This section discusses: • Various weather forecasting methods, their tools, and forecasting accuracy and skill • Images for the forecasting of 6 cities • Applications of weather forecasting
Four forecasting steps: • 1) Assess the present state, called the analysis, by plotting 6 hourly surface and 12 hourly sounding data • 2) Predict a future state by running a computer model of weather changes… • 3) Interpret the model results, called a prognostic chart, given forecasting experience…apply ROTs (Table 14.1 page 369) • 4) Translate forecast into something that is useful
Forecasting methods • Persistence • The future conditions here will be like the present conditions here • Trend • The future conditions here will be like the weather upstream is now • Analogue • The future conditions here will be like weather that historically occurred when similar conditions were present • Climatology • Future conditions here will be near the monthly average for here • Numerical Weather Prediction • Future conditions here will be as predicted by supercomputers running equations of atmospheric motion • “Reading the Sky” • Future conditions here can be predicted by using current conditions and forecasting rules of thumb (ROT)
Persistence Forecast The persistence method works well when weather patterns change very little and features on the weather maps move very slowly. It also works well in places like southern California, where summertime weather conditions vary little from day to day.
Trend Forecasting • Example: frontal movement
Analogue Forecasting Method—Regime Forecasting Figure 14.7
Probability Forecasts Figure 14.6 Climate records can be used to generate probability forecasts for a given event. Florida has less than a 5% chance of snow on December 25th, while northern Minnesota has had snow on that date for each of the past 30 years.
At 00Z and 12Z each day, data is collected and NWP models are run NWP is the 2nd largest user of super computers in the world Numerical Weather Prediction (NWP)
Local Weather “signs” • Understanding how the atmosphere works (“red sky at morning…”, halo around the moon, clear skies at sunset, etc, etc, etc) • Local rules of thumb (ROT) (for example--southerly flow in March) • Table 14.3, page 378; and appendix E (page A11) • If you have good “situational awareness” of the atmosphere, rudimentary wx measurements (temp, DP, pressure, winds), and good ROT—you can make a pretty good 0-24 hour forecast.
Forecast Periods • Nowcasting (less than 6 hours) • Mainly surface observations, weather satellite and radar, and persistence or trend forecasts • Short-range (<3 days) • Trend and analogue methods, weather charts and numerical weather forecast products • Medium range (3-8 days) • Analogue forecasts, numerical weather forecast products, climatology • Long-range (>8 days) • Climatology
Weekly & Monthly Forecasts Stationary weather systems often allow for trend based extended weather forecasts, while multiple runs of numerical weather models, known as ensemble forecasts, allow for 30 to 90 day outlooks. Figure 14.8A Figure 14.8B
Numerical weather models are run multiple times with small changes in the initial conditions Gives forecaster a good estimate of the possible range of weather conditions at a future times Ensemble Modeling
Accuracy and skill • 12-24 hours: very good, 2-5 days: fairly good, above 7 days: slightly more accurate than persistence • Accuracy—what is considered “right”? • Skill—how complicated/hard is the forecast (Southern California in summer…)? • Forecast skill is determined by comparing your forecast against either persistence or climatology • Longer-range climatological forecasting improving • Tornadoes…where they are likely to form: 3 days in advance, but precise area struck is forecastable minutes-few hours in advance
Analysis to Prognosis--NWP Figure 14.1A Figure 14.1B Two forecast model 500 mb progs for the same time. Forecast models lose accuracy over time due to errors in the model, chaos in the atmosphere, and measurement errors
Data Processing and Display Figure 14.2 Viewing weather images, overlays, and graphs in multiple windows is done using the National Weather Service's Advanced Weather Interactive Processing System (AWIPS), which gathers data from the Automated Surface Observing System among other sources.
WSR-88D Doppler Radar Weather Surveillance Radar - 1988 Doppler, also known as next generation radar (NEXRAD), detects severe weather size, movement, and intensity. Data received by the NEXRAD unit are processed by algorithms to assist the forecaster in weather interpretation. Figure 14.3
Meteogram Display Predicted trends in several weather variables are plotted for a 60 hour period on a meteogram. Patterns in variable response, such as rising pressure and a stop in precipitation, are readily observed. Figure 14.4
Probability of Precipitation (PoP) describes the chance of an event occurring at any point in the forecast area, over a certain period of time (usually 12 hours). PoP is defined as: PoP = Pa x Pc, where: Pa =the probability that precipitation will occur somewhere in the forecast area during the forecast period and Pc =the percent of the area that will receive measurable precipitation, if it does occur NWS Probability of Precipitation
Vertical Sounding Profile Radiosonde instruments attached to pilot balloons are launched twice daily to profile weather variables with height. This example shows winds veering (clockwise—warm advection is occurring) from easterly at the surface to southwesterly aloft that may change the freezing rain in the saturated lower atmosphere to non-freezing rain. Figure 14.5 Vertical soundings are also used for strength and timing of convective wx, cloud types and heights, turbulence and icing, etc.
