Summary- Strong, stationary, warm upper high pressure continued across Calif on March 13, but will weaken on the 14th. We are seeing a shift to a wetter pattern with resumption of a broad belt of westerlies across most of Calif in by March 18-19. The troughing pattern is bi-modal. Most troughs develop near the coast of northcentral and N Calif, but a few develop in the Great Basin and Rockies area. Forcing for troughs occurring from the N and Calif coast and WSW-ward is from the sea surface temperature anomaly (SSTA) pattern W of Calif. Forcing for troughs over the Great Basin is related to the currently weak El Niño. The combined effect has been wet in N and northcentral California, and drier than normal in southcentral and S Calif, however this is changing to mostly wet in N and central Calif. We still expect occasional southern storm track conditions, with showers and low snow levels in the Sierras in late Mar and Apr.
In the Near Term – March 18-31… The NOAA/CPC outlook for 21-28 Mar shows wetter than normal conditions in all of Calif, with cooler than normal at the coast. This is a change, but a believable one, from the recent very warm period.
In the southern California avocado growing areas from Santa Barbara County to San Diego County, there will be dry and warm days, with an offshore flow at times. Cold mornings occur in the valleys with frost risk, even if foothills are warm. The best chance for rains to start would be the 19th. Other rains are indicated at times during 21-27th. Watch for a frost briefly about the 27th or 28th, depending on clouds and timing of frontal passages.
Summary – April 1-15… In the southern California avocado growing areas, from San Luis Obispo south, the southern storm track is still present, and able to induce development of cutoff lows or fronts. There are showery episodes which appear minor. April currently has below normal temperatures for southcentral Calif, and some of this could also affect S Calif.
Seasonal Outlook / El Niño Update...April 15 – May 31, 2017… El Niño contribution will continue to some extent, although we’re coming out of the season for El Niño contributions, in general. The usual result would be a little more troughing off Baja and SOCAL, and cont’d offshore flow events San Luis Obispo coast and central Calif. We still have the mid-latitude sea surface temperature anomaly (SSTA) pattern to encourage troughs near the coast of central and N Calif. Currently, in agreement with CFSv2 guidance, we look for a better than average chance for a wet April thru N and northcentral Calif, and in Ventura Co, as well as some late season showers into San Diego Co in April and early May.
Current guidance for temperatures continues near or a little cooler than normal across central Calif and the Sierras. This is good news because it will tend to maintain the current snowpack well into mid-spring.
For SOCAL, April has recurrence of the trend for cooler and possibly a little wetter than normal for San Diego/Orange-San Bernardino Counties.
...Alan Fox, Fox Weather LLC...