METEOROLOGY

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METEOROLOGY

• GEL-1370

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Chapter Nine

• Weather Forecasting

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Goal for this Chapter

• We are going to learn answers to the following
  questions
• Is weather prediction an exact science? If not,
  why not?
• What are the different methods of data
  collection?
• How does weather watch differs from weather
  warning?
• What is a prog and how does it differ from
  analysis?
• How are computer-generated weather forecasts
  prepared?
• What is ensemble forecasting?
• What are the different methods used to predict
  movement of a surface mid-latitude cyclonic
  storm?

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Acquisition of weather information

• Weather forecasting is not an exact science
• Over 10,000 land-based stations and hundreds of
  ships provide surface weather info 4 times/day
  major airports conditions hourly
• WMO World Meteorological Organization over 130
  nations responsible for the international
  exchange of weather data
• NCEPNational Center for Environmental Prediction
  (NCEP) formerly called National Meteorological
  Center, in Camp Springs, MD all meteorological
  data is analyzed, prepare weather maps and charts
  and prediction of weather on a global and
  national scale begins (this transmitted to
  private public agencies)

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Weather Information forecasting tools

• Watches, Warning and Advisories NCEP issues
  advisories in the form of watches warnings
  issued to the public of hazardous driving
  conditions caused by wind, dust, fog, snow, sleet
  or freezing rain
• Watch Indicates that atmospheric conditions
  favor hazardous weather occurring during a
  specified time period
• Warning Indicates hazardous weather is either
  imminent or actually occurring within the
  specified forecast area
• Key Warnings Wind advisory, Wind-chill advisory,
  flash-flood watch and warning, severe
  thunderstorm watch and warning, tornado watch and
  warning, hurricane watch and warning, snow
  advisory, Winter storm warning, blizzard warning,
  dense fog advisory

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Watches, warnings Advisories

• Wind Advisory Issued when sustained winds reach
  25 to 39 mi/hr or when wind gusts are up to 57
  mi/hr
• High wind warning Sustained winds are gt40 mi/hr
  or wind gusts gt57 mi/hr
• Wind-chill advisory When wind-chill temp lt -30
  to -35F
• Flash-flood watch warning May result in flash
  flooding (warning) is occurring or is imminent
  in the specified area (watch)
• Severe thunderstorm watch warning winds gt57
  mi/hr and/or hail gt3/4 inch diameter have been
  visually sighted or indicated by radar

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Watches, warnings advisories contd.

• Tornado watch warning May develop has been
  spotted either visually or by radar take shelter
  immediately
• Snow advisory Expect Snowfall of 2 inch or more
  in 12 hours or 3 inch or more in 24 hrs expected
• Winter storm warning (or heavy snow warning)
  Expect Snowfall of 4 inch or more in 12 hours or
  6 inch or more in 24 hrs expected
• Blizzard warning When falling or blowing snow
  winds of gt35 mi/hr restricts visibility to lt1/4
  mile
• Dense fog advisory visibility lt ¼ mile
• Storm warning Winds gt 54 mi/hr expected
• Hurricane watch and warning When a tropical
  storm or hurricane becomes a threat to a coastal
  area when it appears that the storm will strike
  an area within 24 hrs with wind speeds gt 74 mi/hr

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Flags indicating advisories and warnings in
maritime areas
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Computer and Weather Forecasting Numerical
weather prediction

• Analysis The drawing interpretation of the
  pattern of various weather elements on a surface
  or upper-air chart
• Numerical weather prediction Daily forecasting
  of weather based upon the solutions of
  mathematical equations by high-speed computers
• Atmospheric models Simulation of the
  atmospheres behavior by mathematical equations
  or by physical models
• Method of solution dozens of equations describe
  how atmospheric parameters (temp, pressure,
  humidity, wind speed direction) change with
  time ---each equation is solved a future Dt for
  a large number of locations grid points ---
  Each equation is solved for many (50) levels in
  the atmosphere calculated values are put back in
  to those equations and calculated for the next
  interval of time and so on for 12, 24, 36, 48,
  120 hours so on.

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Computer Weather forecasting contd.

• Prognostic Chart (Prog) A chart showing expected
  or forecasted atmospheric conditions (pressure
  patterns, frontal positions, and so on)
  forecasters use prog as a guide to predict the
  weather
• Variability in weather prediction Variety of
  models, each produce slightly different
  interpretation of the weather mainly due to the
  way the models are used, what model is used,
  distance of grid points, etc. Some models are
  better than others for a specific task (such as
  predicting the position of the trough on
  upper-level chart, position of surface lows, etc)

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Why sometimes forecasts dont work??

• Factors that affect the prediction of weather
  (such as surface temp, winds, precipitation)
  forecasts
• 1) Each models idealize the atmosphere and the
  assumptions made about the atmosphere may be on
  target on some times, but not all the times
• 2) Many models are not global in their coverage
  and thus, the models assumed boundaries could
  lead to errors (e.g., air masses entering from
  western Pacific in to North America, if the
  boundary is set as the west coast not including
  the western Pacific) global model would involve
  longer computation time
• 3) Sparness of data Data from oceans and high
  latitudes are sparse data from satellites have
  helped computers forecast is as good as the
  data fed in to it

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Why sometimes forecasts dont work?? contd.