Forecast Practice Scenarios
Surface Chart Predictions • - 3-hour pressure tendencies plotted on isallobar maps help predict the movement of highs and lows • Lows tend to move toward the region of greatest pressure fall, while highs move toward the region of greatest rise. Figure 14.10
Upper Level Charts Figure 14.11 Upper level winds guide the path of surface pressure systems.
Observed Movement of Fronts Surface weather observations from 6 PM Tuesday and 6 AM Wednesday show how the fronts, pressure systems, and precipitation have moved as predicted. Figure 14.12
Forecast • Using both trend and persistence forecast techniques, what is your forecast for 24 hours from now in: • Denver? Dallas? Chicago? Memphis? • Washington? Augusta? Wed 6am Tues 6am
Observed Weather for 6 AM Wednesday Figure 14.13
Another Scenario: • Forecast for central CA coast for tomorrow • Slowly falling barometer and increasing high clouds • Weak low pressure in Pacific..... • Will it rain, or not??
Surface Weather for 4 PM Sunday Figure 14.14 Look at 500mb chart for clues:
500 mb Chart for 4 PM Sunday Figure 14.15 564 line across the area (local ROT); Omega High present…but cold advection and shortwave may change the stagnant upper pattern; upper-level divergence??
12, 24, & 36 hour Progs Forecaster has access to several forecast models -Which one is best? -- Choice is based on initialization and experience….
Surface Weather Map for 4 AM Sunday Figure 14.17
500 mb Chart for 4 AM Monday Figure 14.18
Surface Weather Map for 4 AM Monday Figure 14.20
88% of all weather-related aircraft mishaps occurred under IMC (Instrument Meteorological Conditions) 83% of fatal crashes involved FAR Part 91 (General Aviation) aircraft Weather Impacts on Aviation Mishaps
Weather and Commercial Aviation • Direct costs due to weather on airline operations can be separated into several categories: diversion, cancellation, delay and insurance. The cost of a diverted flight can be as high as $150,000 and a cancellation close to $40,000 (Irrgang and McKinney, 1992). • A report from the Air Transport Association (ATA) states that the direct annual costs to sixteen member airlines of the first two categories listed above are $47 million and $222 million, respectively (Air Traffic Management in the Future Air Navigation System, 1994).
COMET Module--NAS • http://www.meted.ucar.edu/nas/index.htm
Weather Impacts on Economic Sectors • Oil and gas exploration: • improved forecasts of tropical weather conditions (wind, waves, disturbances) can reduce delays in drilling operations at a cost of up to $250,000 per rig per day (several thousand rigs in the Gulf). • improved hurricane track predictions could reduce days of production shutdown, each day of which costs the industry and the U.S. treasury a combined $15,000,000. • Vegetable processing: • improved temperature and precipitation forecasts can lead to greater efficiency in chemical spraying (e.g., pesticides), which costs $10-$15 per acre per application for hundreds of thousands of acres. • on a national scale the annual cost of lost production to the vegetable processing industry, primarily due to weather, is $42,500,000. • Insurance: • a single hurricane could lead to more than $50,000,000,000 in damages. • weather-related catastrophes have led to more than $48,000,000,000 in property insurance claims over the period 1989-1993.
Wx Impacts on Economy • Rail transportation: • it costs $2,000 per hour to stop a train. A single tornado warning covering 15 miles of track for 15 minutes can lead to seven stopped trains. • most weather-related derailments cost $1,000,000 to $5,000,000. • Electric power: • using improved thunderstorm forecasts could save one utility $200,000 annually in reduced outage time. • using "good QPF forecasts" could save one utility $2,000,000 over five years. • using improved temperature forecasts could save "hundreds of millions annually nationwide for the utility sector". • Aviation: • every avoided cancellation saves $40,000, every avoided diverted flight saves $150,000. • for the 16 members of the Air Transport Association, delays and cancellations cost $269,000,000 annually.
“Own the Weather” • Understand organization’s mission and operations • Decision cycles • Risk adversity • Leadership and their goals • Understand how weather impacts the organization and its competition • Taking the knowledge of weather effects, and weather—develop a weather product in the right format, and deliver to the right person, at the exact right time • Be right more than you are wrong!