30-Day Weather Outlook for November 13, 2017, to December 13, 2017
Summary- The MJO Cycle is beginning the transition from a dry (inactive) phase to favor more active troughing into California and the far west US. The transition should be completed between Nov 17-18th. Support exists in the sea surface temperature anomaly (SSTA) field for a long wave trough in the central N Pacific around 150W, and a second trough near 125-130W.
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 through Nov 21st to an ill-defined pattern, supporting high pressure aloft again to the west of California. The high pressure area appears to continue through the end of Nov, with subsequent fronts and rains redirected into the Pacific Northwest, while California is unusually warm and dry.
For S California, the pattern through Nov 22 will support some cold fronts with rains through about the 21st, but without 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. 17 – Dec. 2… In the southern California avocado growing areas, from San Luis Obispo County to San Diego County, it will be mostly fair with frost/freeze risk in mainly the Edna Valley, Sisquoc, Santa Ynez, and possibly Ventura Co valleys (Ojai etc.) to Paicines, and portions of the Salinas and Pajaro Valleys on Nov 17 -18th.
Rain is possible about Nov 20th with showers ending on the 21st. Significant upslope rains are mostly limited to the Santa Ynez Mountains north, with only light rains in SOCAL proper. Dry and unusually warm with Santa Ana winds possible during Nov 22-23rd, and again on the 26-27th. Continued unusually warm on the 28th. Some rains are possible in Santa Barbara and San Luis Obispo Co’s on Nov 29-30th.
Summary – Dec. 3-20… In the southern California avocado growing areas, from San Luis Obispo south, our CFSDaily guidance suggests possible frontal passages with rains or showers on Dec 4th, and Dec 19-22. Under the usual conditions, we would expect Santa Anas to develop around Dec 6-9th, and sometime in mid-Dec.
Seasonal Outlook / La Niña Update...December 21 – February 15, 2018… Latest solution show either mixed or possibly above normal rainfall in Dec in N and northcentral California, but drier than normal in S California. Jan 2018 solutions are near or drier than normal from both the NMME (ensemble) and CFSv2 forecast 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 Jan. Be ready for frosts on multiple days in a series. Most of our guidance for SOCAL suggests significant rains in the mountains from Santa Barbara Co through L.A. Co and the San Bernardino mountain areas focusing in Jan and Feb 2018.
NMME guidance is showing drier than normal for N California in Jan but near or below normal rain in Feb.
...Alan Fox, Fox Weather LLC...