Weather Studies Introduction to Atmospheric Science American Meteorological Society

1. Chapter 13 Weather Analysis and Forecasting Weather StudiesIntroduction to Atmospheric ScienceAmerican Meteorological Society Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point • Tornado forecasting began in the late 19th century • Started by John P. Finley, U.S. Army Signal Corps • 1884: about 950 “tornado reporters” were gathering data • Finley established criteria for a valid tornado sighting • Finley’s forecasting lasted only 2 years • U.S. Army Signal Corps discontinued the program in 1886 • The word “tornado” was disallowed in Signal Corps forecasts because it might cause the public to panic • Late 1940s and early 1950s, Air Force meteorologists Fawbush and Miller developed a method for forecasting tornadoes • Forecasts were primarily issued for military installations • Interest in tornado forecasting stemmed from tornado that struck Tinker Air Force Base in Oklahoma City, OK on 20 March 1948 • Developed a list of six atmospheric conditions that preceded tornado outbreak • Those conditions reappeared 5 days later • Issued first tornado forecast • Eventually, U.S. Weather Bureau adopted/allowed tornado forecasting for public distribution • Severe Local Storm Warning Center (now the Storm Prediction Center) was established in Norman, OK

3. Driving Question • What is the process involved in making a scientific forecast of the weather? • Weather effects everyone • Forecasts quite accurate out to 7 days • Weather prediction will never be perfect • Incomplete information on initial state of atmosphere • Some scientific questions not yet answered • Short range forecasts still surprisingly accurate • U.S. National Weather Service (NWS) • An agency of the National Oceanic and Atmospheric Administration (NOAA) • Issues 24-hour forecasts with 85% accuracy • This chapter covers: • How forecasts are made • Limits of forecast accuracy • Making your own weather forecasts

4. International Cooperation • International Meteorological Organization (IMO) founded in 1878 - IMO became the World Meteorological Organization (WMO) • Headquartered in Geneva, Switzerland • Agency of the United Nations • Coordinates the efforts of 189 nations and territories in global weather-monitoring program called World Weather Watch (WWW) - WWW makes meteorological information available internationally • Global Observing System • Data from 5 geostationary and 4 polar orbiting satellites,10,000 land stations, 7,000 ships at sea, reconnaissance and commercial aircraft, radar, radiosondes, and buoys • Data transmitted to 3 WMO Centers where maps and charts are created. Forecasts can then be prepared. • Maps/forecasts are sent to Regional Specialized Meteorological Centers. • National Centers for Environmental Prediction (NCEP) responsible for U.S. forecasts

5. International Cooperation - Weather forecasting entails: • Acquisition of present weather data • Graphical depiction of the state of the atmosphere • Analysis of data and maps • Prediction of the future state of the atmosphere • Dissemination of weather information and forecasts to the public

6. Acquisition of Weather Data • Surface Weather Observations • Over 2,000 stations across the U.S. operated by: • National Weather Service (NWS) personnel • Staff of other government agencies, including the Federal Aviation Administration (FAA) • Private citizens or businesses • Automated stations also located in unmanned locations • National Data Buoy Center • Data gathered for preparation of weather maps and forecasts, exchange with other nations, and use by aviation • Observations taken simultaneously • Use Coordinated Universal Time (UTC)

7. Surface Weather Observations, cont. Automated Surface Observing System (ASOS) and communication ports Result of 1990s NWS modernization 862 ASOS units in continual operation Reports temperature (ambient, dewpoint), pressure (sea-level, altimeter setting), wind (direction, speed), precipitation accumulation, visibility, obstruction to vision, present weather, and sky condition Automated Weather Observation System (AWOS) Similar to ASOS Located at 429 smaller airports NWS Cooperative Observer Network Volunteers Provide daily precipitation, and temperature readings Acquisition of Weather Data

8. Acquisition of Weather Data • Upper-Air Weather Observations • Radiosondes • Radio-equipped instrument package • Transmits upper air information to a ground station (rawinsonde observation) Map showing locations of rawinsonde stations. Sites in red are part of the International Global Climate Observing System

9. Weather reported by each observation station is depicted on a map by a station model Weather Data Assimilation, Depiction and Analysis

10. Surface weather maps Isobars Connect points of equal air pressure Isobaric analysis reveals locations of anticyclones (highs) and cyclones (lows), troughs and ridges, and horizontal pressure gradients Weather Data Assimilation, Depiction and Analysis

11. Weather Data Assimilation, Depiction and Analysis • Surface Weather Maps, continued • Cyclone centers are indicated by the symbol L (Low) • Closely spaced isobars around cyclone indicate steep pressure gradient and strong winds • Fronts originate at storm centers • Anticyclone centers are mapped as an H (High) • Usually a relatively weak horizontal pressure gradient, shown by widely spaced isobars, resulting in weak or calm winds • Synoptic surface maps are drawn every 3 hours for North America, and every 6 hours for the Northern Hemisphere

