30-Day Weather Outlook for August 29, 2017, to September 29, 2017
Summary- Strong upper high pressure continues through September 8, and possibly the 9th, producing hot conditions. Hottest conditions are September 1-6 in all of Calif, and 3-8th in N California. Model guidance suggests a tropical cyclone coming N to central Baja California on Sept. 3-5th then stalling southwest of S California. Moisture mostly remains or drifts at sea and turns westward, but some may drift into central California coast/Big Sur area, and SierraNV. Normal early September rainfall is indicated during this period for Arizona.
Warmer than normal sea surface temperatures continue throughout the eastern N Pacific, and in the area of tropical cyclone formation off Mexico thru mid Sep. This places us at risk for subtropical moisture incursions into SOCAL and Arizona.
A trough is expected to develop in the interior Western US and California on Sept. 10-12th, with much cooler conditions and a chance for rain in N California, and mountain areas of N and central California and SierraNV.
Support exists in the sea surface temperature anomaly (SSTA) field for recurrent upper low pressure and frontal developments/rains off Oregon and Washington. Some of these rains may reach the coast with a chance for above normal rainfall in NW Oregon and W Washington.
In the Near Term – Aug. 31– Sept. 14… In the southern California avocado growing areas from Santa Barbara County to San Diego County, there will be patchy coastal low clouds or fog mainly from Aug. 31 - Sept. 1, otherwise partly cloudy and hot with TSTMS in the mountains and deserts. Best chance for TSTMS would be Sept. 1-9.
Summary – Sept. 15-30… In the southern California avocado growing areas, from San Luis Obispo south, deep marine layer and cool drizzly conditions are suggested for Sept. 15-17. Hot weather returns with a possibly very hot Santa Ana occurring from Sept. 18-22. Continued hot and dry 23-26th, then cooler with deeper marine layer returning to the coastal zone and breezy conditions in the mountains from 28-30th.
Seasonal Outlook / El Niño Update...October 1 – December 30, 2017… Wet conditions occur in NW California and the Siskiyou’s at times during Oct, then dry and warm conditions return in Nov. 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...