How come the Tunguska fireball reached the ground before the shockwave?

In this post on his Bad Astronomy blog, Phil Plait describes the Tunguska event as having had a fireball which was followed by a shock wave:

A chunk of rock (or possibly ice) about 30 meters across—the size of a house—barreled in at a speed probably 50 times that of a rifle bullet. Ramming through the Earth's atmosphere, incredible forces compressed it, crumbled it, and when it reached a height of just a few kilometers above the ground, those forces won. In a matter of just a few seconds the energy of its immense speed was converted into heat, and it exploded.

[...] The fireball created a huge forest fire over hundreds of square kilometers of the Podkamennaya Tunguska River region of the Siberian forest ... but then the immense shock wave from the blast touched down. It blew the fire out and swept down those trees like a rolling pin, knocking down untold millions of them.

I guess the forest-fires-then-shock-wave makes sense as an explanation for the large number of scorched and partially scorched trees found at the site. However, I'm confused by the explanation - wouldn't the shockwave be the first thing to hit the ground? Can anyone in the know comment on the anatomy of this explosion?

• In analogy of nuclear explosions I would venture to guess that what is meant here is that the thermal radiation from the fireball reached the ground, first. Since this happens almost at the speed of light, it will definitely outrun the shockwave. Given that the fireball was supposed to be just a few km above ground, the delay between the source and the absorbing ground was only on the order of $10\mu s$. – CuriousOne Sep 23 '15 at 11:53
• @CuriousOne It could be (just a guess) that the nuclear explosion analogy is being wrongly applied here: my understanding is that the fireball in a nuclear blast forms pretty much instantaneously (i.e. with only $c$ delay) as X rays are absorbed by the atmosphere, which almost simultaneously becomes more opaque to them owing to chemical changes. So you have a fireball area heated up in the order of $10\mu s$ and thus the fireball would touch the ground first (nuclear blasts of this size give fireballs big enough to reach through the estimated altitude of the Tunguska bolide) ...... – Selene Routley Sep 23 '15 at 12:16
• ..... But I'm not sure what form of EM radiation is given off by a disintegrating bollide: maybe it wouldn't be as short wavelength and as strongly absorbed by the atmosphere as nuclear blast X rays: it seems to me that knowledge about the bolide's demise would determine the answer to Emilio's question. – Selene Routley Sep 23 '15 at 12:18
• @WetSavannaAnimalakaRodVance: The fireball in a nuclear explosion develops in a matter of microseconds (but it still takes some time after that to peak in total radiated power, I believe), so it would be much faster than the meteor fireball which should develop on the order of $r/v \approx 100m/10km/s = 10ms$. In that sense the delay of the radiation between the meteor and the ground probably won't matter, it's basically instantaneous. One can probably estimate the peak temperature with a black body model. I doubt that it's much more than a few 10,000K and it would cool very fast. – CuriousOne Sep 23 '15 at 12:21
• Apologies if you have already seen this, but if you look at this video, (most of it is drama,) but scroll to 40 mins on and there is a simulation of the event that might be related slightly to your question youtube.com/watch?v=HXfvhJoNi90 – user81619 Sep 23 '15 at 13:50