30-Day Weather Outlook for November 6, 2017, to December 6, 2017
Summary- The MJO Cycle is transitioning from a troughing (wet) phase to a dry one. The transition should be completed between November 14th and 17th. Support exists in the sea surface temperature anomaly (SSTA) field for a long wave trough in the central N Pacific around 160-170W, and second trough near 130W.
The El Niño Southern Oscillation (ENSO) is currently in a stronger ‘La Niña’ phase, i.e. colder than normal sea surface along the east equatorial Pacific to Peru. This favors weaker than normal support for the subtropical jet that normally forms in early to mid-winter, i.e. Dec – Feb.
For N and central California we expect a transition from troughing SW of central California to an ill-defined pattern, with high pressure becoming predominant aloft again to the west of California. The high pressure area will be interrupted at times by troughs and fronts with below normal rainfall.
For S California, the pattern through Nov 22 will support some cutoff lows or dry cold fronts, but there is not much support for significant rains. The warm sea surface temperatures are present, but the pattern is unfavorable for development of subtropical westerlies, or lows and troughs into S California from the W or WSW.
In the Near Term – Nov. 9-26… In the southern California avocado growing areas, from Santa Barbara County to San Diego County, showers will most likely be sparse on Nov 10th. Frost/freeze risk is higher on Nov 11th, 14th and 15th following the frontal passages. The fronts on Nov 10th and 13th are either dry and breezy, or have sparse or low rain amounts. Drier and warmer on Nov 15-16th with some warm NE winds, then another dry NE wind pattern at times during Nov 18-25th. Watch for freezes or wetbulb freezes in the calm periods prior to the start of Santa Ana winds, or after the winds diminish.
Summary – Nov. 27 – Dec. 18… In the southern California avocado growing areas, from San Luis Obispo south, our CFSDaily guidance suggests possible frontal passages with rains or showers on Nov 30th-Dec 1st, 4-5th, and 11-12th. Under the usual conditions, we would expect Santa Anas to develop around Dec 6-9th, and sometime in mid-December.
Seasonal Outlook / La Niña Update...December 19 – February 10, 2018… Latest solutions show mostly above normal rainfall in Dec in N and central California, but drier than normal in S California. Jan 2018 solutions are near or slightly drier than normal from both the NMME (ensemble) and CFSv2 forecast model solutions. For S California, there is some support for light showers, then frosts, occurring between Santa Ana events, with upper lows that develop over Arizona and SE California during several periods in mid to late Dec through most of January. Be ready for frosts on multiple days in a series.
NMME guidance is showing a little below normal temperatures in N California in Jan but warmer than normal in February.
February continues the drier than normal trend over nearly all of California except the NW coast.
...Alan Fox, Fox Weather LLC...