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National Weather Service Tucson Arizona
AREA FORECAST DISCUSSION
FXUS66 KSGX 281551 AFDSGX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN DIEGO CA 850 AM PDT THU AUG 28 2008 .SYNOPSIS...A DEEP MARINE LAYER WILL KEEP SKIES MOSTLY CLOUDY AT THE BEACHES TODAY BUT THE MARINE LAYER WILL SHRINK ON FRIDAY AND MOSTLY SUNNY DAYS AREA LIKELY FOR THE BEACHES THIS LABOR DAY WEEKEND. INCREASING MONSOON MOISTURE WILL BRING A RETURN OF THUNDERSTORMS FRIDAY AND SATURDAY. SOME OF THESE MAY MOVE WEST OF THE MOUNTAINS AND ACROSS THE VALLEYS AND COAST FRIDAY NIGHT AND SATURDAY. DRIER AND MORE STABLE AIR WILL MOVE ACROSS THE REGION SUNDAY AND LABOR DAY. NEXT WEEK AN UPPER LEVEL TROUGH WILL BRING SLIGHT COOLING AND A RETURN OF THE MARINE LAYER. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... .SHORT TERM...TODAY THROUGH SATURDAY... MONSOON MOISTURE PULSING TOWARDS SW CALIFORNIA BUT ALL FACTORS NOT QUITE IN ALIGNMENT TODAY TO GENERATE THUNDERSTORMS OVER MOUNTAINS/DESERTS. THE REMNANT CIRCULATION FROM THE MESO SCALE CONVECTIVE COMPLEX THAT WAS OVER NW ARIZONA LAST NIGHT IS CURRENTLY MOVING WEST SOUTHWESTWARD AROUND 12 KTS ACROSS IMPERIAL/EASTERN SAN DIEGO COUNTY. THIS FEATURE SHOULD TURN MORE TOWARDS THE WEST LATER THIS MORNING ACROSS SAN DIEGO COUNTY AND THEN NORTHWEST THIS AFTERNOON AS IT MOVES OVER THE COASTAL WATERS AND CONTINUES ITS PATH AROUND THE PERIPHERY OF THE HIGH CELL TO THE NORTH. IN ITS WAKE CONVERGENCE ALOFT WILL GENERATE DOWNWARD MOTION WHICH SHOULD BE SUFFICIENT TO OFFSET THE LOW/MID LEVEL CONVERGENCE OVER THE MOUNTAINS THIS AFTERNOON. ALSO TRIGGER TEMPS WILL BE QUITE HIGH...93 FOR 5000 FEET AND 103 FOR 3500 FEET...SO ONLY MODERATE CUMULUS AT WORST EXPECTED OVER HIGHER MOUNTAINS TODAY. PLAN TO TAKE ADVANTAGE OF THIS TEMPORARY BREAK IN ACTIVE CONVECTION TO INSTALL UPGRADES TO WSR-88DS AT KSOX AND KNKX SO THEY WILL BE DOWN FOR A FEW HOURS THIS MORNING. FOR TONIGHT THROUGH FRIDAY THE LOW/MID LEVEL MONSOON MOISTURE INCREASES TO A DEPTH OF AT LEAST 10000 FEET DUE TO THE INCREASING SOUTHEAST FLOW. WITH NO INHIBITORS IN PLACE...DIURNAL HEATING FRIDAY SHOULD LEAD TO SCATTERED TO WIDESPREAD THUNDERSTORMS FORMING OVER MOUNTAINS. FOR FRIDAY NIGHT THROUGH SATURDAY WITH PREVAILING FLOW BECOMING MORE EAST DUE TO SHIFTING OF HIGH CELL...THUNDERSTORMS SHOULD DRIFT WEST OF THE MOUNTAINS AND ANY EMBEDDED SHORT WAVES/REMNANT UPSTREAM CIRCULATION CENTERS CAUGHT UP IN THE FLOW COULD GENERATE SQUALL LINES THAT WOULD PROPAGATE WESTWARD ACROSS THE VALLEYS/COAST/AND COASTAL WATERS...SOMEWHAT ANALOGOUS TO THIS PAST MONDAY. MOVING ON TO THE MORE PRESSING QUESTION...WHEN WILL THE STRATUS CLEAR. MARINE LAYER IS DEEPER AND DEEPENING THIS MORNING FROM