weather geekery.

When the National Weather Service issues a tornado warning — as it did for Allegheny County just a few minutes ago — that means one of two things has happened: someone has actually spotted a funnel cloud and reported it, or NWS meteorologists have seen something ugly on the radar.

Here’s what they’re looking at:

That’s a cap from the NWS taken a couple minutes ago. This isn’t the radar we see on TV; it’s called storm relative velocity, and it shows the forecasters how the air is moving in relation to their radar. The red returns, in general, are moving towards the local NWS station, in Moon Township; the green returns, in general, are moving away. The brighter the colors, the faster the air is moving.

You can see a couple things in this shot. First, the whole storm is rotating; you can tell by how the green has wrapped around the bottom of the red blob in northern Washington County. That’s an indication of a pretty strong storm.

But that red splotch in south-central Allegheny County is the one that prompted the tornado warning. That shows the NWS folks there is a tight area that is rotating rapidly; if it hasn’t already produced a funnel cloud, it could at any time.

We have access to all this stuff, if you’re as interested in the weather as Weather Freak Boy is. The NWS Pittsburgh home page is here; click on the radar bottom of the page and you’ll find a whole mess of different radars, all of which include loops and the ability to zoom on a particular spot, in the menu on the left side of the page.

Or, you could just huddle in the basement. Which is probably what Weather Freak Boy should be doing right now.