30-Day Weather Outlook for September 12, 2017, to October 12, 2017
Summary- Upper low pressure near central and southern California will continue some support for showers in the SierraNV, and possibly also in southern California. Another tropical cyclone is suggested to move into W Mexico on September 17-19th, producing rains in Sinaloa/Sonora, thence into S Arizona and New Mexico. The active trough in California will tend to steer the tropical cyclone into W Mexico and New Mexico rather than into S California.
Support exists in the sea surface temperature anomaly (SSTA) field for a long wave trough in the central North Pacific near 140W, a robust storm track developing from E of Hawaii to the British Columbia coast, but high pressure continuing in the interior W and NW US states. This often associates with troughing near southern California.
In the Near Term – Sept. 15-28… In the southern California avocado growing areas, from Santa Barbara County to San Diego County, it will be cool with troughing and showers from September 13-16th and again from the 18-20th with coastal drizzle and deeper marine layer developing. Hot and dry Santa Ana conditions develop on September 24-25th, followed by the return of troughing and a deeper marine layer again with coastal drizzle.
Summary – Sept. 29 – Oct. 15… In the southern California avocado growing areas, from San Luis Obispo south, a cool period with deeper marine layer and drizzle/light rain or light showers are possible during most of October 1-11th. A period of hot Santa Ana winds is possible during October 13-16th.
Seasonal Outlook / El Niño Update...October 16 – December 31, 2017… After a hot and dry period during mid to late Oct, wet conditions return to NW California and Siskiyou’s at times from the end of Oct through most of Nov. Best chances for rains in Nov are usually during 10-20th, and again around 28-30th. A period of heavy rains and storm events often occurs during December 18-23rd and 27-30.
A La Niña pattern has appeared in the sea surface temperature anomaly field, and this supports a shift to drier than normal conditions in central and S Calif around the end of Dec and into January 2018. There is a suggestion of colder than normal conditions developing for California in early to mid-January 2018.
...Alan Fox, Fox Weather LLC...