30-Day Weather Outlook for February 13, 2021, to March 12, 2021
Summary- A La Niña pattern is expected to continue in February, then weaken a little in March. For California, this argues for drier than normal in central and S California in Feb, but we begin a return to near normal rainfall in mid to late Mar. Occasional wet periods for northcentral California (north Bay Area – Tahoe) in Mar. and on a few dates in Feb.
Heaviest rainfall will tend to focus Mendocino County and north, and in the Pacific Northwest states. On infrequent occasions, cold cutoff upper lows could develop, and draw cold airmasses into SOCAL for brief periods.
Activity of Fronts - We will see seasonably frequent frontal passages into northern California, Oregon and Washington with rains, and a tendency for higher than normal snow levels overall. Mixed in with the predominantly mild pattern will be an occasional cold and snowy event with low snow levels extending south through California and Sierras to S California as some deeper troughs are expected at times. This is typical of a La Niña pattern.
La Niña and Precipitation Trend – La Niñas produce the best support environment for atmospheric rivers into the Pacific Northwest, i.e., Oregon north to Washington, and British Columbia. The main focus of the moisture is into coastal Pacific Northwest. The north parts of the Rockies and Great Basin also see above normal precipitation. Some of these atmospheric river conditions can drift a bit south into northern California at times.
Although more normal conditions for rain may develop in N and NW California in Mar, the long term seasonal trend for most of S California, will tend to drier than normal. This is related, somewhat, to the continued tendency for cyclogenesis in the mid North Pacific (per IVTinit-SSTA contribution).
Current Most Likely NORCAL Precipitation Dates (from our CFSDaily products out 30 days): Dates of larger precipitation systems in N and central California and Sierras: Feb 13, 15-16, 22, 26-27, Mar 2-3, 13-15. Precipitation is a bit small for Feb, but returns closer to normal in mid-March.
In S California - Most Likely Precipitation Dates: A drier pattern is expected for southern California though a few showers are possible: Feb 13, 16, 22, and 27. Mar 13-15.
SOCAL Santa Ana’s and Freezes from our CFSDailyAI - 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.
SOCAL frosts are possible following a cooler trough that moves through from the 13 through the 14 . Best chance of frost/freezes would be between rains on Feb 15 thru the 19 . Another cold trough from the 22 through the 24 will bring a chance of frost/freezes 24- 25.
March: Wetter with a rains into N and Central California Mar 13-15. most of California the region from the Mar 13 through the 15 and another moist system from the Mar 19 through the 23.
Rainfall should approach close to normal as mentioned for central California, and a little above normal for NORCAL coast and northern mountains. Below normal precipitation trend will be in San Diego/Orange Co areas. The appears to be a possible good rain on 13-15 Mar through SOCAL. Temperatures also appear near or below normal for the most part during the late winter and start of early spring (mid Feb-Mar).
The NORCAL Coast Precipitation periods: - (Mendocino County north): Precipitation dates: Feb 13, 15-16, 22, 26-27, Mar 2-3, 13-15. Precipitation is a bit small for Feb, but returns closer to normal in mid-March.
Sierra Nevada best chance for Precipitation: 15-16, 22, 26-27 Dry with high pressure dominating the weather pattern from the 25 through Mar 12 with warmer than normal temperatures. More precipitation with wetter storms and good late season snow in the mountains from Mar 2-3, 13-15, and Mar 19-23.
From our CFSDailyAI system, freezes in north and northcentral California: Cool from Feb 12 through the 14. Frost/freezes are possible from the 14 through the 15 and again from the 17 through the 19 across much of the state.
La Niña seasons are not favorable for subtropical jet formation, or significant rains into SOCAL. However, some cutoff upper lows may develop 20 Mar onward for central and southcentral California, resulting in some late season rains due to above normal sea surface temperatures W of California.
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 Feb 12 - Mar 12: An active storm track will be to the north through northern California through the 24 of Feb. Most of the rain will stay north of southern California but some light showers are possible on the 11, 13, 16. A deeper trough and upper low will bring better chance for rain from the 22 through the 24 into southern California. Turning dry with a stronger high pressure building from Feb 25 and continuing through Mar 12. Dry with no rain and warmer than normal temperatures are expected during this time as the storm track pulls far to the north out of the region. Some light rain is possible from Mar 13 through the 15 but most of southern California will be dry through the rest of the month. Ventura County could see some rain late in Mar around the 28 and 29.
Eastern deserts show dry conditions with warmer than normal temperatures thru most of Feb and likely thru the 1 week of Mar. A little light showers are possible Feb 12 thru the 13 and again from Feb 23 through the 24 .
***Looking Ahead – Long Range Outlook Mar 8- Apr 18 with Comments for late Apr thru June***
Warmer and drier than normal overall especially for southern California. Some near normal precipitation is expected at times across northern California from Mendocino County to the north, but rainfall amounts diminish quickly from the Bay to the south through most of central California.
Drier air will bring less rain but occasional frost/freezes are possible, especially across NW and N California valleys. Best chance for northern California frosts & freezes will be on Feb 14 , 15 and 24 through the 25. Frost/freezes in Mar will be possible from Mar 16 through the 18 and again from Mar 24 through the 26. Some frost into southern California is possible following dry fronts that will occasionally move thru the region. This could affect the early season fruit crop. Troughs could be a bit stronger and deeper from Mar 13 thru the 15 with frost and freezes possible from Mar 16 through Mar 17.
Another wet and deep trough from Mar 28 through the 30 will bring a chance of frost and freezes late in Mar through the first day or two of April. This could be a critical time for growers as plants are becoming active for the start of the growing season.
…Mar 8-Apr 18 2021 N and Central California…
A well defined La Niña is currently indicated for the early spring period but is expected to weaken, especially by the start of April. However, if troughing focuses into the area NW of Hawaii, we may continue to see occasional returns of cold troughs and rains in all of California, with some low snow levels in late Mar through the start of April. This supports predominantly weak westerlies in the southern storm track off south central and SoCal through this normally active period for early spring 2021. Continued rain/snow periods or possibly above normal precipitation in N California and Pacific Northwest states (Washington, Oregon and Idaho), and below normal precipitation, i.e. warmer/drier than normal in Southern California. Usual cold fronts occur in late Mar and April with a few showers of rain… and some snow for the central Sierras.
For SOCAL: in March…even with the up-tick in precipitation expected to develop, it will still be difficult to realize precipitation that approaches normal. There continues a chance for cold troughs to develop with dry cold fronts or upper lows into SOCAL, perhaps some occasional relief from the dry scenario. April….drier than normal for SOCAL.
April-May-June 2021… are still indicated drier than normal in the current CFSv2 model simulation, though with a weakening La Niña, late Spring rains may be observed into central California and possible SoCal later in April into the start of May. 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 in most areas of California, including the coast.
Alan Fox...Fox Weather, LLC
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