30-Day Weather Outlook for December 23, 2020, to January 20, 2021

  • Dec 23, 2020

(December 23 – January 20)

Summary- A La Niña pattern is expected to persist through Jan 2021. It continues in February, although weaker. For California, this argues for drier than normal in the central and south portions, and wetter than normal in N California from late Jan - Feb.  Heaviest rainfall will tend to focus Mendocino  County north, and in the Pacific Northwest states. Occasionally, we may see some troughs develop over Southern California, Arizona, and NW Mexico.

Activity of Fronts - We will see more frequent frontal passages into northern California, Oregon and Washington with rains, and a tendency for higher than normal snow levels, while dry and warm periods persist or recur in Southern California.  Despite the tendency for warm dry upper high pressure, there is a chance for a few arctic surges to develop, with the usual frost/freeze events into S California and San Joaquin Valley.  These are most likely in Jan and Feb. 

La Niñas and Precipitation Trend – La Niñas produce the best support environment for atmospheric rivers into northern California and Oregon, with main focus into coastal areas of the Pacific Northwest. The northern Rockies and Great Basin also see above normal precipitation, but with persistently high snow levels for N and Central California (Sierras). Although more seasonable conditions for rain develop in NW California, the long term seasonal trend for Southern California will tend to be drier than normal. As dry cold fronts come through, there is risk of cold Santa Anas, with damaging winds. In Southern California, La Niñas tend to have long fire seasons, and intermittent recurrence of fire conditions during the winter months (Dec, Jan, Feb).

Current Most Likely Precipitation Dates (from our CFSDaily product out 30 days): Rains Dec 28 , Jan 5-6, 9, 14.  From CFSDailyAI … (CFSDaily results were described above)… are (N California): Jan 14,  Jan 23-25. In S California - Most Likely Precipitation Dates: Dec 28, and Jan 23-26.

SOCAL Santa Anas and Freezes - The main issue for SOCAL is the recurrent downslope/offshore wind events of Santa Anas.  January continues to look significantly dry. With that dryness often occurs Santa Ana winds. The cold Santa Anas of winter often associate with strong damaging winds and cold events. Freezes and wetbulb freezes (wetbulb temperature below 32, often occur in cold Santa Ana events. Hard freezes (temperature 29 or lower) can occur on dry clear nights if wind speeds decrease to calm in valley areas overnight.

The NORCAL Coast Precipitation - (Mendocino County north) and Cascades from Mt Lassen northward have a good chance for below normal precipitation in January.  February appears to have above normal precipitation for NW California, and otherwise near normal precipitation for northcentral and remainder of N California. Temperatures also appear near or above normal for the most part.

La Niña seasons are not favorable for subtropical jet stream formation, or significant rains into SOCAL.

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The listing of dates normally included for hot and cold spells, and precipitation are based on our CFSDaily and CFSDailyAI forecast products, and present generally expected trends in precipitation (both products) and temperature (CFSDailyAI) to 4km. Our system gives some consideration of terrain and coastal influence. We consider the CFSv2 as one of the better ways to represent basic weather down in the sub-monthly time scale beyond the 15 day GFS or monthly maps from  CFSv2 or NMME.

Southern California Deserts Outlook for Dec 25 2020 to Jan 23 2021: Mostly warmer and drier than normal but with the usual risk of frost and cold periods..

A cold, windy showery period occurs on the 28  of Dec. Otherwise, Jan 23-26 appear to be the best dates for showers and gusty winds, with cold weather.

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***Looking Ahead – Long Range Outlook Jan 23-Mar 23 with Comments for Apr-June***

Showers Jan 23-26. Colder than normal 23-26 Jan in particular, with higher than normal incidence of freezes and frost in SOCAL during Feb and Mar 2021, both in southern and eastern deserts as well as SOCAL avocado/citrus areas.

Jan 23-Feb 23 2021 Highlights:

A well defined La Niña is currently indicated for the early spring period. For precipitation in California this is unfavorable news.  With colder than normal sea surface, the energy available to drive the westerlies in the central and eastern N Pacific will tend to be well below normal. This supports predominantly weak westerlies in the southern storm track through this normally active period for late winter and early spring 2021.  Continued seasonably wet or possibly above normal precipitation in Pacific Northwest states (Washington, Oregon and Idaho), and below normal precipitation, i.e. warmer and drier than normal in California. With the dry fall, this could be one of the driest November – February seasons we have seen in a long time. The period of mid to late January. Per the latest CFS Daily 45 day forecast product, rains are currently indicated by the model, but amounts are persistently small.

This pattern also argues for recurrent Santa Ana wind events. The last week of Jan and early Feb. We will hang out a little hope for some rains in the Feb 10-18 time frame, which is normal, but the rains may turn out to be spotty and light, according to current guidance.

For Feb-Mar 2021, NMME guidance suggests: Cooler than normal at the NORCAL coast with recurrent dry N wind events.  Not sure that we’re going to see that near normal rainfall in Feb that we were previously mentioning. Statistically wettest periods of Feb are 8-12 , 13-18 , and 26-28 .

Feb  23-Mar 23…  La Niña conditions persist along the equatorial central and east Pacific, and below normal energy to drive the southern branch of the westerlies.

For SOCAL in Feb, due to La Nina, it may be difficult to realize precipitation approaching normal. There is no definable trend, other than below normal precipitation at this time.

April-May-June 2021

Continued drier than NORMAL. Under these conditions, expect an early start to the 2021 fire season.  In fact, the North American Multi-Model Ensemble shows above normal sea surface temperatures in April-June in the midlatitude eastern N Pacific, and associated drier and warmer than normal for May-June.  June currently looks unusually warm with  persistently hot conditions in N California forest areas at the start of the fire season.

Statistically, the period Jan 2-10 is usually an active one, for storms.  Jan 12-22 is statistically a cold period with freezes between rain events. This Jan in N California, we expect subnormal rainfall, so the  12-22 is more likely to be dominated by frost and freeze events, and persistent valley haze and fog in the San Joaquin Valley. Jan 23-31 can still experience some of the mid-winter pause in storminess in N California as the storm track focuses into the Pacific Northwest. During wet seasons, this is usually a time for the southern storm track to dominate and bring rains into southern and central California.

Alan Fox...Fox Weather, LLC
Copyright © 2020, Fox Weather, LLC, Used by permission.

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