Severe Thunderstorm Warning 58, NWS Phoenix, AZ

AZZ020>028-CAZ030>033-080545-

SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
330 PM MST SAT SEP 7 2002

.SYNOPSIS...
MOIST UNSTABLE AIR WILL REMAIN IN PLACE OVER FAR SOUTHEAST
CALIFORNIA AND SOUTHWEST ARIZONA THE NEXT FEW DAYS. UPPER LEVEL
WINDS WILL BE STRONGER THAN NORMAL SINCE LOW PRESSURE WILL PERSIST
JUST WEST OF THE AREA WHILE A RIDGE OF HIGH PRESSURE REMAINS
CENTERED OVER NEW MEXICO. A SERIES OF WEATHER DISTURBANCES WILL
BRING PERIODS OF SHOWERS AND THUNDERSTORMS TO THE AREA...WITH THE
GREATEST CHANCE FOR STRONG STORMS AND LOCALLY HEAVY RAIN OVER
SOUTH-CENTRAL ARIZONA. SOMEWHAT DRIER AIR IS EXPECTED TO MOVE INTO
SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA BY THE MIDDLE OF NEXT
WEEK AS WESTERLY FLOW DEVELOPS. DAYTIME HIGH TEMPERATURES WILL BE
NEAR OR SLIGHTLY BELOW SEASONAL NORMALS THE NEXT FEW DAYS.

.DISCUSSION...
INTERACTION OF UPPER LEVEL DYNAMICS WITH UNSTABLE AND VERY MOIST
AIRMASS HAS RESULTED IN WIDESPREAD THUNDERSTORM DEVELOPMENT THIS
AFTERNOON OVER AZ PTN OF CWA...ESPECIALLY OVER SOUTH CENTRAL AZ
DESERT. STORM MOVEMENT IS RATHER RAPID...AROUND 30 MPH TOWARD THE
NORTHEAST...BUT RATHER STRONG UNIDIRECTIONAL SHEAR PROFILE SUPPORTS
TRAINING OF STORMS AS WELL AS POTENTIAL FOR MULTICELL SEVERE STORMS.
SEVERE THUNDERSTORM WATCH FOR SW AND SCNTRL DESERT LOOKS GOOD
(THANKS TO STEVE CORFIDI AT SPC FOR CONDUCTING EXCELLENT
COORDINATION CALLS TODAY). FIRST OF TWO WELL-DEFINED UPPER
DISTURBANCES HAS PASSED MAINLY TO THE NORTH OF OUR AREA...BUT
CONFLUENCE ASSOCIATED WITH TAIL OF THIS DISTURBANCE EXISTS ACROSS
SOUTHWEST AND S-CNTRL AZ THIS AFTERNOON. NEXT UPPER DISTURBANCE
APPROACHING AREA FROM THE SOUTHWEST SHOULD PROVIDE CONTINUATION OF
LARGE SCALE ASCENT NECESSARY TO RESULT IN ENOUGH INSTABILITY TO KEEP
STORMS GOING MUCH OF THE NIGHT...MAINLY OVER AND EAST OF THE LOWER
COLORADO RIVER. VERY MOIST AIRMASS IN PLACE...REFLECTED BY DEW
POINTS IN MID TO UPPER 60S AND HIGH PRECIPITABLE WATER VALUES
...ALSO LENDS SUPPORT TO EXCESSIVE RAINFALL POTENTIAL. OVERALL
...FLASH FLOOD WATCH AREA COINCIDES REASONABLY WELL WITH LATEST
QPFERD.

LARGE SCALE PATTERN DOES NOT CHANGE MUCH NEXT COUPLE OF DAYS...SO
POTENTIAL FOR SHOWERS AND THUNDERSTORMS WILL REMAIN UNUSUALLY
HIGH...MAINLY OVER AZ PTN OF CWA. WE REALLY NEED THE RAIN...BUT WE
WILL HAVE TO CONTEND WITH PERIODS OF SEVERE STORMS AND LOCALLY
EXCESSIVE RAIN AT LEAST OVER AND EAST OF LOWER COLORADO RIVER VALLEY
AS AZ PTN REMAINS IN FAVORABLE RIGHT ENTRANCE REGION OF UPPER JET
AND JET STREAKS/DISTURBANCES MOVE NORTHEASTWARD ACROSS THE AREA.
REMAINS OF WEAK TROPICAL DISTURBANCE CURRENTLY NEAR SOUTH END OF
GULF OF CALIFORNIA WILL LIKELY BE ADVECTED NORTHWARD ACROSS AREA
LATE TOMORROW INTO EARLY MONDAY. HAVE LOWERED DAYTIME HIGHS A BIT
NEXT COUPLE OF DAY IN DEFERENCE TO MOISTURE/CLOUDS...BUT DID NOT SEE
THE NEED TO MAKE MORE THAN COSMETIC CHANGE TO FORECAST PACKAGE. SLOW
DRYING TREND NOTED IN WEDNESDAY-SATURDAY PERIOD AS FLOW ALOFT
BECOMES MORE WESTERLY. AS SUCH...WILL STICK WITH IDEA OF REDUCING
CHANCES FOR STORMS FROM WEST TO EAST DURING THAT TIME. DG

.PSR...FLASH FLOOD WATCH UNTIL MIDNIGHT TONIGHT FOR S CENTRAL AZ...
MARICOPA...N CENTRAL PINAL...AND SOUTHERN GILA COUNTIES...FCST AZ
ZONES...22-24 AND 27-28.  SEVERE THUNDERSTORM WATCH UNTIL 8 PM FOR
SOUTHWEST AND S CENTRAL AZ...YUMA...CENTRAL AND EAST LA PAZ...
MARICOPA...NORTHERN PINAL...AND SOUTHERN GILA...FCST AZ ZONES 21-28.