I've been shooting junior athletics & soccer with a K20D, K-5 & K-3 with a DA* 60-250/F4 lens since 2010. In heavy rain the deep lens hood on this lens works well as long as it's not too gusty.
However, after 1-2 hrs, with the internal volume change involved in continual zooming in & out, the lens will start to mist up. (At least for me, it does.) You need to get under cover, take the lens off, wipe the rear lens element and pump the zoom in & out a number to times to reduce the amount of moist air trapped inside the lens.
dosdan's Album: Fun in the Rain - PentaxForums.com
In the processing of shots taken in heavy rain, particularly from a distance (e.g. the discus shots in the album above), you need to boost the contrast and adjust the Black Point when developing the images in an attempt to cut through the murk.
Note: changing lenses in the rain is not a good idea. But if you have to do it, make sure the camera front is always facing down. If any rain spots get on the sensor, the dust/pollution in the rainwater is sufficient to leave spots on the sensor when it dries.
Dan.