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WX-352 Weather Forecasting. PG-13 Weather Joke.
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PG-13 Weather Joke • This had most of the state of Michigan laughing for 2 days and a very embarrassed female news anchor who will, in the future, likely think before she speaks.What happens when you predict snow but don't get any. A true story...A female news anchor that, the day after it was supposed to have snowed and didn't, turned to the weatherman and asked: "So Bob, where's that 8 inches you promised me last night?" Not only did HE have to leave the set, but half the crew did too they were laughing so hard!
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 • 4) Translate forecast into something that is useful
Persistence Forecast Method 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 Forecast Method • Determining motion of systems, and projecting that motion into the future • Frontal and air mass movement
Analogue Forecast Method Figure 14.7
Forecast Using Climatology • Using long-term average weather to predict future conditions • Most useful for long-range forecasting • Includes probability forecast methods
Forecasting Using Probability 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)
While NWP has greatly improved the accuracy of weather forecasts, it still haves significant limitations Usually the best method for forecasting weather 12 hours to 5 days in the future NWP--Limitations
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 “Spaghetti Chart” helps forecaster to know where/when to rely on the NWP model, and where/when to ignore the NWP model
30-90 day outlook of weather conditions Doesn’t give a specific forecast Warmer or cooler (wetter/dryer) than normal Uses ensemble modeling and analogue methods Slightly better than climatology alone These 90 day outlooks were downloaded after Christmas, 2005. How accurate do you think they were? Weekly and Monthly Forecasts Figure 14.8A Figure 14.8B
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; NWP Ensembles
Accuracy and Skill • How good are weather forecasts?? • 0-24 hours: very good • 2-5 days: fairly good • Beyond 7 days: slightly more accurate than persistence • Accuracy—what is considered “right”? • If the TAF called for rain, and it rains in FWB (but not at HRT), the forecast is wrong • Skill—how complicated/hard is the forecast • Southern California in summer…easy or difficult? • Exactly where and when a tornado will touch down…easy or difficult? • Where they are likely to form--not difficult 3 days in advance • Precise area struck--fairly easy a few minutes in advance • Forecast skill is determined by comparing our forecast against either persistence or climatology • If we’re no better than persistence or climatology—why bother?
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
Forecast Practice Scenarios
Forecasting with Surface Charts This simplified 6 AM Tuesday surface weather map is useful for short time interval predictions of fronts and associated weather.
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.
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.
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
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 Look at 500mb chart for clues:
500 mb Chart for 4 PM Sunday 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….
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
< VFR Conditionsceiling (broken or overcast cloud bases) are more than 1,000 feet above the ground, and the visibility is three miles or more, the weather is VFR.
Types of Flight Weather Briefings • Briefings and Advisories: All pilots must obtain a formal weather briefing during preflight. Based upon the following pilot's information a briefing is given: • Type of flight (VFR or IFR), aircraft type, departure and destination airport, flight route, flight altitudes, estimated time of departure (ETD), estimated time of arrival (ETA) • There are three types of briefings that are offered to pilots by "pilot weather briefers" who have been certificated by the National Weather Service, and work at local Flight Service Stations and Weather Service Offices. • Pilots need to know which weather briefing to request: • The Standard Briefing • The Abbreviated Briefing • The Outlook Briefing
1. Standard Briefing is the most complete weather briefing and for the pilot who does not have an opportunity to gather any other weather data. Includes: Adverse weather conditions expected VFR (visual flight rules) flight not recommended Synopsis Current conditions En route forecast Destination forecast Winds aloft Notices to airmen (NOTAM) ATC (air traffic control) delays 2. Abbreviated Briefing is for pilots who need to update previous weather data they have gathered, or who only need a few bits of weather data to complete their weather picture. 3. Outlook Briefing is requested by pilots whose departure time is 6 or more hours away. The briefer then gives forecast information appropriate to the proposed flight route. This will assist the pilot in making an initial decision about the feasibility of such a flight. Types of Flight Weather Briefings
