFS...GFS ENSEMBLE MEAN...AND ECMWF AGREE THAT
TROUGHING OVER THE NORTHEAST PACIFIC/GULF OF ALASKA REGION WILL
SHIFT EASTWARD FOR THE BEGINNING OF A COOLING TREND THURSDAY. THE
MODELS DIVERGE TREMENDOUSLY BY 12Z FRIDAY WITH THE ECMWF CONTINUING
TO DEVELOP A DEEP CLOSED LOW OVER NORTHERN CALIFORNIA/NORTHERN
NEVADA/IDAHO/OREGON WHILE THE GFS AND ENSEMBLE MEAN TRACK AN OPEN
WAVE INTO THE CENTRAL AND NORTHERN ROCKIES. THE ECMWF SOLUTION
EXPANDS AND SAGS THE UPPER LOW A BIT FURTHER SOUTH BY 12Z SATURDAY
AND SUGGESTS A COLD FRONT MOVING ACROSS OUR FORECAST
AREA...ESPECIALLY WESTERN PORTIONS...DURING THE FRIDAY/FRIDAY NIGHT
TIMEFRAME. MEANWHILE THE GFS SHOWS A FOLLOW-ON SHORT WAVE WITHIN THE
MAIN TROUGH MOVING INTO THE FOUR CORNERS AREA. BY 12Z SUNDAY...THE
ECMWF STARTS LIFTING THE LOW NORTHWARD AND THE GFS TRACKS THE
POSITIVE TILT TROUGH EASTWARD. OF NOTE IS THAT THE GFS AND ENSEMBLE
MEAN HAVE TRENDED TOWARD A TROUGH DIGGING FURTHER SOUTH. HPC
BELIEVES THE GFS IS TOO PROGRESSIVE AND PREFERS A SOLUTION SOMEWHERE
IN BETWEEN...FAVORING THE ECMWF BUT NOT AS ANOMALOUSLY DEEP. STILL A
LOT OF UNCERTAINTY IN HOW THINGS WILL DEVELOP BUT SHOWED MORE
COOLING THAN PREVIOUS FORECAST...GENERALLY UNDERCUTTING MEX
GUIDANCE.

TAKING THE ECMWF AT FACE VALUE...IT DOESNT GENERATE MUCH RH FOR OUR
AREA WHICH ISNT SURPRISING SINCE IT EVOLVES OVER LAND AND THE CORE
OF IT STAYS NORTH. GFS SCENARIO IS COMPLETELY DRY. THUS HAVE HELD
OFF ON INTRODUCING ANY MENTION OF PRECIP.

&&

.AVIATION...
HIGH PRESSURE ALOFT WILL CONTINUE TO BUILD ACROSS SOUTHEAST
CALIFORNIA AND SRN AZ...LEADING TO CLEAR SKIES AND LIGHT MOSTLY
DIURNAL WINDS ACROSS ALL TAF SITES FOR THE NEXT 24 HOURS. VFR
CONDITIONS EXPECTED AT ALL AIRFIELDS. NO AVIATION CONCERNS.

&&

.FIRE WEATHER...
FRIDAY INTO THE WEEKEND...
A LARGE STORM SYSTEM MAY DEVELOP OVER THE WESTERN UNITED STATES BY
FRIDAY OR SATURDAY. OUR AREA SHOULD BE ON THE SOUTHERN EDGE OF THE
LOW AND UNDER A DRY WESTERLY FLOW ALOFT. MINIMUM RH VALUES SHOULD
STAY BELOW 15 PERCENT OVER THE DESERTS EACH DAY...WITH MODEST RH
RECOVERY AT NIGHT. THERE IS A POSSIBILITY THAT BREEZY TO WINDY
CONDITIONS WILL DEVELOP SATURDAY AND INTO SUNDAY AS A COLD FRONT
SWEEPS ACROSS THE AREA. GUSTY NORTH WINDS MAY DEVELOP BEHIND THIS
SYSTEM...ESPECIALLY IN ZONES 131 AND 231.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...NONE.

CA...NONE.

&&

$$

NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT

WEATHER.GOV/PHOENIX

DISCUSSION...ROGERS/AJ
AVIATION...CB
FIRE WEATHER...SIPPLE


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