Severe Thunderstorm Warning 42, NWS Phoenix, AZ

AZZ020>028-CAZ030>033-171045-

SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
925 PM MST THU AUG 16 2001

.SYNOPSIS...
HIGH PRESSURE ALOFT WILL STRENGTHEN AND BECOME CENTERED NEAR LAS
VEGAS BY FRIDAY EVENING...THEN DRIFT ACROSS FAR NORTHERN ARIZONA ON
SATURDAY. AS A RESULT...HOT DAYTIME HIGHS AND DRY CONDITIONS ARE
EXPECTED THE NEXT COUPLE OF DAYS OVER SOUTHWEST ARIZONA AND FAR
SOUTHEAST CALIFORNIA. THERE WILL BE A SLIGHT CHANCE FOR A LATE
AFTERNOON OR NIGHTTIME THUNDERSTORM...MAINLY OVER HIGHER TERRAIN
LOCATIONS IN ARIZONA. A BETTER CHANCE FOR STORMS WILL EXIST ACROSS
SOUTHWEST AND SOUTH CENTRAL ARIZONA SUNDAY AND MONDAY AS A
DISTURBANCE MOVES NORTHWEST ACROSS THE AREA. THE RIDGE OF HIGH
PRESSURE IS FORECAST TO MOVE SOUTH OF THE FORECAST AREA EARLY NEXT
WEEK...WHICH WOULD LEAD TO DRIER CONDITIONS AND SOMEWHAT LOWER
DAYTIME HIGH TEMPERATURES.

.DISCUSSION...
ONLY ONE THUNDERSTORM WAS NOTED IN THE CWA AT 0345Z IN SOUTHERN GILA
COUNTY. OTHERWISE...THE OVERALL CONVECTIVE PATTERN WAS NEARLY
SIMILAR TO YESTERDAYS DEVELOPMENT.  T-STORMS IN NORTHERN ARIZONA
WERE ASSOCIATED WITH UPPER LEVEL DIFLUENT FLOW MOVING AROUND A
300/250 MB CALIFORNIA RIDGE...AND T-STORMS IN SOUTHEAST AZ WERE
ASSOCIATED WITH A SOUTHWEST/NORTHEAST ORIENTED DEFORMATION ZONE.
THE SOUTHEAST AZ DEFORMATION ZONE MOVED NORTH FROM YESTERDAYS
POSITION NEAR THE SOUTHEAST CORNER...HOWEVER NOT FAR ENOUGH NORTH
(AS YESTERDAYS MODELS WERE PREDICTING) TO EFFECT THE EASTERN
PORTIONS OF OUR CWA. THE UPPER LEVEL DEFORMATION ZONE AND ASSOCIATED
CONVECTION EXTENDED ALONG A LINE FROM THE WHITE MOUNTAINS IN EAST
CENTERAL AZ...TO ORGAN PIPE NATIONAL MONUMENT IN SOUTH CENTRAL AZ
NEAR THE MEXICAN BORDER.  LARGE CONVECTION DEVELOPED NEAR ORGAN PIPE
NATL MONUMENT...HOWEVER MOST CONVECTION WAS LOCATED IN SOUTHEAST
AZ.

00Z PLOT DATA ALSO SHOWED MONSOON MOISTURE OVER THE SOUTHERN HALF OF
THE STATE CONTINUED TO THIN OVER YESTERDAY.  HOWEVER WHATS
COMPENSATING FOR THE LOSS OF MOISTURE...AND GENERATING INSTABILITY
...WERE THE RELATIVELY COOL 500 MB TEMPERATURES ADVECTING INTO AZ
UNDER NORTHEASTERLY FLOW ALOFT.

BOTH THE NEW ETA AND AVN MODELS FORECAST THE SOUTHEAST AZ
DEFORMATION ZONE TO MOVE A LITTLE FARTHER NORTH FRIDAY...TO A
POSITION THAT SHOULD FAVOR MORE AFTERNOON/EVENING CONVECTION OVER
THE EASTERN MOGOLLON RIM AND WHITE MOUNTAINS.  500 MB NORTHEASTERLY
STEERING CURRENTS...AND CONTINUED COOL ADVECTION... PROMISE TO MOVE
THESE MOUNTAIN STORMS TOWARD THE LOWER DESERTS FRIDAY EARLY EVENING.

FOR SATURDAY...
MODELS FORECAST THE DEFORMATION ZONE TO SHIFT BACK INTO SOUTHEAST
AZ...WITH WARMER MID LEVEL TEMPERATURES. THIS IS DUE TO THE 300/250
MB CA RIDGE NOSING INTO THE NORTHERN HALF OF AZ...FOR A LITTLE MORE
STABILITY OVER THE CWA.

UPDATES TO FIRST PERIOD WILL BE TO CONTINUE A SLIGHT CHANCE FOR
STORMS THROUGH MIDNIGHT.  VASQUEZ