Storms Inside Storms: How Hurricanes Spawn Tornadoes

As Tropical Storm Lee makes landfall over the Louisiana coast and moves inland into southeast Louisiana, Mississippi, and Alabama, primary concerns include extensive flooding and damage from looming tornado weather. Tweet

Tropical cyclones, or hurricanes, actually produce conditions amenable to tornado genesis (Watch a video). As tropical cyclones make landfall, storm forces begin to decay, and wind speeds near the land surface fall off more quickly than wind speeds at higher altitudes due to friction at the ground level. This vertical gradient in wind forces, as well as extreme changes in wind direction (veering winds and shears – McCaul 1991) that occur throughout such a widespread storm system as a tropical cyclone on the scale of hundreds of kilometers in diameter (NOAA FAQ), produces the wind shear necessary for tornado formation. Strong wind shears combined with tropical storm thermal instability at low altitudes translate into the formation of small supercell storms that can spawn tornadoes (NOAA FAQ).

Tornadoes are favorably formed in the front-right quadrant of a Northern hemisphere hurricane that has hit land. This is due to favorable conditions of strong vertical wind shears, strong vertical component gradients, and strong convergence on the cumulus scale (air moving from different directions collides, causing air to spiral upwards in cumulus convection – Engle – Hurricane Science Center) in the outer bands of the storm in the front-right region. Tropical storm-generated tornadoes typically occur during or right after a hurricane center crosses the coast, and occur in regions where cyclone winds and moisture have spent a rather minimal time on land, for example near the coast in the front-right quadrant.

Each tropical cyclone that makes landfall along the U.S. Gulf Coast spawns at least a few tornadoes (McCaul 1991, McCaul 2004), where storms traveling inland from the Gulf Coast typically produce more tornadoes than storms that skirt along the Atlantic Coast, due to a greater percent of the cyclone circulation that moves over land. Intense hurricane-strength tropical cyclones can produce severe tornado outbreaks, with more than 24 individual tornadoes (McCaul 1991). For example, Hurricane Ivan in 2004 caused such a severe outbreak, spawning 117 tornadoes that killed at least 8 people and injured another 17 (NOAA FAQ). Tropical storm-generated tornadoes are often more difficult to predict and detect than traditional tornadoes, due to the weaker nature of minisupercell storms that form them as observed on radar, and a general absence of the severe lightning and thunder seen during typical tornado weather.

Improved forecasts of “tropical cyclone tornado outbreak environments might also have implications for evacuation strategies in coastal areas about to experience a major tropical cyclone landfall.” – McCaul 2004

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References:

1) McCaul, E. W., Jr., D.E.Buechler, S.J.Goodman, and M.Cammarata (2004): “Doppler radar and lightning network observations of a severe outbreak of tropical cyclone tornadoes.” in press Mon. Wea. Rev.
2) McCaul, E. W., Jr. (1991): “Buoyancy and shear characteristics of hurricane—tornado environments.” Mon. Wea. Rev., 119, pp.1954-1978
3) Novlan, D.J. and W.M. Gray (1974): “Hurricane-spawned tornadoes” Mon. Wea. Rev., 102, pp.476-488
4) Gentry, R. C. (1983): “Genesis of tornadoes associated with hurricanes.” Mon. Wea. Rev., 111, pp.1793-1805

Images:
a) Structure of a hurricane – National Science Digital Library
b) Tornadoes can also form over the water, called ‘waterspouts’. Image of a waterspout near the Florida Keys. (wiki Commons)
c) Vertical movements of air associated with convergence and divergence – National Science Digital Library
d) Layers of the Troposphere, showing the Friction Layer: Within the friction layer, circulation patterns of wind will be influenced by friction with land or water. – National Science Digital Library
e) Video Link

McCaul, E. W., Jr., D.E.Buechler, S.J.Goodman, and M.Cammarata (2004). Doppler radar and lightning network observations of a severe outbreak of tropical cyclone tornadoes Mon. Wea. Rev.