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National Weather Service Tucson Arizona
AREA FORECAST DISCUSSION
FXUS65 KTWC 200910 AFDTWC AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TUCSON AZ 210 AM MST SUN JUL 20 2008 .SYNOPSIS...HIGH PRESSURE ALOFT WILL RESIDE NEAR THE FOUR CORNERS REGION TODAY...WHILE A LARGE UPPER LEVEL DISTURBANCE MOVES SLOWLY WEST THROUGH TEXAS. THESE TWO FEATURES...COMBINED WITH PLENTIFUL MONSOON MOISTURE ACROSS SOUTHERN ARIZONA...WILL LEAD TO A SIGNIFICANT UPSWING IN SHOWERS AND THUNDERSTORMS TODAY. SOME OF THESE STORMS MAY BE STRONG AND PRODUCE HEAVY RAIN. HIGH PRESSURE WILL REMAIN OVER THE CENTRAL ROCKIES THROUGH THE MIDDLE OF NEXT WEEK...WHICH WILL KEEP SOUTHEAST ARIZONA IN AN ACTIVE MONSOON FLOW REGIME. && .DISCUSSION...YESTERDAY EVENING A LARGE MCS DEVELOPED OVER SOUTHEAST ARIZONA...SPECIFICALLY OVER COCHISE COUNTY AND PUSHED INTO PIMA COUNTY. IR SATELLITE IMAGERY IS NOW SHOWING THE SYSTEM WEAKENING WITH CLOUD TOPS WARMING DOWN TO AROUND MINUS 60 DEGS C FOR THE BULK OF THE STORM. EARLIER SATURDAY EVENING THE STORM EXHIBITED TOPS OF MINUS 78 DEGS C AND OVER A MUCH LARGER AREA. AT THE PRESENT TIME SATELLITE AND RADAR INDICATE THAT THE WORST OF THE SYSTEM IS NOW PUSHING WEST OF MY CWA (WEST OF AJO)...AND DEVELOPING OVER THE PHOENIX METRO AREA. MANY REPORTS OF HEAVY RAIN WERE RECEIVED YESTERDAY EVENING WITH AMOUNTS TYPICALLY WELL OVER AN INCH...AND SOME 2 TO 4 INCH AMOUNTS NOT UNCOMMON. REPORTS OF SWIFT WATER RESCUES FOR AREAS IN NE PIMA COUNTY WERE RECEIVED YESTERDAY EVENING. AN URBAN AND SMALL STREAM FLOOD ADVISORY CONTINUES IN EFFECT UNTIL 3 AM MST FOR COCHISE COUNTY...AND 4 AM MST FOR MUCH OF PIMA COUNTY. RADAR STILL SHOWING LIGHT TO OCCASIONALLY MODERATE RAIN OVER PIMA COUNTY...BUT THE ACTIVITY IS RAPIDLY DECREASING IN BOTH AREAL COVERAGE AND INTENSITY. WITH SO MUCH ACTIVITY ACROSS MY FORECAST AREA THE BOUNDARY LAYER HAS OBVIOUSLY COOLED CONSIDERABLY. LOOKING AT SURFACE OBS SHOWS THAT TEMPS NOW RANGE FROM THE LOWER 60S TO THE LOWER 70S. THESE VALUES RANGE FROM 2 TO 8 DEGS COOLER THAN THIS TIME YESTERDAY. WE SHOULD ALSO SEE A MASSIVE CLOUD DECK OVERHEAD TO START THE DAY AS THE COMPLEX DECAYS. WITH THESE TWO FACTORS IN PLAY (COOLED BOUNDARY LAYER...AND A THICK CLOUD DECK)...IT SHOULD SERVE TO INHIBIT CONVECTION TODAY. AVERAGE MOS POP GUIDANCE FROM THE 00Z MODEL RUNS INDICATES 71 PERCENT CHANCE OF RAIN FOR TUCSON TODAY. OF COURSE THE MODELS DIDN`T SEE WHAT TRANSPIRED AFTER THE DATA INGEST...SO WILL BE CURIOUS TO SEE WHAT 06Z MODEL RUN HAS TO SAY. GIVEN THE ABOVE SCENARIO I KEPT THE 50 PERCENT POP GOING WHICH UNDERCUTS MOS...BUT PERHAPS NOT ENOUGH. I JUST FIND IT DIFFICULT TO THINK WE WILL SEE ANOTHER DAY TODAY...LIKE YESTERDAY. COULD STILL BE SOME SCATTERED SHOWERS AND THUNDERSTORMS DEVELOP...BUT LIKELY NOT DEVELOPING UNTIL VERY LATE IN THE AFTERNOON OR EARLY EVENING. ALL MODELS SHOW A DECREASE IN POPS FOR MONDAY...AND ESPECIALLY TUESDAY. NAM IS SHOWING PWAT`S DROPPING TO JUST ABOVE THE 1 INCH MARK BY THEN. MOS POP FOR MONDAY AVERAGES AROUND 20 PERCENT FOR TUCSON AND SINGLE DIGIT TO BELOW 20 FOR TUESDAY. DIDN`T GO THAT LOW JUST YET...BUT DID TRIM POPS A BIT. MY FORECAST STILL HAS SCATTERED STORM COVERAGE FOR MOST PLACES FOR BOTH MONDAY AND TUESDAY...WITH THE EXCEPTION OF THE FAR WESTERN DESERTS WITH ONLY ISOLATED COVERAGE ANTICIPATED. MODELS ARE STILL SHOWING THE UPPER LOW OVER TEXAS SLOWLY TRYING TO WORK ITS WAY WEST...BUT STRUGGLES TO DO SO. DIDN`T TOUCH THE EXTENDED FORECAST AT ALL. STILL KEPT LOW END OF CHANCE FOR MOST AREAS. WILL HAVE TIME TO FINE TUNE THE EXTENDED FORECAST WITH SUBSEQUENT SHIFTS. HIGH TEMPS WILL BE A CHALLENGE TODAY. I WENT WITH A HIGH FOR TUCSON OF 90 DEGS...BUT IF CLOUD DECK REMAINS IN PLACE LONG ENOUGH...WE MAY STRUGGLE TO GET OUT OF THE 80S. AFTER TODAY SHOULD SEE A GRADUAL WARMUP INTO THE MID TO UPPER 90S FOR THE REMAINDER OF THE FORECAST PERIOD...WHICH IS SLIGHTLY BELOW TO NEAR NORMAL. && .AVIATION...EXPECT SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS TO CONTINUE TO PUSH WEST THROUGH THE REMAINDER OF THE EARLY MORNING HOURS. CEILINGS SHOULD REMAIN RELATIVELY LOW THROUGH SUNRISE OR SO WITH CIGS BETWEEN 1500 FT TO AROUND 4000 FT. HOWEVER...AS THE DAY PROGRESSES CIGS SHOULD CLIMB TO AROUND 8-10K FEET OR SO. WHEN THUNDERSTORMS DEVELOP LATE THIS AFTERNOON SHOULD ONCE AGAIN SEE CIGS/VSBYS DROP EASILY TO WITHIN MVFR CONDITIONS WHEN CLOSE TO A THUNDERSTORM...AND POSSIBLY TO IFR CONDITIONS. AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS. .TWC WATCHES/WARNINGS/ADVISORIES...NONE.
MOLLERE WEATHER.GOV/TUCSON
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