1998 WAS A YEAR OF WEATHER EXTREMES IN THE SAN FRANCISCO BAY REGION. RECORD RAINFALL AND FLOODING EARLY IN THE YEAR...UNUSUAL TORNADO EVENTS IN MAY AND DECEMBER...AND AN ARCTIC OUTBREAK IN LATE DECEMBER HIGHLIGHTED THE YEAR’S WEATHER.

AS THE YEAR BEGAN...ONE OF THE STRONGEST EL NINO EVENTS EVER RECORDED HELPED PRODUCE A SERIES OF POWERFUL PACIFIC STORMS. THESE STORMS GENERATED STRONG WINDS...RECORD RAINFALL AMOUNTS AND DEVASTATING FLOODING THROUGHOUT THE SAN FRANCISCO BAY REGION AND MONTEREY BAY AREA. HEAVY RAINS SATURATED HILLSIDES...RESULTING IN DEADLY MUDSLIDES.

LOCALLY HEAVY RAIN FELL BETWEEN JANUARY 12TH AND 18TH...BUT IT WASN’T UNTIL EARLY FEBRUARY THAT EL NINO STORMS UNLEASHED THEIR FULL FURY ON CENTRAL CALIFORNIA. CALLED THE “GROUND HOG’S DAY STORM”...THE STORM OF FEBRUARY 2ND AND 3RD PRODUCED TORRENTIAL RAIN AND RECORD FLOODING ON MANY RIVERS AND STREAMS. THE SAN FRANCISQUITO AND PESCADERO CREEKS IN SAN MATEO AND SANTA CLARA COUNTIES BOTH CRESTED AT RECORD LEVELS ...AND THE TOWN OF PESCADERO EXPERIENCED DEVASTATING FLOODING. RECORD FLOODING ALSO OCCURRED ON THE PAJARO AND SAN BENITO RIVERS. DEVASTATING FLASH FLOODING OCCURRED ALONG THE PETALUMA RIVER IN SONOMA COUNTY AND THE TRES PINOS CREEK IN SAN BENITO COUNTY. IN ADDITION TO FLOODING ALONG AREA RIVERS AND STREAMS...MAJOR URBAN FLOODING OCCURRED THROUGHOUT THE SAN FRANCISCO BAY REGION AND CENTRAL COAST. MANY ROADS WERE CLOSED DUE TO WASHOUTS AND MUDSLIDES. SOME ROADS...PARTICULARLY HIGHWAY 1 ALONG THE BIG SUR COAST...REMAINED CLOSED FOR MONTHS.

A SERIES OF STORMS CONTINUED THROUGH FEBRUARY AND BY MONTH’S END... MONTHLY RAINFALL TOTALS HAD BROKEN RECORDS. IN SAN FRANCISCO...14.89 INCHES OF RAIN FELL...BREAKING THE PREVIOUS RECORD OF 12.52 INCHES IN FEBRUARY...1878. OTHER FEBRUARY RAINFALL RECORDS INCLUDED REDWOOD CITY WITH 12.46 INCHES...SAN JOSE’S 10.23 INCHES AND MONTEREY WITH 15.00 INCHES. RAINFALL ACCUMULATIONS DURING FEBRUARY WENT A LONG WAY TOWARD SETTING RECORDS FOR THE 1997- 98 RAINFALL SEASON. SAN FRANCISCO’S RAINFALL TOTAL FOR THE WATER YEAR ENDING JUNE 30...1998 WAS 47.22...THE SECOND WETTEST YEAR ON RECORD.

1998’S ABOVE NORMAL RAINFALL TOTALS WERE NOT UNEXPECTED. EL NINO WINTERS TYPICALLY BRING HIGHER THAN NORMAL RAINFALL TO THE SAN FRANCISCO BAY REGION AND CENTRAL COAST.

DURING THE EL NINO STORMS...THE CALIFORNIA LAND-FALLING JETS EXPERIMENT (CALJET) WAS IMPLEMENTED WITH THE GOAL OF IMPROVING MESOSCALE PREDICTIONS OF HEAVY COASTAL PRECIPITATION AND WINDS IN LAND- FALLING WINTER STORMS. THIS EXPERIMENT INVOLVED NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA) WEATHER RESEARCH AIRCRAFT WHICH FLEW INTO STORMS APPROACHING THE WEST COAST. WEATHER OBSERVING EQUIPMENT AND SCIENTISTS WERE UTILIZED FROM SEVERAL AGENCIES INCLUDING THE NATIONAL WEATHER SERVICE...THE US NAVY AND THE UNIVERSITY OF OKLAHOMA. VALUABLE DATA WERE GATHERED ON THE STRUCTURE AND EVOLUTION OF POWERFUL WEST COAST STORMS...HELPING PRODUCE BETTER FORECASTS OF HEAVY RAIN AND WINDS.

SPRING NEVER SEEMED TO ARRIVE. INSTEAD...THE WET PATTERN ESTABLISHED DURING THE WINTER CONTINUED THROUGH MAY. IN MAY ALONE...SAN FRANCISCO REPORTED 14 DAYS OF RAIN TOTALING 3.92 INCHES. DURING THE ENTIRE 1997-98 RAINY SEASON...SAN FRANCISCO REPORTED MEASURABLE RAIN ON 119 DAYS. THIS SHATTERED THE PREVIOUS RECORD OF 107 RAINY DAYS SET DURING THE 1889-90 SEASON.

