Here are those previously alluded-to mitigating factors:

1.  Deriving consensus estimates for Q2 GDP was a bit of a shot in the dark due to unknown impact of stimulus checks

2.  Factoring in stimulus plus a strong export situation, we knew this would be the strongest GDP reading in quite a while 

3. Remember that this is the ADVANCE reading on growth, the final reading won't be out until 9/26, AND on average, the final is revised downward by half a point.  

4. Analyst expect near zero growth in Q3 and Q4

So who knows!  We may, YET AGAIN, see a day with stronger than forecast economic data, and STILL maintain relatively decent levels in MBS.  Now that little issue we've been talking about--historically wide spreads combined with waning headline risk--really comes in to play.  To whatever extent overly bullish economic data causes selling pressure in fixed-income, MBS are probably looking at slight advantage over treasuries.  Indeed we are seeing a slight tightening this morning after yesterday's day-trading brought us back to roughly even spreads day over day.

Today and tomorrow will shake things up, but as far as today is concerned, with the "biggie" reports already out, it's now up to market reaction (and potential impact of the 5 year UST auction later today) to inform trading patterns.  As long as you have a "refresh" button on your web-browser, and can remember where to find this site, you'll be well-informed.  Stay tuned...