May 23 - July 29
Building a Climatology of Snow Squall Conditions
Ella Hunter (University of Tennessee, Knoxville), Dr. Heather Reeves (OU/CIWRO & NOAA/NSSL), Dr. Andrew Rosenow (OU/CIWRO & NOAA/NSSL), and Daniel Tripp (OU/CIWRO & NOAA/NSSL)
What is already known:
- Snow squalls are generally defined as the sudden onset of snow where the visibility decreases significantly in a short amount of time, but the exact visibility criteria used vary in the literature
- Changes in visibility increase the risk of fatal crashes on roadways
What this study adds:
- Sudden visibility drops are most common in the Intermountain West, New England, and the northern Great Plains
- Locations with the highest frequency of low-visibility snow aren’t always the locations with the most rapid decreases in visibility
- The relative frequency of sudden visibility drops changes its geographic distribution at different minimum visibility thresholds
- Raising the minimum visibility threshold defining a sudden visibility drop from ¼ statute mile to 1 statute mile increased the number of events by three to four times
Abstract:
Snow squalls represent a significant hazard to drivers on the roadways due to the sudden onset of low-visibility snow. With the goal of addressing this threat, the National Weather Service began issuing warnings for snow squalls, but the visibility criteria used for those warnings differ from the criteria in the formal literature, which also contains varying criteria. So long as this disagreement exists, it is impossible to objectively diagnose snow squalls. This study begins the process of addressing the issue of conflicting definitions by stepping back from the issue of snow squalls, and looking at sudden snow-induced visibility drops more generally. Five-minute Automated Surface Observation System observations from commercial airports across the CONUS were examined for visibility drops associated with snow consistent with snow squalls. Observations were classified as sudden visibility drop events if no snow occurred in the hour before the snow started and if the minimum visibility threshold was met within the first hour of snow. At 0.4 km (0.25 mi), the NWS visibility requirement for snow squall warnings, requiring this suddenness definition to be met reduces the number of events per year by 66-90% . Sudden drops in visibility were most common in the Intermountain West, the Northeast, the Great Lakes, and the northern Great Plains. By increasing the visibility threshold from 0.4 km to 1.6 km (1.0 mi), the number of events generally increased by a factor of four to six times, though some regions saw an even greater increase. Lake-effect areas that receive the most amounts of heavy snowfalls generally are not the locations with the most sudden drops in visibility.