SAN FRANCISCO
1999-2000 Rainfall Season in Review

A Normal Season?

       The 1999-2000 rainfall season in San Francisco, and most of northern and central California, began near normal but then fell behind the curve, and there was even talk in some quarters of a drought.  This talk was fueled by a dry December and first half of January, but excessive rains from late January through mid March pushed the seasonal totals above the 30 year normal.  It certainly verified the old expression that we get "normal" by "averaging the extremes".
     After modest near average rain in September and October, November's total of 2.32 fell and inch short of normal.  December started off with just .62 inches during the first two weeks of the month and then it did not rain again until the 4th of January as the West Coast saw a strong ridge of high pressure settle just offshore.  By the second week of January the ridge was replaced by a long-wave trough that would remain in place for most of the next two months.
      From the 10th of January through the 9th of March it would rain 44 out of the 60 days.  During the month of February it rained a record 21 days.  This 60 day period saw a rainfall total of 17.34 inches or 70% of the seasons eventual total of 24.89 inches.
       Spring tried to make an early appearance when it did not rain from 12 of March until the 12 of April, but a series of weak weather systems brought slightly above normal rain for April May and June.

Daily Rainfall vs. Normal

What Normal To Use?

       The common standard from which "normals" are calculated are 30 year periods at the end of each decade.   Consequently, the current normal period is 1961 through 1990 for which the seasonal San Francisco rainfall is 20.52 inches.  However, the average for the most recent 30 year period, 1970 through 1999, has risen to 22.56; and for the past 10 years it's a soggy 25.23 inches.
      It should also be kept in mind that normal should be thought of as a range and not a single value.  One methodology has this "normal range" to be between the 30th and 70th percentile, or from about 16 to 24 inches, thus this past season would rank just into the above normal range.

The Rest of California

      For the most part, California's 1999-2000 rainfall season was characteristic of past La Niñas, with near or above normal rain for Northern and central California and below normal south of the Tehachapis.   Below are this seasons totals, the percent of normal (PON) and the seasonal normals.

     NORMALS ARE     JUL 1-       JUL 1-
     1961-1990       JUN 30       JUN 30
     AVERAGES.       2000    PON  NORMAL
     --------------  -----   ---  ------
BAY AREA
     OAKLAND         25.48   117   21.79
     SFO AIRPORT     20.85   106   19.70
     SAN FRANCISCO   24.89   121   20.52
     SAN JOSE        15.33   106   14.42
     SANTA ROSA      29.90    99   30.30
NORTHERN CALIFORNIA
     EUREKA          36.41    97   37.53
     REDDING         34.16   103   33.30
     SACRAMENTO      23.74   128   18.61
CENTRAL CALIFORNIA
     FRESNO          12.91   122   10.60
     BAKERSFIELD      5.13    90    5.72
SOUTHERN CALIFORNIA     
     LOS ANGELES     11.57    78   14.77
     SAN DIEGO        5.78    58    9.90

	Jul	Aug	Sep	 Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun
1 													1
2 						0.10			0.27				2
3 								0.22					3
4 							0.03		0.52				4
5 				0.01				0.21	0.05				5
6 		0.03		0.01		0.01			0.01		0.03		6
7 			0.01		0.46	0.01		0.01	0.62		0.68	0.11	7
8 			0.07		0.18				0.46		0.30	0.05	8
9 			0.17			0.45		0.03	0.09				9
10							0.03	0.67					10
11 		0.07					0.52	1.23	0.02				11
12 								0.53		0.26			12
13			 			0.05	0.01	2.08					13
14 					0.06			0.61		0.25	0.22		14
15 							0.08				0.17		15
16 					0.41		0.53	0.24		0.54			16
17 					0.10		0.09	0.01		0.51			17
18		 					0.43						18
19 				0.01	0.31		0.12						19
20 								0.41					20
21 					0.07		0.12	0.18					21
22 			0.34				0.14	0.78		0.10			22
23 							1.70	0.07					23
24 							2.20	0.18					24
25							0.04	0.05	 				25
26 								0.52					26
27				0.50				0.42					27
28 				0.12				0.04					28
29					0.20			0.45					29
30 					0.53		0.29						30
31							0.05				 		31
Amount 	0.00	0.10	0.59	0.65	2.32	0.62	6.38	8.97	2.04	1.66	1.40	0.16	
Days 	0	2	4	5	9	5	16	21	8	5	5	2
										Season Total:	24.89 in.
										Days of Rain:	82