I am an amateur astronomer living near the Washington, DC area, but I have a
question about reading weather IR satellite maps. First of all, here is the
IR map for my area:

http://weather.unisys.com/satellite/...=cur&region=at

Here's my question: The above map shows IR colors corresponding to the
highest cloud tops. Obviously, as in the description below the map, and
what I've always thought, is that from the light blue color to the yellow
would indicate precipitation, and especially thunderstorms in the dark blue
to yellow color. However, there have been times when the map was still
fully updated (which occurs hourly) that there was a thunderstorm in my area
when not even the lightest blue showed up on the map, only the gray. It's
caught me by surprise several times in the past. Why is this the case?
Other times, I have seen one of the colored areas (other than gray) over my
area and we weren't getting any precipitation at all (and yes the map was
the latest update).

If someone can explain why these anomalies -if it is one- occur, I would
greatly appreciate it. Is there another map or website I can use that shows
precipitation more accurately?

Thanks