SIGNIFICANT CONVECTION OCCURRED ON SEVERAL DAYS IN MAY. LATE IN THE AFTERNOON ON MAY 5th...AT LEAST 2 TORNADOES OCCURRED IN RESIDENTIAL AREAS ON THE PENINSULA...ONE IN SUNNYVALE AND THE OTHER IN LOS ALTOS. THE FORMER...WHICH WAS EXCEPTIONALLY WELL-DOCUMENTED PHOTOGRAPHICALLY BY LOCAL RESIDENTS...WAS ASSESSED AS BEING OF MARGINAL F2 INTENSITY ON THE FUJITA SCALE AND RESULTED IN MILLIONS OF DOLLARS OF DAMAGE TO RESIDENCES AND OTHER BUILDINGS...UTILITY LINES...VEHICLES...AND LANDSCAPING. INTERESTINGLY...VIDEOTAPE OF THE ASSOCIATED FUNNEL CLOUD SHOWED ANTICYCLONIC ROTATION...THE REVERSE OF THE USUAL CASE. THE LOS ALTOS TORNADO...WHICH OCCURRED JUST A SHORT TIME LATER... ALSO CAUSED SIGNIFICANT DAMAGE AND...UNLIKE THE SUNNYVALE EVENT... RESULTED IN AT LEAST ONE SIGNIFICANT INJURY.

THE FALL OF 1998 WAS CURIOUSLY FREE OF THE WARM SPELLS OFTEN REFERRED TO AS “INDIAN SUMMER.” STRONG OFFSHORE FLOW EVENTS...WHICH USUALLY OCCUR IN EARLY FALL...NEVER MATERIALIZED. THE HIGHEST TEMPERATURE OF THE YEAR IN SAN FRANCISCO (86) OCCURRED IN EARLY AUGUST AND NOT DURING THE FALL. AN EXTENDED RAINY SEASON FOLLOWED BY A RELATIVELY COOL SUMMER AND FALL RESULTED IN A SHORT AND UNEVENTFUL FIRE WEATHER SEASON. NO MAJOR WILDFIRES OCCURRED IN THE BAY AREA.

1998 SAW A TRANSITION FROM A STRONG EL NINO TO A MODERATE LA NINA. DURING THE SPRING AND EARLY SUMMER...STRONG EL NINO CONDITIONS OVER THE TROPICAL PACIFIC RAPIDLY ABATED AND COOLER THAN NORMAL SEA SURFACE TEMPERATURES WERE IN PLACE BY JUNE...TYPICAL OF A MODERATE LA NINA EVENT. DURING LA NINA WINTERS...NORTHERN AND CENTRAL COASTAL CALIFORNIA TYPICALLY EXPERIENCE NEAR NORMAL AMOUNTS OF RAINFALL FOR THE YEAR...WITH HEAVIER AMOUNTS EARLIER IN THE WATER YEAR. TEMPERATURES DURING LA NINA WINTERS ARE GENERALLY COOLER THAN NORMAL.

A COLD SNAP DURING LATE DECEMBER WENT A LONG WAY TOWARD VERIFYING THE COOLER THAN NORMAL LA NINA TEMPERATURE PREDICTIONS. AN ARCTIC AIRMASS PLUNGED SOUTH OUT OF WESTERN CANADA ON SUNDAY...DECEMBER 20TH AND BROUGHT WINTERY WEATHER TO NORTHERN AND CENTRAL CALIFORNIA. FOR THE FIRST TIME SINCE THE WINTER OF 1976...SNOW WAS OBSERVED IN SAN FRANCISCO AND AS FAR SOUTH AS THE MONTEREY PENINSULA. AFTER THE FRONTAL PASSAGE...THE REGION EXPERIENCED A WIDESPREAD HARD FREEZE WITH TEMPERATURES FALLING BELOW FREEZING ON FOUR CONSECUTIVE NIGHTS STARTING ON THE 21ST. ONLY A FEW RECORD LOWS WERE SET DURING THIS COLD SPELL...PRIMARILY BECAUSE IT HAPPENED TO FALL DURING THE SAME PERIOD AS THE BIG FREEZE OF DECEMBER 1990.

COLD WEATHER WASN’T THE ONLY METEOROLOGICAL EVENT OF SIGNIFICANCE DURING DECEMBER. DURING THE EVENING OF DECEMBER 5TH...A VIGOROUS COLD FRONT SPAWNED AN OUTBREAK OF TORNADOES IN THE GREATER BAY AREA. ALTHOUGH THESE TORNADOES WERE RELATIVELY SMALL...SHORT-LIVED...AND WEAK...THEY NONETHELESS WERE THE APPARENT CAUSE OF LOCALIZED WIND DAMAGE IN A NUMBER OF DIFFERENT PLACES...INCLUDING SANTA ROSA... MARSHALL (ON THE EAST SHORE OF TOMALES BAY)...PETALUMA/COTATI... RICHMOND...CASTRO VALLEY...SANTA CRUZ...AND WATSONVILLE. NONE WERE OF GREATER THAN F1 INTENSITY...AND THERE WERE NO REPORTED SIGNIFICANT INJURIES.

1998 WILL BE REMEMBERED AS A YEAR OF EXTREMES. CLIMATIC PATTERNS WORLD-WIDE WERE AFFECTED BY ABNORMAL SEA SURFACE TEMPERATURES IN THE EQUATORIAL PACIFIC. SOME OF THE EXTREME WEATHER EVENTS IN THE BAY AREA COULD BE DIRECTLY ATTRIBUTED TO THE EL NINO AND LA NINA PHENOMENA WHICH METEOROLOGISTS ARE ONLY BEGINNING TO UNDERSTAND.

DD/SSA/WB

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
920 AM PST THU DEC 31 1998