30-Day Weather Outlook for February 26, 2021, to March 29, 2021
Summary- A La Niña pattern is expected to continue in March through August. The North American Multi-Model-Ensemble (NMME) shows La Niña near maximum intensity in July and August. The best chance for near normal precipitation in California is in late Mar and early Apr in NW California, followed by a warm and dry mid to late spring, Apr-May.
Activity of Fronts – A tendency for troughing will continue in Mar in the central Pacific north of Hawaii. This will keep a longwave trough and cold conditions through most of Mar in California. A cold Mar in California suggests above normal incidence of late season frosts. This applies to NW California viticultural areas, the central coastal valleys, and S California.
Precipitation Trend – Below normal precipitation is expected in the last half of Feb, while near or above normal rainfall occurs in Mar for N California, Bay Area and north. Below normal rainfall and warmer than normal conditions occur in April for S California.
It is cold and showery at times in S California during mid to late Mar and early in Apr. The drivers for below cold conditions with below normal rainfall in SOCAL include continued tendency for cyclogenesis in the mid North Pacific per SSTA contributions), and downstream tendency for recurrent high pressure near the coast at 130W, and cold troughing over the Great Basin, Sierras, Arizona, and at times into S California.
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: Mar 5-6, 12, 17-18, 20-23. 25-28.
In S California - Most Likely Precipitation Dates: A drier pattern is expected for southern California although a few showers are possible: Mar 4-5, 11, 17-18, 26-29, and 31 in E Deserts.
SOCAL Warm Spells, 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 frost dates from models GFS and CFSDailyAI: Feb 28, Mar 1-2, Mar 2, 11-12. SOCAL warm to hot spells: coast Orange-Riverside Co’s and San Diego Co: Mar 6, 22-23.
Central Sierra Nevada best chance for Precipitation: Mar 11, 14,17-18, 26-29, and 31.
Freezes in north and northcentral California valley cold spots: Feb 28, Mar 2-3, Mar 7-16, 18, 23-24, 31.
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 rains due to above normal sea surface temperatures W of California over a large area 125-140W.
The listing of dates normally included for 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 28 - Mar 29: Highlights: Possible widespread rain event Mar 4-early 6 spreading through Arizona.
Eastern deserts show dry conditions with warmer than normal Light showers are possible Mar 11-12, and Mar 27-31.
***Looking Ahead – Long Range Outlook Mar 30- Apr 30 with comments for May***
Warmer and drier than normal overall especially for southern California. Some near normal precipitation is possible northwest California with the best chances of rain from Mendocino County north. Frosts and light freezes are possible across NW and N California valleys. Best chance for northern California frosts and freezes will be on Mar 26, 27, and Apr 3-5. Occasional troughs in mid to late April will be followed by a chance of frost and some freezes in Mendocino, Lake and Napa Co’s.
With the occasional late season trough in early to mid May could be followed by a frost or freeze, occurring during the spring bloom to early fruit-set period.
…Mar 26-Apr 26 2021 N and Central California…
The currently well defined La Niña will weaken during this period (April). There may continue occasional cold troughs and showers south through central California, with low snow levels around the beginning of April. The La Nina condition generally supports weak westerlies in the southern storm track off south central and S California through this early to mid spring period. Continued rain/snow periods or possibly above normal precipitation in N California and Pacific Northwest states (Washington and Oregon. Warmer and drier than normal conditions are indicated for central and S California. Usual cold fronts occur in April with a few rain showers, and some snow for the central Sierras. 30-Day temperature guidance for April and May suggest warmer and drier than normal conditions continuing in May. Watch for early season dry and hot spells developing in May (well above normal temperatures) to start the long summer fire season in northern and central California.
For SOCAL: in March…even with the up-tick in precipitation, it will be difficult to recover from the currently large rainfall and moisture deficit.
Although we realize some nice snowstorms during March in the central Sierra, the early onset of recurrent, dry windy cold fronts in April followed by warmer than normal in May and June suggest an early and potentially robust start to the 2021 fire season.
April-May-June 2021… are still indicated drier than normal in the current CFSv2 model simulation.
Alan Fox...Fox Weather, LLC
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