Chapter%209:%20Weather%20Forecasting

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Chapter 9 Weather Forecasting

• Acquisition of weather information
• Weather forecasting tools
• Weather forecasting methods
• Weather forecasting using surface charts

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Acquisition of Weather Information

• World Meteorological Organization
• National Centers for Environmental Prediction
• Watches favorable condition for potential
  hazardous weather what you need to do pay
  attention
• Warnings hazardous weather is imminent or
  actually occurring what you need to do take
  action
• 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

• 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 gt 3 inch 100
probability
Fig. 9-1, p. 238
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Satellites and Weather Forecasting

• 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 earths rotation
Polar-orbiting Satellite 850 km above
surface nearly follow the meridion cover the
whole earth
Fig. 9-5, p. 240
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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

• 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 worlds first computers was built for
  thespecific 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 (L) 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

• 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

• 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) dont 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

• 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
  surprisinglyaccurate forecasting methods
  sometimes.

Q If you forecast clear sky for the next three
days in mid-June 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)

• 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

• 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
500 mb height
Estimate for the next 24 hr
observation for 24 hr later