12. Weather Data Assimilation, Depiction, and Analysis • Upper-Air Weather Maps • Plotted on constant- pressure surfaces • Height contours labeled in meters above sea level, drawn 60 m apart • Altitude of pressure surface varies primarily because of mean temperature differences • Air pressure drops more rapidly in cold air than in warm, due to density differences • Isotherms plotted as dashed lines Sample 500-mb analysis (NOAA)

13. Weather Data Assimilation, Depiction, and Analysis Upper-Air Weather Maps, continued Cyclonic and anticyclonic curvature shown in contours by troughs and ridges in the prevailing westerlies Center of a ridge is relatively warm with high height contours, labeled with an H. Often linked to a warm-core anticyclone at the surface. Center of a trough relatively cold with low height contours, labeled with an L. Often linked to a cold-core extra-tropical cyclone at the surface. Winds that blow across isotherms produce air advection Cold air advection occurs where winds blow from colder to warmer locations Warm air advection occurs where winds blow from warmer to colder locations

14. Deluge of real-time weather information spurred the development of computerized data management systems McIDAS (Man-computer Interactive Data Access System) Ingests weather data and organizes into guidance products for potential users AWIPS (Advanced Weather Interactive Processing System) Used by NWS Offices since 2000 Receives and organizes ASOS data plus analysis and guidance products from NCEP Allows meteorologists to display, process, and overlay images, graphics, and other data Weather Data Assimilation, Depiction, and Analysis AWIPS workstation

15. Weather Prediction • Hydrometeorological Prediction Center • Issues short range 12, 24, 36, and 48-hr forecasts • Medium range 3- to 7-day extended outlooks issued each day, and outlooks up to 10 days are issued 3 times a week • Climate Prediction Center • Generates monthly (30-day), seasonal (90-day) outlooks, and multi-seasonal outlooks

16. Numerical Weather Forecasting Computers are programmed with a numerical model of the atmosphere; a model of mathematical equations that relate wind, temperature, pressure, and water vapor concentration Uses present data to predict values of atmospheric properties for a grid of points on a uniform pressure surface Millions of computations go into 12, 24, 36, and 48-hr forecasts North American Mesosscale Model (NAM) Divides troposphere into 60 vertical layers Forecasts every 6 hours out to 84 hours Nested Grid Model (NGM) Atmosphere divided into 16 vertical layers Issues forecasts twice a day Weather Prediction

17. Weather Prediction • Eta Model • Greater resolution than NGM, consists of 60 levels • Runs 4 times per day • Rapid Update Cycle (RUC) • 50 levels with a horizontal resolution of 20 km • Short range (3–12 hrs) for general and aviation forecasts • Frequency is every 1–3 hrs • Global Forecast System (GFS) • Has two 64 level models operating at different resolutions and forecast periods 17

18. Weather Prediction • Techniques used to optimize the skill of weather forecasting based on numerical models: • Ensemble forecasting • Numerical model generates several forecasts based on slightly different initial conditions • If forecasts are consistent, they are considered reliable • Model Comparison • Comparison is made among forecasts produced by different models • If they agree, the forecast is issued with a high level of confidence • If forecasts are inconsistent using either technique, forecast is considered unreliable

19. Weather Prediction • Forecasting Tropical Cyclones • U.S. Army Signal Corps was in charge of observation and forecasting early on • 1873: gathered reports from Cuba to help detect tropical cyclones • 28 September 1874: first plotting of a hurricane • 1890: forecasting moved to civilians (U.S. Weather Bureau) • Little attention paid to tropical cyclones • War caused increased interest in tropical cyclone forecasting • 1898, Spanish-American War; fear that a hurricane could destroy U.S. fleet • Increased number of weather stations in the Caribbean • Technological advances greatly benefited understanding and monitoring of tropical cyclones • Invention of radio allowed ship-to-shore reports • 1930s: upper air was monitored • 1950s: weather radar at coastal stations observed tropical storms • 1960s: remote sensing via satellites began • Recently, buoys have provided additional information • Aircraft can now deploy dropwindsondes (similar to a radiosonde) to receive sounding from inside storm

20. Weather Prediction • Forecasting Tropical Cyclones, continued • 1940s: Atlantic hurricane forecasting split between Weather Bureau offices in Miami, New Orleans, Washington, DC, Boston, and San Juan • 1967: designated Miami office as the National Hurricane Center (NHC) • Today, forecasting is split between NHC and the Central Pacific Hurricane Center in Honolulu. • NHC responsible for issuing statements for tropical cyclones in Atlantic basin and eastern Pacific basin to 140°W • CPHC activated when tropical cyclone develops in central Pacific • Operates SLOSH model for prediction of storm surges • NHC is a branch of the Tropical Prediction Center (TPC) • TPC, under WMO agreement, has responsibility for prediction of tropical cyclones for 24 nations

