 Weather forecasters use base velocities to detect mesoscale boundaries, which separate contrasting air masses. Mesoscale boundaries are important because convergence along them has the potential to initiate deep-moist convection. A couple of common types of mesoscale boundaries are outflow boundaries and sea and lake breeze fronts. Sea and lake breeze fronts mark the leading edge of cooler maritime air pressing inland from the coast, while outflow boundaries mark the leading edge of rain-cooled air spreading out from the area where thunderstorms down-draft splashes down. So to start by examining mesoscale boundaries on radar, we'll look at this loop of base reflectivity from the radar near Chicago on the afternoon of June 24, 2009, and we'll just watch the animation here for a moment. We can get the sense that there were certainly mesoscale boundaries present on this afternoon. You can see actually some thunderstorms developed along a boundary, and then they threw off their own outflow boundaries as rain-cooled air spread out from those thunderstorms, and you can pick out the boundary here. Now, we'll go back here shortly and take a closer look at some parts of this loop to really dissect what's going on. So if we watch the loop finish here, you can see there's a sort of a complex pattern of outflow boundaries. And what really happened is the outflow boundaries from those thunderstorms merged with the original lake breeze front coming inland. So we'll go back to the very beginning, and we'll watch how this loop progresses. You can see just a few frames in that we certainly have a lake breeze front that has developed. There's this enhanced zone of reflectivity, where reflectivity is a little bit higher. That's because of things like bugs and dirt that was drawn together by the low-level conversions occurring along the boundary, and you can certainly see that boundary highlighted here. Here's a lake breeze front with this linear feature of enhanced reflectivity. As the loop goes on, you can start to see that we get some higher reflectivity, some thunderstorms developed along that boundary, and actually the radar operator switched from clear air mode to precipitation mode, which is why the appearance of the radar looks much different halfway through. And you can see the thunderstorms, and then off to the west of those thunderstorms, you have the outflow boundary, the hybrid boundary, which is the outflow boundary combined with that original lake breeze front, and that continues to press farther inland from the thunderstorms that continue to develop near Chicago on this date. Now, in this case, the boundary showed up pretty nicely on this image of base reflectivity. You can follow these arcs of reflectivity, marking those boundaries, these hybrid mesoscale boundaries, and I have those labeled for you here. You can see the boundaries there, but it really seals the deal when you look at base velocities. So here's the base velocities at this time, and you can see that we have this area of incoming velocities, inbound velocities along the radials, as screen shading meets up with an area of outgoing velocities, these positive velocities along the radials, certainly a zone of convergence here sealing the deal that this was a mesoscale boundary.