Flash Flood in Arizona, August 12, 2001

SOUTHEAST ARIZONA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
920 AM MST SUN AUG 12 2001

.SYNOPSIS...A LOW PRESSURE SYSTEM OVER EASTERN CHIHUAHUA WILL MOVE
NORTHWEST TOWARD ARIZONA TONIGHT. THIS FEATURE WAS BEING STEERED BY
AN AREA OF HIGH PRESSURE BECOMING CENTERED NEAR THE FOUR CORNERS
REGION. THIS HIGH WILL REMAIN SOMEWHAT STATIONARY THE NEXT SEVERAL
DAYS...KEEPING CONSIDERABLE MOISTURE OVER THE REGION.

.DISCUSSION...SE AZ IN BETWEEN SYSTEMS THIS MORNING. INVERTED TROUGH
WHICH IGNITED ISOLATED HEAVY RAINERS LAST NIGHT EXTENDS FROM ORGAN
PIPE INTO THE WESTERN RIM AND IS STARTING TO MAKE THE TURN NNW.
LARGE EASTERLY WAVE CONTINUES TO ROLL NW INTO CHIHUAHUA, WITH MODELS
STILL WAY TOO SLOW AND WEAK WITH IT. 12Z MODELS ARE 6-9HRS TOO SLOW,
AND THE KEY KEPZ SOUNDING TERMINATED TOO LOW FOR THE MODELS TO
INITIALIZE A BETTER FIX. EVEN THOUGH RAINS WERE RATHER SPOTTY LAST
NIGHT, THE CONVECTION GENERATED A MESOHIGH OVER MOST OF THE CWA.
THAT, AND OUR 12Z SOUNDING INDICATES WEVE BEEN SUFFICIENTLY WORKED
OVER TO DELAY DEVELOPMENT. THE OTHER MINUS IS THAT WERE IN THE
SUBSIDENCE FIELD BETWEEN THE TWO SYSTEMS. WELL ISSUE A MORNING
UPDATE TO SHAVE 10-20 PCT FROM POPS IN MOST ZONES THIS AFTERNOON.
THE EXCEPTION WILL BE OVER FAR W PIMA COUNTY AND THE WHITES WHICH
WILL BE ON THE EDGE OF THIS SUBSIDENCE FIELD. NORMALLY, ID DROP
HIGHS A LITTLE, BUT THE SUBSIDENCE FIELD AND DOWNSLOPE FLOW IS
KEEPING TEMPS ALMOST IN STEP WITH YESTERDAYS RISES. IN FACT, I MAY
ACTUALLY RISE HIGHS A BIT IF THE CLEARING CONTINUES AND THE MID
LEVEL CLOUD DECK OVER THE W CWA DOESNT EXPAND E.

THE STORY CHANGES AFTER 00Z AS THE EASTERLY WAVE APPROACHES. ALL
MODELS INDUCE SIGNIFICANT DECREASES IN INERTIAL STABILITY ABOVE
300MB WITH AN INCREASE IN UPPER DIVERGENCE. SINCE THE CIRCULATION IS
MOVING MUCH FASTER THAN PROGGED, ITS LOGICAL FOR THIS FORCING TO
ARRIVE THIS EVENING. WATER VAPOR IMAGERY SEEMS TO BE BEARING THIS
OUT. 700-500MB FLOW IS ALREADY BACKING WITH HEIGHT, BUT IF THE ETA
IS RIGHT, THE FLOW AND SHEAR WILL INCREASE A BIT WITH THE APPROACH
OF THE WAVE, ALONG WITH A CORRESPONDING INCREASE IN MOISTURE
CONVERGENCE AND THETA-E ADVECTION. IF THE CIRCULATION TRAJECTORY
HOLDS, SE AZ WILL BE IN A NICE SPOT FOR MCS DEVELOPMENT TONIGHT. IF
THE 4 CORNER HIGH DEFLECTS IT 50-100MI SOUTH, OR THE WAVE SLOWS
DOWN, THE MCS WILL FIRE OVER SONORA WITH JUST THE REMNANTS LEAKING
INTO THE FAR SE CWA. EITHER WAY, AT LEAST SOME PRECIP WILL CONTINUE
INTO THE OVERNIGHT, SUPPORTED BY PRECIP WATERS OVER 1.5 AND DECENT
MID LEVEL FLOW. AS PART OF THE MORNING UPDATE, WELL DROP THE
EVENING END TO THE STORMS. OVER THE PAST 2 WEEKS, I HAVE HAD AN
ITCHY TRIGGER FINGER ON A FLASH FLOOD WATCH GIVEN SO MUCH MOISTURE
LAYING AROUND. HOWEVER, AS HAS BEEN TYPICAL FOR THIS MONSOON SEASON,
WHILE SOME AREAS HAVE BEEN REALLY DUMPED ON, ORGANIZATION HAS PROVEN
ELUSIVE. THE BEST COURSE OF ACTION HAS BEEN TO WAIT FOR THE WHITES
OF ITS EYES AND PUT IN HEIGHTENED WORDING IF NEEDED ONCE WE SEE HOW
THINGS DEVELOP. THE SAME STRATEGY IS THE BEST COURSE OF ACTION
TODAY. PYTLAK

.TWC...NONE.

