Not a March Miracle, but Mighty Marvelous

rainy season not over yet

As we await the arrival tonight of a rare April "atmospheric river", let's take a quick look back at March.

News outlets across the state (Sacramento Bee, San Diego Union-Tribune, San Francisco Chronicle) have been reminding us last month was no "March Miracle". Fair enough, but it was mighty fine - even "marvelous".

1991 was the year when late winter storms forever ensconced the term "March Miracle" into the lexicon of hydrologists and their groupies. 1991 was the fifth year of the 1987-1992 drought and, through February, was the driest year of the century (yes, drier than 1977 or 2015 at that point). Then came a series of very welcome March storms, drenching California from Calexico to Happy Camp. March 1991 was not just a wonderfully wet March in an otherwise drought year, it was the wettest March ever in some parts of the State.

March 2018 is modest by comparison, but marvelous nonetheless. It doesn't rank among the wettest of all Marches in recorded history, but it does rank near the top in how much it has changed an otherwise dismal water supply forecast.

Figures 1, 2 and 3 below show years in the hydrological record where March rain has been particularly plentiful in a relative sense - when the amount of rainfall in March has increased annual rainfall by the greatest percentage. Each bar shows the magnitude of precipitation for the water year to date (October-March), by month, for the years where March rainfall accounted for the highest proportion of the total. The percent values, shown in the label along with the year, represent the percent of rainfall in March compared to that between October 1 and the end of February.  These percentage values do not rank as high in 2018 as they did in 1991, but they are the second highest values recorded in the San Joaquin Valley and the Tulare Basin.

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Ah, but the astute among you, who have read down this far, are wondering how the prognosis for the snowmelt season, April-July, has changed. Wonder no further, simply check out Figure 4. Note that for many rivers in the central Sierra, the forecast has doubled. And that's not counting the gully washer that starts tonight

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Finally, we are pleased that San Francisco's junior water rights on the Tuolumne have already reached the point where its storage will be maxed out in 2018. Note that Restore Hetch Hetchy's legal case for restoration has nothing to do with whether this year or any other is wet. But we are always happy for San Francisco, as well as all communities, farmers, fisheries and wildlife when we have a wet winter.