What are outflow boundaries?

Question

What are outflow boundaries?

Answer 1

Coming out mainly from large-scale thunderstorm complexes, also known as mesoscale convective complexes (called MCCs), outflow boundaries are imaginary bounaries often ahead of a cold front with a jet stream that allows for low-level bulk shear and causes and allows extremely rapid destabilization in areas that got hit by the first MCC, which often stabilize the atmosphere. The stabalization is short-lived and the outflow boundaries quickly destabilize the atmosphere as soon as it is stabilized because the outflow boundaries pump up very high amounts of low-level moisture at the surface and hence thunderstorms can strike or develop again as fast as one MCC pulls out.

Here is a scenario that involves outflow boundaries.

Say, one MCC hits Chicago, IL by way of a large southwest to northwest cold front. Now add a distinct area of a low-level jet stream...the right rear quadrant, or RRQ, that hits Chicago as soon as the MCC has passed, and add 70-80 degree dew points and peak heating of the sun. RRQs of low-level jets are the places, especially in the Midwest, for the highest level of bulk shear and strong buoyancy and updrafts as the warm air condenses very rapidly into first cumulus...and then cumuloninbus clouds that produce thunderstorms....especially tornadic thunderstorms.

The end result is an

outflow boundary, where, even with rain-cooled air, parameters responsible for thunderstorms are still there, even after the MCC had passed because, as I mentioned before, things like the RRQ of the low-level jet and high dew points make the atmosphere, especially at the surface, still very unstable. You can blame WAA (warm air advection) for 62 percent of this atmospheric instability....and hence, stabilizing of the atmosphere is very short-lived and is ready to be "juiced" once more and once again into quick destabilization to create more thunderstorms (hence another MCC) again, often explosively developed into supercells which can produce tornadoes.

This will create a rapidly destabilizing atmosphere as fast as 2 hours, creating another MCC in Chicago that can even be worse than the previous MCC. This is how one severe weather outbreak or tornadic thunderstorms can take place.

Answer 2

When a thunderstorm develops it possesses a cycle in which one side of the storm "sucks" in air (inflow) and the other side pushes it out (outflow).

If a storm is strong enough or has a decent mesocyclone associated, then it will likely produce an "outflow boundary".  This boundary is a cool pool of air that rushes at low elevations ahead of the oncoming storm.  This is the reason you usually feel a cool breeze that hits a good 15-30 minutes before the storm.

Answer 3

To add to what Baseball said, outflow boundaries can propagate large distances from the storms that generated them, perhaps one hundred miles or more.  If two outflow boundaries intersect that's a good place for new storms to form.