Tornado in California, April 14, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
3 PM PDT SAT APR 14 2007

.DISCUSSION...COLD FRONT WHICH PASSED THROUGH THE AREA THE MORNING
AND VERY EARLY AFTERNOON PRODUCED SOME LOCALLY HEAVY RAIN AS IT
SLOWLY MADE ITS WAY EAST.  AS NOTED THIS AM THE UPPER TROUGH WAS
SPLITTING AND THE SOUTHERN END WAS BEGINNING TO CLOSE OFF.  THIS
ALLOWED THE FRONT TO SLOW AS THE UPPER TROUGH BEGAN A NEGATIVE
TILT.  CENTER OF UPPER CIRCULATION APPEARS TO BE RIGHT OVER MONTEREY
BAY AS OF THIS HOUR WITH WV IMAGERY CONTINUING TO DARKEN AS
UPPER LOW DEEPENS AND BEGINS TO MOVE SOUTH. LOOKS LIKE THE HEAVIEST
RAINFALL WITH THIS SYSTEM WAS IN THE EAST BAY HILLS AND SANTA LUCIA
RANGE WHERE NEARLY A HALF-INCH FELL.  THE SOUTHERN HALF OF THE
DISTRICT DID QUITE WELL GIVEN THE DYNAMICS DROPPED FURTHER WEST AND
SOUTH WITH THIS EVENT.  MODELS AGAIN DID A POOR JOB IN HANDLING THE
PRECIP WITH THIS TROUGH.  MAY NOT BODE WELL FOR HOW IT IS HANDLING
PRECIP FOR TUESDAYS AND FRIDAYS EVENT OF NEXT WEEK.  MORE ON THAT
IN THE EXTENDED DISCUSSION.

EXPECT SOME SHOWERS TO LINGER ESPECIALLY IN THE EASTERN SECTIONS AND
FAR SOUTHERN AREAS OF THE DISTRICT INTO THIS EVENING WITH A FEW
SHOWERS POSSIBLE IN THE FAR SOUTHEAST INTO TOMORROW MORNING. MUCH
LESS CHANCE OF THUNDERSTORMS THIS AFTERNOON AS CLOUDS HELD ON LONGER
BEHIND FRONT CUTTING OFF HEATING.

NEXT BIG PROBLEM IS WINDS.  ALL HIGH RESOLUTION MODELS INCLUDING WRF
AND COAMPS SHOW STRONG WINDS OVER THE OCEAN AND NEAR THE COAST
BEGINNING TONIGHT AND ACTUALLY INCREASING THROUGH THE DAY TOMORROW
AS STRONG DEEP NORTH TO NORTHWEST FLOW DEVELOPS.  NORTHERLY
GRADIENT INCREASES FROM CRESCENT CITY TO SFO TO 10 MBS BY MORNING.
BL WINDS OVER THE OCEAN ARE SHOWING 40 TO 45 KTS. WINDS OVER LAND
ARE FORECASTS TO BE ONLY 15 TO 25 KTS.  THUS BELIEVE WE WILL NOT SEE
ADVISORY LEVEL WINDS OVER THE EAST BAY HILLS OR NORTHERN MTNS.  BUT
PERSONS TRAVELING TO THE COAST TOMORROW AND TOMORROW EVENING SHOULD
BE PREPARED FOR STRONG AND GUSTY WINDS WITH GUSTS TO 40 MPH
POSSIBLE ALONG THE ENTIRE COASTLINE. WINDS WILL GRADUALLY DIMINISH
OVERNIGHT SUNDAY AS WEAK HIGH PRESSURE BUILDS IN ALOFT. TEMPERATURES
SHOULD BE NEAR SEASONAL EXCEPT ALONG THE COAST THROUGH MONDAY AS
WARM AIR MOVES IN RAPIDLY ALOFT.

