30-Day Weather Outlook for February 27, 2017, to March 27, 2017
Summary- Westerlies occur across most of Calif. The troughing pattern is bi-modal, with some troughs developing just off the coast of central and N Calif, and some focusing into the Great Basin. Forcing for troughs occurring from the Calif coast southwestward is from the sea surface temperature anomaly (SSTA) pattern W of Calif. Forcing for troughs over the Great Basin are related to the current new El Niño that has begun. Periods of WNW flow with troughing occur, bringing more occurrence of cold conditions with lower snow levels into N and central Calif, due to troughing over the Great Basin. Individual troughs contain moisture but lack strength as they reach the coast, but intensify while moving inland. This new pattern of development, related to El Niño favors development of troughs in S and SE Calif, NW Mexico, and Arizona, and a colder and occasionally showery pattern in central and N Calif. This maintains a colder than normal pattern with periods of snow showers thru the Sierras. As El Niño becomes more active during late March thru early May, the southern storm track continues. The southern storm track will induce showers and continued snowy conditions in the Sierras in late March thru April. The southern storm track will tend to support the development of severe weather events (thunderstorms and tornadoes), with above normal rainfall in Oklahoma, north Texas in April and Kansas-Nebraska in May.
In the Near Term – March 2-16… After the rains that we expect thru March 7th, the NOAA/CPC outlook for March 7-13 shows drier and warmer than normal for all of Calif, with a trough W of Calif, a weak ridge of high pressure over the N Great Basin, and a weak southern branch of the westerlies thru S Calif to southern Rockies. We suggest that the warm and dry pattern for Calif will not last that long, and revert to wet for central and N Calif about March 9-15th.
In the southern California avocado growing areas from Santa Barbara County to San Diego County, rains or showers occur from March 4-5. It will be dry and warmer, but with some cold mornings in the valleys about March 7-8. Periods of showers are suggested for March 9 and 13-15.
Summary – March 17 – 31… In the southern California avocado growing areas, from San Luis Obispo south, it appears to be mostly dry, but with some rains about March 27-28. The best chance for frosts would be about the 17-19, then a warmer period develops. The oddity unique for SOCAL is the southern storm track which may generate more periods of showers and troughing than we currently see in the numerical guidance.
Seasonal Outlook/ El Niño Update...April 1 – May 31, 2017… El Niño contribution will continue to increase, inducing troughs in S and central Calif and Arizona. We still have the mid-latitude SSTA pattern that will tend to support troughs near the coast of central and N Calif. Currently, in agreement with CFSv2 guidance, we look for a better than average chance for a wet April thru N and central Calif to Ventura Co, as well as some late season rains into San Diego County.
Current guidance for temperatures continues near or a little cooler than normal across Calif and the Sierras. This is good news because it will tend to maintain the current snowpack well into mid-spring with slightly slower than normal snowmelt.
For SOCAL, in April there will be a recurrence of the trend for it being cooler and possibly a little wetter than normal. May is a little wetter than normal during the first week, then the hot conditions inland may be slow to develop.
...Alan Fox, Fox Weather LLC...