• 4) Inadequate representation of many pertinent
  processes (e.g., interactions of water, ice
  local terrain on weather systems, such as
  radiation budget locally, etc.)
• Some models take large geographic features
  (oceans, mountain chains) neglecting small scale
  features (hills and lakes) and could have a
  marked influence on local weather
• 5) Inherent chaotic behavior within the
  atmosphere Several small, unpredictable
  atmospheric fluctuations and disturbances (and
  the uncertainties) get amplified with time in
  projecting the weather farther into the future

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500-mb progs for 7 PM EST, May 4, 1999 48 hrs
in to the future Prog (a) MRF Model Prog (b)
ETA Model solid lines height contours 552
5520 m
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Tools for Forecasting the Weather

• High-speed data modeling systems are used
• AWIPS (Advanced Weather Interactive Processing
  System) processes information received from the
  Doppler radar system and the Automated Surface
  Observing System (ASOS) operated in airports
  ASOS systems provide continuous information on
  wind, temp, pressure, cloud-base height, runway
  visibility
• Soundings A two-dimensional vertical profile of
  temperature, dew point winds
• Several meteorological indexes can aid in
  determining the likelihood of thunderstorms,
  tornadoes hail

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The AWIPS work station provides various weather
maps
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Doppler Radar data during a severe hailstorm in
Orlando, FL 100 probability that thunderstorm
was producing hail algorithm estimated hail size
gt 3
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Satellites Weather Forecasting

• Satellites provide photographs of areas where
  there are no ground-based observations
• Geostationary Satellites (Geosynchronous
  satellites) Orbit the equator at the same rate
  as the earth spins (remains at 36,000 km above a
  fixed spot on earths surface) allows continuous
  monitoring of a specific region
• Pictures received from Geostationary satellites
  to determine the cloud movement, dissipation, or
  development associated with weather fronts
  storms --- useful to forecast the progress of
  large weather systems
• Polar-orbiting satellites Pass over the north
  and south polar regions on each revolution
  provide sharp pictures in polar regions circle
  the earth at 850 km above the earth provides
  detailed photographic information about violent
  storms cloud systems

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Geostationary satellite monitors one area
constantly, as it remains stationary with respect
to an observer on earths surface
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Polar Orbiting satellites scan from north to
south
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Satellites contd.

• Satellites use radiometers to observe clouds both
  day night (by detecting radiation from top of
  the clouds)
• Imager Advanced radiometer that satellite
  pictures with much better resolution
• Sounder Advanced radiometer that provides a more
  accurate profile of temp, moisture at different
  levels in the atmosphere
• Infrared satellite picture can distinguish warm,
  low clouds from cold, high clouds (warm objects
  radiate more energy than cold objects)

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Infrared satellite picture can distinguish warm,
low (Gray) clouds from cold, high (white) clouds
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A visible image (left) and an infrared image
(right) of the eastern Pacific taken at the same
time
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Satellites contd.

• Clouds appear white in the visible image in the
  IR image, clouds appear to have many shades of
  gray
• Along the elongated band of clouds associated
  with the occluded front, the clouds appear white
  and bright in both pictures, indicating thick
  heavy clouds
• Time-series photographs provide info on the
  movement of the clouds and hence are useful to
  predict arrival of clouds and storms and the
  passage of weather fronts
• Other Forecasting Methods
• Persistent Forecast A prediction that future
  weather will be the same as present weather
• Steady-state or trend method Surface weather
  systems tend to move in the same direction and at
  the same speed as they have been moving

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Other forecasting methods contd.

• Nowcasting Extension of trend method from
  minutes to a few hours
• Analog Method Existing features on a weather
  chart may strongly resemble features that
  produced certain weather conditions
• Weather types can be used to long-range weather
  forecasting do not adequately predict specific
  weather elements
• Ensemble forecasting A technique based on
  running several forecast models (or different
  versions of a single model), each beginning with
  slightly different weather info to reflect errors
  in the measurements agreement forecasters can
  place a high degree of confidence in the
  forecast low degree of confidence means that the
  models do not agree

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Forecasting contd.

• Climatological Forecast Forecast based on the
  climatology of a particular region e.g., in LA,
  rainfall amounts more than a trace occur once in
  90 days (1 of the time) we can predict with
  99 confidence of what can happen!
• What does it mean 70 chance of rainfall?
• 60 of the forecast area, it will rain??
• 60 chance it will rain within the forecast
  area??
• 60 chance that any random place in the forecast
  area will receive measurable rainfall

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Forecasting words used by the National Weather
Service (precipitation gt 0.01)
probability of preci. Forecast wording for steady precipi. Forecast wording for showery precipi.
20 Slight chance of precipitation Widely scattered showers
30 to 50 Chance of precipitation Scattered showers
60 to 70 Precipitation likely Numerous showers
gt80 Precipitation, rain, snow Showers

• .

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Probability of a white christmas one inch or
more snow on the ground (30 yr average)
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Accuracy and skill in weather forecasting

• Forecasts made for 12-24 hrs are usually accurate
• 1-3 day forecasts are reasonably good not
  perfect better than 50
• With longer time, the accuracy decreases and gt7
  days, accuracy falls off rapidly
• The forecast must be better than current weather
  or the normal (climatology) weather in LA
  example
• Teleconnection prediction Warming of the
  equatorial tropical Pacific affects weather in
  different regions of the world the interactions,
  where a warmer tropical Pacific can influence
  rainfall in CA are called teleconnections
  aids to predict seasonal changes, but not diurnal

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Predicting weather from local signs

• A halo around the moon portends rain Why?
• Halo is caused by by the bending of light as it
  passes through ice crystals and that ice
  crystals-type clouds (cirrostratus) are often the
  forerunners of an approaching storm
• Few forecasting rules that may be applied when
  making a short-range local weather forecast
• Observation Indication Local weather forecast
• Surface winds from Possible cool Possible
• S or SW clouds front thunder- showers
• Building to the west storms approaching possibly,
  
• Warm (hot) humid from the west turning cooler
• windy

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Forecasting from weather signs
Observation Indication Local weather forecast
Winternight Clear, relatively calm with low humidity Rapid radiational cooling A very cold night
If clear, relatively calm, low humidity, snow on the ground Rapid radiational cooling A very cold night
Cloudy, relatively calm with low humidity Clouds will absorb and radiate IR to surface Not as cold when the sky is clear
Summer night Clear, hot, humid Strong absorption/emission of IR High minimum temp

• .

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Contd.
Observation Indication Local weather forecast
Clear and relatively dry More rapid radiational cooling Lower min temp
If surface winds are from N and become NE, then E, then SE Surface high pressure area moving to E low pressure approaching from W Increasing clouds with the possibility of precipitation within 24 hours
If surface winds are from NE then N, then NW Surface low pressure area is moving to the east, surface high press. Approaching from W Possible showers or thunderstorms with gusty winds
Afternoon cumulus clouds with flat bases, and tops at just about the same level Stable layer above the clouds Partly cloud, with no precipitation

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Weather forecasting using surface charts

• Thumb Rules for forecasting
• 1) Storms fronts tend to move in the same
  direction and speed during the previous 6 hrs,
  over a short-time intervals
• 2) Lows tend to move in a direction that
  parallels the isobars in the warm air ahead of
  the cold front
• 3) Lows tend to move toward the region of
  greatest surface pressure drop, whereas highs
  tend to move toward the region of greatest
  surface pressure rise
• 4) Surface pressure systems tend to move in the
  same direction as the wind at 500 mb level. Speed
  at the surface ½ the speed at aloft

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Forecast example

• Projection of pressure systems, fronts and
  current weather in to the future by assuming
  steady-state conditions (12- 24 hours projection)
• The storm center over the Central Plains and
  should move northeast the 500-mb chart suggests
  that the surface low should move northeast at a
  speed of 25 knots
• Six Cities
• Augusta (GA), Washington D.C., Chicago, Memphis,
  Dallas, and Denver

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Surface weather map for 600 AM Tuesday. Dashed
lines positions of weather features 6 hrs ago
shaded green precipitation
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500-mb chart at the same time L position of the
surface low
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Weather Forecast

• Augusta, GA cP associated with a high pressure
  center brought freezing temp and fair weather
  southerly winds on the western side bring warmer
  moist air clear and cold in the morning,
  increasing high clouds, wind direction will be
  light barometric pressure falling slowly
• -Reality fog was found --- temp of the ground
  was not considered warm, moist air over the cold
  surface --- fog
• Washington D.C. Storm approaches from the west
  slowly increasing cloudiness

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Weather prediction contd.
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Weather prediction contd.

• Sleet was found not predicted Why??
  Intensification of the storm influx of warmer
  air from the ocean slowly raised the surface temp
  and the sleet soon became rain
• Chicago Major snowstorm total snowfall is 12
  inches actual snowfall is 13 inch problem
  storm system slowed as it became occluded
• Memphis, TN Predicted thunderstorm did not
  materialize winds were not strong enough to mix
  the cold, moist air that had settled in the
  valleys with the warm air above

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Weather prediction contd.

• Dallas Gulf of Mexico effect
• Denver Rocky mountain effect
• Problems associated with the prediction

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Surface weather map for 600 AM wednesday
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Summary Chapter -9

• Warning and Watch
• Problem with modern day weather prediction
• Prog, numerical weather prediction
• Geosynchronous satellites
• IR images vs visible images
• Persistent forecast, Steady-state forecast,
  nowcasting, analogue weather prediction
• Ensemble weather forecasting probability
  forecasting
• Problems associated with forecasting