Matt,
As you know, the Hendrickson hatch can be in full swing by April 15th on the lower Main but may not end in Deposit until Memorial Day.
As for timing of hatches, I use plant activity as a general marker for hatches. I just watched bit on the weather channel and based on 165 years of data, the cherry blossoms are still 5-7 days ahead of schedule. Locally, I use forsythia as an indication the Hendrickson hatch has started.
I wouldn't bet that the hatches will be too far off of normal timing.... barring any freakish weather. Flow volumes have just as much impact on timing and intensity.