Flash Flood in Arizona, July 26, 2002

SOUTHEAST ARIZONA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
217 PM MST FRI JUL 26 2002

.SYNOPSIS...HIGH PRESSURE HAS RELOCATED FROM THE FOUR CORNERS INTO
THE SOUTHERN PLAINS. THIS WILL ALLOW A DRIER WESTERLY FLOW ALOFT TO
FILTER INTO THE REGION THIS WEEKEND...LEADING TO A DOWNTURN IN
THUNDERSTORM ACTIVITY. THE HIGH IS THEN EXPECTED TO SHIFT BACK INTO
THE FOUR CORNERS BY EARLY NEXT WEEK...WHICH WILL RESULT IN A RETURN
TO MORE TYPICAL THUNDERSTORM COVERAGE.

.DISCUSSION...AS EXPECTED, STORMS SO FAR HAVE BEEN HUGGING THE
MOUNTAINS AND PRODUCING BRIEF BUT HEAVY RAINS (1-2/HR). THE
RESULTING ANVILS HAVE SPREAD INTO SOME OF THE VALLEYS AND SQUASHED
ADDITIONAL CUMULUS DEVELOPMENT. WE HAVE SEEN NO TENDENCY FOR THE
STORMS TO ORGANIZE OR MOVE INTO THE VALLEYS EN-MASSE, SO WELL
RETAIN CLIMO POPS THIS EVENING DESPITE THE OTHERWISE FAVORABLE
CONDITIONS. WELL ALSO RELUCTANTLY HOLD ONTO CURRENT LOWS, ALTHOUGH
IF THE VALLEYS DONT PICK UP SOME RAIN THIS EVENING AND INSTEAD
REMAIN CLOUDY, EVENING SHIFT MAY HAVE TO RAISE THEM A NOTCH.

DESPITE A PRETTY GOOD GULF SURGE (AT ONE POINT 30KTS JUST BEFORE
12Z), THE DRIER AIR ALOFT IS INVADING AND GRADUALLY WINNING OUT OVER
NW AZ. ONCE THE SURGE BREAKS UP IN THE DEEPENING W FLOW, THE DRIER
AIR WILL START TO MIX DOWN MAKE MORE RAPID PROGRESS INTO OUR CWA.
EVEN IF WE MANAGE TO HOLD ONTO A SHALLOW SURGE OVERNIGHT, THE WEAK
LOW LEVEL FLOW, INCREASINGLY UNFAVORABLE UPPER FLOW, AND GRADUAL
WARMING ALOFT WILL TEND TO REDUCE TSTM COVERAGE. WELL CONTINUE THE
TREND OF MY PREDECESSORS BY LOWERING POPS SAT. WE ORIGINALLY THOUGHT
THAT WED BE NEARLY SHUT DOWN ON SUN, BUT BY THEN THE HIGH OVER THE
SOUTHERN PLAINS WILL BE MOVING BACK INTO POSITION AS THE WEST COAST
TROUGH DEAMPLIFIES A BIT. THUS THE 18Z ETA STARTS BRINGING MOISTURE
BACK IN FROM THE SE AS EARLY AS SUN AFTERNOON. ANOTHER ITEM TO
WATCH, THIS TIME FOR MON, WILL THE THE EASTERLY WAVE OVER THE GULF
OF MEXICO WHICH HAS BEEN CAPTURED UNDERNEATH THE RIDGE. MODELS ARE
NOT IN VERY GOOD AGREEMENT AS TO WHERE IT WILL TRACK, BUT EVEN IF IT
HEADS ACROSS SONORA OUR MID LEVEL FLOW WILL INCREASE OUT OF THE E OR
NE WHILE WE PICK UP SOME MID LEVEL COLD ADVECTION. IN BOTH
INSTANCES, WELL PLAY CLOSE TO THE VEST ON POPS AND LEAN A BIT BELOW
CLIMO UNTIL WE SEE HOW FAST THE MOISTURE WILL REALLY RETURN.
STANDARD MONSOON FORECAST LOOKS GOOD AGAIN BY DAY 4, ALTHOUGH WELL
KEEP THE W IN CHANCE CATEGORY IN CASE THE NEXT TROUGH OFF THE W
COAST ENDS UP BEING A LITTLE STRONGER. PYTLAK

.TWC...NONE.

