Meteorological forcing of extreme winds along continental U.S. coasts

Extreme winds have important impacts on the natural and built environment, such as uprooting trees, contributing to storm surge, and damaging buildings. Most extreme winds derive from extratropical and tropical cyclones, but mesoscale features such as thunderstorms can also produce extreme winds. In this research we examine the meteorological features associated with extreme winds and how those features vary seasonally and spatially within the continental U.S. We focus on coastal areas because these regions are routinely subject to extreme winds (for example: hurricanes and nor'easters), they have a high population density, and because the coastal population (and accompanying infrastructure) is expected to continue to grow into the next decades.

We define "extreme wind" as the fastest 2-minute wind speed and direction in a given month, as reported in the Local Climatological Data publication. We compiled monthly extreme wind speeds for 50 ASOS stations along the Pacific, Gulf, Atlantic, and Great Lakes coasts from the beginning of the ASOS record (typically the mid-1990s) through December 2012. We subsequently identified 11 "representative stations" that are characteristic of the seasonal and spatial pattern in extreme winds along these coasts. For each site we selected the top-5 and bottom-5 of the fastest 2-minute winds by season (winter, spring, summer, fall) to bracket the observed range of extreme speeds. We drew on the NCEP/NCAR Reanalysis, NOAA Daily Weather Maps, radar archives, and hourly weather records to help us characterize the synoptic- and mesoscale features associated with extreme winds at these 11 stations.

Our research to date suggests that, for all seasons, extratropical cyclones are the primary driver of extreme winds along the Pacific coast. The top-5 extremes often arise from strong surface systems with distinct upper-level troughs; bottom-5 extremes may occur with strong upper-level pressure gradients not accompanied by well-defined surface systems. Similar to the Pacific coast, top-5 extremes along the Gulf and Atlantic coasts in winter and spring typically derive from extratropical cyclones with accompanying upper-level troughs; bottom-5 extremes coincide with a range of features, including surface cyclones with distinct upper-level troughs, strong upper-level pressure gradients without surface cyclones, and (in the south) from thunderstorm winds. In the summer and fall, the top-5 extremes along the Gulf and Atlantic coasts are often associated with tropical cyclones and hurricanes but can also arise from non-hurricane thunderstorms and (in the fall) nor'easters. The strongest (non-hurricane) thunderstorm winds are usually associated with extratropical cyclones with distinct frontal boundaries, suggestive of large-scale storm organization (such as mesoscale convective systems). Bottom-5 extremes in summer also arise from thunderstorms but do not always show clear synoptic-scale organization. In the Great Lakes region, the top-5 extreme winds often occur with summer thunderstorms and fall- and early-winter cyclones, when the relatively warm Great Lakes can enhance cyclogenesis or frontogenesis. In the fall and winter, bottom-5 extremes can occur with well-defined surface and upper-level systems but also with weaker surface systems with or without strong upper-level pressure gradients. Bottom-5 extremes in spring and summer typically derive from individual thunderstorms or weak synoptic-scale systems.

Because most extreme winds in fall, winter and spring are derived from extratropical cyclones, poleward shifts in storm tracks as projected under climate change may reduce the intensity of the strongest cool- and cold-season winds along U.S. coasts. Increased spring and summer temperatures, on the other hand, could enhance atmospheric instability and promote more frequent and/or stronger thunderstorms, potentially increasing the magnitude of summer wind extremes. Any increase in hurricane frequency and/or intensity also would increase the average magnitude of extreme winds along the Gulf and Atlantic coasts.