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Rules of Thumb for Weather Forecasting

Bad GEMPAK analysis. Rules of Thumb for Weather Forecasting. There’s a 50% chance of (insert weather condition here)…. AOS 452 11 October 2005. “Rule of thumb” definition.

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Rules of Thumb for Weather Forecasting

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  1. Bad GEMPAK analysis Rules of Thumb for Weather Forecasting There’s a 50% chance of (insert weather condition here)… AOS 452 11 October 2005

  2. “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

  3. 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

  4. Forecasting clouds and precipitation • Moisture • Clouds: > 70% RH at 700 mb • Precipitation: > 90% RH at 700 mb • Dew point depression less than 10°C indicates moisture availability • Lifting mechanism • Fronts • Convergence zones • Daytime heating • Orography • Stability • Amplitude modulator • No thunderstorms when 700 mb temperature >12°C and/or CIN > 50 J/kg

  5. Forecasting movement of weather systems • Front’s 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

  6. 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

  7. Forecasting methods • Climatology

  8. Forecasting methods • Climatology • Persistence • Today equals tomorrow

  9. 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

  10. Forecasting methods • Climatology • Persistence • Trend • Analog • Pattern recognition

  11. Forecasting methods A forecaster on a roll gathers no MOS… (Prof. Morgan) • Climatology • Persistence • Trend • Analog • Numerical • Model output statistics (MOS)

  12. Forecasting methods • Climatology • Persistence • Trend • Analog • Numerical • Ensemble • Spaghetti plots • Probability density functions

  13. 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

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