Drylines

[Dean Baron]

Just wondering why it is that drylines usually produce a large amount of severe weather. Other than the sharp change in dewpoints, i dont see any other reason why these are such big producers of severe weather, compared to a cold/warm front which drastically change dewpoint and temps.

[Bobby Prentice]

Just wondering why it is that drylines usually produce a large amount of severe weather. Other than the sharp change in dewpoints, i dont see any other reason why these are such big producers of severe weather, compared to a cold/warm front which drastically change dewpoint and temps.

Thunderstorm formation requires three ingredients:

1. Sufficient water vapor (http://amsglossary.allenpress.com/glossary/search?id=water-vapor1) ("dewpoints") to produce condition 2.
2. Unstable air (http://amsglossary.allenpress.com/glossary/search?p=1&query=unstable&submit=Search). Thunderstorms are almost always associated with conditional instability (http://amsglossary.allenpress.com/glossary/search?p=1&query=conditional+instability&submit=Search).
3. A lifting mechanism to "break the cap" and release the conditional instability. These lifting mechanisms include: fronts, troughs, drylines, upslope flow, local winds (sea breeze, land breeze, lake breeze, valley breeze) and outflow boundaries from prior storms.

Thunderstorm type (multicell, supercell, squall line, etc.) is primarily determined by vertical wind shear, the type of lifting mechanism, and mesoscale (http://amsglossary.allenpress.com/glossary/search?p=1&query=mesoscale&submit=Search) influences. Supercell is the storm type which produces virtually all giant hail (1.75 inches or larger) as well as most strong and virtually all violent tornadoes. Also, a large percentage of the extreme (65kt / 74mph and greater) thunderstorm wind events are produced by supercells.

Fronts (http://amsglossary.allenpress.com/glossary/search?id=front1)are density (temperature) discontinuities that produce deep, linear lift in elongated zones (see cold front (http://geog.arizona.edu/%7Ecomrie/geog230/coldf.jpg) and warm front (http://geog.arizona.edu/%7Ecomrie/geog230/warmf.jpg)). In general, the strong the front, the stronger the lift. Frontal lift tends to produce lines of storms which are prone to competition and interference with each other. These storms often grow upscale into mesoscale convective systems (MCS) (http://amsglossary.allenpress.com/glossary/search?id=mesoscale-convective-system1) which retard the formation and longevity of supercell (http://amsglossary.allenpress.com/glossary/search?id=supercell1)storm modes.

On the other hand, the lift produced by a dryline (http://amsglossary.allenpress.com/glossary/search?p=1&query=dryline&submit=Search) tends to be concentrated in preferred point locations tied to mesoscale (http://amsglossary.allenpress.com/glossary/search?p=1&query=mesoscale&submit=Search) circulations ("dryline waves"). The lift is produced by convergence (http://amsglossary.allenpress.com/glossary/search?id=convergence1) of the low-level wind flow. Dryline lift tends to produce isolated to widely scattered thunderstorms with little or no competition or interference between storms. This supports the formation and longevity of supercell (http://amsglossary.allenpress.com/glossary/search?id=supercell1) storm modes.