30-Day Weather Outlook for September 5, 2017, to October 5, 2017
Summary- High pressure continues through about September 16 in the Pacific Northwest and Great Basin. Unusually warm or hot conditions continue likely during this period in Oregon and most of California with support for dry conditions in the north. Upper low pressure off California will continue the support for rains in southern and central California from TSTMS, while NW California starts to see some rain from a few fronts arriving from the west.
Model guidance suggests a continued active period for tropical cyclones during the middle part of September. Warmer than normal sea surface temperatures continue throughout the eastern N Pacific, and in the area of tropical cyclone formation off Mexico through mid Sep. This places us at risk for subtropical moisture incursions into SOCAL and Arizona.
Support exists in the sea surface temperature anomaly (SSTA) field for a long wave trough in the central North Pacific near 140W, and high pressure continuing in the interior W and NW US states. This often associates with troughing near southern California.
In the Near Term – Sept. 8-22… In the southern California avocado growing areas from Santa Barbara County to San Diego County, the upper low, after retreating to the N of southern California on September 6th will make another showery visit to the Southland on the 9-10th, inducing TSTMS in the mountains and deserts.
Summary – Sept. 23 – Oct. 6… In the southern California avocado growing areas, from San Luis Obispo south, the warmer than normal conditions continue through September 24th, then there is a chance for cooler conditions with coastal drizzle during the 25-30th. Another period of cutoff low activity is suggested for October 2-6. A period of hot Santa Ana winds is possible during October 8-15th.
Seasonal Outlook / El Niño Update...October 7 – December 30, 2017… After a hot and dry period during October 8-15, wet conditions return to NW California and the Siskiyou’s at times during mid to late Oct, followed by a seasonably wet November. After a dry break during the latter half of November, wetter than normal conditions develop during the middle half (10-25th) of Dec.
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, during late Dec and into January 2018.
...Alan Fox, Fox Weather LLC...