Friday, January 15, 2016

Hey - What's up El Nino?

I don't mean to sound ungrateful, but this El Nino is pretty grudging about giving us the rain that we were told would be coming… Mind you, I love every drop we get and the snow in the Sierras gives me hope for snow melt in the reservoirs next summer. However the amount of rain fallen in the SF Bay Area in this season is actually less than the same time last year, which was less than normal.  (Due to one good storm that came barreling through last year.) 

What we havent had yet this year is a rollicking good Pineapple Express storm sequence… Remember back a few years when we get a steady sequence of wet warm air direct from the tropical oceans south of Hawaii? It made an atmospheric river hooked in with jet stream influence.  When this moist warm air is forced to rise over the coastal hills and Sierra Mountains it cools it down and hard steady rains were released.  Its tricky when you ask for a certain weather pattern – because you might get more than you wanted –With ‘expresses’ we have received substantial flooding  - and the warm rain can melt the snow pack resulting in early run off… much of which flows out to sea...not good! 

So a Pineapple express is a mixed blessing. What then are we likely to get from a nice well-formed El Nino system?  The weather pattern that we call El Nino is not completely understood.  Some years the temperature of the tropical Pacific oceans become warmer than normal  making a plume of warmer water that extends far out into the Pacific Ocean from the coast of South America. The strength of an El NiƱo is linked to just how warm the sea waters get, and this year in November and December, they reached record highs. The name El Nino in Spanish means “the baby boy” referring to Baby Jesus – which relates the time of year when this weather pattern is likely to happen. Due to the warmer than usual sea water, we are currently in one of the strongest El Nino year ever recorded. 

This  El Nino year heating has been influenced by the greenhouse effect on earth brought about by high levels of greenhouse gases in the atmosphere  (such as CO2, Ozone, Methane), that trap sun heat energy and don't permit it to escape into space  as happens normally. 

 Even though the seawater heating happens in the tropics, it affects the weather thousands of miles away. By changing the location of where warmer ocean waters can be found in the Pacific, regions of seawater where heat is being given off into the tropical atmosphere are also changed.  Normally, the warmer Asian Pacific is where convection and rainfall are found, while the atmosphere over the Mexico and California Pacific coast remains is more stable. Events in the earth’s atmosphere make a kind of domino effect – what happens one place affects what happens elsewhere – which in turn changes something else… but the end result for us is more moist warmer than normal  air being driven ashore to bring us more rain than usual.

Although there have been no major storms so far this season there has been a non-stop series of medium sized  storms every few days since early December.The Sierra Mts. have been plastered with heavy snowfall and, for the first time in several years, the water content of the snowpack is above normal for the season at this time.  Good news for skiers to!

weather map 1.14.15 9 PM

My favorite weather prediction site in Weather Underground  (Just enter your ‘zipcode’ and you can get local information. ) You can even see a video clip of the last half hour of Doppler radar images showing changing precipitation patterns in your area. From this you can tell the direction of rainfall patterns – and make a good guess whether you have time to ‘walk the dog’ before the rain will hit your area.
Everybody loves to complain about the weather predictions they receive but when you look at a Doppler storm map, you see that such things as the vageries of cloud patterns, topography, rain shadows… causing there to be a mosaic of ever changing rainy patches that are quite random and unpredictable.  When they say 20% chance of rain - that means that, on 10 days with conditions like this there will most likely statistically be 2 days with rain - in the region - but not necessarily on your rooftop... its all statistical probability.

I appreciate the rain we have received (and there may be more in February and March)…but our water shortages will not go away with one season – Look at the state of our mostly empty reservoirs…

So at this point we have about 8.2“ of precipitation in Fremont … Normal average for this date is 10.3”… Season normal 16.7 inches…  'They say' we might get a good storm Sunday and Monday (see chart above) but weather people have a pattern of being really optimistic about the likelihood of future rain - only to change it and easy "well, you should at least get .5 inches.."  We will see...

So 40 days and 40 nights might do it...