[photochemsyn]

That's a highly controversial claim that would need a whole host of published peer-reviewed research papers to support it. Physics-based simulations (initial state input, then evolve according to physics applied to grids) have improved but not really because of smaller grids, but rather by running several dozen different models and then providing the average (and the degree of convergence) as the forecast.

Notably forecast skill is quantifiable, so we'd need to see a whole lot of forecast predictions using what is essentially the stochastic modelling (historical data) approach. Given the climate is steadily warming with all that implies in terms of water vapor feedback etc., it's reasonable to assume that historical data isn't that great a guide to future behavior, e.g. when you start having 'once every 500 year' floods every decade, that means the past is not a good guide to the future.

[yorwba]

Given 50 states and independent weather in each state, on average one state would experience each "once every 500 years" extreme weather event every decade. Of course in reality weather is not independent across political borders, but there are also many more locations where flood levels can be measured than just one per state. So depending on the details "once every 500 years" may not be as rare as it sounds, even without deviation from historical patterns.