Good point. The problem with Sandy (which occurred in October) is that the normal seasonal variation in death rates is likely to mask any longer term effects on death rates. A difference with storms in Puerto Rico is that there is no winter weather, so even a small change in death rates is more likely to be noticeable.

I suspect that a January cold wave may result in more deaths than the claimed figures for Maria. Here is a link that shows that cold waves that last more than a day or two can significantly increase death rates: