Earlier this week, I wrote about the format and wording of severe weather warnings in order to most effectively communicate the necessary information to the public. In that post (and the excellent discussion that took place in the comments), I referred several times to the problems that can arise from the shape of warnings. Before I let loose on that, let’s set some historical context. From 1953-2005, the National Weather Service issued warnings on a by-county basis. This lead to over-warning unaffected areas.
In 2005, the NWS began a pilot program to issue warnings with a forecaster-defined polygon shape. In this way, the warnings could be issued such that they reflect actual threats and not political boundaries. All forecast offices began using these “storm-based warnings” in 2007, but the system still isn’t perfect.
Even though forecasters can issue warnings that are based on the atmosphere, they still must consider the political boundaries. Many warning dissemination systems (more on this in a future post) don’t support polygon warnings, so if a warning would normally clip a corner of a county, the forecaster must consider whether or not to cut a notch out of the warning. (I asked for clarification from a friend who is an NWS forecaster. He said there’s a setting that optionally excludes tiny slivers of counties. “We try to do our best to serve two masters between county based communication systems and scientifically based warnings. The main focus at all times, however, is getting information to people who are threatened as fast as possible and in as useful a manner as possible.”)
The problem is further compounded when a storm exists along the boundary between the County Warning Areas (CWAs) of two forecast offices. Current NWS practice does not allow a warning to extend outside an office’s CWA. Since CWAs edges are determined by the county borders, they are frequently uneven. A storm may clip a small portion of another office’s CWA, and the issuing forecaster must shape the polygon to avoid that portion. In order to include said portion, the other office must issue its own warning. While offices will often coordinate when storms are near a CWA boundary, I seriously doubt that any forecasters will take the time to make sure their polygons match precisely. The resulting discontinuity can be confusing to the public and makes absolutely no sense from a threat perspective. The storm does not respect political boundaries.
Removing the county-border issues from the polygon system still doesn’t lead to perfection. Polygons themselves suffer from some issues. Notably, they’re ripe for being over-large in order to improve verification scores, as Patrick Marsh posted earlier this week. The other key concern is that, as I mentioned above, some warning systems have no concept of polygons. NOAA All-Hazards Radio (also known as “weather radio”) is a prime example., as are outdoor warning sirens (some locations have the ability to sound sirens selectively, but it is by no means ubiquitous). Until these systems are modernized, even the best polygons will still lead to over-warning.