The 3-day period ending at 1200UTC 16 December ranked fourth on our list which may reflect somewhat that the data coverage in 2002 was better than during some of the earlier events.   According to the Environmental News Service, California’s northern coast received 26 inches of rain from December 12 through December 16 and more than 20 inches was recorded at Shasta.  In parts of the San Francisco Bay region received more than 17 inches in the same period.  The flooding in California killed 3 people. 


The heaviest rainfall during this period was concentrated over the northern portion of the area (see figure below) which may also explain why it did not have a larger impact on the general public as the most populated areas were south of the heaviest rain. 

December 2002 event

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 16 Dec 2002. 

Higher resolution analyses of the 3 individual days that were included in the 3-day rainfall event (in mm).

The synoptic pattern showed many of the same characteristics of other major events but also showed one difference.   The similarities were the negative height anomaly off the Pacific Northwest coast and the positive anomaly to its south at 500-hPa and presence of an atmospheric river with anomalously high PW within it.  Another similarity with other events was the strong pressure or height gradient that extended into northern California which implied strong southwesterly geostrophic winds.  The one notable difference was the lack of a positive height anomaly over Alaska or the Bering Sea. 

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 13 Dec. 2002 (top panel), 1200 UTC 14 Dec. 2002 (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 13 December 2002 (top panel), 1200 UTC 14 Dec. 2002 (bottom panel).


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 14 Dec. 2002.  The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Anomalous PW and 850-hPa moisture flux were present through much of the 3-day event.  During the heaviest 24 hour period, the atmospheric river or moisture plume impinged upon northern California though most of the 24-h period.  The normalized moisture flux anomaly increases from around 3 early in the period to over 5 sigma by 0600 UTC 14 Dec. (not shown) with the over 5 sigma departure continuing through 1200 UTC 14 Dec.  The greater than 42 mm PW maximum and greater than 3 sigma PW departure that was present off the coast at 1800 UTC 13 Dec..  The anomalous PW continued to shift eastward with the greater than 42 mm PW maximum being located just off the California coast and the greater than 3 sigma PW departure having spread across northern California southward to San Francisco by 0600 UTC 14 Dec (not shown).  The normalized anomalies decreased somewhat during the 24 hour period after 1200 UTC 14 Dec. before again increasing during the following 24 hours with normalized anomalies of 850-hPa moisture flux again exceeding 5 sigma by 0600 UCT 16 Dec.  Such high normalized anomalies suggest a return period for such a strong moisture plume would be around 3 years.