  An analytical model of transverse convective instability of a bunch at
transition crossing is presented for the microwave case, when the wake is
sufficiently short compared with the bunch length. The space charge is assumed
to be strong, as it typically is for the low and medium energy synchrotrons. A
simple formula for the intensity threshold is obtained and its prediction is
compared with available CERN PS data; without a single fitting parameter, a
good agreement is demonstrated.
