30-Day Weather Outlook for August 22, 2017, to September 22, 2017
Summary- A trough in N California on August 25th will be replaced by persistent upper high pressure and hot conditions on August 27th – the first part of September. The hottest conditions are from August 27-30th, and September 3-5, and in N California from September 5-9. Model guidance suggests a tropical cyclone coming N to central Baja California on September 1-2, then turning inland over Sonora and producing a potential for heavy rains into E Arizona-New Mexico and SW Texas on September 4-5th. Warmer than normal sea surface temperatures continue throughout the eastern N Pacific, and in the area of tropical cyclone formation off Mexico thru mid-September. This places us at risk for subtropical moisture incursions into SOCAL and Arizona for this period. 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. 24 – Sept. 6… In the southern California avocado growing areas from Santa Barbara County to San Diego County, there will be night/morning coastal low clouds or fog mainly from August 24-26th, otherwise partly cloudy and hot with TSTMS in the mountains and deserts. Best chance for TSTMS would be from September 1-7.
Summary – Sept. 5-20… In the southern California avocado growing areas, from San Luis Obispo south, TSTMS occur on September 6-7 in SOCAL mountains and deserts. Well above normal temperatures prevail.
Seasonal Outlook / El Niño Update...September 20 – November 30, 2017… TSTM activity occurs in the SierraNV during September 20-30. Wetter than normal conditions develop in Oct, followed by a dry Nov.
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, and a turn to wetter than normal in Oct for NW Calif in late fall-early winter.
It appears that, at least in the current set of forecasts from CFS, there is support for cutoff lows, and this throws inconsistency into the pattern. There are some indications of cutoff low activity over Arizona, which would favor dry N wind events in SOCAL with dry and hot conditions occurring in mid-October.
...Alan Fox, Fox Weather LLC...