Thursday, June 9, 2011

Yosemite Flood Watch

NWS has issued a Flood Watch for our part of the Sierra starting Friday evening. We are getting warmer weather this week and the Merced River has started rising again, reaching 4000 cubic feet per second (cfs) at Pohono Bridge (the west/downstream end of Yosemite Valley) last night. The average annual high water flow for the Pohono gauge is just 2500 cfs.
As has been said elsewhere, on top of our tremendous snowpack, it's been an unusually cool and wet spring. The normal warming (and therefore snowmelt) trajectory has not happened; there is still a huge quantity of snow in our higher elevations. We have just a little less snow now than what we usually have in April; the combination of June heat and April snowpack suggests powerful possibilities. The rate of Merced runoff may get interesting in the next few days.
We had two spikes of just under 5000 cfs in early May. Then we had another cool stretch and on the typical annual highest water date of May 20, the river was running only about 1800 cfs, 72% of average volume.
NBC Nightly News aired a feature last week about Yosemite's big waterfalls during a period when they were running right about average for springtime. The exceptional flows may have already happened, or may be coming within this next week. Though we're now 3 weeks past the usual highest flow date, we may not yet have seen our biggest water volume for the season. In 2010 we topped out at near 8000 cfs; if it stays warm (like June usually does; our forecast continues warming for the next 5 days) we may surpass that and see the Merced back up through Wosky Pond on to Northside Drive.

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