30-Day Weather Outlook for December 11, 2017, to January 11, 2018

  • Dec 12, 2017

Summary- The MJO Cycle has been in an active state for about a week. GFS model solutions are suggesting support for more energetic sub-tropical westerlies around 30N, coming into S California. However, the active phase of the MJO cycle appears near maximum on Dec 11, and some transition to less activity and energy into the subtropical westerlies appears to begin after the 17th.

Support continues in the sea surface temperature anomaly (SSTA) field for troughing well to the west of California, centered about 135-140W. Therefore the SSTA contribution keeps most of California in a drier than normal pattern.

The El Niño Southern Oscillation (ENSO) is currently in a well-developed ‘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 branch of westerlies.

There is support for troughing and cold conditions in the 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 early Jan 2018. There is a chance that this may continue the dry pattern for part or most of Jan.

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 would continue intermittent in California, and totals of precipitation below normal in the Bay Area and central coast areas, and showery or meager rains and snows through the N and central Sierra during the rest of Dec and the first part of Jan 2018. There is a chance for some snowy periods in the N and central Sierra during mid Jan.

For S California, warm sea surface temperatures are present to the southwest, but the La Niña pattern tends to discourage development of subtropical westerlies. Lows and troughs will tend to develop in the Great Plains, and S Rockies/SE California, with a continued higher number of Santa Anas.

In the Near Term – Dec. 16 – Dec. 30… In the southern California avocado growing areas, from San Luis Obispo County to San Diego County, we still have a risk of offshore flow and crisp cold nights after mild or warm days. Watch for frosts/freezes, long nights, short daytime heating period, predominantly dry airmasses through the latter part of Dec. There is a chance for some unusual cold on about 2-3 nights, probably around Christmas and the 28-30th.

Summary – Dec. 31 – Jan 10, 2018… In the southern California avocado growing areas, from San Luis Obispo south, rains will be mostly occurring in N California and the SierraNV. However, a cutoff low is always possible in such an erratic pattern, with some showers.

Seasonal Outlook / La Niña Update...January 11, 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. The current exception is locally heavier snow in the Tahoe-Yosemite areas. After a brief shot of good snowfall in the SierraNV in January, February currently appears to turn dry again but with plenty cold events (freezes) in N and Central California.

N and Central California, of greatest concern is the chance for below normal temperatures indicated for January through some of N and central California, along with dry conditions. This type of configuration points to cold dry N winds in central California as well as cold and damaging Santa Anas in SOCAL during late Jan, 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 for most of California, including SOCAL.

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

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