Rules of Thumb for Weather Forecasting

1
Rules of Thumb for Weather Forecasting

• AOS 452
• 11 October 2005

Theres a 50 chance of (insert weather condition
here)
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 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
• Daytime heating
• Orography
• Stability
• Amplitude modulator
• No thunderstorms when 700 mb temperature gt12C
  and/or CIN gt 50 J/kg

5
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

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

Critical thickness Rain/snow line
1000-500 mb 5400 m
1000-700 mb 2840 m
1000-850 mb 1300 m
850-700 mb 1540 m
850-500 mb 4100 m
700-500 mb 2560 m