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30-Day Weather Outlook for April 10, 2017, to May 10, 2017

Posted: April 11, 2017

Summary- The tendency for troughing near and west of N California continues, even as we approach the normal end of the season.  The sea surface temperature anomaly (SSTA) pattern in the central N Pacific is strong and persistent enough to continue the tendency for troughing through May and possibly into June.  Forcing for troughs over the southwestern US may still be related to the currently weak El Niño, although seasonal factors that affect El Niño’s influence decrease with approach of summer.  A classic, moderate to strong El Niño anomaly pattern is starting to develop, as shown by the current SSTA pattern, and the forecast developments shown by the North American Multi-model Ensemble (NMME) in the early fall.

In the Near Term – April 14-28…  The NOAA/CPC outlook for April 18-24 shows a cooler and wetter than normal condition in N and central Calif, and continued occasional rain and snow events for the Sierras. Wet periods cluster around April 13th and again on the 16-20th.

In the southern California avocado growing areas from Santa Barbara County to San Diego County, the heaviest rains remain north of SOCAL.  A few light showers are possible on April 13th in W Santa Barbara Co to Ventura Co. Showers again develop about April 18-20th in S Calif.

Summary – April 29 – May 13… In the southern California avocado growing areas, from San Luis Obispo south, rain, other than the usual marine drizzle, is unlikely in late April – early May.  The southern storm track still is expected to redevelop, but its influence will focus further north, into central Calif, and is less likely in SOCAL.

Seasonal Outlook / El Niño Update...May 13 – July 15, 2017… Troughing is still possible to some extent, with some drizzly mornings in late May, and again about June 7-12.  El Niño continues to strengthen during the early to mid-summer.  In addition, the SSTA pattern in the mid-latitude NE Pacific will continue to support troughing near the coast of Calif in late May and early June.  Troughing near the Calif coast N of the Bay Area will tend to encourage showers and TSTMS to develop in the Siskiyou’s, NW Calif mountains, and in the N Sierras. Troughing off N Calif will come into phase with the summer monsoonal SE flow from Mexico during  the first part of July. This would tend to encourage TSTMS in Yosemite and central SierraNV areas.

For the hurricane season west of Mexico, it currently appears that there is a better than even chance for tropical cyclones to develop off central America, then turn N and NNW, going into S Baja or Jalisco coastal areas/Manzanillo.

A few preliminary comments are in order regarding the next wet season: Nov 2017-Mar 2018.  During  the last two seasons, we saw, in 2015-16, a moderate to strong El Niño that encountered the opposing influence of the SSTA pattern in the north Pacific west of Calif.  The result was an inconsistent response to El Niño, and above normal rainfall in N Calif, but below normal rainfall in S Calif. This season 2016-17, we had heavy rains in N and central Calif, and heavy snow with an exceptional snow pack in the Sierras, but a normally dry La Niña pattern. The La Niña was characteristically wet in N Calif, but was also wet in portions of SOCAL. 

For rainy season (winter) 2017-18, there is a strong El Niño coupled with the residual SSTA pattern associated with the current troughing.

The bottom line is that with both El Niño (ENSO) and the mid-latitude SSTA pattern working together, finally, after two years at odds with each other, we may have a better chance for an upcoming wet winter through all of Calif in 2017-18.

...Alan Fox, Fox Weather LLC...