30-Day Weather Outlook for March 16, 2021, to April 14, 2021

  • Mar 16, 2021

Summary- A La Niña pattern is expected to continue in March through August according to the North American Multi-Model-Ensemble (NMME).  La Niña Continues in place with some variation at times. La Niña should continue this spring with a maximum intensity in July and August.  This will keep a wetter than normal pattern across  thru the middle of April with some tapering off of the precipitation during the end of the month.  This latest forecast is more wet with the current models showing better Northern California rain through even thru the middle of April.  Below normal precipitation will continue across most of Central California & Southern California thru this period though near normal precipitation is likely from Paso Robles to the north to the Bay.

Activity of Fronts – A tendency for troughs will continue in mid to late Mar in the central Pacific north of Hawaii.  This will keep a longwave trough and below normal temperatures and wetter conditions from N Central California to the N through Northern California. NE California along with N Nevada will also be wetter than normal with good snow expected thru the N SierraNV range, at least into the middle of Apr.  Between fronts  with these colder troughs, frost and freezes will be possible.  The storm track will gradually progress to the N from the middle of Apr thru the start of May with weaker fronts affecting Northern and Central California.

Precipitation Trend – Near to above normal rainfall occurs in mid to late Mar through the start of Apr for N California.  Well above normal precipitation later in the month from Mendocino County to the north.  Near normal rainfall for N parts of Central California from about Paso Robles to the N with below normal precipitation across S parts of Central California and Southern California. Below normal rainfall and warmer than normal conditions occur in late Mar through Apr for S California.  Even drier for Southern California from mid Apr through the end of the month.  Occasional deeper troughs will bring some upper lows into Southern California with some rain and showers at times, but well below normal precipitation is expected.  The drivers for cold conditions with below normal rainfall in Southern California 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 troughs 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): Mar 18-19, 22, 28-30 (showers), Apr 1-4 (wet), Apr 5 (light rain), 6-10 (wet), 15-20 (light rain), 21-22 (wet), 23-24 (light rain).  Dry from Mar 23-26, Apr 11-14 and again from Apr 25-30. Light rain amounts for Central California occur on most of these dates with the best chance of rain from the Mar 28-30, Apr 5, 7-10, 18-21, 22-23.  

In S California - Most Likely Precipitation Dates:  Mar 19 (light rain), 28-31 (light), Apr 3-5, possibly moderate, 8-10 (light) and 22 (light).  This pattern will keep rainfall across Southern California well below normal for this time of year with only occasional deeper troughs with some upper lows developing bringing some rain into Southern California.  Turning very dry in late Apr.

Southern California Warm Spells, and Freezes from our CFSDailyAI - Southern California frost dates from models GFS and CFSDailyAI:  Warm spells: Mar 21-25, Apr 12-17, 26-30.  May looks very dry and warm, although May generally sees an increase in marine stratus influence at the coast and coastal influence zone.  Frost/freeze events: Mar 20-21, 29-31, Apr 5-6, 11-13, 23-24.  Frost/freezes are usually not as strong for Southern California after the middle of April due to the coastal marine layer.  

Central Sierra Nevada best chance for Precipitation: Mar 26-31, Apr 4-5, 6-10, 16-20, 21-23.  Decent snow accumulation in the SierraNV, especially across the N SierraNV is expected thru the middle of Apr.  Less snow accumulation will occur as the storm track pulls farther to the N during mid to late Apr.

Freezes in north and northcentral California valley cold spots:  Mar 20, 22-24, Apr 10-13, 20, 25-27.

La Niñas usually do not bring good rain to Southern California. Cut-off lows are possible at times. These increase the chance for rain and showers on occasion.  The next deep trough and cut-off low is most likely Mar 19.  A deeper cut-off low is possible Apr 4-5.  As the storm track pulls farther to the N later in Apr and in May, these lows become less likely with generally drier conditions expected across Southern California.

