9. TOPOGRAPHY AND CLIMATOLOGY

A forecaster should have a through knowledge of local topography and it's influence(s) on severe weather. This section will briefly touch on some aspects of topography. It is not meant as a thorough treatment of the subject.

A. Topography

1. Mountains

Mountain thunderstorms can form due to a variety of mechanisms such as orographic lifting, lee side convergence, channeling, and wake effects. Investigation of the terrain associated with each genesis region, as well as an understanding of the ridge top flow, allows the identification of the predominant mechanisms activating these regions (Barker and Banta 1985). A variety of severe weather can occur with mountain convection, including significant tornadoes (Evans and Johns 1996).

2. Desert

Over the southwestern United States thunderstorms tend to occur either in association with baroclinic disturbances in the late fall, winter, and spring or during the summer monsoon season. Mesoscale features such as the Gulf of California surge and diurnal wind circulations near complex terrain, influence the development and evolution of convection (Stensrud 1996).

3. Sea Breeze/Lake Breeze

Without the effects of topography, a sea/lake breeze develops a very frontal nature, reflecting the shape of the coastline. Sufficient surface convergence is the primary focusing mechanism responsible for development of precipitation as the boundary moves inland. Many studies have been accomplished on this warm season phenomenon, most recently Kelly et al. (1998) concluded for undisturbed east coast sea breeze days, the K index was found to best discriminate thunderstorm days from other days.

B. Climatology

There are a number of resources for severe weather climatology available . In Missouri, Hatch (1996) did a county-by-county severe weather analysis over 100 years for the Springfield, Missouri County Warning Area (CWA). His paper is available at: http://www.crh.noaa.gov/sgf/papers/svrtxt.htm. Darkow (1996) also did an extensive analysis of tornado climatology for Missouri for the years 1916 through 1994. Additional tornado climatology on a national scale can be found in Otsby (1993), Livingston and Schaefer (1993), and Concannon et al. (2000).

While climatological knowledge of severe local storm events is useful in the overall evaluation of the severe weather threat, the atmosphere on any given day will not likely conform to the statistical normal (Sturtevant 1995).

10. SEVERE WEATHER STABILITY INDICES

 


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