30-day weather outlook for September 24, 2018 – October 24, 2018

  • Sep 24, 2018

Summary- Late September through the first half of October is a transition time from summer high pressure to a stronger onshore flow. This year should be no exception as a strengthening of the westerly flow is expected over the next few weeks.

Some high pressure which is also typical of early fall is expected at times. The latest models bring high pressure from Sept 25th through the 27th and again from Oct 8th through the 12th. Warmer than normal temperatures are expected with dry conditions during these high pressures.

Sea surface temperatures (SSTs) continue warmer than normal off the coast of southern California and west of Baja, Mexico. This will give an abundant moisture source with higher evaporation expected off the coast. Tropical cyclones or upper lows that develop off the west coast of Mexico should be wetter and potentially stronger due to these warmer SSTs.

An El Niño is still likely with warmer than normal SSTs from late fall through mid-winter. This is the time we see greater influence from El Niño (warm eastern equatorial SST anomalies).

Rains still appear weak through the region but a couple of upper lows/troughs are forecast to bring some rain into various parts of California over the next 30 days. An upper low will move into northern California from Sept 29th through the 30th bringing a chance of light rain to mainly northern Calif and the northern part of central Cal. Another trough will bring more light rain to mainly northern California from Oct 2nd through the 4th.

This is the season for the highest tropical activity on average through the eastern Pacific with this year looking to be near normal or above normal in activity. Tropical depressions, and at times stronger tropical storms and hurricanes are forecast to bring above normal rain to northern Mexico and Arizona with some of these showers and thunderstorms possible into southern Calif at times. One tropical depression will bring a chance for significant rains to the deserts of southern California and through Arizona about Oct 2-4.

As tropical systems are more active, the westerlies in general may be a bit farther to the north through the next 30 days with less than normal precipitation through northern California and a probable late start to winter rains along the northern California coast.

In the Near Term – Sept 30 – Oct 13…Salinas Valley-San Luis Obispo Co…Cool but dry with an upper low moving to the north on Sept 29th and 30th. A little light rain in the northern-most areas are possible with this system. Another trough will be dry but will keep temps cooler than normal from Oct 2nd through the 5th. High pressure builds from the 7th through the 13th with dry and warmer conditions.

…S Calif Avocado Area, San Luis Obispo Co to San Diego Co…High pressure remains mainly to the south but warmer sea surface temps will continue to build the high into southern Calif with warmer than normal temps continuing thru the middle of October.

The tropics are becoming more active in the eastern Pacific with moisture from a remnant tropical system likely arriving into southern Calif around Oct 2nd or 3rd with heavy rain possible in Arizona and S California’s southern mountains and deserts.

Another tropical system is possible towards Oct 12th or 13th.

Summary – Oct 14 – October 28… In the southern California avocado growing areas, from San Luis Obispo south, high pressure will dominate the region but along with the high pressure will be some subtropical moisture streaming into the region from remnant tropical storms and systems. With the warmer than normal SSTs there will be an increase in tropical activity off the coast of Baja Mexico with remnants from some of these systems bringing moisture into southern Calif.  Looking seasonal to slightly wetter than normal during the last half of October as the upper lows and tropics will be active with the warmer than normal SSTs.

Seasonal Outlook / El Niño Update...Oct 20 – Dec 31… Occasional weak storms will bring some rain across mainly northern Calif with some remnant tropical activity into southern Calif possible in late October, possibly into early November. But the westerlies will remain weak with likely a late start to the main northern and central California winter rains.

Most of the state will stay below normal for precipitation through November with near normal or slightly below normal rain likely for December. Near normal or slightly higher than normal precipitation is possible for SE California due to an increase in remnant tropical moisture in the area which should continue through October and possibly the first half of November.

Seasonal to warm temperatures across northern Calif will continue through the end of the year, especially through November with some cooling to near normal in December. Warmer than normal temps are expected across southern Calif likely through the middle of December.

For southern California, some upper lows and remnant tropical system and moisture are possible at times through the end of October with a more active than normal end to the tropical season across the eastern Pacific. This is likely due to the warmer than normal SSTs off the Mexican and southern Calif coast. Turning drier later in November with a dry pattern likely through most or all of December.

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

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