Spatial and Temporal Distributions of North American and U.S. Surface Fronts using NOAA/NCEP WPC Surface Analyses

In graduate school, I was fortunate to have Daniel Keyser as a PhD advisor and take courses about mesoscale dynamics and mid-latitude cyclones from him. These courses outlined the history of knowledge about fronts and cyclones from the early investigations by Norwegian scholars in the 1900s through to the 1990s. At the time there were several significant studies of the distribution and frequency of cyclones/anticyclones and rapid cyclogenesis, but very little climatological information about fronts. Over the last two decades, several studies have analyzed the spatial distribution of surface frontal boundaries on the global and regional scales primarily using objective techniques for frontal identification, but none has presented an analysis of these features over the U.S. Understanding the climatological distributions and fronts has provided motivation for a number of projects that students, colleagues, and I have worked on over the last 5-10 years to study various aspects of subjectively analyzed surface fronts.

In this study the spatial distribution of NOAA/NCEP Weather Prediction Center (WPC) surface fronts in North America is presented for a 10-year period (January 2009–December 2018). Archived High-Resolution Coded Surface Bulletins consisting of the latitude and longitude points of each front are used in the analysis. Frontal segments are plotted for cold, warm, occluded, and stationary fronts. The spatial frequency of each front type is assessed for a 1.0° grid over North America and a 0.25° grid over the contiguous U.S. Additionally, the temporal evolution of occluded and warm-season stationary fronts has been subjectively identified from the archived WPC surface analyses over the eastern U.S. for smaller subset of years (6 years and 9 warm-seasons respectively).

In North America the maximum spatial frequency of all front types occurred offshore in the western Atlantic from the U.S. coast northeastward to Greenland and in the Pacific in the Gulf of Alaska corresponding to areas of high cyclone frequency. Over the U.S. the location and magnitude of maximum frontal frequencies are approximately 45 cold frontal segments per year in the lee of the Appalachian Mountains and offshore of the mid-Atlantic coast, 25 warm frontal segments per year over the northern Great Plains and along the Pacific Northwest, 110 stationary frontal segments per year in the lee of the Rocky Mountains, and 20 occluded frontal segments per year offshore in the Pacific Northwest. A seasonal analysis of frontal segments demonstrated a winter maxima of cold fronts over the Atlantic and southeast U.S., with a notable poleward retraction of the maxima during the warm season into the Great Plains.

The temporal evolution of warm season stationary fronts showed that they are a ubiquitous feature in the eastern U.S. occurring on 89% of all warm season days. A significant portion of these fronts (~52%) appeared on only one surface analyses while 26% and 9% lasted for at least 12 and 24 hours respectively. The maximum frequency of the stationary fronts occurred in July and August with minimums in April and September. In contrast, occluded fronts are most frequent during the cold season and least frequent during the late summer. However, summer occluded fronts have a slightly longer mean duration (20 h) than fronts in other seasons (~18 h). Similar to warm-season stationary fronts, most occluded fronts have a short duration (< 12 h) with 10% having a duration exceeding 36 h.