An Investigation of February 1999 Chinooks Introduction Washington State Wyoming Colorado Data Synoptic Overview Mesoscale Discussion ConclusionAn Investigation of February 1999 Chinooks Laura A. Betker Department of Atmospheric and Oceanic Sciences, University of Wisconsin – Madison, Undergraduate, Madison, Wisconsin. Abstract: A downslope windstorm event in the Rocky Mountains, referred to as a Chinook occurred in early February of 1999. This paper investigates this Chinook and the high and low pressure systems on the synoptic scale that led to their formation. Cool air pools formed by high-pressure systems to the east and west of the Colorado Rockies led to a mid-level inversion that trapped lee side waves, eventually breaking into winds of more than 100 mph. This also cause a warming to the east in the Denver area and lee cyclogenesis to create a small scale low-pressure system over eastern Colorado. Introduction Over the two-day period from February 2-3, 1999 a severe downslope windstorm severely affected a large part of the western part of the United States with major damage occurring in Washington State, Wyoming, and Colorado, and minor power outages and/or damage in western Idaho and eastern Montana. Inversions are either side of the mountains were responsible for inversion formations at mid-levels that led to trapped mountain waves eventually breaking to form downslope wind storms. These windstorms occur all over the world and in the Rockies are given the special name of Chinooks. Chinooks typically occur from December through March due to the strong presence of the jet stream is frequently located directly over the region. A map of Chinook locations in combination with other strong winds found throughout the region is provided in figure 1. Found throughout the Rocky Mountains Chinooks are made of warm, westerly winds, as opposed to the East Winds, North Winds, Wasatch Winds and the Santa Ana Winds, each associated with unique wind directions, locations and temperatures.Figure 1: Typical regions affected by downslope windstorms, including Chinooks. Image Source: Jonathan Vigh. Casualties were solely made up of a litter of puppies who perished in a house fire located in Toppenish, Washington as the “wind blew papers against a heat lamp that was keeping the puppies warm of the back porch of the house,” (CNN.com). The storm moved from the Pacific Northwest into the Rocky Mountains bringing down trees power lines and roof materials with wind gusts were reported to have reached over 100 mph at various locations in the Rockies. Washington State Sustaining the most damage, Washington State recorded a wind gust of 73 mph at Otis Orchards just east of Spokane. Other gust reports around the state included 53 mph at Spokane International Airport, the highest ever recorded for that location, and 63 mph in downtown Spokane (National Weather Service). Otis Orchards……………………..73 mph Spokane International Airport……53 mph Downtown Spokane………………63 mph valanche control included the closing of ower outage occurred in N mph AU.S. 2 through Stevens Pass and ski resorts were shut down due to the high winds. Thousands of homes in Northwest Washington were without power due to fallen trees; as much as 60,000 home at one point reported by Puget Sound Energy as the high volt transmission line was knocked down (CNN.com). Wyoming Far less pOregon as some trees fell due to the winds in Cheyenne. Wind gusts included a 93 mph report northwest of Rawlins and 71 mph at the Cheyenne Airport (National Weather Service). Sixty miles of Interstate 80 was closed for a period spanning both February 2 and 3 because of blowing snow. Despite the closures, dozens of trailers were blown over along Interstates 80 and 25 (CNN.com). W of Rawlins…………………...93Cheyenne Airport…………………71 mphColorado strongest gust reported for the h The focus of this paper will be on Many sources were used to compile the data needed for this analysis of the February 2-3, 1999 downslope windstorm map cally cited as originating from nother source were created using GARP Thestate of Colorado was 119 mph at Wondervu, southwest of Boulder, and gusts of 100 to 103 mph were recorded at Longmont, Blue Mountain, and Rocky Flats (National Weather Service, UCAR). Wondervu………………..………119 mphCarter Lake………………...…….107 mph Rocky Flats…………………...….103 mph Blue Mountain……….…………100 mph Longmont………….……...……100 mph CU/Boulder…...……….………….98 mpGunbarrel……...………………….80 mph Broomfield………...……………...80 mph Second only to the damage in Washington State, damage for Colorado in and around the Boulder area, reported by the Boulder County Sheriff’s Office, included roofs blown off homes, windshields blown out of cars, and trees and power lines knocked down (CNN.com). There were large areas of power losses especially in the western suburbs of Denver. Although the National Center for Atmospheric Research (NCAR) lost its power the battery back up of the automated sensor recorded a four-second average of 93 mph just after 11:30 pm local time, 0630 UTC. This is shown in figure 2 with a meteogram of wind speed and gusts. The peak seen just past 11:30 local time is the 93 mph gust and is the largest recorded for that location throughout the storm. Damage along the front range of the Rockies totaled approximately three million dollars. Figure 2: Meteor gram of wind speed and gust speed for the NCAR facility in Boulder, Colorado. the downslope windstorm in Boulder ounty, Colorado, although similar event in the Rocky Mountains. Allbackgrounds and upper air data that are not Cdiagnoses may be applied to Washington State as well as Wyoming. Data specifiaprogram under the influence of the UW-NMS model. Unysis data was used for surface and upper air maps as individually noted. Soundings were gathered from the University of Wyoming website. Facts onwind totals and damage reports were taken from the National Weather Service and CNN.com, respectively. Individuals giving advice and/or guidance throughout the paper are cited in the section entitled “References and Acknowledgments.” Synoptic Overview The storms that had formed over the Pacific Northwest and along the west coastof the United States was brought inland bythe jet stream