Understanding the meteorology of winter storms
Winter storms can significantly disrupt commerce, and, in extreme situations, they can result in loss of life and extensive property damage. To prevent such losses, public works and public safety officials must know where to get the most accurate meteorological forecasts — and they must be able to interpret them.
The National Center for Environmental Prediction (NCEP), a division of the National Weather Service (NWS), uses current numerical weather prediction models with horizontal grid spacings of 80 kilometers to map the location and forecasted path of winter storms. Winter cyclones (so called because, in the continental United States, the horizontal winds in winter storms circulate counter-clockwise) are large enough to cover an area entailing many 80 kilometer-square grids, so they are easy to predict. However, if a prediction places an otherwise perfectly forecasted cyclone just one grid point east or west of where it actually goes, huge differences in forecast accuracy result.
In addition to cyclone path variability, other characteristics of cyclones conspire against accurate precipitation forecasts. If a forecast model has a horizontal resolution of 80 kilometers, it may miss the important air motions and precipitation production that are actually occurring in the frontal regions. Thus, many precipitation-producing processes that occur in frontal zones are “invisible” to the numerical forecast models.
The explosion of computer technology in the past decade has brought the science of numerical weather prediction to a new level of complexity and accuracy. Numerical forecast models with smaller horizontal distance between grid points can overcome some of the limitations. But, because those models employ more grid points to cover the same area, they are much more expensive to operate.
Sources of forecast information
Despite the fact that some of the most important forecast elements (such as precipitation type) are among the least skillfully predicted by numerical forecast models, the picture is not bleak. Useful information concerning precipitation probability, amount, location and time of onset is available from a number of sources in the public and private sector.
The NWS, which employs scientists who treat the numerical forecast data as “guidance” with which to make more accurate forecasts for local areas, is the main source of meteorological information. Considerable value is added to the numerical product when it is interpreted by an experienced, knowledgeable forecaster who has access to up-to-the-minute satellite, radar and aircraft observations with which to gauge the accuracy of the numerical guidance. Potential cyclone path errors and precipitation intensity in frontal regions can be tracked in the observations, thus allowing the forecaster to make accurate short-term (six- to 12-hour) forecasts even in the face of occasionally poor numerical guidance.
The NWS’s primary responsibility in the event of a winter cyclone is to issue advisories and warnings. Winter weather advisories are notices that the conditions are right for the development of hazardous conditions. A warning (such as a heavy-snow warning or blizzard warning) is issued when the threat from winter weather is imminent and usually unavoidable.
A large number of private sector forecasting firms and other businesses also provides weather information (a list can be found in the monthly publication, Bulletin of the American Meteorological Society). The Internet also is a repository for weather information. Each of the more than 100 NWS Forecast Offices (available at http://www.nws.noaa.gov/) has its own Web site with a voluminous amount of local climate and forecast information. Weather trackers should begin to create a list of appropriate sites while the weather is benign.
Optimum roadway maintenance during winter storms results from prediction of adverse weather conditions, not simply reaction to them. Knowledge about winter storms and the ability to predict them has never been higher. That, coupled with increasing access to information technology, can help cities and counties develop a solid base in effective roadway maintenance strategies.
Jonathan Martin is an assistant professor in the Department of Atmospheric and Oceanic Sciences at the University of Wisconsin, Madison. This article was adapted from an article first published in Grounds Maintenance (www.grounds-mag.com). It is reprinted with permission.