When is The Worst Weather in the Central Valley of Costa Rica?
The worst weather in the central valley of Costa Rica, San Jose, Alajuela, Grecia, Heredia, and Atenas. is September and October when the rainfall if very very heavy. You can expect heavy rainfall every day and with good luck you will get some sunny mornings with the rain coming in the afternoons and into the night. Also the weather is very cool and you will need a jacket and of course a big umbrella. The rain will sometimes cover parts of the streets, gushing from the city gutters.
So my recommendation is to try to avoid vacationing in the central valley in these months of September and October.