Analyzing Air Masses: Severe Storm Fronts and Risks Explained

Air masses, fronts, and storms (40 points) On August 22-23, 2015, a strong frontal system moved across Minnesota triggering severe thunderstorms. The purpose of this lab is to analyze convective outlook maps to assess risk from this system, document how surface weather variables changed over time, and analyze radar images as this frontal system moved across Minnesota. The Storm Prediction Center (part of NOAA and the National Weather Service) generates convective outlook maps that highlight areas at risk for thunderstorms over the upcoming one to eight days. These maps highlight risk for thunderstorms, tornadoes, damaging winds, and large hail. Go to the website: https://www.spc.noaa.gov/misc/SPC_probotlk_info.html 1. The Storm Prediction Center describes thunderstorm risk using six color-coded levels. Match the levels below with the color and description by drawing lines between each level and color and between each color and description. Level Color Description________________________________ Thunderstorm Dark Green Greater concentration of organized severe storms with varying intensity Marginal Red Limited organization/longevity/coverage Slight Orange Any area with a 10% or higher risk of thunderstorms Enhanced Light Green Severe weather outbreak with numerous intense and long- lived tornadoes Moderate Yellow Organize storms expected, but limited coverage and varying intensity High Magenta Widespread potential for severe weather including tornadoes and intense storms In advance of the passage of the frontal system you are analyzing in this lab, the Storm Prediction Center issued convective outlook maps highlighting the risk with this system. Open this website: https://www.spc.noaa.gov/products/outlook/archive/2015/day1otlk_20150822_1630.html 131

2a. Examine the “Categorical Graphic” which highlights thunderstorm risk associated with this frontal system. What was the highest level of risk forecast for this system? List all the states had this risk level. b. What was the level of risk in the Minneapolis/St. Paul region associated with this frontal system? 3a. Scroll down to examine the probabilistic tornado, damaging winds, and large hail graphics. What was the probability of tornadoes in the Twin Cities area? b. What was the probability of damaging winds in the Twin Cities area? c. What was the probability of large hail in the Twin Cities area? Examine the series of surface weather maps provided (Figures 1-9) that station model weather data and frontal locations from 21:00Z Saturday, August 22, 2015 to 21:00Z Sunday, August 23, 2015. The data shown for most station models include the wind speed and wind direction (blue), cloud coverage (blue), air temperature (red), and dew point temperature (green). Most station models also show atmospheric pressure (gold). All maps were compiled from the Surface Analysis Archive of the Weather Prediction Center (part of the National Weather Service). 4. All consecutive maps are separated by the same amount of time. Examine any two consecutive figures (such as Figures 1 and 2). How much time is there between these two maps? 5. Figure 1 is dated at 21:00Z on Saturday, August 22, 2015. When times are designated by a “Z”, it represents universal time (UTC) at the Prime Meridian that runs through Greenwich, England. During daylight savings time (CDT) Minnesota is five hours behind UTC. What time and date does Figure 1 represent in Minnesota? Examine Figure 1 (21:00Z on Saturday, August 22, 2015) to answer questions 6-15. Locate the most southeastern station model in Minnesota that matches the one shown below. You will be using this station position for each map you examine throughout the lab. 132

6. Determine the weather variables listed below based on this station model. Make sure to label units for numbers. temperature_______________ dew point temperature_______________ wind direction_______________ wind speed_______________ sky coverage_______________ pressure_______________ 7a. Approximate the range in temperatures across all of Minnesota on Figure 1. b. Approximate the range in dew points across all of Minnesota on Figure 1. 8. Is wind direction across all of Minnesota highly variable or mostly consistent? 9. A frontal boundary is shown near the western border of Minnesota on this map (the southern 2/3 of the western border). Which type of front is shown? 10a. Are temperatures west of the front (in South Dakota) higher or lower than temperatures in southeastern Minnesota? b. What is the approximate difference in temperature between southeastern Minnesota and central South Dakota? 11a. Are dew points west of the front (in South Dakota) higher or lower than dew points in southeastern Minnesota? b. What is the approximate difference in dew points between southeastern Minnesota and central South Dakota? 12. Describe the wind direction west of the front. 133

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