Rules of Thumb for Weather Forecasting AOS 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-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

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 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 mb forecasts from January- March 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)  Not applicable to mountainous regions Critical thickness Rain/snow line mb 5400 m mb 2840 m mb 1300 m mb 1540 m mb 4100 m mb 2560 m