Class #29: Friday, November 61 Weather Forecasting

Class #29: Friday, November 62 Types of Weather Forecast 1. Folklore –Some is superstition Shows no skill Groundhog day: February 2; If the groundhog sees his shadow, six more weeks of winter –Some is based on facts Can show some skill “When spiders’ webs in air do fly, The spell will soon be very dry” Were often used by mariners

Class #29: Friday, November 63 Types of Weather Forecasts 2. Rules of Thumb –Show skill –More scientific than folklore –Began in the 19 th century (1800s), when meteorology developed into a science –Can be quite modern

Class #29: Friday, November 64 More about rules of thumb Rules of thumb are often based on the Norwegian cyclone model, and the typical behavior of air masses and fronts Rules of thumb can be very modern –Example: Dewpoint > 50°F necessary for supercell storms –Example: Decision tree in Fig for our area

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8 Types of Weather Forecast 3. Control Forecasts –These are used to measure skill. –Any method that works better than these shows skill –Climatology The average of this day for the past 30 years, updated every 10 th year –Persistence What already happened, does it continue?

Class #29: Friday, November 69 Examples of Climatology and Persistence In the tropics near oceans, climatology and persistence can be excellent forecasts, and hard to show improvement over them –Southern Florida, Hawaii, islands in the trade winds In middle latitudes under strong jet streams, weather shows wide variations and be far from persistence and climatology. –If a cold front is approaching, neither persistence nor climatology would make a good forecast

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Class #29: Friday, November 611 More Sophisticated Forecasting Methods 4. Trend forecasts –Benjamin Franklin pioneered in 1743 –Assumes weather patterns move –Assumes weather patterns otherwise unchanged –Like an extrapolation –Used today for brief times and called nowcasting 5. Analog forecasts

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Class #29: Friday, November Analog Forecasts Assume (incorrectly) that “history repeats itself” Categorizes weather into a small number of weather types Popular in the U.S. from 1935 to 1950 Involved finding a similar situation in past weather events Occasionally worked, but overblown claims led to the decline of analog forecasts for many years

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Class #29: Friday, November 617 The test of analog vs. trend forecasting D-Day, June 1944, the Invasion of Normandy Overall, trend forecasts did a better job Also, overall, weather caused a lot of damage to Allied operations in WWII. There had to be a better way, and there was. Surprisingly, analog forecasts have made a comeback.

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Class #29: Friday, November 625 The Comeback of Analog Forecasts ENSO has characteristic weather patterns that are statistically more common in El Niño, La Niño, and neutral conditions. Some of the uncertainty of numerical weather forecasts (the 6 th type of forecasts) can be assessed using a type of analog forecast (to be discussed in more detail later).

Class #29: Friday, November Numerical Weather Forecasting Rely on extensive calculations. That’s where “numerical” comes from. Use equations, including the gas law, conservation of mass, conservation of energy (1 st law of thermodynamics), conservation of momentum (Newton’s 2 nd law), and conservation of water vapor. Conservation means “all accounted for”.