Part 5. Human Activities

Chapter 13

Weather Forecasting and Analysis

IntroductionWeather Forecasting – Both Art and Science

National Weather Service• Began as Army Signal Corps, then National Weather Bureau• Part of the National Oceanic and Atmospheric Agency

(NOAA)• National Centers for Environmental Prediction (NCEP)

– Automated Surface Observing Systems (ASOS)Data acquisition and dissemination

• World Meteorological Organization (WMO) – 170 member nations with 10,000 land stations, 7000 ships,

300 buoys, weather satellites, and 1000 weather balloons (twice daily)

A National Weather Service Office

Advanced Weather Interactive Processing System (AWIPS)

An automated weather recording station

– Upper atmosphere– Radiosondes, rawinsondes

A radiosonde launch

Forecasting methods• Climatological forecasts • Persistence forecasts • Analog approach • Numerical weather forecasting

Types of forecasts• Qualitative forecasts • Probability (PoP) forecasts • Quantitative forecasts

Numerical Modeling and Weather Forecasting (supercomputer-based forecasts)

Analysis phase -- create or update a computer code of a numerical model of the atmosphere

• Numerical model -- mathematical model of atmospheric circulation and moisture; includes equations of motion, continuity, energy, etc.

Prediction phase– Start by inputting current and recent atmospheric

conditions into the model/computer program– Solve equations to obtain prediction of future

atmospheric conditions– Output maps and other prediction products– After some time elapses (hours or days), repeat

the process with the latest current data

(a)-(c) = Three numerical models, (d) = Final forecast, (e) = Observed

Forecast represents some type of weighted average of the individual forecast maps

Precipitation forecast Observed precipitation

24-hr forecast skill for various NCEP modelsForecast results from 1995. Forecasts of heavy precipitation were less accurate than forecast of light precipitation. Here, the EDAS model was most consistently accurate.

24-hr, 1-in. NCEP precipitation forecast skill average

Forecast skill from 1983-1993 for manual forecasts. Winter precipitation forecasts were better than summer forecasts.

MOS Example

Forecast skill is steadily improving with time.

Medium range forecasts (MRF) -- 72 hours to a few weeks• Several different numerical models are used

to make medium range forecasts– Ensemble forecasting is a forecast in

which the predictions from several different computer model forecasts are combined to form an ensemble average

A 10-day ensemble forecast of the 500 mb layer

Higher forecast reliability is expected where the model forecasts all agree.

Lower forecast

reliability is expected

where the model

forecasts disagree.

Long range forecasts cover times from several weeks to several months or more • Use climatology, statistics, numerical modes,

and subjective judgment• Usually involve coupled ocean-atmosphere

models• Are produced in the USGS by NOAA’s

Climate Prediction Center (CPC)

Weather Maps and Images• Forecasting done by experienced

meteorologist– Examination and analysis of many

weather charts and models

Surface maps• Large-scale features depicted• Pressure cells, winds, station models

Weather maps are an important part of weather forecasting

Early morning 850 mb maps can be used to predict surface high air temperatures; the surface high temperature is usually about 27° higher than the early morning air temperature at 850 mb in summer (22° in spring and fall, 16° in winter).

700 mb maps can be used to help forecast cyclogensis and thunderstorm movement

500 mb maps help with forecasts of cyclone development and movement

An omega high on a 500 mb map

300 mb and 200 mb maps show the locations of the jet stream and strong jets; these help determine Rossby wave movements

Convergence/divergence associated with jet streaks

Faster moving air

Slower moving air

Slower moving air

Visible satellite image

Infrared image

Water vapor image

Thermodynamic diagrams• Temperature and humidity values plotted• Indicates stability

A Stuve diagram

Sounding plotted on a Stuve diagram

Stability as indicated on a Stuve diagram

Lifted index• Thunderstorm potential• Negative values indicative of thunderstorms

K-index• Indicates air mass thunderstorm potential• High positive values (> 15) indicate

thunderstorm potential

End of Chapter 13

Understanding Weather and Climate

4th Edition

Edward Aguado and James E. Burt