30-Day Weather Outlook for December 17, 2022 to January 17, 2023

Dec 16, 2022

BASIC PATTERN:

Large Scale Pattern –

The westerlies continue further south now, but the onshore-moving systems are tending to be weaker than normal. Despite some moist airmasses coming east-northeastward from the Pacific and subtropical areas, there will continue significant interruptions to the wetter pattern during the remainder of Dec. We expect recurrence of a cool and wet pattern in 24-26 Dec with potential for heavy upslope rains to the N California coastal mountains. New Years Day, 4-6 and 9-11 appear mostly dry.
We note that the sea surface temperature anomalies from the NOAA/NESDIS Coral Reef Watch Program continues to show limited El Nino conditions focusing near the Mexico coast, i.e. east of 130W and affecting
southern and western Mexico in Dec. If this trend continues, it may result in deep cold troughs at times in California, but a return to dry conditions with freezes coast of N and Central California in Jan.
Precipitation Trend: After the rainy period during the Christmas holiday weekend, we return to overall drier than normal conditions. In Jan 2023, the tendency for below normal precipitation spreads to nearly all of California during Jan 4-6, 9-11, and 14-20. This includes the Sierras, and drier airmasses than normal coast and S California, and above normal freeze risk (cold temperatures) in central coast and SOCAL Cold windy conditions with freezes coastal valleys alternate with showery rain episodes. These rain episodes alternate with periods of hard freezes with very dry, breezy cold events with unusually dry airmasses. Watch for cold Santa Anas with significant freeze risk due to low dew points (dry airmasses), as arctic air comes south from the Yukon.
SOCAL precipitation trend for the first period of 30 days: Rains Dec 20-21, then dry 22-23, and rains/showers on 25-26th. The first half of Jan 2023 appears to be drier than normal.
In Feb 2023, from NMME and CFSv2 models, there is a tendency for us to continue to mostly below normal precipitation, except for possibly some rains and Sierra snows during 12-19 Feb, and 27 Feb – 2 Mar.
March 2023 looks currently (from NMME Model) colder than normal, but significantly wet, with plenty of rains and mountain snows for N and Central California (including Sierras) and the North and Central coast areas.

FORECASTS FOR CALIFORNIA

Forecast for Northern and Central California:

Norcal RAINS: Dec 20-22, 24-27, 29, 30-Jan 1, Jan 10-14.
Norcal MILD SPELLS: Dec 21-22, Jan 3-8 and 17-19.
Norcal COOL SPELLS: Dec 20-21, 24-27, Dec 29-Jan 2 and Jan 10-14.
Norcal WINDS (dry cold winds): Dec 20-22, 26-27 29-Jan 3, and Jan 14.
Norcal FROSTS: Dec 22-23, Jan 2-5, 14.

Central Calif RAIN: Dec 21-22,Dec 24-27, Dec 31-early Jan 1, Jan 12-13.
Central Calif MILD SPELLS: Dec 21-23, Jan 4-9 and 16-18.
Central Calif COOL SPELLS: Dec 24-27, Dec 31-Jan 1-2 and 12-14.
Central Calif DRY COLD WINDS/FREEZE RISK: Dec 28, Jan 2 and Jan 15.

Forecast for Southern California:

SOCAL RAINS: Dec 24-25, and Jan 13-14.
SOCAL MILD TO WARM SPELLS: Dec 20-22, Jan 4-11 and 17-19.
SOCAL COOL OR COLD SPELLS: Dec 24-27, Dec 30-31, Jan 2-3 and 12-20.
SOCAL FROSTS.FREEZES: Dec 30-31, Jan 4-6, 7-9 and 16-18.
SOCAL SANTA ANAS: Dec 22-23 brief, 29-30, Jan 6-8.

Forecast for Central Sierra Nevada:

Central Sierra Nevada Precipitation: Dates of rain or snow: Dec 20-21, 24-26, Dec 31- Jan 1 and Jan 13-15.

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The listings of dates normally included for hot and cold spells, and precipitation are very approximate. They are based on our CFSDaily and CFSDailyAI products; and present expected trends in precipitation and temperature (CFSDailyAI) to 4km. Our system gives some consideration to 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.

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Southern California Deserts Outlook for December 17 - January 17, 2023

Highlights: In 9-15 Dec, and possibly a showery windy cold front about 25-26 Dec and 31 Dec – New Years Day. As usual in late Dec-early Jan, watch for frosts and freezes.

Looking Ahead – Long Range Outlook - January 17 - February 17, 2023

NORCAL and Central California coastal valleys, and Central Valley: In general, NMME has been appearing a little drier than the CFSv2 for California as a whole. Watch for above normal risk of Santa Anas in Jan-early Feb 2023. Near normal precipitation in Jan 2023 from the CFSv2 (wetter than normal in Siskiyou Mountains and Cascades in California, with plenty of snow), but drier than normal per NMME, in west portions of N and Central California.

For SOCAL mountains westward to coast: January 2023 currently appears seasonably cold with above normal risk of frost and freeze. Rainfall in the eastern deserts may actually go above normal in Jan 2023, while snowfall amounts in mountains are near normal. Watch for above normal frequency of Santa Anas, warm days but abnormally cold nights. Although cutoff lows may develop on occasion to bring rains to Southern California, the supply of surface and ground water will be heavily dependent on strategic planning of water storage in Northern California and Sierras with the expected rains in NORCAL, and snows in Sierras and
Siskiyou’s. We expect a return to abnormally dry conditions in SOCAL and at the central coast in mid Jan onward, with recurrent Santa Anas. Dry also in Feb, with recurrent frosts/freezes. The CFSV2 Model at this time shows a wet and cold March for California. However, the challenge continues to plan usage of whatever snow water equivalent (SWE) we have. The season for Atmospheric Rivers to beneficially add to lakes and rivers this year has been Dec. We saw good rains and snowpack in Nov from a couple of storms. The rains and snows in Dec will also help to maintain or increaseSierra snowpack. A wet and cold March would also help water supply for the summer. The dry and unusually cold (frost-freeze events) in Feb for Central California will impact challenge to water storage in all of California, including SOCAL from this winter’s rain season.

(Terms and Definitions Used In This Weather Outlook)