What are precursors to Yellowstone eruption?

In all the interviews with geologists regarding the Yellowstone situation, they always describe several indicators that the magma chamber may be rising. Precursors have been described as earthquake swarms, harmonic tremors, hydrothermal changes (geysers going off at different times or new geysers), lake bottom building or rising, to name the strongest indicators. Of course, they do remind us that they don't really know, because humans have not been around long enough to track supervolcanic eruptions, and these indicators could just as easily mean the restless giant is only yawning and may go back to sleep.

So far at Yellowstone there have been swarms, harmonic tremors, hydrothermal changes, and also ground swell.

"The Yellowstone “supervolcano” rose at a record rate since mid-2004, likely because a Los Angeles-sized, pancake-shaped blob of molten rock was injected 6 miles beneath the slumbering giant, University of Utah scientists report in the journal Science." Source

"The upward movement of the Yellowstone caldera floor – almost 3 inches (7 centimeters) per year for the past three years – is more than three times greater than ever observed since such measurements began in 1923, says the study in the Nov. 9 issue of Science by seismologist Robert B. Smith, geophysics postdoctoral associate Wu-Lung Chang and colleagues."

""Our best evidence is that the crustal magma chamber is filling with molten rock," Smith says. "But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again," he adds."

Now we have this, above. A recent graph from USGS illustrating discharge cubic feet per second from Yellowstone Lake.

The trend upward from the past 79 years of historical data started at the same time as the swarm of quakes underneath the lake. The triangles on the graph indicate the average for the date over the years. Why is there more water pouring out of Lake Yellowstone? Some say melting snow. The problem is the temp there has been in the 20's. The lake bed rising would be one explanation. Click chart to enlarge, source data here

Alternately, and more likely, may be a pending hydrothermal explosion in which underground water encounters a hot spot and blasts through the surface. Small hydrothermal explosions producing craters a few feet wide occur in Yellowstone perhaps once or twice a year. Large hydrothermal explosions leaving craters the size of a football field occur every 200 years or so, according to a 2007 paper co-authored by Heasler, Lowenstern and others, says this news article.

Whatever is going on there...is still going on. What it adds up to is anybody's guess..including the geologists.