30-day weather outlook for August 14, 2018 – September 14, 2018

  • Aug 14, 2018

Summary- Strong upper high pressure will continue over the central Great Plains WSW to central and S California. This supports monsoonal moisture into S California at times.

Troughs and upper lows will form near Washington, and off the N California coast. SW or WSW flow is mostly dry at the coast of Oregon and California.

Upper high pressure will tend to develop south of Alaska and into the eastern Gulf of Alaska at 45-55N, 140-152W. This recurrent pattern will tend to encourage troughing near the Pacific Northwest and California coasts, and on brief occasions into the northern Great Basin -Idaho.

Recurrently warmer than normal sea surface off Mexico, and strong high pressure in the SW U.S. will tend to support tropical cyclones. Moisture will be steered near SOCAL during this more active part of the season.

Although less hot than in the recent past; dry, breezy conditions with higher risk of wildland fires will continue into Sept.

As troughs develop further south in late Aug and Sept, there appears to be increased risk for thunderstorms (TSTMS) in the Sierras. TSTM-related ignition of fires in the Sierras will tend to increase.

The El Niño (warmer than normal sea surface) along the equatorial central and east Pacific, continues to show some development through the fall.

In the Near Term – Aug 16 – Aug. 31…Salinas Valley-San Luis Obispo Co…It will be hot from Aug 18-24th then cooler from the 25th – Sept 1st. Deeper marine layer and coastal drizzle are possible in coastal valleys during the 27th – 1 Sept.

…S Calif Avocado Area, San Luis Obispo Co to San Diego Co…Sea surface temperatures along the SOCAL coast will continue to be unusually warm, with temperatures in the mid-70s still possible. Generally, very warm conditions occur through Aug 24th, then it turns cooler with a coastal eddy from the 25th through Sept 1. There is a chance for some warm rains into the SOCAL coast to mountains areas on Aug 31-Sept 1-2. Otherwise, precipitation consists of mostly drizzle accompanying coastal eddies during the cooler periods. A few TSTMs reach westward from Arizona at times during Aug 16-24th, affecting the mountains and deserts.

Summary – Sept 1-15… In the southern California avocado growing areas, from San Luis Obispo south, it will be cool with light drizzly rain possible from a tropical rain event on Sept 1-2, then it turns dry and warmer during Sept 3-4. Cool again with another drizzly period from the coast to mountains areas on the 8-9th due to another tropical rain event. Dry and warmer from Sept 12-15th.

Seasonal Outlook / El Niño Update...Sept 15 – Dec… We are already seeing warming of the equatorial sea surface in the far east Pacific near Peru/Ecuador. This trend is expected to continue. The El Niño, (warm phase of the ENSO pattern), typically adds extra energy to drive the subtropical westerlies during the winter rain season. The effect of El Niño to drive the subtropical westerlies is not limited to winter, but also can start early, e.g in Sep and Oct. In the early part of the season, it tends to assist development of troughs and upper lows along the California coast, and stronger than normal cutoff lows near S California. Because the sea surface this year is significantly warmer (+4 to +5F) than normal off Baja, some of those cutoff lows have greater potential to produce significant rains during the early season, including Oct.

For Nov and Dec, we see the possibility of below normal rainfall for N and northcentral California, but above normal in S California.

For southern California, occasional surges of tropical moisture are possible, especially as troughs dig further south to tap remnants left by tropical cyclones near Baja in Sept – mid Oct. Because of proximity to tropical cyclones, SOCAL may see some rains in mid to late Sept as troughs deepen southward near the coast. Near or perhaps a little above normal rainfall could occur in SOCAL in late Sept.

The probability of rain from cutoff lows is greater in Oct than Sep.

Usually we see a Santa Ana or two in Oct. However, with expected El Niño forcing, the chances for Santa Anas may be lower during this Oct due to upper lows, and some attempts to develop a southern storm track.

There continues to be some suggestions of development of early season rains into S California during Sept, sometime in Oct, mid Nov, and in Dec.

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

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