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Weather Forecasting


Rumor has it that the TV station still has not hired another Weather Forecaster. ... From 1991 to the overthrow of Sadaam, Iraq did not transmit weather data. ... – PowerPoint PPT presentation

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Title: Weather Forecasting

Chapter 9
  • Weather Forecasting

A dangerous Job?
  • Francisco Arias Olivera was a popular TV
    personality in a small Peruvian town (Sicuani)
    Population (21K). One day Francisco forecast a
    two-inch rainfall but instead they got 19 inches
    of rain. The local river flooded the town
    washing away 250 homes and killing 17. Outraged
    citizens stormed the TV station, and lynched
    Francisco. Six people charged with his murder
    were released after pleading justifiable
    homicide. Rumor has it that the TV station still
    has not hired another Weather Forecaster.

Why make a forecast?
  • Forecasts are issued to save lives, save property
    and crops and to tell us what to expect in our
    atmospheric environment.
  • Who among us does not look at the weather at some
    time or other during the week?

Science or Art?
  • Weather forecasters are responsible for
    predicting weather accurately so thousands (if
    not millions) of people will know what to expect
    in the weather.
  • But weather forecasting is not an exact
    scienceevery now and then we fail to get it
    right. There is hard science behind every
    weather forecast primitive equations, computer
    models, calibrated equipment etc. However, there
    is an art in making a forecast, interpreting the
    current and forecast conditions. The atmosphere
    is not a Closed system. We cannot track every
    short wave or minor perturbation in the
  • When we are wrong we are subject to jokes and
    ridicule and even anger.
  • Oh but when we get it rightit can be the best
    feeling in the world.

Acquisition of Weather Information
  • Over 130 nations
  • Over 10,000 land sites and hundreds of ship
    observations report each day
  • All in a common format
  • WMO is responsible for the international exchange
    of weather data and certifies that observation
    procedures do not vary among nations
  • Not everyone plays ball all the time. From 1991
    to the overthrow of Sadaam, Iraq did not transmit
    weather data.

  • Weather info from all over the world is
    transmitted electronically to NCEP located in
    Camp Springs Maryland.
  • NCEP relays the weather info to various public
    and private agencies for use in preparing local,
    regional and global forecasts.

  • Comprised of 9 centers which provide a wide
    variety of national and international weather
    guidance products to National Weather Service
    field offices, government agencies, emergency
    managers, private sector meteorologists, and
    meteorological organizations and societies
    throughout the world. NCEP is a critical national
    resource in national and global weather
    prediction. NCEP is the starting point for nearly
    all weather forecasts in the United States.
  • Office of the DirectorAviation Weather
    CenterClimate Prediction CenterEnvironmental
    Modeling CenterHydrometeorological Prediction
    CenterNCEP Central OperationsOcean Prediction
    CenterSpace Environment CenterStorm Prediction
    CenterTropical Prediction Center

Climate Prediction Center
The Modern Forecaster
  • Has access to hundreds of maps, charts, vertical
    atmospheric profiles (soundings), satellite
    images, Doppler Radar, etc.
  • Many radio and TV stations hire private
    meteorologists or meteorological companies to
    make their own forecasts with the aid of NCEP
  • Some stations use untrained announcers to simply
    read forecasts from the National Weather Service
    or private companies (Accu Weather)

National Weather ServiceWatches, Warnings and
  • Watch indicates that atmospheric conditions are
    favorable for hazardous weather to occur over a
    particular region during a specified time
  • Warning indicates that hazardous weather is
    either imminent or actually occurring within a
    specified forecast area.
  • Advisories are issued to inform the public of
    hazardous driving conditions caused by wind,
    dust, fog, snow, sleet, rain or freezing rain.

