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AS-201 Weather Forecasting. This section discusses: Various weather forecasting methods, their tools, and forecasting accuracy and skill Images for the forecasting of 6 cities Applications of weather forecasting. Four forecasting steps:.

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Presentation Transcript

This section discusses:

  • Various weather forecasting methods, their tools, and forecasting accuracy and skill
  • Images for the forecasting of 6 cities
  • Applications of weather forecasting
four forecasting steps
Four forecasting steps:
  • 1) Assess the present state, called the analysis, by plotting 6 hourly surface and 12 hourly sounding data
  • 2) Predict a future state by running a computer model of weather changes…
  • 3) Interpret the model results, called a prognostic chart, given forecasting experience…apply ROTs (Table 14.1 page 369)
  • 4) Translate forecast into something that is useful
forecasting methods
Forecasting methods
  • Persistence
    • The future conditions here will be like the present conditions here
  • Trend
    • The future conditions here will be like the weather upstream is now
  • Analogue
    • The future conditions here will be like weather that historically occurred when similar conditions were present
  • Climatology
    • Future conditions here will be near the monthly average for here
  • Numerical Weather Prediction
    • Future conditions here will be as predicted by supercomputers running equations of atmospheric motion
  • “Reading the Sky”
    • Future conditions here can be predicted by using current conditions and forecasting rules of thumb (ROT)
persistence forecast
Persistence Forecast

The persistence method works well when weather patterns change very little and features on the weather maps move very slowly. It also works well in places like southern California, where summertime weather conditions vary little from day to day.

trend forecasting
Trend Forecasting
  • Example: frontal movement

Probability Forecasts

Figure 14.6

Climate records can be used to generate probability forecasts for a given event.

Florida has less than a 5% chance of snow on December 25th, while northern Minnesota has had snow on that date for each of the past 30 years.

numerical weather prediction nwp
At 00Z and 12Z each day, data is collected and NWP models are run

NWP is the 2nd largest user of super computers in the world

Numerical Weather Prediction (NWP)
local weather signs
Local Weather “signs”
  • Understanding how the atmosphere works (“red sky at morning…”, halo around the moon, clear skies at sunset, etc, etc, etc)
  • Local rules of thumb (ROT) (for example--southerly flow in March)
  • Table 14.3, page 378; and appendix E (page A11)
  • If you have good “situational awareness” of the atmosphere, rudimentary wx measurements (temp, DP, pressure, winds), and good ROT—you can make a pretty good 0-24 hour forecast.
forecast periods
Forecast Periods
  • Nowcasting (less than 6 hours)
    • Mainly surface observations, weather satellite and radar, and persistence or trend forecasts
  • Short-range (<3 days)
    • Trend and analogue methods, weather charts and numerical weather forecast products
  • Medium range (3-8 days)
    • Analogue forecasts, numerical weather forecast products, climatology
  • Long-range (>8 days)
    • Climatology

Weekly & Monthly Forecasts

Stationary weather systems often allow for trend based extended weather forecasts, while multiple runs of numerical weather models, known as ensemble forecasts, allow for 30 to 90 day outlooks.

Figure 14.8A

Figure 14.8B

ensemble modeling
Numerical weather models are run multiple times with small changes in the initial conditions

Gives forecaster a good estimate of the possible range of weather conditions at a future times

Ensemble Modeling
accuracy and skill
Accuracy and skill
  • 12-24 hours: very good, 2-5 days: fairly good, above 7 days: slightly more accurate than persistence
  • Accuracy—what is considered “right”?
  • Skill—how complicated/hard is the forecast (Southern California in summer…)?
  • Forecast skill is determined by comparing your forecast against either persistence or climatology
  • Longer-range climatological forecasting improving
  • Tornadoes…where they are likely to form: 3 days in advance, but precise area struck is forecastable minutes-few hours in advance

Analysis to Prognosis--NWP

Figure 14.1A

Figure 14.1B

Two forecast model 500 mb progs for the same time.

