30-Day Weather Outlook for January 26, 2020, to February 27, 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 central and S California in Feb and Mar, while a wet and active period for the Pacific Northwest and Northern California, and Northcentral California (north Bay Area – Tahoe). Heaviest rainfall will tend to focus Mendocino County to the north, and in the Pacific Northwest states. On infrequent occasions, cold cutoff upper lows 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 the state to S California as some deeper troughs are expected at times.
La Niña and Precipitation Trend – La Nina’s 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. Some of these atmospheric river conditions can move a bit south into northern California at times.
Although more seasonable conditions for rain may develop in N and NW California for short periods, the long term seasonal trend for most of the state, and especially SOCAL, will tend to be 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): Wet and cold storm January 26th thru 29th. More good rain from the 31st thru the 2nd. Drier and cold from the 3rd through the 11th with some warming but dry from the 12th thru the 16th. A weak system will bring light rain from the 17th thru the 19th. A wetter trough from the 22nd thru the 25th.
In S California - Most Likely Precipitation Dates: 28th thru the 29th with another rain from the 2nd thru the 3rd. A cold trough with some showers from the 5th thru the 8th. Turning dry as the storm track pulls north with some high pressure building from the 9th thru the 17th. Cooler but dry on the 18th and 19th. Another chance of rain from the 23rd thru the 25th of February.
SOCAL Santa Anas 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 freezes occur on Jan 27th, 31st, Feb 1st. Freezes are likely with very cold air after the rains on the 5th with frost/freezes likely from the 6th thru the 10th.
February appears to have support for above normal precipitation during the first half of the month from Tahoe-Bay Area northward, then a drier than normal scenario from the 9th thru the start of March. Jan and Feb are normally the best months for winter precipitation. Temperatures also appear near or above normal for the most part during the early spring (mid Feb-Mar).
The NORCAL Coast Precipitation periods: - (Mendocino County north): Precipitation dates: Jan 26th thru 28th and 29th thru 30th. A wetter system from the 31st thru Feb 2nd. Cold but dry most areas from the 5th thru the 8th of Feb. Maybe a few showers are possible with this cold and deep trough. Dry with high pressure building from the 10th thru the 16th. Light rain from the 17th thru the 19th. Better rain and wetter from the 22nd thru the 25th.
Sierra Nevada best chance for Precipitation: Jan 26-28, 1-3, Feb 7-8th. Dry from the 9th thru the 16th. A weak trough will bring light precipitation from the 17th thru the 19th with a wetter trough from Feb 22-25. Dry again late in the month into the start of March.
From our CFSDailyAI system, freezes in north and northcentral California: several occasions within the period Jan 26th, Feb 3-4, Feb 6-9 with very cold temps expected!
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 and the occasionalformation of upper lows that can bring rain at times to SoCal.
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 27-Feb 28: Periods of rain and showers Jan 28th & 29th. Some good rain and showers with wet conditions as a deep trough and developing upper low move thru the region. Another system will bring more rain/showers from the 2nd thru 3rd. Cold showers with some arctic air from the 5th thru the 8th.
Eastern deserts could see some showers on the 29th, 3rd, 5th-7th.
***Looking Ahead – Long Range Outlook Feb 21- Apr 04 with Comments for Apr-June***
Warmer and drier than normal overall. Exceptions appear to be: Colder than normal Feb 26th thru Mar 2nd. Good chance of frost/freezes are expected the end of the Feb thru the start of March in SOCAL and throughout the state. This could affect the early season fruit crop. If lows are strong enough, then rains, instead of frosts/freezes are possible. In either case, troughing is expected to be more frequent in early March for California as a whole.
…Mar 2-Apr 6 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 continue to see occasional returns of cold troughs and rains in all of California, with some low snow levels in late Feb and Mar. This supports predominantly weak westerlies in the southern storm track off southcentral and SoCal through this normally active period for early spring 2021. Continued rain/snow periods or possibly above normal precipitation in Pacific Northwest states (Washington, Oregon and Idaho), and below normal precipitation, i.e. warmer/drier than normal in California, interrupted, as mentioned above, by an occasional cold troughs 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 offshore wind events as fronts with small rain amounts push through from NW to SE.
For late Feb and Mar 2021, NMME guidance suggests: near normal temperatures at the NORCAL coast with recurrent dry N wind events. The statistically wet periods of Mar are 1-4, 10-14, and 26-31.
Mar 2-Apr 6… the pattern is basically unchanged. Strong La Niña conditions with cold sea surface temperature anomalies are forecasted along the equatorial central and east Pacific. This pattern supports below normal energy to drive the southern branch of the westerlies again in April and May. Potentially seasonal to wet across northern California and wetter than normal thru the Pacific NW.
For SOCAL in March, due to La Niña, 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 with occasional upper lows developing off these troughs.
April-May-June 2021… are still indicated 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
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