Types of Watches, Warnings and Advisories
  • Wind Advisory sustained winds reach 25 to 39
    mi/hr or gusts up to 57 mi/hr
  • Wind Chill Advisory wind chill of -30F to
  • Winter Storm Warning- expect snowfall of 4 or
    more in 12 hours or 6 or more in 24 hours
  • Flash flood watch/warning
  • Snow advisory
  • Severe thunderstorm watch/warning
  • Blizzard Warning falling or blowing snow and
    winds of at least 35 mi/hr frequently restrict
    vsby to less than ¼ mile for several hours
  • Others on page 233 of text

Forecasting Methods and Tools
  • In the 50s many short ranged forecasts where made
    by moving the existing systems along at a steady
    rate (up two over one)
  • In late 70s and into the mid 80s many weather
    maps and charts were still plotted and analyzed
    by hand
  • Modern Computers and present observing techniques
    make todays forecasts much better than those of
    the past.

Numerical Weather Prediction
  • Each day many thousands of observations
    transmitted to NCEP are fed into high speed
    computers that are used to prepare Numerical
    Weather Predictions.
  • Computers analyze data and use it to predict the
  • Routine daily forecasting of weather by the
    computer has come to be known as Numerical
    Weather Prediction
  • Atmospheric models simulation of the
    atmospheres behavior by mathematical equations

  • 1755 Euler develops first equation of fluid
  • 1827 and 1845 Navier-Stokes add molecular
    viscosity terms to fluid equations.
  • 1904 Bjerknes proposes a paradigm shift from
    empirical forecasts to ones applying basic laws
    of physics.
  • 1922 L. F. Richardson calculates using crude
    method first numerical forecast using the
    primitive equations (PE).
  • 6 weeks to crank out a 6 hour forecast
  • Pressure calc an order of magnitude in error

  • Computers arrive in 1940s
  • Von Neumann, Charney, Rossby, Eliassen, and
    Platzman attempt to use computer resources to
    solve PE.
  • Realized need to simplify due to scarce computer
  • 1950 first successful numerical forecast
  • Took 24 hours to crank out a 24 hour forecast
  • 1952 Computer technology advances cut 24 hour
    forecast time to 5 minutes
  • 1954 Services for JNWPU. Attempt baroclinic
    model run and not successful (no skill over
    empirical methods)
  • 1963 First successful baroclinic model 6-level
  • 1976 Limited Fine Mesh model started up. Some
    skill, phased out in 1994.

  • 1980 12-level Global Spectral Model
  • Consists of 2 parts
  • Aviation forecasts in support of aircraft out
    to 5 days
  • MRF Medium Range Forecast out to 15 days
  • 14 minutes runtime/day 3.5 hours for 15 day
  • 1990 Nested Grid Model (NGM) introduced
  • Early 90s ETA Model introduced
  • 32 km resolution and 45 layers
  • 1999 Meso-ETA introduced
  • 10 km horiz grid, 60 layers

ECMWF European Center for Medium Range Weather Fo
Reading England
Each ob must be corrected for
Time Location Quality Compared to those around
it and to previous forecasts This is done for ove
r 6 million obs per day
O Model Grid Points
X Observations
(No Transcript)
Pronostic Chart (PROG)
  • NWP provides a series of charts or numerical
    output for a specified period, 12, 24, 36, 48, 72
    hours etc out to 7 to 10 days in some cases
    longer 16-30 days.
  • The final forecast chart representing the
    atmosphere for a specified time in the future is
    called a PROG

Two surface pressure and precipitation progs for
2000 EST, September 29, 2003 48 hours into the
future. Prog on left is Navy Operational Global
Atmospheric Prediction System (NOGAPS) model,
whereas prog on right is the Global Forecast
System (GFS) model from NCEP
Why Forecasts go Wrong_at_!!
  • Flaws in computer models limit the accuracy of
    the forecast
  • Models idealize the atmosphere, making certain
    assumptions that may be correct in some
    instances, but wrong in others.
  • Data sparse regions of the world leave gaps in
    our observations
  • Models cannot accurately interpret many factors
    that influence weather, may not handle terrain
    features well, may not have the resolution to
    get the local weather right
  • Chaos Theory a butterfly flaps his wings in a
    rainforest in South America and your forecast in
    Asheville goes to pot. Small disturbances in the
    atmosphere, not picked up by the model, can ruin
    your forecast