Forecast models lose accuracy over time due to errors in the model, chaos in the atmosphere, and measurement errors


Data Processing and Display

Figure 14.2

Viewing weather images, overlays, and graphs in multiple windows is done using the National Weather Service's Advanced Weather Interactive Processing System (AWIPS), which gathers data from the Automated Surface Observing System among other sources.


WSR-88D Doppler Radar

Weather Surveillance Radar - 1988 Doppler, also known as next generation radar (NEXRAD), detects severe weather size, movement, and intensity.

Data received by the NEXRAD unit are processed by algorithms to assist the forecaster in weather interpretation.

Figure 14.3


Meteogram Display

Predicted trends in several weather variables are plotted for a 60 hour period on a meteogram.

Patterns in variable response, such as rising pressure and a stop in precipitation, are readily observed.

Figure 14.4

nws probability of precipitation
Probability of Precipitation (PoP) describes the chance of an event occurring at any point in the forecast area, over a certain period of time (usually 12 hours).

PoP is defined as: PoP = Pa x Pc, where:

Pa =the probability that precipitation will occur somewhere in the forecast area during the forecast period and           

Pc =the percent of the area that will receive measurable precipitation, if it does occur

NWS Probability of Precipitation

Vertical Sounding Profile

Radiosonde instruments attached to pilot balloons are launched twice daily to profile weather variables with height.

This example shows winds veering (clockwise—warm advection is occurring) from easterly at the surface to southwesterly aloft that may change the freezing rain in the saturated lower atmosphere to non-freezing rain.

Figure 14.5

Vertical soundings are also used for strength and timing of convective wx, cloud types and heights, turbulence and icing, etc.


Surface Chart Predictions

  • - 3-hour pressure tendencies plotted on isallobar maps help predict the movement of highs and lows
  • Lows tend to move toward the region of greatest pressure fall, while highs move toward the region of greatest rise.

Figure 14.10


Upper Level Charts

Figure 14.11

Upper level winds guide the path of surface pressure systems.


Observed Movement of Fronts

Surface weather observations from 6 PM Tuesday and 6 AM Wednesday show how the fronts, pressure systems, and precipitation have moved as predicted.

Figure 14.12

  • Using both trend and persistence forecast techniques, what is your forecast for 24 hours from now in:
  • Denver? Dallas? Chicago? Memphis?
  • Washington? Augusta?

Wed 6am

Tues 6am

another scenario
Another Scenario:
  • Forecast for central CA coast for tomorrow
  • Slowly falling barometer and increasing high clouds
  • Weak low pressure in Pacific.....
    • Will it rain, or not??

Surface Weather for 4 PM Sunday

Figure 14.14

Look at 500mb chart for clues:


500 mb Chart for 4 PM Sunday

Figure 14.15

564 line across the area (local ROT); Omega High present…but cold advection and shortwave may change the stagnant upper pattern; upper-level divergence??


12, 24, & 36 hour Progs

Forecaster has access to several forecast models

-Which one is best?

-- Choice is based on initialization and experience….

weather impacts on aviation mishaps
88% of all weather-related aircraft mishaps occurred under IMC (Instrument Meteorological Conditions)

