Massive rain coming to Southern California 20 inches

[link to www.weather.com]
Issued by The National Weather Service
Los Angeles, CA
2:39 pm PST, Fri., Jan. 15, 2010

... A MAJOR PATTERN CHANGE IS EXPECTED NEXT WEEK WITH THE POTENTIAL FOR EXCESSIVE RAINFALL AND STRONG WINDS...

A MAJOR CHANGE IN THE UPPER LEVEL PATTERN IS EXPECTED TO BEGIN ON SUNDAY AND CONTINUE THROUGH NEXT WEEK. A VERY POWERFUL WEST TO EAST JET STREAM SAGGING SOUTHWARD THROUGH THE PACIFIC WILL BREAK DOWN THE PERSISTENT RIDGE ACROSS THE WEST COAST. THIS WILL ALLOW A SERIES OF STORM SYSTEMS TO TRACK INTO SOUTHERN CALIFORNIA WHILE MAINTAINING A RICH SOURCE OF SUBTROPICAL MOISTURE.

CONFIDENCE IS GROWING THAT SIGNIFICANT AND POSSIBLY EXCESSIVE RAINFALL WILL OCCUR OVER SOUTHWESTERN CALIFORNIA FROM LATE SUNDAY THROUGH MOST OF NEXT WEEK. LIGHT RAIN COULD DEVELOP ACROSS SOUTHERN CALIFORNIA BY LATE SUNDAY... WITH INCREASING RAIN COVERAGE AND INTENSITY MONDAY INTO MONDAY EVENING. THIS FIRST SYSTEM IS EXPECTED TO BRING .50 TO 2 INCHES ACROSS MOST COASTAL AND VALLEY AREAS... WITH 2 TO 4 INCHES ACROSS FOOTHILL AND MOUNTAIN AREAS. THE STATION BURN AREA CAN EXPECT THE HEAVIEST RAINFALL WITH THIS FIRST SYSTEM FROM MONDAY AFTERNOON INTO MONDAY EVENING.

THE SECOND STORM SYSTEM IS EXPECTED TO AFFECT THE REGION ON TUESDAY. THE THIRD AND MOST LIKELY STRONGEST STORM SYSTEM IS EXPECTED WEDNESDAY INTO EARLY THURSDAY. A COLD AND UNSTABLE AIR MASS WILL CONTINUE TO KEEP A THREAT OF SHOWERS INTO AT LEAST FRIDAY. FOR THE ENTIRE WEEK... THERE IS THE POTENTIAL FOR RAINFALL TOTALS OF 4 TO 8 INCHES IN COASTAL AND VALLEY AREAS... AND 8 TO 16 INCHES IN THE FOOTHILLS AND MOUNTAINS. LOCAL RAINFALL AMOUNTS OVER 20 INCHES WILL BE QUITE POSSIBLE ACROSS FAVORED SOUTH FACING COASTAL SLOPES.

THE STRONG SOUTHERLY WINDS THAT ARE PROJECTED AT TIMES NEXT WEEK COULD BRING SIGNIFICANT OROGRAPHIC ENHANCEMENT OF RAINFALL ACROSS SOUTH FACING SLOPES. EARLY INDICATIONS ALSO SHOW THE POSSIBILITY OF EMBEDDED THUNDERSTORMS AT TIMES DURING THE WEEK. RAINFALL RATES BETWEEN ONE HALF INCH AND ONE INCH PER HOUR WILL BE POSSIBLE DURING TIMES OF HEAVIEST RAINFALL... ESPECIALLY ACROSS SOUTH FACING SLOPES. IF THUNDERSTORMS DEVELOP... THERE WILL BE THE POTENTIAL FOR ISOLATED RATES IN EXCESS OF ONE INCH PER HOUR.

THE LARGE AMOUNT OF EXPECTED PRECIPITATION... AS WELL AS THE POTENTIAL FOR HIGH INTENSITY RAINFALL... WILL BRING A SIGNIFICANT THREAT OF FLASH FLOODING AND DEBRIS FLOWS... ESPECIALLY TO THE RECENT BURN AREAS. AS SOILS BECOME MORE SATURATED THROUGH THE WEEK... THERE COULD ALSO BE AN INCREASED RISK OF URBAN AND SMALL STREAM FLOODING. IF THE STRONG STORM MATERIALIZES FOR THE WEDNESDAY INTO THURSDAY TIME FRAME... THERE COULD ALSO BE SOME MAIN STEM RIVER FLOODING CONCERNS.

SNOW LEVELS ARE GENERALLY EXPECTED TO RANGE BETWEEN 4000 AND 6000 FEET NEXT WEEK. AS STRONG SOUTHERLY WINDS PRECEDE THE STRONG SYSTEMS... SNOW LEVELS SHOULD BRIEFLY RISE ABOVE 6000 FEET ON MONDAY AND WEDNESDAY. THERE IS THE POTENTIAL FOR SIGNIFICANT SNOW IN THE MOUNTAINS NEXT WEEK... WITH SEVERAL FEET OF NEW SNOW POSSIBLE AT RESORT LEVELS ABOVE 6000 FEET.

THERE WILL LIKELY BE PERIODS OF STRONG SOUTHERLY WINDS NEXT WEEK... WITH THE POTENTIAL FOR WIDESPREAD GALE FORCE WINDS ACROSS THE COASTAL WATERS AND DAMAGING WINDS ACROSS PORTIONS OF SOUTHWEST CALIFORNIA. AS THESE STORMS MOVE INTO THE AREA NEXT WEEK... THEY WILL ALSO BRING THE POTENTIAL FOR A LONG DURATION OF HEAVY SURF CONDITIONS AND DANGEROUS RIP CURRENTS TO LOCAL BEACHES. THE COMBINATION OF STRONG WINDS AND HIGH SEAS COULD ALSO BRING SOME COASTAL FLOODING ISSUES... ESPECIALLY NEAR TIMES OF HIGH TIDE.

RESIDENTS OF SOUTHWESTERN CALIFORNIA ARE URGED TO STAY TUNED FOR THE LATEST INFORMATION ON THESE POTENTIALLY STRONG STORM SYSTEMS.