Chapter 9: Weather
Forecasting
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Acquisition of weather information
Weather forecasting tools
Weather forecasting methods
Weather forecasting using surface charts
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Acquisition of Weather Information
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World Meteorological Organization
National Centers for Environmental Prediction
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Watches: favorable condition for potential hazardous
weather; what you need to do: pay attention
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Warnings: hazardous weather is imminent or actually
occurring; what you need to do: take action
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Advisories: like warning but for less hazardous weather,
what you need to do: take action if necessary
• The internet plays a crucial role in the global
communication of weather information.
• Go to:
http://www.weather.gov
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Weather Forecasting Tools
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AWIPS: Advanced Weather
Interactive Processing System
used by forecasters and can
process satellite, radar, surface
observation, radiosonde data and
weather forecasting model output
Sounding
Meteorgram
Q: Is there fog based on the
sounding? a) yes, b) no
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Near Orlando, FL
Hail size > 3 inch
100% probability
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Fig. 9-1, p. 238
Satellites and Weather Forecasting
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Geostationary satellites
Geostationary Operational Environmental Satellite (GOES)
Polar orbiting satellites
infrared images: estimate height; 3-D image
visible images: not useful at night
water vapor images: particularly useful for clear sky
Satellite sounder data of temperature and humidity
Q: Can a geostationary satellite cover polar regions?
a) yes, b) no
Q: Can a polar-orbiting satellite cover the whole earth?
a) yes,
b) no
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Geostationary Satellite:
cannot see polar areas;
36,000 km above
equator at fixed
location;
Same speed as earth’s
rotation
Polar-orbiting
Satellite:
850 km above
surface;
nearly follow
the meridion;
cover the
whole earth
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Fig. 9-5, p. 240
3-D infrared imagery
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Visible
Infrared
Enhanced Infrared
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Infrared water vapor channel
3-D TRMM satellite image
Go to: www.weather.gov
for real-time satellite information. Use 24 hr loop
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The Computer and Weather Forecasting:
Numerical Weather Prediction
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Analysis: final chart using available data at present
numerical weather prediction (NWP): based on computer
models, starting from initial time with “analysis” data
atmospheric models: fluid dynamics and atmospheric
physics
Progs: prosnostic chart for weather forecast for a specific
future period
• One of the world’s first computers was built for the
specific purpose of performing weather forecasts.
Q: Which one is for the current condition?
a) analysis, b) prog, c) both
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48-hr NWP
forecast of 500
mb height
12km vs 60km
grid spacing
Q: which model forecasts
the low off the west
coast better?
a) 12 km regional model;
b) 60 km global model
analysis
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Why NWS Forecasts go Awry and Steps
to Improve Them
grid spacing: models cannot resolve features within a
grid cell (e.g., 40 km for global models)
 incomplete data coverage (e.g., over remote regions)
 model deficiencies due to subgrid processes (e.g.,
clouds, land surface)
 Chaos: weather forecasts are highly sensitive to our
ability to observe the weather. Since it is impossible to
observe the weather at all places at all times, weather
forecasts will never be perfect. This is the reason that
we cannot predict the weather 1 month or 1 year from
the forecasting day.
Q: For global model with a grid spacing of 40 km, can you see
the Mt. Lemmon or Tucson in the model?
a) yes, b) no
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Why NWS Forecasts go Awry and
Steps to Improve Them
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ensemble forecasting:
spaghetti plot to
indicate the
robustness of forecast
Q: where do you have
more confidence in the
forecasting?
a) Northeastern Pacific
b) Northwestern Atlantic
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Other Forecasting Methods
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persistence forecast: using current state to predict
future; not bad for Tucson in June
trend forecast: assuming constant change rate
analogue method: search for similar chart in history
statistical forecast: routinely used; Model Output
Statistics (MOS)--correct known model errors
probability forecast: particularly for precipitation
climatological forecast: using climatology to predict
future; good for Tucson rainfall in June
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Q: What does it mean by
`chance of (steady) rain
is 60% for one area’?
a) It will rain over 60%
of the area
b) 60% chance that any
random location in
the area will receive
measurable rainfall
Q: Your friend claims that
the forecast of 50% chance
of rainfall is meaningless as
it is the same chance for
head in coin tossing. How
do you respond? a) agree,
b) disagree, c) don’t know
Pay attention to the last note
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Probability for a `white Christmas’ – 1 inch or more of snow
Q: The probability for a white Christmas is 20% in Northern
Arizona. It means a white Christmas would occur:
a) once every 5 years, b) once every 20 years
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Types of Forecasts
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very short range forecast or nowcast: 0-6 hr
short range forecast: 6 hr – 2.5 days
Medium-range (or extended) forecast: 3-8.5 days
long range forecast: 8.5 days – 2 weeks
Monthly and seasonal outlooks: above, near, or below
normal conditions
• Long-range forecasts are less specific than short range
forecasts.
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Use this figure to answer the two questions below:
Q: Precipitation outlook (left panel) for Arizona is
a) above normal, b) near normal, c) below normal
Q: Temperature outlook (right panel) for Arizona is
a) above normal, b) near normal, c) below normal
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Accuracy and Skill in Forecasting
Forecasts show skill only when they are more accurate than
a straightforward forecast (e.g., only using persistence or
climatology)
• Both persistence and climatology are surprisingly
accurate forecasting methods sometimes.
Q: If you forecast clear sky for the next three days in midJune for Tucson, and those forecasts turn out to be
accurate. Does it means that your forecasts have skill?
a) absolutely yes, b) absolutely no, c) not necessarily yes
Q: Do we have forecasting skills in predicting the weather at
noon 2 weeks from the forecasting day?
a) yes, b) no
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Predicting the Weather from Local Signs
Halo: `a halo around the
moon portends rain’ (folklore)
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• To see a halo: block out the sun
with your hand and
look at the cirrostratus clouds.
Wear polarized sunglasses if
possible.
Use typical changes
of wind, T, Td, clouds,
and precipitation
associated with cold
fronts
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Use typical changes of
wind, T, Td, clouds, and
precipitation associated
with warm fronts
Using information related to longwave cooling to predict
nighttime temperature:
Q: if you predict a clear and dry night, the nighttime
temperature would be:
a) relatively cold; b) relatively warm
Q: if you predict cloudy night, the nighttime temperature
would be: a) relatively cold; b) relatively warm
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Determining the Movement of
Weather Systems
forecasting rules of thumb:
surface pressure systems tend to move in the same direction as
the 500 mb wind; the speed at which surface systems move is
about half the wind speed at 500 mb
 using the surface chart
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• Internet now provides much of the weather information
http://www.weather.gov (briefly discuss “warnings &
forecasts”, “graphical foreasts”, …)
http://www.atmo.arizona.edu
http://www.wrh.noaa.gov/twc
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Current front and front 6 hr ago
Estimate for the next 24 hr
500 mb height
observation for 24 hr later
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