AZZ019-029>035-270700-
SOUTHEAST ARIZONA HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE TUCSON AZ
1140 AM MST FRI JUL 26 2002

.SYNOPSIS...THE MAIN THREAT ACROSS SOUTHEAST ARIZONA TODAY WILL BE
HEAVY PRECIPITATION AS AMPLE LOW AND MID LEVEL MOISTURE REMAINS
ACROSS THE AREA. A WEAK FLOW ALOFT WILL RESULT IN NEARLY STATIONARY
OR A SLOW AND RANDOM STORM MOTION TODAY. THIS WILL INCREASE THE
THREAT OF HEAVY RAINFALL...ESPECIALLY IN THE MOUNTAINS. THEREFORE...
THE PROBABILITY OF RECEIVING PRECIPITATION IN EXCESS OF ONE INCH
WILL BE ABOVE CLIMATOLOGY. THE PROBABILITY OF A SEVERE THUNDERSTORMS
BE SLIGHTLY BELOW TO NEAR CLIMATOLOGY.

.DISCUSSION...THE GOES-10 SATELLITE IMAGERY INDICATED THAT PLENTY OF
MOIST AIR IS RESIDING ACROSS ALL OF SOUTHEAST ARIZONA TODAY WITH THE
MORNING TUCSON UPPER AIR BALLOON SOUNDING INDICATING A PRECIPITABLE
WATER ESTIMATE OF 1.66 INCHES. IN ADDITION...MOISTURE ACROSS THE
SOUTHWESTERN DESERTS WERE REINFORCED LAST NIGHT AS A WEAK SURGE OF
MOISTURE PUSHED INTO THE COLORADO RIVER VALLEY FROM THE GULF OF
CALIFORNIA.

THE UPPER AIR SOUNDING INDICATED TWO IMPORTANT FACTORS THAT WILL
INFLUENCE THUNDERSTORM ACTIVITY TODAY. FIRST...THE ATMOSPHERE IS
MORE UNSTABLE THAN YESTERDAY...WITH COLDER AIR AT MID LEVELS AND
WARM AIR IN THE LOW LEVELS. THIS WILL ALLOW FOR DEEP CONVECTION TO
OCCUR. ALTHOUGH CONVECTIVE INITIATION WILL ALSO BE AIDED BY A
STRETCHING DEFORMATION FIELD IN THE UPPER ATMOSPHERE...NO
SIGNIFICANT DYNAMICS ARE PRESENT TO FOCUS DEVELOPMENT IN ANY
PARTICULAR AREA.

THE SOUNDING ALSO INDICATED AN EXTREMELY WEAK FLOW ALOFT. THIS WILL
NOT ONLY RESTRICT THUNDERSTORM ACTIVITY MAINLY TO THE MOUNTAINS...
BUT THE RANDOM AND SLOW MOVEMENT OF THE STORMS WILL KEEP MOST STORMS
FROM PUSHING INTO THE VALLEYS.

THIS WEAK FLOW AND THE MOIST ATMOSPHERE WILL SIGNIFICANTLY INCREASE
THE THREAT FOR EXCESSIVE PRECIPITATION...ESPECIALLY IN THE MOUNTAINS.
THEREFORE...THE PROBABILITY OF RECEIVING RAINFALL IN EXCESS OF ONE
INCH WILL BE ABOVE CLIMATOLOGY TODAY...WITH HIGHER PROBABILITIES IN
THE MOUNTAINS.

THE MOISTURE PROFILE WILL HOWEVER REDUCE THE THREAT OF SEVERE LEVEL
THUNDERSTORM OUTFLOWS. THEREFORE...THE PROBABILITY FOR A SEVERE
THUNDERSTORM WILL BE SLIGHTLY BELOW TO NEAR CLIMATOLOGY.