000
FXUS65 KPSR 070310
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
810 PM MST SAT SEP 6 2008
SOUTH-CENTRAL AND SOUTHWEST ARIZONA...AND SOUTHEAST CALIFORNIA
FORECAST DISCUSSION
.SYNOPSIS...
A DRY NORTHWEST FLOW ALOFT WILL KEEP THE DESERT SOUTHWEST GENERALLY
CLEAR AND HOT THROUGH SUNDAY. EARLY NEXT WEEK A LOW PRESSURE SYSTEM
WILL ENCROACH ON THE DESERT SOUTHWEST RESULTING IN INCREASING
CLOUDS...COOLER TEMPERATURES...AND INCREASED CHANCES FOR MOUNTAIN
THUNDERSTORMS MAINLY ACROSS ARIZONA. THE LOW PRESSURE SYSTEM WILL
REMAIN A CONSTANT WEATHER FEATURE THROUGH THE REMAINDER OF THE WEEK.
&&
.DISCUSSION...
MOSTLY CLEAR SKIES COVERED THE FORECAST AREA (SOUTHEAST CA TO SOUTH
CENTRAL AZ) AT 03Z...BUT A FEW SPOTTY MID CLOUDS REMAINED. SCATTERED
CLOUDS IN SOUTHERN AZ SATURDAY AFTERNOON WERE THE LEADING EDGE OF
MUCH DEEPER MONSOON MOISTURE IN NORTHWEST MEXICO. THERE WERE EVEN
ONE OR TWO HIGH CLOUD BASED THUNDERSTORMS NOTED SATURDAY AFTERNOON
JUST SOUTHEAST OF TUCSON...AND OVER THE WHITE MOUNTAINS.
AS MENTIONED ABOVE...MOISTURE IS MUCH DEEPER IN MEXICO. BASED ON THE
PHOENIX AND TUCSON BALLOON SOUNDINGS...IN ORDER TO DEVELOP
MEANINGFUL C.A.P.E. FOR THUNDERSTORMS...700 MB DEWPOINTS WOULD NEED
TO INCREASE TO 2 DEGREES C ...AND 800 MB DEWPOINTS AROUND 4 C. THOSE
VALUES WERE NOT TO FAR AWAY IN MEXICO.
AS THE PREVIOUS DISCUSSION BELOW MENTIONS...A LARGE PACIFIC TROF IS
FORECAST TO MOVE INTO THE ENTIRE WESTERN U.S. BY TUESDAY MORNING.
THIS WILL ENABLE BACKING LOW LEVEL SOUTH WINDS FROM MEXICO...AND A
POTENTIAL RESURGENCE OF MONSOON MOISTURE NORTH INTO AZ LATE
MONDAY...BUT PARTICULARLY TUE. INCREASING JET STREAM WINDS AND
COOLER MID LEVEL TEMPERATURES PORTEND A VERY UNSTABLE ATMOSPHERE
TUESDAY FOR INCREASING THUNDERSTORMS...POSSIBLY SEVERE IN CENTRAL
AND NORTHERN AZ.
OTHERWISE...DRY WEATHER WILL CONTINUE OVER THE FORECAST AREA THROUGH
MONDAY. CURRENT FORECASTS REFLECT THIS...AND ALSO REFLECT AN
INTRODUCTION OF THUNDERSTORMS BACK INTO THE FORECAST FOR SOUTH
CENTRAL AZ TUESDAY. NO FORECAST UPDATES NEEDED. PREVIOUS
DISCUSSION BELOW STILL APPLIES.
ADDITIONALLY...THE EXCESSIVE HEAT WARNING ISSUED EARLIER TODAY FOR
SOUTHEAST CA EXPIRED AT 03Z. FORECAST AFTERNOON TEMPERATURES ON
SUNDAY ARE BELOW WARNING CRITERIA.
PREVIOUS DISCUSSION...
MONSOON IS ALREADY SHOWING SIGNS OF RETURNING...AS CU (ALBEIT
LOW-TOPPED) HAS DEVELOPED ALONG THE MOGOLLON RIM AND THE HIGHER
TERRAIN OF SOUTHERN AZ. ENVIRONMENT GENERALLY REMAINS TOO DRY FOR
THUNDERSTORMS THIS AFTERNOON. THIS IS REFLECTED IN LOCAL K-INDICES
WHICH ARE ONLY PEAKING AROUND 20 ACROSS MUCH OF THE FORECAST AREA.
