Storms firing on an outflow boundary

There's a moderate risk of severe storms with a PDS (particularly dangerous situation) tornado watch out for parts of Nebraska and Kansas this afternoon.  And storms are indeed exploding across the central plains.  Here's a reflectivity image from earlier this afternoon in southern Nebraska.

Fig 1 -- 0.5 degree base reflectivity from KUEX at 2121Z, June 20, 2011.

Several powerful storms are visible in the northern part of the image, but what's also rather interesting are those two "lines" visible in the reflectivity near the radar.  These lines mark the leading edges of outflow boundaries from those powerful storms up north.  As these storms moved along, cool air sinking in the rear-flank downdraft (the back part of the storm that defines the "hook echo" in rotating storms) spreads out once it hits the ground, creating a miniature cold front along its leading edge.  This creates a boundary where air flowing out of the storm from that rear flank downdraft runs into the ambient air which is often moving in a different direction.

These boundaries and the pockets of cooler air behind them can linger around for several hours after a storm has passed.  Furthermore, since winds tend to be converging along these boundaries, they provide a focal point for new storms to fire.  Notice in the image above I circled several smaller storms that were just then forming right ahead of the eastern outflow boundary.  It's likely that convergence of the low-level winds along that boundary forced air to move upward and set the convection in motion.

We can see this convergence on the base velocity image.

Remember that the base velocity image only tells us whether air is moving toward the radar (green colors) or away from the radar (red colors).  I drew small white arrows showing the direction of motion implied by the colors in the radar velocity image above.  Notice that all the arrows should point directly toward or directly away from the radar.  In the green area, all the arrows point toward the radar.  In the red area, all the arrows point away from the radar--matching what I just said.  We can see that the arrows point toward each other on either side of the outflow boundary line.  This implies that the air is indeed converging there.

And, as an added bonus, you can see a strong velocity couplet indicating a tornado associated with one of those powerful storms to the north!

Outflow boundaries like these are often the focal point for new storms to fire, particularly on days when conditions are very unstable.  Meteorologists use the radar to see these boundaries in advance so that they know areas where storm development is increasingly likely.

But why can we see these boundaries on radar?  They're often far from where it's raining, so what's reflecting the radar beam back?  There's a lot of debate about just what makes up these radar returns, and several factors contribute.

1. The air density tends to change rapidly from the cooler air on one side of the boundary to warmer air on the other side of the boundary.  When the radar beam goes through this abrupt density change, it bends the beam somewhat--kind of like that old trick of putting a pencil in a glass of water and it looks like the pencil is broken right where it enters the water.  This bending of the beam can direct the beam into the ground, other obstacles, or even bend some of the beam back toward the radar.  This change in what is called the refractive index across the boundary is some of what makes it visible.
2. Abrupt changes in winds on either side of the boundary can kick up a lot of dust  and leaves into the air.  This puts more stuff in the air for the radar beam to bounce into.
3. A lot of people believe that insects can get "collected" on the leading edge of these boundaries.  There  can be strong winds behind the boundary that act as a short of shovel, collecting lots of insects that are flying around along the leading edge of the cooler air.  The increased concentration of insects can also increase the radar returns.

So, there are outflow boundaries.  If you're looking for places that are good for storms to start firing, outflow boundaries provide a good place to start.