Transcript Class #34: Monday, April 7

Class #29: Friday, November 6
Weather Forecasting
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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
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Types of Weather Forecasts
• 2. Rules of Thumb
– Show skill
– More scientific than folklore
– Began in the 19th century (1800s), when
meteorology developed into a science
– Can be quite modern
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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. 13.1 for our
area
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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 10th year
– Persistence
• What already happened, does it continue?
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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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More Sophisticated Forecasting
Methods
• 4. Trend forecasts
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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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5. 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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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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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 6th type of forecasts) can be assessed
using a type of analog forecast (to be discussed in
more detail later).
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6. 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 (1st law of thermodynamics),
conservation of momentum (Newton’s 2nd
law), and conservation of water vapor.
Conservation means “all accounted for”.
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