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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 19 - APR 16: 

Highlights:  A trough and weak upper low will bring light rain and showers on Mar 19 with some showers into Mar 20.  Scattered showers are possible Apr 4-5 in the S California deserts with light showers into Arizona on Apr 5-6.  Another deep trough and low could bring showers into the desert areas Apr 9-10.  Dry from Mar 21 to 31.  Dry again Apr 7-8. Dry during Apr 11-21.  Late Apr looks much drier with no rain of consequence into the desert regions from Apr 24-30.

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***Looking Ahead – Long Range Outlook Apr 18- May 16 with comments for late May into Jun *** 

The storm track remains active north of the region with occasional deep troughs bringing some light showers into Southern California and the desert region through Apr 23.  Turning dry starting Apr 24 with little or no chance of rain and showers expected for Southern California according to the latest models.  May looks very dry with currently no rain of consequence expected.  May rain 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 monsoon moisture including moisture from Mexico generally starts in mid to late July.  So June through early July should also remain very dry according to the current pattern.  Central California will be wet with a series of troughs from Apr 17-24.  Central California turns drier with high pressure building from Apr 25-May 5.  A weak system could bring light rain to Central California on May 6-7.  Dry with the storm track to the N with high pressure blocking rain from the region May 8-13.  A trough will bring light rain on May 14-15.  Dry May 16-20.  The best chance of Central California rain will be from Paso Robles to the N with only light rain and showers moving into S areas of Central California.  N California rain from Apr 17-24 will keep wet conditions across the region.  Wetter days are expected Apr 21-22.  High pressure and dry from Apr 25-May 4 with above normal temperatures throughout the region.  Light rain reaches into Northern California May 5-8 and again May 13-17.  Turning drier with below normal precipitation with high pressure later in the month.  Some troughs could bring light rain late May through early Jun.  Frosts and light freezes are possible across NW and N California valleys between storm events.  Best chance for northern California frosts and freezes will be on Apr 20, 25-28, May 9-11, 18-20.

…Apr 18-May 16 N and Central California…  

The currently well-defined La Nina will weaken slightly during this period (April into early May). The storm track should be active across Northern California with above normal precipitation from Apr 17-24.  Drier from the Apr 25-May 4 with above normal temperature due to high pressure locking over the region.  Light rain May 5-8 with seasonal to cool temperatures.  Warmer with high pressure and dry conditions from May 9-12.  Weak troughs will bring a chance of light rain with cool temperatures May 13-17.  Dry and warmer with high pressure building May 18 and likely continuing through the rest of May.  Some weak systems could bring some clouds and possible light rain late in the month but conditions look a lot drier.  Light rain across northern parts of central California are possible Apr 16-24 with wetter days and some rain into S areas of the region Apr 22-23.  Dry Apr 25-May 5 with high pressure building throughout the region.  Weaker troughs with light rain from King City to the N May 6-8 and again May 15-17.  Drying out across central California from May 18 through the end of the month.  Drying conditions will likely bring an early start to the fire season this year.  Dry with warmer than normal conditions late May through the start of June.

For Southern California:  Below normal precipitation in general will continue but some scattered showers are possible Apr 19.  A slight chance of light rain and showers May 7-8 but most will stay north of the region.  Another trough will bring clouds but little or no rain May 16-18.  Late May looks dry with rain unlikely unless a deeper troughs and developing upper lows form which are possible but less likely late May through Jun.

Some late season snow accumulation in central and northern SierraNV range is expected during the middle and end of April.  Best snow accumulation will be across the northern SierraNV.  Wetter from Apr 17-25 as a series of troughs moves through the region with colder than normal temperatures.  Weaker troughs could bring lighter snow May 6-8 and May 14-17 though snow levels will be higher in the mountains, especially with the later storm.  Turning drier with warmer temperatures late May with the storm track pulling more north of the region with only weaker lighter snow possible at times.  This will bring drying conditions in the mountains bringing an early start to the 2021 fire season by late May into the early summer.

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

Alan Fox...Fox Weather, LLC
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