Chapter 9: Weather
Forecasting
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Acquisition of weather information
Weather forecasting tools
Weather forecasting methods
Weather forecasting using surface charts
Acquisition of Weather
Information
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To make accurate forecasts, we need to
know what the weather is right NOW
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A network of 10,000 land and sea based
weather stations report data at least 4 times a
day
World Meteorological Organization (WMO)
Responsible for keeping weather data from
175 nations in the same format
 Example : Baghdad History
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Acquisition of Weather
Information
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Observations come to National Center for
Environmental Prediction (NCEP)
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Analyze the data, then transmit to local public and
private offices via the National Weather Service
(NWS)
All forecasters in the country use information provided
by NCEP and NWS
Watch - issued when hazardous weather is
expected for a specific period of time
Warning – indicates hazardous weather is
imminent of occurring already
Acquisition of Weather
Information
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Advisory – to inform public is less hazardous
conditions
For example: High wind warning is issued when
sustained winds are at least 40 mph of gusts
have reaches 57 mph.
Severe thunderstorm watches are issued
thunderstorms with strong winds and large hail
are possible
Full list of watches and warnings on page 237.
Weather Forecasting Tools
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Advanced Weather Interactive Processing
System (AWIPS)
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Allows forecasters to put all tools in one program
Can look at observations, radar, maps, etc.
Meteogram – a charge that shows how variables
change for a given time
Sounding – two-dimensional vertical profile of
temperature, dew point, and winds
Weather Forecasting Tools
Weather Forecasting Tools
Satellites and Weather
Forecasting
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Before satellites, tropical storms and hurricane
went undetected. Now, they provide advanced
warning
Geostationary satellites – orbit at the same rate
as the Earth is spinning. Example : Here
Satellites and Weather
Forecasting
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Polar orbiting satellites – parallel median
lines and move from pole to pole
Orbit at a lower altitude (greater detail)
 Can get the poles with no distortion
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Satellites and Weather
Forecasting
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Geostationary operational environmental
satellite (GOES)
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New generation satellite
Can detect amount of radiation coming from clouds
Can obtain info about moisture at different levels
Infrared images – shows how much radiation
clouds are emitting. (Diff between high and low
clouds?)
Visible images – Detect radiation that is reflected
to satellite (like a photograph)
Satellites and Weather
Forecasting
Fig. 9-9a, p. 242
Fig. 9-9b, p. 242
Fig. 9-10, p. 243
The Computer and Weather
Forecasting: Numerical
Weather Prediction
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Analysis – a final interpretation of a weather chart
after human correction
Numerical weather prediction – the use of
computers to predict the weather using complex
math
Atmospheric models – mathematical models that
create a picture of the current atmosphere
Progs (prognostic chart) – a chart weather forecast
for a time in the future
• One of the world’s first computers was built for the
specific purpose of performing weather forecasts.
Why NWS Forecasts Go Awry
and Steps to Improve Them
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Bad assumptions by a
model (sometimes good
and sometimes bad)
Grid spacing varies for
each model
Chaos – small,
unpredictable changes
Ensemble forecasting
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Running multiple models
to obtain a consensus
Other Forecasting Methods
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Persistence forecast – forecast for
tomorrow will be what happened today
Trend forecast – atmos movements will
remain the same in the future
Analogue method – current patterns will
provide similar weather to similar patterns
in the past
Statistical forecast – forecasts weighted
and made by a computer
Other Forecasting Methods
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Probability forecast – gives a percent
change of something (what does 40%
change of rain mean?)
Climatological forecast – forecast based
on climatology
Other Forecasting Methods
Types of Forecasts
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Very short range forecast – forecast for up
to few hours (nowcast)
Short range forecast – generally a forecast
for up to a few days
Medium range forecast – 3 to 8 days out
Long range forecast – beyond 8 days out
• Long-range forecasts are less specific than short range
forecasts.
Types of Forecasts
Accuracy and Skill in
Forecasting
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Forecasts of a few hours to a few days are good
Forecast of a few days are better than “flipping a
coin”
Long range forecasts are generally very bad
What is a bad forecast? Is a forecast off by a
few degrees bad?
If I say that it will not rain in the summer in San
Jose, does that show skill?
The skillful forecast would be to say when it
WOULD rain in San Jose in the summer
Accuracy and Skill in
Forecasting
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Forecasts have gotten better over the past
15 years due to computer models
7-day forecast now are as skillful as 3-day
forecast 10 years ago
Forecasting thunderstorms and tornadoes
are extremely difficult
Predicting the Weather From
Local Signs
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Book of Signs was written in 300 B.C. to
try to predict the weather
Used color and shape of clouds. Also
intensity of which a fly bites
Halo around the sun – a halo around the
sun could mean rain is on the way. Halo
made by bending light through cirrostratus
Predicting the Weather From
Local Signs
Table 9-2, p. 254
Determining the Movement of
Weather Systems
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Forecasting rules of thumb
Storms are fronts tend to move in the same
direction with the same speed as previous 6
hours
 Low pressure systems move parallel to
isobars in warm air region
 Lows move where pressure drops fast, highs
where pressure rises fast
 Systems move in same direction as wind in
500-mb level
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Determining the Movement of
Weather Systems
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forecasting rules of thumb
using the surface chart
A Forecast for Six Cities
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Augusta, Georgia
Washington, DC: rain or snow?
Chicago: major snowstorm
Memphis: mixed bag
Dallas: cold wave
Denver: clear and cold
Stepped Art
Fig. 9-21, p. 256