Tools for Forecasting the Weather
Advanced Weather Interactive Processing System -
Can process DOPPLER, ASOS, ingest models, etc.
Handles and displays obs
and weather maps.
WSR-88 Doppler Radar
  • Doppler radar data from Melbourne, Florida, on
    March 25, 1992, during a severe hailstorm that
    caused 60 million in damages in the Orlando
    area. In the table near the top of the display,
    the hail algorithm determined that there was 100
    percent probability that the storm was producing
    hail and severe hail. The algorithm also
    estimated the maximum size of the hailstones to
    be greater than 3 inches. A forecaster can
    project the movement of the storm and adequately
    warn those areas in the immediate path of severe

  • WSR-88D
  • Doppler Radar

  • Meteogram illustrating predicted weather elements
    at Buffalo, New York, from 1 P.M. January 18,
    1999, to 7 P.M. January 20, 1999. Notice that at
    7 A.M. on January 19, the weather is projected to
    be sea-level pressure 1007 mb cloud height 2000
    ft. southwest winds at 15 knots visibility 10
    miles light snow air temperature 33F dew
    point 24F and the minimum temperature for the
    day should be 30F. The forecast is derived from
    the model output statistics (MOS) of the Nested
    Grid Model (NGM). 

Profiler winds for California
Upper air sounding
  • A sounding of air temperature, dew point, and
    winds at Pittsburgh, PA, on January 14, 1999.
    Looking at this sounding, a forecaster would see
    that saturated air extends up to about 820 mb.
    The forecaster would also observe that
    below-freezing temperatures only exist in a
    shallow layer near the surface and that the
    freezing rain presently falling over the
    Pittsburgh area would continue or possibly change
    to rain, as cold easterly surface winds are
    swinging around to warmer southwesterly winds

Major Types of Satellites
  • Geostationary

Polar Orbiting
Very important for covering vast areas of globe.
70 of earth is watervery few surface
observations over the oceans
GOES (Geostationary Operational Environmental
  • Orbital period exactly matches the rotation of
    the earth
  • Altitude of (35,800 km) or 22,000 miles
  • Appears to "hover" over one spot on the Earth's

Global GOES Coverage
GOES Images
  • Altitudes usually range from 700 to 800 km
  • Satellites in this category include NOAA, DMSP,
    Landsat, and SPOT
  • Slightly more than 14 orbits in a single day.

Defense Meteorological Satellite Program (DMSP)

Meteorological Uses
  • Cloud cover, types, obstructions to visibility
  • Tropical Cyclones
  • Low pressure systems at sea
  • Winds velocity
  • Rain rates
  • Sea heights

Other Forecasting Methods
  • Persistence Forecast easiest way to make a
    forecast. Simply predict future weather will be
    the same as present weather.
  • Steady-state or trend Forecast uses the
    principle that surface weather systems tend to
    move in the same direction and at approximately
    the same speed as they have been moving. Today
    using this method for very short duration
    forecasts is called nowcasting.
  • Analogue Method relies on the fact that the
    existing features on a weather chart (or series
    of charts) may strongly resemble features that
    produced certain forecast conditions in the past.
    Ive seen this situation before, and this is
    what happened

Other Forecasting Methods
  • Ensemble forecasting approach based on running
    several forecast models or different versions
    (simulations) of a single model each beginning
    with slightly different weather information to
    reflect errors inherent in the measurements. If
    at the end of a specified time, the models match
    each other fairly well, the forecaster can issue
    a forecast with high confidenceif they dont

Other Forecasting Methods
  • Climatological forecast a forecast based on the
    climatology (average weather) of a particular
    region. I.e. It rarely ever rains in Los
    Angeles in July and August. Stats from all past
    years indicated it is not prudent to forecast
    much rain in these months.
  • Probability forecast a forecast of the
    probability of occurrence of one or more of a
    mutually exclusive set of weather conditions.

