30-day weather outlook for July 9, 2018 – August 9, 2018

  • Jul 10, 2018

Summary- High pressure is expected to continue in the northcentral and northeast Pacific and W of N California and Oregon 140-150W. Closer to the California coast, recurrent troughing is indicated, focusing off SOCAL.

There continues to be a risk of very hot conditions alternating with very warm and humid weather due to strong upper high pressure in the SW U.S, and weak low pressure off the central and S California coast. Monsoonal moisture and thunderstorms (TSTMS), will increase, following the normal seasonal progression in late July and first part of August.

High pressure will continue to produce very hot and dry conditions over most of Mexico and all of Central America during late July through about 20 August.

Sea surface temperatures have been colder than normal off Baja California, but a shift to unusually warm anomalies (SSTAs) appears to be returning. 

The El Niño (warmer than normal sea surface along the equatorial central and E Pacific, is developing. This long term process is expected to continue through the fall and upcoming winter.

In the Near Term – July 14 – July 28…Salinas Valley-San Luis Obispo Co…Dry and hot in the coastal valleys and foothills most days, with tendency for recurrent shallow marine layer or breaks in the coastal low cloud/fog regime. Monsoonal TSTMS are possible July 12-13, and 17-23rd in the central SierraNV.

…S Calif Avocado Area, San Luis Obispo Co to San Diego Co…Continued well above normal temperatures through end of July. We had the extremely hot event last weekend that broke records.  We had a report from Carpinteria of the temperature at 2am on July 8 of 100 degrees within 6 miles of the beach, combined with gusty northerly (sundowner) winds. Although extremely hot conditions such as this near the coast are not normally indicated, we continue at risk for some very hot conditions, as strong upper high pressure with E flow aloft recurs, and squashes the marine layer to a minimal zone of fog at the immediate coast at times during the rest of July.

Yes, the climate has been changing to warmer, and we should adjust expectations, crop practices, and water supply requests accordingly.

Summary – July 30 – August 15… In the southern California avocado growing areas, from San Luis Obispo south, dry midsummer conditions prevail in coast and valley areas.

Monsoonal TSTMS increase through Arizona, and come through the S Deserts and into the SE SierraNV, and San Bernardino Co mountains and deserts. TSTMS with rain are most likely in the southern and eastern Deserts, lower Colorado River area, mountains of E San Diego Co north to the Mt San Gorgonio area, and possibly Palm Springs-Rancho Mirage/San Jacinto Mountains during this period.

Seasonal Outlook / El Niño Update...August 15 – October 15… A few comments about the upcoming (Oct-Feb) El Niño pattern suggested for this winter’s rain season.  Guidance from the NMME (North American Multi-Model Ensemble) for Oct 2018 through Feb 2019 suggests a marginally wet but unusually warm El Niño.  This translates to some periods of heavy rains in Nov, Dec, and late Jan-Feb.  The bottom line-take-home message from the current NMME forecast solution is a few good rains, interspersed with unusually warm periods.  Some rains may also be accompanied by warm subtropical airmasses, producing minimal snowpack.

There has been much discussion about Atmospheric Rivers (ARs), and their association with a warming climate.  Guidance from the NMME suggests that the Bay Area north through Mendocino Co are those areas most likely to receive heavy rains, as are El Dorado Co to Sierra Co, along the SierraNV west slope.

During late Aug through Sept and into early Oct, mostly dry conditions are suggested. Later in the fall and early winter (Dec), movement and landfall of Atmospheric River systems appear a little more difficult to predict during Nov-Dec than in previous seasons.

SOCAL appears to be warmer (hotter) than normal overall in late Aug and Sept. There appears an increased risk for lightning in SOCAL mountain areas in Aug-early Sept. Most tropical moisture will be recurving across NW Mexico and Baja, thence into Arizona. The best chance for above normal rainfall in SOCAL appears to be late Oct-Nov. After Nov, the focus of rains appears to shift back to N and northcentral California, leaving SOCAL drier and warmer than normal (Dec).

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

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