In Depth Review of April 12-13th Wind Event

Gravity waves are a relatively common meteorological phenomenon that typically develop and dissipate in the mid to upper levels of the atmosphere without causing any readily apparent effects on our daily weather.  In many ways, they can be thought of as ripples moving through the atmosphere which cause a rapid fall and rise in pressure along with a corresponding strong downward and upward motion. Gravity waves often appear in satellite images as shockwave or ripple patterns in the clouds. The figure on the right shows a gravity wave, captured by satellite imagery, causing a rippled cloud pattern off the Texas Coast.   

Gravity waves can be initiated by many different phenomena including deep convection, shear instability, momentum surges, geostrophic adjustment, unbalanced frontogenesis, and topography. However, a very special combination of atmospheric factors is required to maintain the waves and duct them to the Earth’s surface. The critical ingredient for the wave’s survival is a deep low-level inversion or stable layer, such as is found north of a surface warm front. This inversion layer must be very stable but also deep enough to contain at least a quarter of the vertical wavelength of the gravity wave. Furthermore, there must also be a conditionally unstable layer above the inversion layer to prevent the wave energy from propagating upwards and dissipating. When these conditions are met, the inversion layer provides a good medium for the continued horizontal propagation of the trapped gravity wave. Trapped gravity waves typically move at roughly the speed of the mean flow in which they are embedded.  In the April 12-13 wind event, the mean wind was likely in excess of 60 mph based on the low-level RAP forecast wind speeds.  

Gravity wave wind events are very difficult to forecast and are often ongoing before forecasters are aware of their presence. One of the key tell-tale signs of a gravity wave is a sudden drop in surface pressure ahead of the approaching wave trough followed by the rapid onset of strong winds, peaking with the passage of the trough. Behind the trough, surface pressures rise rapidly and the winds diminish. A sharp back edge or break developing in a precipitation shield, indicative of sinking motions, can be another tell-tale sign of a gravity wave.

Example of a gravity wave just off the coast of Texas caught by visible satellite

Here are some things to look for in a pattern that may support gravity waves:

• Negatively tilted, highly diffluent trough in 300 mb height field.

• Observed jet approaches a downstream axis of inflection.

• Observed jet max propagates ahead of geostrophic jet max at base of trough.

• Surface frontal boundary to the south or southeast.

Here are some of the more notable areas of damage:

• Pikeville where widespread tree and structural damage occurred, including to a marina on Fishtrap Lake and to the roof of a grocery store. An amateur radio and radio station tower were also blown over. 
• Roof damage in Hi Hat in Floyd County.
• A tree blown onto a car in Cumberland in Harlan County.
• Trees blown onto homes in Barbourville.
• Uprooted/snapped trees, power poles shifted several inches, and roof damage near Smilax in Leslie County.
• A barn destroyed in Stinnett in Leslie County.
• Roofs blown off of homes/apartments from Hollyhill to Whitley City, multiple trees downed on homes and across roadways, and hardwood trees snapped in McCreary County.
• A tree falling into a house in Corbin and one on top of a car in Williamsburg.
• A garage having its roof blown off and door blown out in Hazard.
• A tree falling onto a home in Pippa Passes in Knott County.
• A tree falling onto and destroying a home in Blackey in Letcher County.