New Years 1997 event. 

24-h HPC precipitation analysis with contours of  0.50, 1, 2, 3, 4, 5 and 6 inches  ending at a) 1200 UTC 31 Dec 1996, b) 1200 UTC 01 Jan 1997, c) 1200 UTC 02 Jan 1997.  

The CDC .25 by .25 degree precipitation analysis ranked this as the 17th highest 3-day total which is very misleading as this was the most impressive event during a 10-year study by Junker et al. (in press) that included the 1995 events.  The New Years flood produced nearly 2 billion dollars in property damage and forced more than 100,000 people from their homes (Baird and Robles 1997). December 1996 set the stage by being an exceptionally wet month prior to the 3-day event.   The flooding associated with the event may have rivaled the legendary flood of 1862 (Roos 1997).  The flooding of the Feather and Cosumnes Rivers was the highest on record while the flooding on the American River almost equal to the flooding in 1986 (see figure below). 

From Roos 2007

There were numerous levee failures along the San Joaquin and Sacramento valleys.  More that 120,000 people were evacuated from their homes. Nine people were killed and 48 counties were declared disaster areas.  For additional information on the impact of the floods, click here. High snow levels and rain led to serious flooding in Idaho where the Weiser river exceeded flood stage by 7 feet and in Montana along the Musselshell, Yellowstone and Little Salmon Rivers.

A picture to show the scope of the flooding in California. From Roos 2007

The synoptic pattern was very similar to that of the majority of the top ten ranked events.  A 500-hPa ridge was located in the vicinity of the Bering Sea with an area of anomalously high heights (see below).  A strong negative anomaly with greater than 3 sigma departures from normal was located to its south and east. A positive anomaly was located to its south.  The strong negative anomaly west of northern California and the long fetch of deep southwesterly flow helped bring warmer than normal air into California and much of the west with an over 2 sigma temperature departure extending from northern California northeastward into Idaho and Montana. The deep southwesterly flow also tapped a very strong and unusually wide atmospheric river.  The synoptic pattern promised to bring anomalous moisture and warmth from California northeastward into Idaho and Montana.

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 31 Dec. 1997.  The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 02 Jan 1997. 

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure)  valid 1800 UTC 31 Dec. 1996 (top panel), 1800 UTC 01 Jan. 1997 (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 1800 UTC 31 Dec. 1996 (top panel), 1800 UTC 01 Jan. 1997 (bottom panel).

The maximum PW within this “river” was the highest found in any plume during a 10 year period (1991-2000).  The width of the moisture plume also was among the largest found.  Normalized PW anomalies exceeded 3 sigma on all three days included of the HPC precipitation analyses shown above.  During January 1st the PW anomaly reached greater than 5 sigma, a return period of  more than 120 months which probably translated to over 20 years given the 6-month length of the extreme rainfall threat season.  The normalized 850-hPa moisture flux  anomalies (NMFA) were also high through the event.  At 0000 UTC 30 December the 850-hPa moisture flux exceeded 5 sigma (not shown). The very high NMFA continued on December 31 and increased to over 6 (figure not shown) signaling that the MF within this atmospheric river was rare.   The rainfall maximum was generally located where the maximum moisture flux was aimed for the longest time.    For a more in depth discussion of the normalized anomalies associated with this event, see Junker et al 2007.