National Weather Service Tucson Arizona
AREA FORECAST DISCUSSION
FXUS65 KTWC 032200 AFDTWC AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TUCSON AZ 300 PM MST FRI JUL 3 2009 .SYNOPSIS...THE COMBINATION OF SOLAR HEATING...A MOIST AIRMASS IN PLACE ACROSS ALL OF SOUTHEAST ARIZONA...WEAK STORM MOTIONS AND A LIFTING MECHANISM IN THE MIDDLE LEVELS OF THE ATMOSPHERE PUSHING INTO THE AREA FROM THE SOUTH WILL RESULT IN NUMEROUS SHOWERS AND THUNDERSTORMS WITH AN ENHANCED THREAT OF PRODUCING HEAVY RAINFALL THIS AFTERNOON AND TONIGHT. OTHERWISE...ABUNDANT MOISTURE WILL BRING SCATTERED AFTERNOON AND EVENING THUNDERSTORMS THIS HOLIDAY WEEKEND. AS A TROUGH PROGRESSES INTO THE WEST COAST...DRIER AIR WILL MOVE INTO ARIZONA NEXT WEEK FOR A DECREASE IN MOISTURE AND THUNDERSTORM ACTIVITY. && .DISCUSSION...KEMX RADAR INDICATED MAINLY ISOLATED THUNDERSTORMS OVER THE HIGHER TERRAIN OF SOUTHEAST ARIZONA EARLY THIS AFTERNOON... WITH SCATTERED THUNDERSTORMS ACROSS SOUTH-CENTRAL ARIZONA. A LOOP OF THE RADAR IMAGERY INDICATES THE WEAK MCV/MID-LEVEL FEATURE THAT WAS OVER COCHISE COUNTY THIS MORNING IS NOW TRACKING WEST ACROSS SANTA CRUZ COUNTY. I WOULD EXPECT THIS FEATURE WILL ENHANCE THUNDERSTORM DEVELOPMENT AS IT TRACKS FURTHER WEST INTO THE LATE AFTERNOON HOURS. THE 12Z KTWC UPPER AIR SOUNDING SHOWED A VERY MOIST AIRMASS RESIDING ACROSS THE AREA...WITH A PRECIPITABLE WATER VALUE OF 1.56 INCHES. THIS MOISTURE IS ALSO EVIDENT AT THE SURFACE...WITH SURFACE DEWPOINTS IN THE UPPER 50S TO LOWER 60S. WEAK FLOW IS ALSO EVIDENT ON THE UPPER AIR SOUNDING FROM THE SURFACE TO 500 MB. THIS WOULD SUGGEST A WEAK MOTION TO STORMS TODAY ACROSS THE FORECAST AREA. THIS FACT COMBINED WITH A VERY MOIST AIRMASS WOULD SUGGEST THAT THE MAIN THREAT TODAY WILL BE VERY HEAVY RAIN...WITH A LOWER RISK FOR STRONG THUNDERSTORM OUTFLOWS. THE 500 MB HIGH CENTER IS OVER THE SOUTHERN PLAINS TODAY. WE SHOULD CONTINUE TO SEE A MID-LEVEL DEFORMATION ZONE ROTATING NORTHWARD INTO THE FAR SE CORNER OF THE STATE INTO THE EARLY EVENING. THIS FEATURE WILL SERVE AS AS AN MECHANISM FOR THUNDERSTORM CONVECTION ACROSS THE FORECAST AREA. GIVEN THE WEAK STORM MOTION...THE VERY MOIST AIRMASS AND THIS FOCUSING MECHANISM ROTATING NORTHWARD ACROSS OUT NECK OF THE WOODS...I CONSIDERED THE POSSIBILITY OF ISSUING A FLASH FLOOD WATCH FOR PORTIONS OF MY FORECAST AREA. HOWEVER...I BELIEVE THAT THE WEAK FEATURES MOVING ACROSS THE FORECAST AREA ARE NOT SUFFICIENT ENOUGH TO ALLOW STORMS TO ORGANIZE INTO AND SIGNIFICANT THUNDERSTORM CLUSTERS. GIVEN THIS...I DO NOT BELIEVE THAT THE HEAVY RAIN WILL BE WIDESPREAD ENOUGH TO WARRANT THE WATCH. THE 12Z NAM AND GFS MODELS TRY TO PICK UP A MID-LEVEL DISTURBANCE CURRENTLY FIRING STORMS OVER EAST-CENTRAL SONORA MEXICO AND SHIFTS IT INTO SOUTH CENTRAL AZ LATE THIS EVENING INTO THE OVERNIGHT PERIOD. THIS FEATURE...IF IT INDEED MAKES ITS WAY UP INTO OUR NECK OF THE WOODS COULD HELP ORGANIZE CONVECTION ACROSS THE WESTERN HALF OF THE CWA AFTER MIDNIGHT AND HELP PRODUCE HEAVY RAINFALL. IN ANY EVENT...I HAVE REFLECTED THIS THINKING IN THE 06Z-12Z DIGITAL GRIDS. OTHERWISE...THE UPPER TROUGH PUSHING INTO THE PACIFIC NORTHWEST SQUASHES THE RIDGE...EVEN RESULTING IN A WEST/NORTHWEST FLOW INTO SOUTHEAST ARIZONA LATE THIS WEEKEND INTO EARLY NEXT WEEK. ALTHOUGH THIS WONT ENTIRELY REMOVE OUR MOISTURE...IT WILL REDUCE IT ENOUGH TO LIMIT THE OVERALL COVERAGE OF STORMS ACROSS OUR FORECAST AREA. && .AVIATION...THE COMBINATION OF SOLAR HEATING...A MOIST AIRMASS IN PLACE ACROSS ALL OF SOUTHEAST ARIZONA...WEAK STORM MOTIONS AND A LIFTING MECHANISM IN THE MIDDLE LEVELS OF THE ATMOSPHERE PUSHING INTO THE AREA FROM THE SOUTH WILL RESULT IN NUMEROUS SHOWERS AND THUNDERSTORMS WITH AN ENHANCED THREAT OF PRODUCING HEAVY RAINFALL THIS AFTERNOON INTO TONIGHT. THESE STORMS WILL LOWER VISIBILITIES TO 1-3 NM IN HEAVY RAIN...WITH LOWER THAN NORMAL CLOUD CEILINGS. ALTHOUGH THE RISK FOR STRONG AND GUSTY THUNDERSTORM WINDS TO 45 MPH STILL EXISTS...THE OVERALL THREAT OF STRONG WINDS IS MINIMAL. AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS. && .FIRE WEATHER...A MOIST ATMOSPHERE ACROSS SOUTHEAST ARIZONA WILL RESULT IN SCATTERED THUNDERSTORMS THROUGH THE WEEKEND. A TROUGH MOVING TOWARDS THE WEST COAST WILL SHIFT A DRIER WESTERLY FLOW INTO ARIZONA NEXT WEEK...ALTHOUGH ENOUGH MOISTURE SHOULD REMAIN IN PLACE TO MINIMIZE ANY FIRE WEATHER RISKS AND CONCERNS. && .TWC WATCHES/WARNINGS/ADVISORIES...NONE.
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