March 8-11, 1995 event

The 3-day period ending at 1200UTC 11 March 1998 ranks as number 8.  The event produced heavy rain and flooding in the Napa and Sonoma valleys as well as in the Sierra range.  The Napa River peaked at 5.3 feet above flood stage in St. Helena which lead to nearly 400 residents having to be evacuated because of the flood waters.  At Napa,  the river peaked at 30.5 feet, 5.5 feet above flood stage which brought flood waters into the city.  However,  the biggest impact was along the Pajaro River Basin in Santa Cruz and Monterey counties.  The flood control along the river is designed to handle a 19000 cfs discharge (a 25-year event).  During the March event, the band of heavy rainfall associated with the storm stalled over Pajaro River Basin. The two red arrow are pointing to small areas where more than 6 inches fell during 24-hours, the HPC analysis indicates over 12 inches of rain fell over the basin.  The differences between the 72-hr CDC analysis with a maximum rainfall of slightly more than 150 mm over the basin while the maximum implied by the two HPC analyses would be in excess of 300 mm. 


A levee break near the upstream from Watsonville led to flooding of a over 300 acres of prime farmland in Monterey county including the town of Pajaro.  The area of severe flooding was declared a Federal Natural Disaster. 

HPC rainfall analysis for the 3-heaviest days of the event (in inches). 

Greater than 6 inches

The synoptic pattern was again very similar to the majority of the other events.  Ridging and a relative geopotential height maximum was present over the Bering Sea and western Alaska at 500-hPa with a potent negative height anomaly off the California coast.  The anomalously low heights associated with the trough helps maintain a potent geopotential height  gradient implying stronger than normal southwesterly geostrophic winds.  The deep southwesterly flow tapped into subtropics and pulled a strong atmospheric river northeastward where it impinges upon the California coast.  In this case,  the left exit region of an upper level jet streak was located where the heaviest rain fell along the coast.  850 temps were anomalously warm,  another common trait associated with cases exhibiting strong atmospheric rivers. 

Large area 4-panel chart,  200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 10 Mar. 1995.  The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure)  valid 0000 UTC 09 Mar. 1995 (top panel), 1200 UTC 09 Mar. 1995 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure)  valid 0000 UTC 09 Mar. 1995 (top panel), 1200 UTC 09 Mar. 1995 (bottom panel).

The atmospheric river impinged upon the coast of northern and central California for an extended period on March 9 and 10.  During the 48-hour period the normalized PW anomalies exceeded 3 sigma.  By 0000 UTC 9 March,  a band of 850-hPa that exceeded 3 sigma moved into the region with a small area of greater than 3 being located just on the coast.  During the next 12 hours,  the 850 moisture flux (MF) increased until the normalized anomaly exceeded 5 sigma.  The moisture plume slowly sand southward during the next 24 hours with the magnitude of the normalized MF anomaly decreasing somewhat at 0000 UTC 10 March (not shown) but then re-strengthening again to over 5 sigma by 1200 UTC.  The combination of the slow movement of the plume and the very high normalized anomalies within it, are a common trait of these multi-day extreme rainfall amounts. 

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 08 Mar 1995-1200 UTC 11 March 1995.