Merging Storms

[Mike Krzywonski]

What generally happens when 2 storms from different directions intercept each other?

 

[Glen Romine]

Storm mergers are still a bit of a mystery. Often in the early stages of a severe weather event, many cells will form, but sometimes the number of storms decreases as the cells move away from the initiation region. Storm environment characteristics (shear orientation relative to the boundary and forcing; boundary layer humidity; etc...) and forcing play a significant role in convective evolution, such as whether individual cells or a squall line are favored. One of the factors that can play into cell isolation is cell mergers, as mergers many times will result in two cells becoming only one cell after the storm's cross paths (though not always). Sometimes, when two cells merge, the surviving storm can actually become more intense, though this is not typically the case apparently. The age of the two cells, angle of collision, and how well the updrafts phase together are all likely players in what happens when the cells merge. If the end result is fewer storms though, cell isolation has been shown to be an important factor in significant tornado potential.

 

[Gene Moore]

There are many different types of cell mergers, some are nothing more than storm a bumping into storm b. It's likely you're interested in supercell mergers, or storms that are within a supercell environment. If the merging cell is upstream then it tends to strengthen that storm during the merger. For example, storm a north storm -- storm b south storm. Reasons include...storm b will intensify as it merges because it comes in contact with storm a core, or rain area. The rain area generally contains outflow thus acts as a boundary to storm b. Also, it effectively increases the core size of storm b and that usually has a slowing effect. Any time a storm slows within its flow regime the convergence is increased. Also, a merger can cause storm b to move east or right of storm a, thus increasing low level shear. With regard to supercells all these are factors will likely intensify storm b. Finally storm b will become the dominant cell as it blocks the inflow to storm a.

Now here comes the hard part. If storm a has a very large mesocyclone (nearing synoptic scale) it may ingest storm b by rotating it into storm a. In this case storm b may merge as it rotates into the east side of storm a. Thus preserving the inflow area of storm a and increasing the core area of storm a. This is rare, but if you can catch it you'll likely see tornadoes. First time I saw this was during a stationary caprock supercell in 1975 that produced 10 tornadoes (that we know about). I've never put the image set from this day on my site, but it is an interesting case. We later figured out what was going on when we looked at the radar loop. Most of the merging storms also produced tornadoes. We were on the south side photographing tornadoes with some other OU chase teams, while long time chaser Jim Leonard was well northeast of us also shooting tornadoes from the merging cells.