WEAK LIFTING AHEAD OF ABOVE MENTIONED CIRCULATION CENTER. RULE OF THUMB USING A 1000 FOOT CLOUD LAYER WOULD SAY STRATUS NOT BREAKING UP UNTIL 1020 PDT. A NAM12 MODEL CROSS SECTION ALONG THE COASTS SHOWS THE MEAN LAYER RELATIVE HUMIDITY IN THE LOWER 2000 FEET STAYING ABOVE 90 PERCENT ALONG SAN DIEGO COAST ALL DAY...WITH ORANGE COUNTY COAST DROPPING DOWN TO AROUND 75 PERCENT FOR ONLY A BRIEF 3 HOUR PERIOD...BETWEEN 1400-1800 PDT...AND THEN HUMIDITY ROCKETS BACK UP TO +90 PERCENT...CLOSER TO 100 PERCENT IN LOWER 1000 FEET AFTER 2000 PDT. SO LOOKS LIKE LIMITED/LATE PATCHY CLEARING FOR SAN DIEGO COAST TODAY AND EARLY TO MID AFTERNOON CLEARING FOR ORANGE COUNTY COAST...WITH HAZY SUNSHINE THIS AFTERNOON FOR THE VALLEYS. MUCH BETTER PROSPECTS FOR SUNSHINE ALONG THE COAST ON FRIDAY AS INCREASING INSTABILITY MENTIONED ABOVE WILL BE SPREADING WESTWARD ACROSS THE ENTIRE AREA...ERODING/ELIMINATING THE CAPPING INVERSION AND MIXING OUT THE MARINE LAYER SO STRATUS SHOULD BREAK UP MUCH EARLIER ON FRIDAY...AROUND 0900 PDT...AND GONE BY 1000 PDT WITH NO RETURN FRIDAY NIGHT/SATURDAY...OR AT WORST PATCHY/SCATTERED STRATUS CONFINED TO BELOW 300 FEET OVER WATER AND SOUTH SAN DIEGO COUNTY COAST/NORTHERN BAJA CALIFORNIA. .LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY... ALL MODELS IN AGREEMENT WITH RELATIVELY STRONG TROUGH MOVING INLAND TO THE NORTH SATURDAY NIGHT...RESULTING IN INCREASING WEST TO SOUTHWEST WINDS AND DRIER/STABLE AIR MASS MOVING OVER SOUTHWEST CALIFORNIA SUNDAY THROUGH AT LEAST MONDAY AND POSSIBLY LINGERING THROUGH REMAINDER OF NEXT WEEK. MARINE LAYER SHOULD BEGIN TO REBUILD BY SUNDAY NIGHT AIDED BY A COASTAL EDDY CIRCULATION...HOWEVER LABOR DAY LIKELY TO BE MOSTLY SUNNY AT THE BEACHES WITH ANY STRATUS/FOG CLEARING WELL BEFORE 1000 PDT. AFTER MONDAY A WEEK UPPER LOW SLOWLY APPROACHES FROM THE WEST SOUTHWEST...AND THE RESULTING WEAK CYCLONIC FLOW ALOFT SHOULD AID IN MAINTAINING A COASTAL EDDY AND DEEPENING THE MARINE LAYER. FAIR WEATHER EXPECTED ELSEWHERE. && .AVIATION... 271520Z...MARINE LAYER STRATUS THICKNESS THIS MORNING ABOUT 1K FT WITH BASES NEAR 1K FT MSL AND TOPS TO FL020. THIS SOLID MARINE DECK SHOULD DELAY CLEARING AT THE COASTAL AIRPORTS UNTIL AROUND 19Z TIME FRAME. FARTHER INLAND...EXPECT THE CIGS TO LIFT BETWEEN 16Z AND 17Z...ALTHOUGH 3-5SM VIS RESTRICTIONS WILL CONTINUE IN HAZE UNTIL AROUND 20Z. AT THE BEACHES...AREAS OF CIGS/HAZE ARE LIKELY TO LINGER THROUGH THE AFTERNOON. ABOVE THE MARINE LAYER VIS WILL BE UNRESTRICTED WITH SOME SCT TO LOCALLY BKN CLOUDS AT FL150. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. &&
PUBLIC/FIRE WEATHER...BALFOUR AVIATION/MARINE...JAD NWS ON THE WEB AT WEATHER.GOV/SANDIEGO
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