HOWEVER...LATEST NAM-WRF SUGGESTS DEEPER MOISTURE CURRENTLY OVER
CHIHUAHUA WILL LIFT NORTHWESTWARD INTO SE AZ SUN. THIS SHOULD HELP
TO PRIME THE LOWER LEVELS BY MON ACROSS SOUTH-CENTRAL AZ WITH
DEWPOINTS IN THE LOWER-MID 50S ACROSS THE LOWER DESERTS.
MODELS IN GENERALLY GOOD AGREEMENT THAT AN AREA OF LOW PRESSURE WILL
SLIDE SLOWLY SOUTHWARD DOWN THE CA COAST THIS WEEK. THIS WILL SET UP
A SOUTHWESTERLY FLOW...FAVORING THE HIGHER TERRAIN (EVEN ACROSS THE
SOUTHWEST DESERTS) OF THE FORECAST AREA FOR SHOWER AND THUNDERSTORM
ACTIVITY. WESTERN EXTENSION OF THE SEMI-PERMANENT ATLANTIC RIDGE IS
EXPECTED TO DRIFT WESTWARD AS TROPICAL CYCLONE IKE MOVES WESTWARD
TOWARDS THE FL STRAITS AND POSSIBLY INTO THE GULF OF MEXICO. MOIST
SUBTROPICAL FEED WILL CONTINUE ACROSS THE DESERT SOUTHWEST THROUGH
MIDWEEK AS THE SOUTHWESTERLY FLOW STRENGTHENS BETWEEN THE EASTERN
PACIFIC LOW (WHICH MAY ULTIMATELY CUT OFF) AND A BUILDING RIDGE
ACROSS MEXICO.
BEST CHANCE FOR PRECIPITATION IN THE VALLEY APPEARS TO BE LATE IN
THE WEEK AS THE LOW-LEVEL FLOW ACQUIRES MORE OF AN EASTERLY
COMPONENT AROUND THE RIDGE...CENTERED GENERALLY ACROSS THE FOUR
CORNERS OR EAST OF THE AREA. DISCERNIBLE THREAT FOR THUNDERSTORMS
WILL ALSO EXIST ACROSS THE LOWER CO RIVER VALLEY (PER THE LATEST
ECMWF) AS THE CA LOW EVENTUALLY SLIDES EASTWARD WHILE DISSIPATING
NEXT WEEKEND.
MADE SOME MINOR ADJUSTMENTS TO THE DIGITAL DATABASE...MAINLY TO
INCREASE POPS THROUGH THE WEEK. TEMPERATURE-WISE...INCREASE IN
MOISTURE COMBINED WITH DECREASING THICKNESSES WILL RESULT IN COOLER
TEMPS...THOUGH STILL REMAINING ABOVE AVERAGE THROUGH AT LEAST THU OR
FRI.
&&
.AVIATION...
SOUTHEAST CA TO SOUTH CENTRAL AZ...INCLUDING KPHX...KIPL...KIWA...
KNYL...AND KBLH AIRFIELDS. MOSTLY CLEAR SKIES AND LIGHT WINDS ARE
EXPECTED THROUGH AT LEAST 04Z MON.
AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.
&&
.FIRE WEATHER...
MONDAY THROUGH THURSDAY...ABOVE NORMAL TEMPERATURES CAN BE EXPECTED
MONDAY...BEFORE A GRADUAL COOLING TREND WILL BRING TEMPERATURES TO
NEAR SEASONAL NORMALS BY WEDNESDAY. THE ATMOSPHERE WILL REMAIN
RELATIVELY DRY...WITH AFTERNOON RH VALUES IN THE TEENS MONDAY AND
TUESDAY...RISING A BIT BY WEDNESDAY INTO THE UPPER TEENS AND LOWER
TWENTIES ACROSS THE LOWER DESERTS. WINDS WILL GENERALLY BE LIGHT
WITH ONLY A FEW AFTERNOON GUSTS. A WEAK LOW PRESSURE SYSTEM WILL
DEVELOP OFF THE COAST OF SOUTHERN CALIFORNIA...RETURNING A SLIGHT
CHANCE OF THUNDERSTORMS TO SOUTHERN GILA COUNTY EACH DAY.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...NONE.
CA...NONE.
&&
$$
NATIONAL WEATHER SERVICE PHOENIX IS ON THE INTERNET AT
WEATHER.GOV/PHOENIX
DISCUSSION...VASQUEZ/HIRSCH
AVIATION...VASQUEZ
FIRE WEATHER...ROGERS
