Derecho (Bow-Echo) Review

(Much of this is a summary of Coniglio, M.C., D.J. Stensrud, and M.B. Richman, 2004: An Observational Study of Derecho-Producing Convective Systems. Wea. Forecasting, 19, 320–337.)

A. Definition

A derecho-producing convective system (DCS) is a type of mesoscale convective system (MCS; Zipser 1982) that produces large swaths of severe "straight-line" winds at the Earth's surface. Straight-line winds attributed to convection are typically grouped into those generated by "mesohighs" (Johnson and Hamilton 1988), "gust fronts" (Goff 1976; Wakimoto 1982) or "downbursts" (Fujita and Wakimoto 1981).

Johns and Hirt (1987; JH87 hereafter) define a derecho as a family of downburst clusters (Fujita and Wakimoto 1981) across an area whose major axis is at least 400 km (see JH87 for a more complete definition). Many types of extratropical MCSs (Maddox 1980; Zipser 1982; Parker and Johnson 2000) can produce derechos. Through a study of 70 warm season (May-August) cases, JH87 show that on radar the downbursts and strong surface winds are usually associated with "line echo wave patterns" (Nolan 1959) and/or "bow echoes" (Fujita 1978) in the leading convection.

The radar echo presentation for a DCS will include a quasi-linear convective system with bowed segments. In some cases, one prominent bow will be observed. That prominent bow is the region behind of which the strongest straight line winds associated with a downburst are surging outward.

B. Ingredients-based Methodology is Best Approach

This study shows that there are many types of flow patterns associated with DCSs. These results should make forecasters aware that derechos can develop under a variety of large-scale flow patterns during all months of the year and that the idealized dynamic and warm-season patterns discussed by Johns (1993) only depict a portion of the full spectrum of the possibilities of large-scale flow patterns associated with the development of DCSs.

Here are the important findings that can be connected to "ingredients":

Fujita, T.T., 1978: Manual of downburst identification for Project NIMROD. SMRP Res. Paper 156, University of Chicago, 104 pp

Johns, R.H., and W.D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 1, 32-49.

Johnson, R.H., and P.J. Hamilton, 1988: The relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line. Mon. Wea. Rev., 116, 1444-1463.

Maddox, R.A., 1980a: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61, 1374-1387.

Nolan, R.H., 1959: A radar pattern associated with tornadoes. Bull. Amer. Meteor. Soc., 40, 277-279.

Parker, M.D., and R.H. Johnson, 2000: Organizational modes of midlatitude mesoscale convective systems. Mon. Wea. Rev., 128, 3413-3436.

Wakimoto, R.M., 1982: The life cycle of thunderstorm gust fronts as viewed with Doppler radar and rawinsonde data. Mon. Wea. Rev., 110, 1060-1082.

Zipser, E.J., 1982: Nowcasting. Academic Press, p 191-221.