30-Day Weather Outlook for December 4, 2017, to January 4, 2018
Summary- The MJO Cycle has been in an inactive state, but is progressing in a more active state during Dec 5-12th. This may facilitate slightly more active subtropical westerlies developing towards central California, but this is not confirmed by current model guidance. Support continues in the sea surface temperature anomaly (SSTA) field for troughing well to the west of California, centered about 140-150W. This maintains a dry and warmer pattern for California. Shower activity occurring in California still is considered minimal if any.
The El Niño Southern Oscillation (ENSO) is currently in a strengthening ‘La Niña’ phase, i.e. colder than normal sea surface along the equatorial Pacific from Peru. This provides weaker than normal energy to drive the subtropical jet, which normally develops in Dec-Jan.
There is support for troughing and cold conditions in the central and east-central US. There is support for near or above normal snowfall in the Great Lakes area. This supports continued troughing in the eastcentral US through the end of Dec and in Jan 2018. There is a chance that it could lock us into a very dry pattern for most of Jan as well.
For N and central California, if the current pattern of high pressure over and W of central and S California becomes difficult to break, rains will continue to be intermittent in California, and precipitation well below normal in the northcentral and central coast areas, and predominantly dry through the N and Central Sierra during the rest of Dec. There is a slight chance for some snowy periods in the central SierraNV during mid Jan.
For S California, warm sea surface temperatures are present to the southwest, but the La Niña pattern is generally unfavorable for development of subtropical westerlies. Lows and troughs will tend to develop in the Great Plains, and S Rockies/SE California, and recurrent Santa Anas/NE-E flow.
In the Near Term – Dec. 7 – Dec. 21… In the southern California avocado growing areas, from San Luis Obispo County to San Diego County, the strong Santa Anas will diminish, leaving a period of light offshore flow, sunny warmer days crisp cold nights. Watch for frosts/freezes, long nights, short daytime heating period, predominantly dry airmasses through the middle part of Dec.
Summary – Dec. 22 – Jan 6, 2018… In the southern California avocado growing areas, from San Luis Obispo south, for the most part, the December holiday period appears mostly dry at this time. There may be some rains during Dec 28-31st in central or S California from an upper low.
Seasonal Outlook / La Niña Update...January 7, 2018 – February 28… Latest solutions show colder and a possible return to near normal rain and snow in the central SierraNV during mid Jan especially in the Tahoe-Yosemite region. Mostly drier than normal in Jan 2018. The current exception is locally heavier snow in the Tahoe-Yosemite areas, which would be appreciated in an otherwise very dry early winter period. After a brief shot of good snowfall in the SierraNV in January, February currently appears to turn dry again.
In Northern and Central California, of greatest concern is the well below normal temperatures indicated for January through all of N and central California, along with dry conditions. This type of configuration points to cold dry Santa Anas in central California as well as in SOCAL, and accompanying frequent wetbulb freeze events. February is not as cold, but drier, with risk of wetbulb freezes.
For S California, there is support for dry Santa Anas, and accompanying frequent wetbulb freeze events. February is not as cold, but drier, with risk of many wetbulb freezes. Other than a possible showery period or two, associated with upper lows, both January and February are unusually dry for S California. The ensemble (NMME) model suggests wet conditions in late Feb and the first part of March 2018.
...Alan Fox, Fox Weather LLC...