  We report observations of energetic electron flux and Phase Space Density
(PSD) to show that a relatively weak magnetic storm with $Sym-H_{min} \approx
-50 $nT, resulted in a relativistic and ultra--relativistic electron
enhancement of two orders of magnitude similar to the St. Patrick's event of
2015, an extreme storm with $Sym-H_{min} \approx -235 nT$. This enhancement
appeared at energies up to $\approx 10$ MeV, lasted for at least 24 days and
was not recorded in geosynchronous orbit where most space weather alert data
are collected. By combined analysis of PSD radial profiles and Fokker--Planck
simulation, we show that the enhancement of relativistic and
ultra--relativistic electrons is caused by different mechanisms: first, chorus
waves during the intense substorm injections of April 21--25 accelerate the
seed electron population to relativistic energies and redistribute them while
inward diffusion driven by Pc5 ULF waves further accelerates them to
ultra--relativistic energies.
