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As I understand it, the F- levels are determined, not by the wind speed or size of the tornado, but by the damage it causes. So the F-4, F-5, F-whatever, is only anounced AFTER the tornado has passed and the damage is assessed.

Willing to be corrected on this, but that's what I heard a local weather dude say.

You are probably correct. I am going by what the experts said months after the tornado had struck. Everything above ground where it hit was totally destroyed.

The Fujita Scale

F-Scale Number Intensity Phrase Wind Speed Type of Damage Done F0 Gale tornado 40-72 mph Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.

F1 Moderate tornado 73-112 mph The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.

F2 Significant tornado 113-157 mph Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.

F3 Severe tornado 158-206 mph Roof and some walls torn off well constructed houses; trains overturned; most trees in forest uprooted

F4 Devastating tornado 207-260 mph Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.

F5 Incredible tornado 261-318 mph Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-enforced concrete structures badly damaged.