84th AMS Annual Meeting

Wednesday, 14 January 2004: 1:45 PM
A climatological study of 100 mm or greater days in northern California
Room 607
Norman W. Junker, NOAA/NWS, Camp Springs, MD; and R. H. Grumm, R. Hart, and L. F. Bosart
Poster PDF (59.5 kB)
A 10-year climatology (1991-2000) of 10 cm (4-inch) and greater daily rainfall events was constructed and analyzed over California. Eighty-one cases were identified, all but one occurred in the period from November through April. Sixty-one of the cases occurred north of 37oN. However, significant month-to-month and year-to-year variation in frequency was observed. For each case, the mean-sea level pressure and 500 hPa height analyses at 0000 UTC and 1200 UTC were examined to establish which time was most likely closest to the beginning of the event. Gridded fields for a subset of the these cases where the rainfall maximum was located in the northern Sierra mountain range of northern California (within a very focused geographic region: 39 to 40oN and 120.5 to 121oW) were then compared to the daily climatological means from the 30-year period from 1961-1990. Composites were then constructed for the eighteen 10 cm or greater events where the maximum rainfall was located within this very focused region.

Monthly mean charts of 500 hPa heights, 850 hPa winds and precipitable water were also constructed for the two months with the most observed 10 cm or greater events in northern California (Dec. 1996 and Jan. 1997), and the two months with the fewest number of events (Dec. 1991 and Jan. 1992). The two dry months were associated with above normal 500 hPa heights across the Pacific Northwest and central and northern Rocky mountain states, while also having a below normal zonal component of the 850 hPa winds over northern California. The two wet months had an axis of stronger than normal zonal 850 mb wind extending into California. The composite mean 500 hPa of each wet month exhibited a negative anomaly centered near 30oN 160oW that extended to the west or southwest past the dateline, while having a positive anomaly to its east or southeast.

The eighteen case 10 cm or greater event composites displayed a synoptic pattern with below normal heights at 850 and 700 hPa off the Pacific Northwest coast and above normal heights over Baja California. Southwesterly winds were present at 850 and 700 hPa with stronger than normal 850 hPa wind. The composites also displayed anomalously high values of 850 hPa and 1000-700 hPa moisture transport and precipitable water in a band extending from 30oN 135oW northeastward into northern California with the moisture transport vectors being approximately perpendicular to the axis of the Sierra Nevada range. A composite of the 250 hPa wind field suggested the presence of the entrance region of a jet streak over northern California.

Similar fields were examined for three multi-day, extreme rainfall events that produced major flooding or flash flooding in Northern California (each was associated with a federal disaster declaration) and were then compared to the 10 cm or greater composites. The width of the axis of above normal moisture transport was significantly wider for each day of these three more extreme events than was found for the eighteen-case composite. Climatological anomalies were also greater, especially for moisture transport and precipitable water. The latter had anomalies that were 2 to 5 standard deviations from the mean. The strength of the height, wind, moisture flux and precipitable water anomalies during these major events suggests that anomalies of these fields could be used to identify cases that have the potential to produce major flooding or flash flooding in northern California from model forecast fields even if the modelís explicit forecasts of precipitation prove to be relatively poor.

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