NEXT CHANCE OF RAIN IS FOR TUESDAY AND TUESDAY NIGHT. GFS AND ITS
ENSEMBLES ARE HAVING A HARD TIME GENERATING ANY PRECIP OUT OF THIS
NEXT SYSTEM EVEN THOUGH THERE ARE CONSIDERABLE HEIGHT FALLS AND JET
DYNAMICS OVER THE DISTRICT DURING THE DAY TUESDAY.  LOOKS LIKE MAIN
PROBLEM IS  THE TRAJECTORY OF THE LOW IS DOWN THE COAST FROM CANADA
TO OFF NOR CAL.  THUS THE SYSTEM SEEMS TO BE SOMEWHAT STARVED FOR
MOISTURE.  HOWEVER THE MODELS HAVE DONE VERY POORLY WITH THE LAST
TWO SYSTEMS THROUGH OUR AREA SO FORECAST NEEDS CLOSE MONITORING.
RIGHT NOW HAVE ONLY INCLUDED THE FAR NORTH IN ANY CLOUDS AND PRECIP
FOR NEXT TUESDAY AND TUESDAY NIGHT.

EXTENDED...SLIGHT MODIFICATION TO THIS MORNINGS DISCUSSION...

IN LOOKING AT WATER VAPOR LOOP OVER THE PACIFIC AND EAST ASIA CAN SEE
A SERIES OF WAVES COMING OFF THE ASIAN CONTINENT EMBEDDED IN A FAST
WESTERLY JET. WOULD APPEAR THE MAIN RIDGE POSITION IS SLOWLY
RETROGRESSING BACK TO THE WEST NEAR 150 TO 155W. THIS SHOULD ALLOW
SHORTWAVES TO BEGIN DIGGING A LITTLE FURTHER SOUTH AND WEST AS THEY
APPROACH CA. THE KEY WILL BE THE DISTANCE BETWEEN SUBSEQUENT
SHORTWAVES EMBEDDED IN THIS WESTERLY FLOW OVER THE PACIFIC. RIGHT
NOW WAVES ARE ABOUT 72 HRS APART. WE HAVE THE TUESDAY EVENT
AND THEN ANOTHER SYSTEM IS FORECAST BY THE EURO AND GFS FOR FRIDAY.
THE THIRD SYSTEM IS FORECAST FOR SUNDAY OF NEXT WEEK.  GIVEN THE
CURRENT PATTERN OF WEAK BLOCKING OVER CENTRAL CANADA AND LONG
WAVE RIDGE OUT NEAR 155W WOULD EXPECT CURRENT STORM TRACK TO
CONTINUE AND THUS ABOVE NORMAL CONFIDENCE IN FORECASTING
UNSETTLED WEATHER FOR THE NEXT WEEK TO TEN DAYS.  THE TOUGH PART
IS IN THE DETAILS OF OF EACH PASSING SYSTEM AS TO ITS RAINFALL
POTENTIAL.  WILL NEED TO REFINE THESE FORECASTS AS EACH SYSTEM GETS
CLOSER.

&&

.AVIATION...STRONG WEST WINDS HAVE DEVELOPED AFTER THE FRONT WENT
THROUGH AND AN AIRPORT WEATHER WARNING WAS ISSUED FOR SFO UNTIL 03Z.
WINDS GRADUALLY DECREASING DURING THE NIGHT BUT THERE SHOULD BE
ENOUGH WIND TO PREVENT ANY FOG FORMATION. CUMULUS CLOUDS WITH LOCAL
CEILINGS AROUND 3500 FEET WILL DISSIPATE AFTER SUNDOWN.

&&

.MARINE...NW WINDS INCREASING RAPIDLY BEHIND FRONT AND WILL REACH
GALE FORCE CONDITIONS TONIGHT. SEAS COME UP TO 13-14 FOOT RANGE
AS WELL WITH PERIODS AROUND 12 SECONDS BY SUNDAY MAKING IT A
ROUGH DAY ON THE WATER.

&&
.MTR...WATCHES/WARNINGS/ADVISORIES...
       .TNGT...SCA ALL WATERS CONVERTING TO GALES ALL OCEAN WATERS
       LATE TONIGHT.

PUBLIC FORECAST: REYNOLDS
AVIATION/MARINE: SOROKA/W PI

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO