Weather Forecasting

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Chapter 9

• Weather Forecasting

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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.

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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?

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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
  atmosphere.
• 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.

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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.
  

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• 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.

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NCEP

• 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

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Climate Prediction Center
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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
  products.
• Some stations use untrained announcers to simply
  read forecasts from the National Weather Service
  or private companies (Accu Weather)

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National Weather ServiceWatches, Warnings and
Advisories

• Watch indicates that atmospheric conditions are
  favorable for hazardous weather to occur over a
  particular region during a specified time
  period.
• 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.

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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
  -35F
  
• 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

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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.

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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
  weather.
  
• 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

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SOME HISTORY

• 1755 Euler develops first equation of fluid
  mechanics.
  
• 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

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THE COMPUTER AGE

• 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
  resources.
  
• 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
  PE.
  
• 1976 Limited Fine Mesh model started up. Some
  skill, phased out in 1994.
  

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THE COMPUTER REVOLUTION

• 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
  forecast
  
• 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

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ECMWF European Center for Medium Range Weather Fo
recast
Reading England
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OPTIMUM INTERPOLATION
X
X
X
X
X
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
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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

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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
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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

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Tools for Forecasting the Weather
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Advanced Weather Interactive Processing System -
AWIPS
Can process DOPPLER, ASOS, ingest models, etc.
Handles and displays obs
and weather maps.
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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
  weather.

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NEXRAD

• WSR-88D
• Doppler Radar

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Meteogram

• 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). 

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Profiler winds for California
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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
  aloft

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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
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GOES (Geostationary Operational Environmental
Satellite)

• 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
  equator.
  

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Global GOES Coverage
GOES-10
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GOES-EAST (GOES-8)
GOES-WEST (GOES-10)
GOES-CENTRAL (GOES-11)
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GOES Images
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POES

• 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.

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POES ORBIT COVERAGE
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Defense Meteorological Satellite Program (DMSP)

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Meteorological Uses

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

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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

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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
  matchwell

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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.

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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.

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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)

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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
  bites.
  
• Our own folklore halo around the moon portends
  rain
  
• 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.

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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
  precipitation.

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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
  otherwise.
• 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.

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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.

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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.

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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
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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.
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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.
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Surface weather map for 600 A.M. Wednesday.