30-Day Weather Outlook for April 9, 2018, to May 9, 2018
Summary- Most rains focus on the SierraNV (rain and snow), and N California as far S as the Bay Area. Troughs and fronts alternate with periods of warm dry upper high pressure from April 13th through the 26th. Our CFSDailyAI system suggests some showers in the central and N SierraNV from April 28-30th, then into S California on May 1-2. Another rain with some TSTMS is suggested in the SierraNV from May 5-9.
Support continues in the mid-latitude Pacific sea surface temperature field for high pressure near 145 160W 37-43N and a trough near 135-128W.
Warm sea surface temperature anomalies persist off to W and SW of San Diego. This will tend to provide moisture for upper lows troughs, and fronts that develop or move mainly into central California/SierraNV.
In the Near Term – April 13-27… In the southern California avocado growing areas, from San Luis Obispo County to San Diego County, the cold sea surface temperature anomaly off N California may increase the risk of cold nights in the normally coldest valleys on about April 13-15th, and 19-22nd. The storm track will be migrating gradually further north, so rains of consequence in SOCAL are unlikely.
During the periods of troughing, a coastal eddy and cool days with morning drizzle are more likely this time of year. Cool days with best chance for drizzle would be April 23-24, 28th – May 1.
Summary – April 27 – May 9… In the southern California avocado growing areas, from San Luis Obispo south, a cool period with showers is suggested for May 1-2, followed by a warmer period from the 3-4th as the low moves east. The next period of showers or deep marine layer with coastal drizzle occurs about May 6-10th.
Seasonal Outlook / La Niña Update...May 10 – September… This report is longer than usual. However, the Aug-Sept period deserves special coverage.
A basically normal pattern of minimal rainfall occurs during this period for California as a whole. There is no significant anomaly of precipitation indicated until the Aug-Sept period. During Aug and Sept, the effects of the warmer than normal sea surface to the W, SW, and S of SOCAL may produce higher risk of rainfall from tropical waves in general, including tropical cyclones.
The positive rain anomaly is currently indicated by both CFSv2 and NMME in Sept for W Mexico, Baja California, and SOCAL to Arizona, as a general trend.
Keep in mind, however, that rains into California from tropical cyclones are sensitive to movement and behavior of cutoff upper lows near central and S California. Cutoff lows behave and move erratically. All that can be said at this point is that if sea surface temperatures continue warmer than normal through the summer, we will have a warm and humid Aug and Sept period. There will be more ambient moisture available to produce rains, if upper lows develop. Warmer than normal sea surface off Baja California would provide additional moisture and energy to maintain tropical cyclones that move northwestward near Baja California during the normally active Aug- Sept period.
In general, summers such as this, with warm sea surface temperature anomaly, tend to have more interruptions in the normal marine layer, resulting in hot periods near the coast and coastal hills.
...Alan Fox, Fox Weather LLC...