notes.chap12

Weather Analysis & Forecasting
Chapter 12

A weather forecast is a scientific estimate of the weather conditions at some future time.
National Weather Service (a branch of National Oceanic and Atmospheric Administration, NOAA)
- Provides the following forecasts and warnings for the US and its territories, free:
  - Weather
  - Hydrologic
  - Climate
- Most important services include forecasts and warnings of hazardous weather including:
  - thunderstorms
  - flooding
  - hurricanes
  - tornadoes
  - winter weather
  - extreme heat
- Emergency Management Agency says 80% of all declared emergencies are weather related.
- DOT states that more than 6000 fatalities per year can be attributed to the weather.
- Heat waves claim approximately 1000 lives per year.
- The 1993 Mississippi River Valley flood claimed 48 lives and caused more than $15 billion in damages.
- Property damage due to wind, hail, snow and tornadoes increased more than 500% from 1986 to 1995.
The process of providing weather forecasts and warnings occurs in three stages:
- collect and analyze global atmospheric data to provide a picture of its current state.
- apply a variety of techniques to establish the future state of the atmosphere - weather forecasting.
- Disseminate the forecasts to the public, mainly through the private sector (e.g., The Weather Channel, AccuWeather, CNN).
Weather forecasting is EXTREMELY important to us.
Weather Analysis (current atmospheric conditions)
- World Meteorological Organization (WMO) is responsible for international exchange of weather data.
- Data collected four times each day (at 0000, 0600, 1200 and 1800 Greenwich Mean Time) from:
  - 10,000 land surface stations.
  - 7,000 ships.
  - 3,000 data buoys.
- Data also collected from:
  - weather satellites
  - radiosondes - 900 balloon-borne instrument packages are launched daily at 6:00AM and 6:00PM (CST). Fly to about 115,000 feet. Parachute to earth. Can be reused.
- US Weather Data Collection
  - International and domestic weather data collected by the National Center for Environmental Prediction near Washington, DC. This branch of the NWS prepares weather maps, charts and forecasts on a global and national scale.
  - FAA operates weather stations at most major airports
  - NWS and FAA operate 900+ Automated Surface Observing Systems (ASOS).
  - Weather information is disseminated to the 119 Weather Forecast Offices where it is used to produce local and regional weather forecasts. These offices also gather and submit weather data to the NWS.
  - The same information is provided free to private weather companies such as:
  - National Weather Service
  - Weather Channel
  - AccuWeather
  - Weather Underground
- Hunter College, Geography Department Weather Station
Debris Flow Forecasting - San Francisco
Synoptic (coincident in time) Weather Maps display weather conditions at a given moment in time.
Weather Forecasting - complex and quantitative.
- Synoptic Weather Forecasting - techniques are employed to extrapolate future conditions from current conditions.
  - Techniques include:
    - persistence forecasting
    - trend forecasting
    - analog method
  - Analog Method - looks at a developing cyclonic storm and compares it to the behavior of similar weather systems from the past. Was the primary method of weather forecasting in the 1950s and 1960s. Today, had been replaced by other methods although still used for 24-48 hour forecasts and to forecast specific events, such as when a rain storm will arrive at a specific location.
- Numerical Weather Forecasting:
  - Relies on the physical laws of the atmosphere to predict its future state.
  - Complex computer models are used that require billions of calculations.
  - Slight variations in initial conditions can produce very different forecasts so multiple runs of the model are run.
  - Forecasters make the final decision on which model run to use.
  - The final forecast is a blend of different model data and input from satellite images, RADAR and other sources.
  - Common Numerical Weather Forecasting Models
    - The European Centre for Medium-Range Weather Forecasting (ECMWF) itself has half a dozen different models.
    - NOAA's GFS, the Global Forecast System, replaced the AVN in 2002, which itself absorbed the MRF (Medium Range Forecast). The GFS is a 35 to 70 km resolution, medium-range, global model. Because all output from its various products is freely available over the Internet, it is widely used by commercial forecasters.
  - The NWS produces generalized forecast charts of the future condition of the atmosphere called prognostic charts.

National Weather Service Prognostic Charts

NOAA Weather Forecasts

Meteorologists use the prognostic charts, their professional experience and "rules of thumb" from the analog method to project future sites of cyclogenesis.
The general prognostic charts are sent to the 119 weather forecasting offices where the data are blended with local conditions, numerical predictions and the weather quirks of of the region into a site-specific forecast.

Statistical Methods - can be used to predict a single aspect of the weather such as the daily high temperature.
Techniques Used in Short-Range Forecasting
- Persistence Forecasts - simplest forecasting technique based on the tendency of the weather, at a given site, to remain unchanged for several hours, or even a day or so.
- Trend Forecasting - related to persistence forecasting, assumes that the weather upstream will persist and move on to affect the area in its path. Not good for predicting short-lived weather situations or small-scale weather such as tornadoes, hail storms or thunderstorms.
Long-Range Forecasts - not a weather forecast in the usual sense. Predicts whether an area will have near-normal temperature and precipitation conditions or not.
Forecast Accuracy - have improved significantly during the past 20 years. Beyond 7 days, forecasts are no more accurate than projections made from climatic data.

Tools of Weather Forecasting
- Automated Surface Observing Systems (ASOS) - about 1000 will be deployed in places currently outside the observational network.
- Advanced Weather Interactive Processing Systems (AWIPS) - computer network
- NEXRAD (Next Generation Weather RADAR) - not used to predict small scale weather phenomena but to detect and track.
- Satellites in Weather Forecasting
  - 1960 - TIROS (Television & Infra-Red Observation Satellite) launched.
  - 1964 - Second generation Nimbus satellite.
  - Polar orbiting weather satellites with 100 minute orbits.
  - 1966 - Geostationary satellites
  - Types of imagery:
    - Visible
    - Emitted IR - used to view cloud tops. Higher clouds are colder and have more precipitation.
    - Water-vapor - 6.7 micrometer wavelength energy emitted by water vapor. Bright regions mean high water vapor content.
  - GOES (Geostationary Operational Environmental Satellites) - provide visible, IR and water vapor images every 30 minutes.