As I recall from when I followed the supercomputer space in more detail, weather was one of the areas that tended to use specialized supercomputers like IBM BlueGene rather than big clusters because the weather models were harder to parallelize. Go back in time and I'm sure you'll see quite a few high-end IBM pSeries. Go further back and there will be things like Cray vector machines.
No it is inherently a well parallelizible problem, it is just that code was originally written for the original vector supercomputer CPUs (Cray, NEC SX) and had to be rewritten for MPI style clusters.
It ironic because now we have to rewrite everything to use SIMD/vector accelerators (GPUs).
