Severe Thunderstorm Warning 37, NWS Phoenix, AZ

AZZ020>028-CAZ030>033-170545-

SOUTHWEST ARIZONA/SOUTHEAST CALIFORNIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
320 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...
HOT TEMPS ARE ON TAP THE NEXT FEW DAYS AS A RIDGE OF HIGH PRESSURE
STRENGTHENS OVER THE SOUTHWEST UNITED STATES. HIGHS MAY COME CLOSE
TO RECORD LEVELS NEXT COUPLE OF DAYS OVER PORTIONS OF THE LOWER
DESERT...AS MANY DESERT LOCATIONS WILL SEE MAX TEMPS IN THE 110 TO
115 RANGE. HOTTEST TEMPS SHOULD OCCUR OVER FAR SOUTHEAST CALIFORNIA
AND ALONG THE LOWER COLORADO RIVER VALLEY. 300-250 MB DEFORMATION
AXIS HAS NOT MOVED MUCH SINCE MID-MORNING...AND REMAINS ALONG THE
SOUTHEAST PERIPHERY OF OUR CWA. RELATIVELY STRONG NE STEERING FLOW
...15 TO 25 KT IN 700-400 MB LAYER...IN PLACE OVER SOUTH CENTRAL AZ
WITH SOMEWHAT WEAKER NORTHEAST FLOW OUT WEST. 850-500 MB LAPSE RATES
ARE IMPRESSIVE...AND SHOULD REMAIN THAT WAY NEXT COUPLE OF DAYS AS
WARMER THAN NORMAL LOW LEVELS ARE OVERLAIN BY COOLER THAN NORMAL MID
LEVELS. AS OF 3 PM...ISOLATED THUNDERSTORMS HAVE FORMED OVER SEVERAL
LOCATIONS IN CWA THIS AFTERNOON...MAINLY OVER ELEVATED TERRAIN...AND
STORMS APPEAR TO BE PROPAGATING/MOVING RATHER QUICKLY TOWARD THE
SOUTHWEST.  GIVEN THE DEGREE OF INSTABILITY AND STRENGTH OF
FLOW...ANY STORMS THAT FORM WILL HAVE A DECENT POTENTIAL TO PRODUCE
SEVERE WEATHER...WITH HIGH WINDS THE PRIMARY THREAT.

SOME DRYING IN LOW AND MID LEVELS EXPECTED FRIDAY AND SATURDAY AS
NORTHEAST FLOW PERSISTS AND HIGH BUILDS OVER AREA...BUT LOW LEVEL
FLOW STILL FAVORABLE TO BRING MOISTURE NORTH...THEN EAST...FROM THE
GULF OF CALIFORNIA...AND MODELS FORECAST ONLY MINIMAL LOW LEVEL
DRYING...SO THERE SHOULD STILL BE JUST ENOUGH MOISTURE FOR CAPE TO
EXIST...AND ISOLATED THUNDERSTORMS CANNOT BE RULED OUT...MAINLY OVER
THE FAVORED HIGHER TERRAIN OF SOUTH CENTRAL AZ BUT POSSIBLY OVER
LOWER TERRAIN OF SOUTH CENTRAL ARIZONA AS WELL...MAINLY OVER
MARICOPA AND NORTHERN PINAL COUNTY. LOWERED POPS SLIGHTLY FRIDAY
EVENING COMPARED TO THIS EVENING BASED ON IDEA THAT SOME DRYING WILL
OCCUR WHILE DEFORMATION AXIS IS FORECAST TO SHIFT A BIT EAST IF ITS
CURRENT POSITION.

IN EXTENDED...CONFIDENCE IN THIS PORTION OF THE FORECAST IS LOW
...ESPECIALLY BEYOND MONDAY AS SIGNIFICANT DISAGREEMENT EXISTS
AMONGST MEDIUM RANGE MODEL SOLUTIONS. FOLLOWED AVN GUIDANCE TODAY
...SO FEEL THAT SOUTHWEST AND SOUTH CENTRAL AZ COULD SEE AN
INCREASED CHANCE FOR STORMS BY SUNDAY AND MONDAY AS THE STRONG UPPER
HIGH SHIFTS EAST TO 4 CORNERS OR NRN NEW MEXICO AND EAST OR
SOUTHEAST FLOW DEVELOPS OVER AREA. WITH MID TROPOSPHERIC RIDGE
FORECAST TO SHIFT TO SOUTH OF CWA BY TUESDAY AND PLACE AREA IN DEEP
SOUTHWESTERLY FLOW BY MIDWEEK...WENT WITH A GRADUAL DECREASE IN
THUNDERSTORM ACTIVITY OVER LOWER DESERT...ENDING CHANCE WORDING
TUESDAY OVER SOUTHWEST AZ AND DOING LIKEWISE FOR LOWER DESERT OF
SOUTH CENTRAL AZ WED AND THURS. SHOULD SEE A BIT OF A DECREASE IN
DAYTIME HIGHS NEXT WEEK IF AVN/MRF IS CORRECT. IF ECMWF/UKMET TURN
OUT TO BE MORE ACCURATE...LATE AFTERNOON/NIGHTTIME THUNDER WILL BE
POSSIBLE OVER AT LEAST THE AZ PORTION OF THE CWA THROUGH THURSDAY. DG

AZZ022>024-027-028-170300-

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE PHOENIX AZ
400 PM MST THU AUG 16 2001

...ISOLATED SEVERE THUNDERSTORMS POSSIBLE OVER SOUTH-CENTRAL AND
SOUTHWEST ARIZONA LATE THIS AFTERNOON AND THIS EVENING...

COUNTIES IN THE RISK AREA INCLUDE:
     IN ARIZONA...MARICOPA...NORTHWEST PINAL...SOUTHERN GILA...AND
     YUMA.

THE PRIMARY HAZARDOUS WEATHER RISKS INCLUDE:
     STRONG STRAIGHT LINE WINDS. DENSE BLOWING DUST IS POSSIBLE WITH
     STORMS OVER THE LOWER DESERT.

OTHER THREATS INCLUDE: CLOUD TO GROUND LIGHTNING AND BRIEF HEAVY
RAIN.

STORM SPOTTER ACTIVATION MAY BE NECESSARY LATE THIS AFTERNOON AND
TONIGHT.

DG