tl;dr: Great Improvement – Looking Solid for Above Normal Water Year on the South Fork American River
The April Water Report was released late Friday April 8th, with the Water-Year forecast leaping into the Above Normal range. Specifically it’s forecasting the American River below Folsom at 2,880 thousand acre-feet, with 2,600-3,500 being Above Normal. It’s a huge 22% jump from the March forecast of 2,365 with an unimpaired runoff of 812 thousand acre-feet in March versus the forecasted 469, combining with a slight increase in future expectations. The 80% probability range is 2,592-3,580 which is about as centered within the Above Normal range as it can get.
While the numbers are promising, be careful to not let percentages play tricks with your mind. Let’s assume the remaining 20% is evenly split above and below that range. So 10% chance of Wet, and a 10% chance of Below Normal (or worse). Putting that in perspective, if we had this exact forecast ten separate years, one of those years would end as Below Normal. Which with all things considered, isn’t that bad either.
How could it drop from Above Normal to Below Normal in just one month? Of the forecasted 2,880 thousand acre-feet, only 1,678 is in the bank (having already occurred) while 42% (1,202) is based on long range weather forecast. This includes a forecast for April to have 430 thousand acre-feet of unimpaired runoff. All it would take is for April to fall short by 65% (we’ve seen that far too often, and already the first 10 days of April have been pretty dry), or a combination of April falling significantly short plus future month expectations getting reduced. Any combination that reduces the forecast by more than 280 thousand acre-feet (a 23% drop in future expectations) would cause us to drop into Below Normal, and loose Wednesday guaranteed flows.
April now has flow 7 days a week, with a little more on the weekends. Same for May, but the if the May report changes, those changes will take effect around May 13th.
Summer flows (and actually all the way through October) will lock in based on the May report, which should be released around May 10th. If the forecast stays in the Above Normal range, the South Fork of the American River will have the following guaranteed minimum flows from Saturday May 28th (Memorial Day Weekend) through Labor Day Monday September 5th: