Download Forecasting Rules of Thumb

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