30-Day Weather Outlook for January 11, 2020, to January 31, 2021
Summary- A La Niña pattern is expected to persist through Mar 2021. It continues in February and well into March. For California, this argues for drier than normal in most of California, and seasonably wet in the Pacific Northwest and far NW California during at least Jan and most of Feb. Heaviest rainfall will tend to focus Mendocino County north, and in the Pacific Northwest states. On infrequent occasions, zonal flow conditions (wet west flow) periods occur, and may produce some rains of significance into Central and Southern California, Nevada and Arizona.
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 overall. Mixed in with these overall dry and warm conditions will be an occasional unusually cold and snowy event with low snow levels in central and 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 mid to late Jan and mid Feb.
La Niñas and Precipitation Trend – La Niñas produce the best support environment for atmospheric rivers into NW California, Oregon-Washington, and British Columbia. Main focus is into coastal Pacific Northwest. The north parts of the Rockies and Great Basin also see above normal precipitation, but with recurrently high snow levels for N and Central California (Sierras).
Although more seasonable conditions for rain may develop in N and NW California for short periods, the long term seasonal trend for most of California, and especially SOCAL, will tend to be drier than normal. As dry cold fronts come through, there is risk of Santa Ana wind events, some with damaging winds, mainly late Jan and in first half of Feb. In Southern California, La Niñas tend to produce long fire seasons, and intermittent recurrence of fire conditions during the winter months (Jan, Feb, Mar).
Current Most Likely NORCAL Precipitation Dates (from our CFSDaily products out 30 days): Rains Jan 13. Rainy again Jan 24-27, In Feb, current rain dates are Feb 6-7-8 , and Feb 12-13 and Feb 19-20.
In S California - Most Likely Precipitation Dates: Jan 26-28, and Feb 6-8, 12 . Other rains are currently suggested for 19-20 Feb.
SOCAL Santa Anas and Freezes - The main issue for SOCAL is the recurrent downslope/offshore wind events of Santa Anas. January is fairly dry for SOCAL. With that dryness often occur Santa Ana winds. The cold Santa Anas of winter often associate with strong damaging winds and freeze events. Freezes and wetbulb freezes (wetbulb temperature below 32), often occur in cold Santa Ana events. Hard freezes (ambient air temperature 29 or lower) can occur on dry clear nights if wind speeds decrease to calm in valley areas overnight. Freezes then Santa Anas then freezes occur in S California after the rains of 24-27 Jan.
The Cascades from Mt Lassen north to Oregon border have a good chance for below normal precipitation in second half of Jan. February appears to have support for frequent dryness or subnormal rain. Exception is the snows of 23-27 Jan. Jan and Feb are normally the best months for winter precipitation. Temperatures also appear near or above normal for the most part during this coming spring (Feb-Mar).
The NORCAL Coast Precipitation periods: - (Mendocino County north): Precipitation dates: Jan 12-13, 22-23, 24-27, Feb 12-13. Feb 19-22.
Sierra Nevada best chance for Precipitation: 24-27 Jan, 6-8 Feb, and 18-29 Feb.
Freezes in north and northcentral California: 15 Jan, 21-22 Jan, several occasions within the period 25-30 Jan, 9-11 Feb. Freezes in central coast region…suggested by cold daytimes outside of rain occurrence:, 24, 28-30 in Jan, and Feb 8.
Freezes in central and southern California 21-22 Jan, (including San Joaquin Valley and Delta). Coldest periods are: 28-30 Jan, 7-10 Feb from our CFSDailyAI model.
La Niña seasons are not favorable for subtropical jet formation, or significant rains into SOCAL. However, this winter, the anomalously warm SST’s (positive SSTA) continuing to the west of California and out into the central Pacific may contribute additional support for upper high pressure to persist at times in Feb as well as contribute moisture for infrequent SOCAL storms (see Figure 3).
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 Jan 15-Feb 15: Mostly warmer and drier than normal, but less warm than earlier in the winter. The is also more risk for frost as well as showery/windy/cold periods.
Jan 24-27 continues to consistently have the highest probability of significant rains and mountain snows, with gusty winds with cold weather in the SOCAL southern and eastern Deserts.
***Looking Ahead – Long Range Outlook Feb 7-Mar 31 with Comments for Apr-June***
Warmer and drier than normal overall. Exceptions appear to be: Colder than normal 7-10 Feb 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. If the upper lows are strong enough, rains, instead of frosts/freezes are possible. In either case, troughing is expected to be more frequent in late Feb and Mar for California as a whole.
…Feb 8-Mar 31, 2021 Highlights:...
A well defined La Niña is currently indicated for the early spring period. However, if troughing focuses into the area NW of Hawaii, we may see a return of troughing in the early spring period (Feb-Mar) as mentioned above, with cold troughs, and showers into central and southcentral California. In a La Niña situation, this would be our best chance for retrieving some rain and mountain snow (SierraNV).
This supports predominantly weak westerlies in the southern storm track off southcentral and S California through this normally active period for late winter and early spring 2021. Continued seasonably rainy/snowy 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, interrupted, as mentioned above, by an occasional trough in California with showers and low snow levels.
From the latest CFSDaily 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 as fronts with sparse rain amounts push through from NW to SE.
We will offer a little hope for some rains in the Feb 12-18 time frame, which is normal, but rains may turn out to be spotty and light, according to current guidance.
For Feb-Mar 2021, NMME guidance suggests: near normal temperatures at the NORCAL coast with recurrent dry N wind events. Drier than normal conditions are indicated by CFSv2 for Feb. The statistically wet periods of Feb are 13-18 , and 26-28 – 4 Mar.
Feb 23-Mar 23…the pattern is basically unchanged. Strong 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 continues a chance for cold troughs to develop in California and perhaps some occasional relief from the stark dry scenario.
April-May-June 2021… continue drier than normal in the current CFSv2 model simulation. Under these conditions, we would suggest 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 conditions for May-June.
Alan Fox...Fox Weather, LLC
Copyright © 2020, Fox Weather, LLC, Used by permission.