30-Day Weather Outlook for April 2, 2018, to May 2, 2018

  • Apr 05, 2018

Summary- Fronts come through with significant rains on April 5-7th, rains on 10th, showers in N California on 12th, and some more showers in the N SierraNV and N California on the 15th and 18th.

Support continues in the mid-latitude Pacific sea surface temperature field, for high pressure near 145-160W 37-43N, a trough near 135-128W with a shift of active westerlies further north to focus on NW California and Oregon.

Warm sea surface temperature anomalies persist off to W and SW of San Diego. This will tend to provide moisture for upper lows troughs, and fronts that develop or move into California from the W.

In the Near Term – April 6 - April 20… In the southern California avocado growing areas, from San Luis Obispo County to San Diego County, chances for showers return about April 6-7th, then again about 10-11th. Although some rains are indicated on the 6-7th the heaviest rains remain north of the Thomas Fire burn area (Santa Barbara Co).

The cold sea surface temperature anomaly off N California may increase risk of a cold night in the normally coldest valleys in early to mid-April, during clear periods between fronts and rains. Rains are possible about the 6-7th, 10-11th, mainly from Ventura Co north, but only showers or sprinkles in southern counties, including Riverside, Orange, and San Diego.

Summary – April 21 – May 5… In the southern California avocado growing areas, from San Luis Obispo south, rain appears unlikely during this period. The best chance for cold nights occurs during April 22-25th followed by a general transition to hot conditions in the foothills and inland valleys and SOCAL deserts during April 25-30th – May 5.

Seasonal Outlook / La Niña Update...May 6 – June 10… Below normal rainfall is suggested by CFSv2 for May. After the hot period around the first few days of May, in southern California we should turn fairly quickly into the usual marine stratus pattern while it heats up inland. This means basically night and morning coastal low clouds and fog, partly cloudy or mostly sunny and mild afternoons near the coast of southern and central California/San Luis Obispo Co. As southern California experiences coastal eddies and marine layer conditions, N California and the Bay Area tend to have more sunshine.

With hot conditions expected inland and in the foothill areas of central California and SierraNV, snowmelt will tend to accelerate during May 5-20th in the central SierraNV.

In the first week or so of June, CFSv2 shows a tendency for upper lows and troughs near the N California coast. This trend is consistent with the prevailing sea surface temperature anomaly pattern. If this area of above normal rainfall occurs, as suggested in June, it may associate with above normal thunderstorm (TSTM) activity, at the beginning of the hot and dry season. This is currently shown to follow a hot and dry May.

For S California it appears to be warmer and drier than normal inland during this period. However the hot conditions inland may tend to increase onshore flow into the coastal valleys, resulting in mild conditions near the coast.

May is the season for fog and stratus in coastal and valley areas. The usual fog/stratus regime will be occasionally interrupted by a warm or hot day, with minimal fog at the coast, and shallow marine inversion.

Warm and dry conditions in May are followed, according to CFSv2, by some possible troughing during the first week or so of June. This troughing would tend to support TSTMS in N California, but a deeper marine layer and seasonally cool coastal marine layer conditions in SOCAL coast and valleys.

Looking out further into the summer, NMME (ensemble) guidance is suggesting conditions that support an unusually hot summer. If warmer than normal conditions develop in mid to late June, some of the hottest days of the year can occur in the SOCAL deserts. These are supported by a high solar angle, and long periods of daytime heating during high sun, together with the highest probability of possible sunshine.

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

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