Wet October -  Dry November...
Climatologically, What Does It Mean?

  Jan Null, CCM
Golden Gate Weather Services


Precipitation Climatology for Sequences of a Wet October and Dry November

What has happened in past rainfall seasons when we have had a relatively wet October followed by relatively dry November?  And does it matter?

I have looked at San Francisco's rainfall, which has the longest consecutive record in the state and is a fairly representative proxy for northern and central California, and defined a "wet October" as having more than 2 inches of rain and a "dry November" with less than 1 inch.  Using this criteria there are nine occurrences since San Francisco rainfall records began in 1849.

San Francisco Rainfall

October November Season Ended With:
1854 2.43 .34 23.83
1858 2.74 .69 22.22
1876 3.36 .25 11.04
1884 2.55 .26 18.10
1962 5.51 .60 22.15
1969 2.61 .45 20.80
1975 2.75 .43 7.95
1991 2.35 .50 19.20
2000 2.36 .85 ??

Furthermore, let's define a "wet season" as one falling in the top 30% of the cases (i.e., 70th percentile) as explained by Steffens, and a "dry season" as falling below the 30th percentile.  For the San Francisco rainfall dataset from 1849 to 1999, the 30th percentile is 17.50 inches and 70th is 24.70 inches.  Therefore "normal" rainfall for San Francisco can be thought of as the range between these two values; and not just the 151 season mean which is 21.79 inches.

A quick survey of the above chart shows that no season with the wet-dry October-November sequence ended as "wet"; however 6 of the 8 cases were "normal" and 2 would be classified as "dry".  Not much of a signal.  (Ironically, if we look at the 20 seasons since 1849 which had BOTH a dry October and a dry November, there were wet years on 4 occasions.  There were also 10 that were dry years and 6 that were normal).

Note also that none of the wet-dry seasons were one of the "no-Niņo" years discussed in "Winter 2000-2001 Precipitation & Temperature Climatology and Outlook California & the US".

Also, remember last year there was only about 50% of seasonal rainfall as of January 10th, before finishing with about 121% of normal!  See San Francisco 1999-2000 Rainfall Season in Review.

November Jet Stream Analysis and Climatology

The position of the Jet Stream plays a key role in distribution of rainfall.  The two graphics in this panel were produced by Steve Holets (Meteorologist, PG&E) and depict the relationship of the November 2000 Jet Stream position to the mean and recent years; and also to the most similar cases since 1948.

Nov. 2000 Jet Stream Position vs. Mean, 1999, 1998:  MeanJet.gif (15908 bytes)

Nov. 2000 Jet Stream vs. Most Similar Cases:   Top5Jet.gif (11119 bytes)
(1948, 1952, 1958, 1964, 1977)

San Francisco Rainfall (inches)

Year 1948 1952 1958 1964 1977 2000
Nov. 1.18 2.42 0.09 3.99 2.22 0.85
Season Total 18.28 21.10 10.46 22.29 26.87 ??
1948-1999 Nov. Mean = 3.00 inches
1948-1999 Seasonal Mean = 21.51 inches