We study the formation of thermal fronts in an active scalar equation that
is similar to the Euler equation. For a particular initial condition, an ea
rlier candidate for finite-time blowup, the front forms in a generalized se
lf-similar way with constant hyperbolicity at the center. The behavior belo
ngs to a class of scenarios for which finite-time blowup is impossible. A s
ystematic exploration of many different initial conditions reveals no evide
nce of singular solutions. [S1063-651X(99)10107-7].