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Rules of Thumb for Weather Forecasting AOS 452 11 October 2005 There’s a 50% chance of (insert weather condition here)… Bad GEMPAK analysis “Rule of thumb” definition ►A rule of thumb is an easy-to-remember guideline that is not necessarily a hard-andfast 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 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 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 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 D(prog) / Dt ► Track time the performance of models through 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 JanuaryMarch over 23-year period Forecasting methods ► Climatology Forecasting methods ► Climatology ► Persistence Today equals tomorrow 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 Forecasting methods ► Climatology ► Persistence ► Trend ► Analog Pattern recognition Forecasting methods ► Climatology ► Persistence ► Trend ► Analog ► Numerical Model output statistics (MOS) A forecaster on a roll gathers no MOS… (Prof. Morgan) Forecasting methods ► Climatology ► Persistence ► Trend ► Analog ► Numerical ► Ensemble Spaghetti plots Probability density functions 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 Critical thickness 1000-500 mb 1000-700 mb 1000-850 mb 850-700 mb 850-500 mb 700-500 mb Rain/snow line 5400 2840 1300 1540 4100 2560 m m m m m m