Rules of Thumb for Weather Forecasting

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Rules of Thumb for Weather Forecasting

• AOS 452
• 11 October 2005

Theres a 50 chance of (insert weather condition
here)
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• Rules of Thumb for Weather Forecasting
• from U. of Wisconsin AOS 452
• (used by permission of Prof. Martin)?

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Also contains common sense techniques which are
more than Thumb Rules.
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Rule of thumb definition

• A rule of thumb is an easy-to-remember guideline
  that is not necessarily a hard-and-fast rule or
  scientific formula but more than just a dumb
  guess
• Oxford English Dictionary first finds an instance
  of the phrase in 1692
• What he doth, he doth by rule of Thumb, and not
  by Art. Sir William Hope, The compleat
  fencing-master

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Forecasting temperature

• Cloud cover
• Wind speed and direction
• Advection of cold/warm air
• Mixing
• Local effects (sea/lake breezes, downslope flow)?
• Low-level moisture
• With no air mass change, overnight low will be
  dewpoint at 5 PM
• Surface characteristics
• Vegetation
• Wetness
• Snow cover
• Urban vs. rural

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Thermal Wind/Thickness (dc)

• Really, geostrophic shear
• Veering/Backing advection rules are a
  straightforward application of this concept you
  learned in Dynamics and they work. Backing with
  increasing height CAA. Veering WAA.
• Thermal wind blows along thickness contours
  (cold to the left)
• Thickness is a useful representation of layer
  mean temperature.

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Forecasting clouds and precipitation

• Moisture
• Clouds gt 70 RH at 700 mb
• Precipitation gt 90 RH at 700 mb
• Dew point depression less than 10C indicates
  moisture availability
• Lifting mechanism
• Fronts
• Convergence zones (and boundaries dc)?
• Daytime heating
• Orography
• Stability
• Amplitude modulator
• No thunderstorms when 700 mb temperature gt 12C
  and/or CIN
• gt 50 J/kg

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Forecasting movement of weather systems

• Fronts speed is 125 of ground-level
  cross-frontal wind behind the front
• Caution near topography
• Isallobars provide direction of future movement
  of lows and highs
• Move in a line along max/min couplet
• Lows tend to travel in the general direction and
  at 70 of speed of the 700 mb wind
• Lows move parallel to isobars in the warm sector
• Large disturbances tend to move more slowly than
  smaller disturbances
• Adjacent lows tend to merge

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D(prog) / Dt

• Track the performance of models through time
• Is the model handling the situation well?
• Extrapolation of forecast trends shown to have
  little forecast value
• (Hamill 2003, Wea. Forecasting)?
• Used 2070 850 mb forecasts from January-March
  over 23-year period
• Note that this is different from known model
  tendencies
• (which should always be considered)

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Forecasting methods

• Climatology

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Forecasting methods

• Climatology
• Persistence
• Today equals tomorrow

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Forecasting methods

• Climatology
• Persistence
• Trend
• Determining the speed and direction of movement
  for fronts, high and low pressure centers, and
  areas of clouds and precipitation to predict
  where those features will be at some future time

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Forecasting methods

• Climatology
• Persistence
• Trend
• Analog
• Pattern recognition

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Forecasting methods
A forecaster on a roll gathers no MOS (Prof.
Morgan)?

• Climatology
• Persistence
• Trend
• Analog
• Numerical
• Model output statistics
• (MOS)?

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Forecasting methods

• Climatology
• Persistence
• Trend
• Analog
• Numerical
• Ensemble
• Spaghetti plots
• Probability density functions

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Forecasting winter precipitation type

• From A Comprehensive Winter Weather Forecast
  Checklist by John Gordon (NWS-SGF)
  http//www.crh.noaa.gov/sgf/papers/wwchklst.htm
• Not applicable to mountainous regions