83% of fatal crashes involved FAR Part 91 (General Aviation) aircraft

Weather Impacts on Aviation Mishaps
weather and commercial aviation
Weather and Commercial Aviation
  • Direct costs due to weather on airline operations can be separated into several categories: diversion, cancellation, delay and insurance. The cost of a diverted flight can be as high as $150,000 and a cancellation close to $40,000 (Irrgang and McKinney, 1992).
  • A report from the Air Transport Association (ATA) states that the direct annual costs to sixteen member airlines of the first two categories listed above are $47 million and $222 million, respectively (Air Traffic Management in the Future Air Navigation System, 1994).
comet module nas
weather impacts on economic sectors
Weather Impacts on Economic Sectors
  • Oil and gas exploration:
    • improved forecasts of tropical weather conditions (wind, waves, disturbances) can reduce delays in drilling operations at a cost of up to $250,000 per rig per day (several thousand rigs in the Gulf).
    • improved hurricane track predictions could reduce days of production shutdown, each day of which costs the industry and the U.S. treasury a combined $15,000,000.
  • Vegetable processing:
    • improved temperature and precipitation forecasts can lead to greater efficiency in chemical spraying (e.g., pesticides), which costs $10-$15 per acre per application for hundreds of thousands of acres.
    • on a national scale the annual cost of lost production to the vegetable processing industry, primarily due to weather, is $42,500,000.
  • Insurance:
    • a single hurricane could lead to more than $50,000,000,000 in damages.
    • weather-related catastrophes have led to more than $48,000,000,000 in property insurance claims over the period 1989-1993.
wx impacts on economy
Wx Impacts on Economy
  • Rail transportation:
    • it costs $2,000 per hour to stop a train. A single tornado warning covering 15 miles of track for 15 minutes can lead to seven stopped trains.
    • most weather-related derailments cost $1,000,000 to $5,000,000.
  • Electric power:
    • using improved thunderstorm forecasts could save one utility $200,000 annually in reduced outage time.
    • using "good QPF forecasts" could save one utility $2,000,000 over five years.
    • using improved temperature forecasts could save "hundreds of millions annually nationwide for the utility sector".
  • Aviation:
    • every avoided cancellation saves $40,000, every avoided diverted flight saves $150,000.
    • for the 16 members of the Air Transport Association, delays and cancellations cost $269,000,000 annually.
own the weather
“Own the Weather”
  • Understand organization’s mission and operations
    • Decision cycles
    • Risk adversity
    • Leadership and their goals
  • Understand how weather impacts the organization and its competition
  • Taking the knowledge of weather effects, and weather—develop a weather product in the right format, and deliver to the right person, at the exact right time
  • Be right more than you are wrong!
own the weather1
Own the Weather
  • USAF adopted this, and applied to weather support to US Army, USAF, and Special Operations forces in the mid-90’s
    • USAF weather personnel taught to forecast early in career
    • Most experienced technicians and officers trained in the concepts, and assigned to tactical level to tailor weather products
garske chart
“Garske Chart”

Synchronizes all significant environmental data into an operational timeline

  • Over past 10 years, there was a large demand for new meteorologists--currently about 10,000 BS-level or higher graduates employed
    • Government (National Weather Service; military)
    • Private companies
    • Consulting
    • Broadcast weather
    • Other related fields (physical and earth science; environmental fields)
key points and concepts
Key Points and Concepts
  • Persistence forecast is a prediction that future wx will be the same as the present.
  • The trend method is based on the assumption that systems continue presently movement
  • The analogue method (regimes) makes a wx prediction by comparing past wx maps and wx patterns to those of the present.
  • Climatological forecasts are based on the climatology (average weather) of a particular region
  • NWP is a useful tool for forecasting, but has limitations
con t
  • Ensemble forecasting is a technique based on running several forecast models, each beginning with slightly different weather information to reflect errors in the measurements. If they agree—forecaster can have a high confidence in the model forecast.
  • For a forecast to show skill, it must be better than a persistence forecast or climatological forecast
  • After a number of days, the atmosphere’s chaotic behavior, along with flaws in computer models and small errors in measurements, limits the accuracy of forecast models
  • Surface lows tend to move in direction parallel to the isobars (in the warm sector), and toward the direction of greatest pressure fall (isoallobaric)
con t1
  • Weather conditions have a huge effect on commercial and civil aviation
    • Most GA crashes occur in IMC conditions
  • The majority of flight delays in the National Airspace System (NAS) are due to weather
  • Weather can have a significant impact on the economy—resulting in billions of dollars in lost revenues; and can kill thousands
    • Companies are learning to “own the weather” to minimize losses and increase competitiveness
  • The US armed forces introduced “own the weather” concepts now in use throughout the meteorological community
comet module nws weather forecasting
COMET Module—NWS Weather Forecasting