30-Day Weather Outlook for April 24, 2017, to May 24, 2017

  • Apr 25, 2017

Summary- There is a tendency for troughing near and just west of N California, before and after May 1st. 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 early June near and over the Calif coast. Forcing for troughs over western Mexico will continue. A classic, moderate to strong El Niño anomaly pattern is expected to develop per the current SSTA pattern and the NMME (North American Multi-Model Ensemble): Time frame- later in the summer and in autumn.

In the Near Term – April 28 - May 11…  In the southern California avocado growing areas from Santa Barbara County to San Diego County, periods of partly cloudy conditions occur on April 30th and May 1 with minimal, if any, showers. Cold mornings occur on April 27-30th, and May 1-2. Otherwise dry and warmer conditions occur.

Summary – May 12-31… In the southern California avocado growing areas, from San Luis Obispo south, there is risk of a return to cool and showery conditions across central and S Calif during May.  This currently appears most likely about May 21-26. Temperatures averages a little above normal in SOCAL overall. The rainy or showery periods will most likely be cooler than normal, while the intervening dry and warm periods may be well above normal in May (lack of coastal low clouds). 

Seasonal Outlook / El Niño Update...June 1 – July 15, 2017… Troughing is still possible to some extent, with some drizzly mornings in early to mid-June. After hot periods in May, June is closer to normal temperatures. 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 SierraNV. Troughing off N Calif may start to 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, colder than normal sea surface temperatures SW and S of southern Baja Calif may inhibit tropical cyclone development in the first part of summer, i.e. late May thru July. Cyclones that develop will tend to move into Jalisco/Colima/Michoacan during June. It will most likely be July before cyclones maintain intensity when moving WNW towards southern Baja.

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. During this past February – April 2017, we had heavy rains in N and central Calif, and heavy snow with an exceptional snowpack in the SierraNV, but without significant contribution from El Niño or La Niña.

For rainy season (winter) 2017-18, there is an active El Niño coupled with the residual SSTA pattern associated with the current troughing of this late winter and spring. The bottom line is that when both El Niño (ENSO warm phase) and the mid-latitude SSTA pattern begin working together as the next rainy season begins, after opposing each other in the previous two years, we may have a better chance for an upcoming wet winter in 2017-18 for all of Calif.

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

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