The three-character sequence number for SIGMETs will change from 25th March. Every phenomena will now receive a specific letter according to AMC1.MET.TR.250(c) . (see also table below).
As an example: The first SIGMET after 00:00 due to turbulence will have the sequence number U01. The second SIGMET will get U02 (even if that occurrence is in a different part of the FIR/CTA) etc.