All the coal could switch back and there will still be a shortfall this year. Remember: we're 1 TCF less than a year ago in inventory and production will be down another 750-1000 BCF between now and the end of the year or a shortfall of up to 2 TCF. All the nat gas can switch and we'd still have a big hole to fill.
I'm not sure its an exact number but more around the perception of the EIA reports. Possibly we gonna get another "better than expectations" this thurs. The key is to control the expectations by setting them as low as possible.