Unlike a hurricane, which can be observed from within, a tornado is so

One problem with E is that "impractical of study" is not correct. We could say that something is "impractical to study," but with "of," the whole expression makes no sense.

So let's see, would E work if we said "Unlike a hurricane, which can be observed from within, a tornado is so small as to be impractical to study"? Not really. We use "so X as to Y" when we want to emphasize that something goes so far in the direction of X that it reaches Y. Look at the example Ashish provides above: a shift was "so gradual as to be indistinguishable." A few other examples:

The offer was so low as to be insulting.
The difference was so fine as to be imperceptible.

So basically, when we use this construction, we are simply adding detail to the original adjective. How low was the offer? Insultingly low. How fine was the difference? Imperceptibly fine. Basically, in these cases we have a long-winded way of saying something simpler: "The offer was insultingly low." "The difference was imperceptible." Note that in the official example, "so gradual as to be . . . ," the sentence is a bit harder to simplify. Perhaps the construction is used there because there's not really a clearer way to say that the shift was "imperceptibly gradual." (It doesn't make much sense when phrased like that.)

Let's compare this to "so X that Y." In that case, Y is a consequence of X. Because tornadoes are small, they are impractical to study. "Impractical to study" doesn't really amplify or build on "small"; rather, it shows a consequence of the tornadoes' size.