Probability Forecast
  • Probability of a "White Christmas" - one inch or
    more of snow on the ground - based on a 30-year
    average. The probabilities do not include the
    mountainous areas in the western United States.

Accuracy and skill in forecasting
  • 12-24 hour forecasts are quite accurate
  • 1-3 day forecasts are fairly good
  • 7-10 days are getting better but still often fall
    into the smoke and mirrors category.
  • What qualifies as a right or wrong forecast? If I
    forecast 65oF for a max temp tomorrow and we get
    66oF did I blow it??
  • We say a forecast shows skill when it is more
    accurate than a forecast based only on
    persistence or climatology.
  • Today we often accurately predict large scale
    weather events many days in advance (major snow
    storms, blizzards, hurricane strikes)

Book of Signs
  • Theophrastus, student of Aristotle, 300 BC
    compiled all sorts of weather indicators in his
    Book of Signs
  • Ways to foretell the weather by examining natural
    signs shape of clouds, intensity at which a fly
  • Our own folklore halo around the moon portends
  • See Table 9.2 on page 248 of book
  • Discuss how knowing model of a mid latitude low
    can help foretell weather. SW winds before
    front, NW after, etc.

Weather forecasting using surface charts
  • Surface weather map for 600 A.M. Tuesday. Dashed
    lines indicate positions of weather features six
    hours ago. Areas shaded green are receiving

Determining the Movement of Weather Systems
(Rules of Thumb)
  • For short intervals, storms and fronts tend to
    move in the same direction and at the approx.
    same speed as they did during the previous 6
    hours unless there is reason to indicate
  • Lows tend to move in the direction that
    parallels the isobars in the warm air ahead of
    the cold front
  • Lows tend to move toward the region of greatest
    surface pressure drop, highs tend to move toward
    the region of greatest surface pressure rise.
  • Surface pressure systems tend to move in the same
    direction as the wind at 5500 m (18,000 ft) the
    500 mb level. The speed at which the surface
    systems move is about ½ the speed of the winds at
    this level.

Lows move toward pressure falls
  • Isallobars - lines of equal 3-hour pressure
    change - for 600 A.M. Tuesday. The "F"
    represents the region of greatest pressure fall,
    while the "R" shows the region of greatest
    pressure rise. A plus 2 indicates a rise of 2 mb.

Movement based on 500 mb
  • A 500-mb chart for 600 A.M. Tuesday, showing
    wind flow. The light red L represents the
    position of the surface low. The winds aloft tend
    to steer surface pressure systems along and,
    therefore, indicate that the surface low should
    move northeastward at about half the speed of the
    winds at this level, or 25 knots.

Rain of snow for DC on Wednesday?
  • Projected 12- and 24-hour movement of fronts,
    pressure systems, and precipitation (shaded green
    area) from 600 A.M. Tuesday until 600 A.M.
    Wednesday. Tuesdays weather chart on right.

Increasing clouds today and continued cold. Snow
beginning by early Wednesday Morning possibly cha
nging to rain. Winds will be out of the
southeast. Pressures will fall
Chicago on Wednesday?
  • Projected 12- and 24-hour movement of fronts,
    pressure systems, and precipitation (shaded green
    area) from 600 A.M. Tuesday until 600 A.M.
    Wednesday. Tuesdays weather chart on right.

Cloudy and cold with light snow beginning by noon
becoming heavy by evening and
Ending by Wednesday morning. Total accumulations
will range between six and ten
Inches. Winds will become strong and gusty out
of the east or northeast today,
Becoming northerly tonight and northwesterly by
Wednesday morning. Pressures will
Fall sharply today and rise tomorrow.
How about Denver?
  • Projected 12- and 24-hour movement of fronts,
    pressure systems, and precipitation (shaded green
    area) from 600 A.M. Tuesday until 600 A.M.
    Wednesday. Tuesdays weather chart on right.

Clear and cold tomorrow. Northerly winds today
becoming light and variable by
Tonight. Low temperatures tomorrow morning will
be near Zero. Pressure will Rise.
Surface weather map for 600 A.M. Wednesday.