SIGNIFICANT STORM SYSTEM TO IMPACT FORECAST AREA EARLY NEXT WEEK...

http://www.srh.noaa.gov/data/MTR/AFDMTR

000
FXUS66 KMTR 091556
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
900 AM PDT FRI OCT 9 2009

...SIGNIFICANT STORM SYSTEM TO IMPACT FORECAST AREA EARLY NEXT
WEEK...

.DISCUSSION...LARGE SCALE SYNOPTIC PATTERN SHOWS HIGH AMPLITUDE RIDGE TROUGH PATTERN WITH FULL LATITUDE RIDGE LOCATED ALONG 135W WITH COLD UPPER LOW DIGGING SOUTH THROUGH EASTERN MT AND NORTHERN HIGH PLAINS. VERY DEEP 967 MB LOW 45.6/156W MOVING NORTH INTO THE GULF OF ALASKA.

GIVEN HIGH PRESSURE ALOFT AND AT THE SURFACE OVER CA THIS AM HAVE WELL DEFINED MARINE LAYER NOW AT 2200 FT BASED ON SFO SODAR. EARLY VIS SATL IMAGERY SHOWS MARINE STRATUS HAVING PENETRATED WELL INLAND WITH SOLID STRATUS COVERING MOST OF THE EASTERN PACIFIC. WITH SUN ANGLE A LITTLE LOWER AND STRATUS ABOUT 800 FT THICK EXPECT SLOW BURN OFF WITH STRATUS LINGER MOST OF THE DAY AT THE COAST. HIGH TEMPERATURES MAY BE SLIGHTLY COOLER TODAY GIVEN EXTENT OF STRATUS. THIS IS NOT REFLECTED IN THE CURRENT FORECAST SO WILL REVIEW THIS FOR POSSIBLE UPDATE.

MODELS SHOWING DEVELOPMENT OF A WEAK UPPER LOW BY SAT MORNING JUST OFF THE CENTRAL CA COAST. THIS ENERGY IS CURRENTLY MOVING DOWN THE BACK SIDE OF THE HIGH AMPLITUDE RIDGE AND SHEARS OFF UNDERNEATH THE RIDGE AS IT BEGINS TO MOVE EAST. EXPECT THIS WILL CONTINUE TO DEEPEN THE MARINE LAYER BUT NOT ENOUGH TO MIX IT OUT FOR TOMORROW. SO EXPECT POSSIBLE EARLY RETURN OF THE LOW CLOUDS THIS EVENING AND WIDESPREAD LOW CLOUDS OVER THE AREA TOMORROW MORNING AND POSSIBLY WELL INTO THE DAY TOMORROW. THIS MAY COOL HIGH TEMPERATURES EVEN MORE FOR SAT SO WILL REVIEW THIS AS WELL.

BY SUN WE BEGIN TO SEE THE TRANSITION TO WHAT WELL BECOME A VERY STORMY PERIOD INTO THE MIDDLE OF NEXT WEEK. FIRST OF WHAT MAY BE THREE INDIVIDUAL UPPER TROUGHS UNDERCUTS THE HIGH AMPLITUDE RIDGE LATE SUN INTO MONDAY AT ABOUT 35N. WOULD EXPECT ENOUGH LIFTING OF THE MARINE LAYER TO BEGIN MIXING OUT THE LOW CLOUDS BY LATE SUNDAY BEFORE THE MIDDLE AND HIGH CLOUDS BEGIN TO MOVE IN FROM THIS FIRST TROUGH. TIMING OF THESE EVENTS IS GOING TO BE BASED ON THE 00Z EURO AND NOT THE 06Z GFS WHICH HAS DEVIATED FROM WHAT THE LAST SEVERAL DAYS OF MODEL RUNS HAVE BEEN SHOWING. WILL UPDATE THIS IN THE AFTERNOON DISCUSSION IF 12Z EURO AND GFS INDICATE OTHERWISE.

