30-Day Weather Outlook for March 31, 2021, to April 28, 2021

  • Mar 31, 2021

Summary- The La Niña pattern is weakening, especially in the southern Pacific near Peru and the W coast of South America as some warmer than normal SSTAs are developing.  Still not sure if this warming will end the La Niña or just weaken it over the next few months. This weakening La Niña pattern teleconnects with a building high pressure near and just off the west coast of North America bringing a deep trough pattern into the Great Basin.  Still cooler water temperatures continue off the west coast of the U.S. as a La Niña generally continues.  La Niña continues in place with some variation at times but the general pattern appears to be weakening.  La Niña should continue, though weaker, late Spring into the summer but changes may be starting to switch the pattern.  We will monitor for changes, updating the forecast each week with future outlooks. This high pressure pattern is making most of California dry with even northern California expected to be drier than normal through most of April according to the latest models.  Very dry across CenCal and SoCal with an occasional deep trough to the E, backing into S California with some showers occasionally reaching into the region as upper lows are possible at times.  Most of the rain from these backdoor systems will only bring light rain to the southern half of the state.

Precipitation Trend – Well below normal precipitation is expected across the state due to a strong ridge and high pressure pattern that appears to be nearly stationary across the coast and just W of California.  Very dry with little chance of rain into central and SoCal at the start of Apr.  Some rain arrives with a weakening of the high pressure through the middle of Apr but most of the rain will be across NORCAL and most will be light and below normal for this time of year.  

Current most likely NORCAL Precipitation Dates (from our CFSDaily products out 30 days): Apr 5-7, 10-12 (light rain), 14-15 (better rain), 18-20 (light rain), 21-22 (wetter),  May 7-8, 12-14 (light rain).  Most of the rain looks light with some heavier rain on Apr 14 and the 21st.  Turning drier in May with only light rain at times with a lot of dry periods also expected.  Not much rain reaches south of NorCal though a little light rain at times into S areas of NorCal are possible with some deeper storms.  Dry periods from Apr 1-4, 16-17, Apr 23 – May 11.

In S California – Little or no rain is expected due to a strong high pressure that will be locked over the region.  Possible light rain reaches SoCal on Apr 6, 14-15 and possible scattered showers on the 22.  Little or no chance of rain thru the start of May with a slight chance of light rain reaching into SoCal on May 14.  Well below normal precipitation is expected across the region with a lot of dry days and warmer than normal temperatures.  This is due to high pressure dominating the weather pattern through most of the period.

SOCAL Warm Spells, and Freezes from our CFSDailyAI - SOCAL frost dates from models GFS and CFSDailyAI:  Warm spells: Apr 1-5, Apr 16-18, Apr 24 – May 6, 15-20.  May looks very dry and warm with only weak systems bringing light rain and not much rain of consequence into SoCal.  SoCal Frost/freeze events: Apr 8-9, 16-17 and 23-24.  Possible May frost and freezes on May 9, 15-16. Frost/freezes are usually not as strong for SoCal after the middle of April without a deep trough moving into the region.

Central Sierra Nevada best chance for Precipitation: Apr 5-7, 11-12, 14-16 (wetter), 19-20 (light precipitation) and 21-23 (wetter). May rain into CenCal SierraNV is possible on May 8 and 13-14 (light rain).  Late season snow accumulation will be below normal due to high pressure being locked over the region.  Some snow at times but due to warmer than normal temperatures, snow accumulation will be higher in the mountains with rain in the lower elevations.

Freezes in north and northcentral California valley cold spots:  Apr 8-9 (light freezes), 13 (light freezes), 16-17 (light freezes), 23-25 (hard frost/freezes possible).  May frost/freezes on May 9-10 (light freezes), 15-16 (light freezes).

The La Niña along with high pressure will keep precipitation well below normal across most of the state but especially across S 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 MAR 31 - APR 28:  Highlights:  The pattern remains dry through Apr 5  with some isolated showers possible on Apr 6.  Slightly wetter and cooler from Apr 10  thru the 22  though most of the showers into the region will be light and not much precipitation of consequence is expected.  The best chance of decent rain will be Apr 15th and 22 .  Turning very dry with a stronger high pressure setting up near the W coast with little storm penetration into S California after Apr 23 .  May looks dry with storm systems generally staying well N or the desert region.  Possible May light rain on the 9  and 15.

***Looking Ahead – Long Range Outlook May 2- May 30 with comments for Jun *** 

The storm track remains well north with high pressure ridging over the region.  Very dry across most of the state with well below normal precipitation expected.  Above normal temperatures across the state are forecast due to the high pressure and ridge that will generally stay locked over the region.  Some light rain at times will move through northern California but little is expected south of Mendocino County through the month of May.  May rain south of NorCal usually only comes with deep troughs and developing upper lows off the S end of these troughs.  This is unlikely this year so a very dry pattern is expected. Summer monsoonal moisture including moisture from Mexico generally starts in mid to late July.  The start of June will likely begin dry with some increase in showers later in the month as a little pre-monsoonal moisture arrives.  This Jun appears to follow this pattern with likely little or no rain across most of the state before the 23 .  Temperatures will remain above to well above normal temperatures across most of California with occasional frost/freezes though less than normal due to the high pressure dominating the weather pattern.

…May 2 - May 30 N and Central California...  

High pressure is strengthening across the region which will shove the storm track well to the N.  Some weaker systems will bring occasional anemic light rain when they do move through the region and into the high pressure.  Warm to very warm temperatures with above normal conditions are expected late Apr through most of May.  Dry with warmer than normal conditions continuing late May and likely through the middle of Jun.  The systems that do move into the high pressure will weaken with most being minor rain events through the area.  A chance of light rain is possible May 7-8, 12-14.  Dry with little chance of rain on other dates.

For SOCAL:  Below normal precipitation will continue with current models showing no rain of consequence in May.  Overall, a very dry pattern is expected through most of May with continued dry conditions through at least the first half of Jun.  If any light rain occurs in SoCal it will be around May 9 and possibly on the 14 -15. Possible frost at the coldest valleys May 9 and 15th.

Snow accumulation has been good across the northern SierraNv mountains late Winter but moisture is lacking now as high pressure is the dominant weather feature.  Occasional warmer than normal systems in May will bring light snow to the higher mountains. The best chance of snow will be May 7-8 and 12-14.  Above normal temperatures will keep snow levels higher than normal for this time of year.  This extended dry period will bring a lot of drying for the hillside and mountain vegetation with a likely early start to a moderate to potentially severe fire season!  Above normal temperatures and very dry conditions will likely increase the fire risk as early as early May with dangerously dry conditions by mid to late May and definitely through most of June.

Late June-July-Aug 2021… are still indicated drier than normal conditions in the current CFSv2 model simulation.  Monsoonal moisture could finally bring some decent rain into SoCal this summer but the general monsoon season usually starts in the middle or end of July.

Alan Fox & Zane Stephens...Fox Weather, LLC
Copyright © 2021, Fox Weather, LLC, Used by permission.

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