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for a specific type of severe weather are favorable or expected but not occurring or imminent. Warnings are issued when conditions conducive to a particular kind of severe weather are occurring or impending. These warnings and watches are the primary means of communicating the risk of severe weather to the public.
In analyzing data from January 1, 2003, through July 31, 2019, our goal was to find areas that have high numbers of watches and warnings. The typical GIS workflow would be to intersect the 7.5 million storm watch and warning polygons. The solution: create a grid of hexagons (approximately 10,000 hexagons, each one about 1,000 km2) covering the United States, then intersect the hexagon grid with the 7.5 million warning and watch polygons using the ArcGIS Pairwise Intersect tool. Pairwise Intersect distributes the intersection work across multiple logical computer cores and employs an efficient
overlay algorithm. The final step is to use the Sort and Summary Statistics tools to calculate the number of warning and watch polygons under each hexagon and the predominant warning or watch event type.
Because the NWS issues 19 different advisories, watches, and warnings, using an intuitive color palette is necessary (e.g., winter events are blue, wind events are orange and pink, and floods are mostly in muddy water hues). These colors must look cohesive and yet differentiate from one another without individually dominating the map (e.g., dense fog, represented by the color purple, must not appear to outcompete high surf, represented by the color turquoise). This need is particularly true with severe thunderstorms. A significant portion of the map has this predominance, but by selecting a navy blue instead, the color is present on the map without dominating it.
THE PREDOMINANCE MAP
This map confirms that many warnings and watches along the coasts are primarily small craft or marine advisories. Within the continental United States, thunder- storm watches and warnings dominate the eastern half of the country. Winter weather dominates in the northern Great Plains, transitioning to red flag (indicating extreme fire danger) and flood warnings in the more arid South.
Data courtesy of the Iowa Environmental Mesonet.
Technology Showcase 201