EXTENDED...
SO USING THE 00Z EURO AS THE BASIS FOR THIS SIGNIFICANT UPCOMING EVENT HERE IS THE SEQUENCE OF EVENTS CURRENTLY EXPECTED. THIS FIRST TROUGH MOVES IN UNDER THE RIDGE WITH RAIN BEGINNING ALONG THE COAST JUST BEFORE SUNSET ON MONDAY. APPEARS THIS FIRST WAVE WILL ROTATE NORTH INTO NORTHERN CA MONDAY NIGHT WITH MOST OF THE RAIN FROM ABOUT SANTA CRUZ NORTH. WINDS WILL BEGIN TO INCREASE ALONG THE COAST AND OVER THE COASTAL WATERS MONDAY NIGHT BUT NOT EXPECTED TO REACH CRITERIA FOR HIGH WINDS UNTIL THE DAYLIGHT HOURS TUESDAY. THE SECOND MAJOR TROUGH DROPS INTO THE MAIN UPPER TROUGH THAT SETS UP UNDER THE CUTOFF HIGH OVER SOUTHERN AK. THIS ENERGY IS ASSOCIATED WITH WHAT WAS WESTERN TYPHOON MELOR. THIS WILL BE THE MOST SIGNIFICANT PERIOD OF RAIN AND WIND AT APPEARS TO BE FOCUSED ALONG THE CENTRAL COAST FROM MARIN DOWN TO BIG SUR. HEAVY RAIN STRONG GUSTY WINDS SHOULD LAST THROUGH WED EVENING THEN SHOULD SEE WINDS AND RAIN DIMINISH AS THE FRONT PASSES SOUTH. EURO AND 00Z GFS INDICATING PWS APPROACH 2 IN OFFSHORE CENTRAL CA WITH WINDS AT THE 925 MB LEVEL OF 55 TO 60 KTS. THIS IS A VERY POTENT COMBINATION AND SHOULD PRODUCE COPIOUS AMOUNTS OF RAIN AT THE HIGHER ELEVATIONS WITHIN THE FORECAST AREA. FIVE TO EIGHT INCHES OF RAIN FOR THE PERIOD MONDAY NIGHT THROUGH WED MORNING ARE CERTAINLY POSSIBLE WITH LOCALIZED AMOUNTS OF CLOSE TO 10 IN. THE KEY WILL BE HOW LONG THIS ATMOSPHERIC RIVER OF VERY HIGH MOISTURE LINGERS OVER ANY GIVEN LOCATION. WITH SOME GOOD UPPER DYNAMICS ASSOCIATED WITH 155KT 200 MB JET SHOULD EXPECT LOWER ELEVATIONS OUTSIDE ANY SIGNIFICANT RAIN SHADOWING TO RECEIVE 1 TO 3 INCHES OF RAIN AND POSSIBLY MORE. TRYING TO DRAW ANALOGIES TO PREVIOUS EVENTS IS ALWAYS TRICKY. WE HAVE LOOKED AT RECENT EVENTS LIKE JAN 4-5 2008 AND DEC 12, 1995 AS CLOSE COUSINS TO WHAT MODELS ARE FORECASTING. THIS STORM HAS HIGHER MOISTURE SUPPLY BUT MAY NOT HAVE AS SIGNIFICANT A WIND COMPONENT. AGAIN WE ARE STILL SEVERAL DAYS OUT AND TRYING TO PINPOINT EXACT IMPACTS ARE DIFFICULT. WILL UPDATE THE SPS THIS AFTERNOON TO TRY AND HIGHLIGHT EXPECTED IMPACTS OF WHAT IS EXPECTED TO BE A VERY SIGNIFICANT EVENT...ESPECIALLY FOR THE FIRST STORM OF THE YEAR.



&&

.AVIATION...AS OF 4:40 AM PDT FRIDAY...MARINE LAYER IS ROUGHLY 2 THOUSAND FEET DEEP. LIGHT ONSHORE WINDS ARE HELPING TO BRING IN STRATUS ACROSS THE AREA. MVFR CEILINGS ARE REPORTED AT AREA TERMINALS EARLY THIS MORNING.

VICINITY OF KSFO...MVFR CEILINGS THROUGH 19Z-20Z THEN VFR THIS AFTERNOON. GENERALLY LIGHT WEST WINDS BUT A FEW GUSTS UP TO 15 KNOTS ARE POSSIBLE FROM MID-LATE AFTERNOON.

KSFO BRIDGE APPROACH...LOW CLOUDS WILL CLEAR OUT BY LATE MORNING. MOSTLY CLEAR SKIES THIS AFTERNOON AND EARLY EVENING.

MONTEREY BAY TERMINALS...MVFR THIS MORNING WITH GRADUAL CLEARING LATE MORNING AND EARLY AFTERNOON. STRATUS WILL LIKELY MAKE A RETURN EARLY THIS EVENING.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
.NONE

$$

PUBLIC FORECAST: REYNOLDS
AVIATION/MARINE: CANEPA/BLIER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO




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