21. Weather Prediction • Forecasting Tropical Cyclones, continued • Biggest challenge is predicting track and intensity • Forecasts issued every 6 hours. Covered 72 hr periods until 2001 when period was extended to 96 and 120 hrs • Track forecasts based on climatology, numerical models, and experience of forecaster Track forecast for Hurricane Katrina issued at 5 p.m. EDT on 26 August 2005. The white area (“cone of uncertainty”) encompasses possible variations in the track based on output of numerical models. (NOAA, NHC)

22. Weather Prediction

23. Weather Prediction • Forecasting Tropical Cyclones, continued • SLOSH (Sea, Lake and Overland Surges from Hurricanes) model predicts location and height of storm surge • Probability forecast included in advisory statements since 1983 • Hurricane Watch: winds of at least 119 km (74 mi) possible with in the next 36 hours • Hurricane Warning: hurricane conditions expected in 24 hours or less • Watches and warnings also issues for tropical storms

24. Weather Prediction During the 20th century, tropical cyclone fatalities in the U.S. trended downward, whereas property damage trended upward

25. Storm Prediction Center (SPC) Located in Norman, OK Forecasts severe storms Also monitors fire weather, blizzards Other Forecasting Centers • Aviation Weather Center (AWC) • Located in Kansas City, MO • Supports FAA • Forecasts for aviation interests

26. River Forecast Centers (RFC) 13 centers located nationwide Develops river, reservoir, and flood forecasts Monitors and forecasts river discharge and stage Marine Forecasting Ocean Prediction Center is located in Camp Springs, MD Issues forecasts, warnings, and guidance for mariners, fisheries, and recreational boaters Other Forecasting Centers Locations of RFCs and their regions of responsibility

27. Weather Prediction • Forecast Skill • Weather forecasting skill declines rapidly for periods longer then 48 hrs, and is minimal beyond 10 days • Missing or inaccurate observational data • Failure to detect all meso-scale and micro-scale circulation systems • Imprecise equations in numerical models • 1- to 5-day forecasting has shown slow but steady improvement • Better understanding of atmospheric processes • Larger and faster computers • More reliable and sophisticated observational tools • Including Doppler radar and remote sensing by satellite • Denser weather observational networks worldwide

28. Weather Prediction • Computers aren’t likely to replace humans • The best forecasters rely on knowledge, experience, and intuition • Begin with previous and current observations • Must analyze and interpret computerized predictions • Adapt those forecasts to regional and local circumstances

29. Weather Prediction • Long-Range Forecasting • Climate Prediction Center • Camp Springs, MD • Prepare 30-day (monthly) 90-day (seasonal) and 15-month (multi-seasonal) generalized climate outlooks • Long range forecasting relies on teleconnections: a linkage between changes in atmospheric circulation occurring in widely separated regions of the globe • 30-day (monthly) outlooks rely on circulation patterns at 700 mb level • Identifies areas of persistent warm and cold air advection • 90-day (seasonal) outlooks rely on long-term trends and recurring events • Computer attempts to match past trends with present conditions • 15-month (multi-seasonal) outlooks began in 1995 • Each month 13 forecasts are issued, each covering a 3-month period. Subsequent 3-month forecast overlaps the previous by two months.

30. Weather Prediction

31. Weather Prediction • Single-Station Forecasting • Short-term weather prediction based on observations at one location • Forecasts usually generalized and tentative • Fair-weather bias • Fair-weather days outnumber stormy days almost everywhere • Predicting all fair-weather days would be correct more then half the time • Persistence • Weather episodes persist for some period of time (i.e., if it has been cold and storm for several days, it may continue that way for awhile) • Climatology • Forecast prepared based on previous years weather

32. Weather Prediction

33. Weather Prediction • Private Sector Forecasting • Television and radio stations, some newspapers, and private forecast services • Some private meteorologists tailor forecasts to specific needs of their commercial, agricultural, or industrial clients • Supplement the efforts of government forecasters

34. Communication and Dissemination • NCEP maps and charts transmitted to local NWS Forecast Offices to guide meteorologists in preparing local forecasts • Weather information then distributed to the public • When hazardous weather threatens, NWS issues: • Outlooks: provided for advanced notice • Watches: hazardous weather is possible based on current or anticipated conditions • Warnings: hazardous weather is occurring in the region or imminent • Advisories: anticipated weather hazards; less serious then those covered by warnings

35. NWS also issues: Tornado Warning: detection of a thunderstorm that is known or likely to produce a tornado Heavy Snow Warning: snowfall of at least 10 to 15 cm (4 to 6 in.) expected in less then 12 hrs Blizzard Warning: blowing or falling snow with sustained winds of 56 km (35 mi) per hr or higher, reducing visibility to less then 400 m (1300 ft) Flash Flood Watch: flash flooding possible within watch area Flash Flood Warning: dangerously rapid rise in river level is imminent or occurring Public receives weather reports and forecasts via radio, NOAA weather radio, TV, Internet, newspapers Communication and Dissemination