AZZ019-029>035-130700-

SOUTHEAST ARIZONA HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE TUCSON AZ
1145 AM MST SUN AUG 12 2001

.SYNOPSIS...A DISTURBANCE SOUTH OF THE TEXAS BIG BEND WILL MOVE
NORTHWEST TOWARD ARIZONA...WHILE HIGH PRESSURE WILL BECOME CENTERED
NEAR THE FOUR CORNERS REGION. ANOTHER WEAK DISTURBANCE OVER CENTRAL
PIMA COUNTY WILL SHIFT OFF TO THE NORTHWEST THROUGH THE AFTERNOON.
THESE WEATHER FEATURES...ALONG WITH AMPLE MOISTURE AND DAYTIME
HEATING WILL LEAD TO AFTERNOON AND EVENING THUNDERSTORMS ACROSS
SOUTHERN ARIZONA.

.DISCUSSION...A WEAK VORT MAX OVER CENTRAL PIMA COUNTY HAS PRODUCED
SOME MORNING SHOWERS OVER FAR WESTERN PIMA COUNTY AND ALONG THE
LOWER COLORADO RIVER VALLEY. THIS FEATURE WILL ENHANCE THE CHANCES
OF THUNDERSTORM DEVELOPMENT OVER FAR WESTERN SECTIONS OF PIMA COUNTY
EARLY THIS AFTERNOON. HOWEVER IN THE WAKE OF THIS DISTURBANCE...MID
LEVEL CLOUDS AND WEAK SUBSIDENCE ACROSS CENTRAL/EASTERN PIMA...SANTA
CRUZ AND SOUTHEAST PINAL COUNTIES WILL LEAD TO A LATE START FOR
THUNDERSTORM DEVELOPMENT. OTHERWISE...AN EASTERLY WAVE CENTERED OVER
SOUTHEAST CHIHUAHUA WILL CONTINUE ITS NORTHWEST PROGRESSION TODAY...
ENTERING FAR SOUTHEAST PORTIONS OF THE REGION LATE THIS AFTERNOON
AND EVENING. THIS WILL ENHANCE THE CHANCES FOR THUNDERSTORM
DEVELOPMENT OVER EASTERN PORTIONS OF THE CWA (COCHISE/GRAHAM AND
GREENLEE COUNTIES) LATE THIS AFTERNOON AND EVENING. THIS AREA IS
ALSO RELATIVELY CLOUD FREE...WHICH WILL ALLOW FOR MAXIMUM DAYTIME
HEATING...ALSO INCREASING THE CHANCES FOR AN EVENING EVENT.
PRECIPITABLE WATERS ARE A LITTLE LOWER THAN THE LAST COUPLE OF DAYS
OVER THE EASTERN HALF OF SOUTHEAST ARIZONA...WITH GOES PRECIPITABLE
WATER IMAGERY INDICATING VALUES IN THE 1 TO 2 INCH RANGE ACROSS THE
CWA (THIS MORNINGS TUCSON SOUNDING SHOWS 1.56 INCHES).

THEREFORE...THE RISK OF SEVERE THUNDERSTORMS AND FLASH FLOODING WILL
BE BELOW NORMAL OVER WESTERN PORTIONS OF THE CWA...NEAR NORMAL IN
CENTRAL SECTIONS OF THE CWA (INCLUDING TUCSON) AND ABOVE CLIMATOLOGY
OVER EASTERN PORTIONS OF SOUTHEAST ARIZONA. GRAHAM AND COCHISE
COUNTIES WILL HAVE THE HIGHEST RISK OF BOTH FLASH FLOODING AND
SEVERE WEATHER...WITH THE FULL COMPLEMENT OF SUNSHINE TODAY
INCREASING ATMOSPHERIC INSTABILITY...BEING ON THE LEADING EDGE OF
THE EASTERLY WAVE LATE THIS AFTERNOON/EVENING...AND HAVING SATURATED
SOILS FROM HEAVY RAINS OVER THE PAST COUPLE OF DAYS.

STORM MOVEMENT WILL BE GENERALLY TO THE NORTHWEST AROUND 10 MPH.

ZELL