Chapter 9: Chapter 9: Weather Weather

ForecastingForecasting

Acquisition of weather informationAcquisition of weather information Weather forecasting toolsWeather forecasting tools Weather forecasting methodsWeather forecasting methods Weather forecasting using surface Weather forecasting using surface

chartscharts

Acquisition of Weather Acquisition of Weather InformationInformation

World Meteorological OrganizationWorld Meteorological Organization National Centers for Environmental National Centers for Environmental

PredictionPrediction Watches: Watches: favorable condition for potential hazardous favorable condition for potential hazardous

weatherweather Warnings: Warnings: hazardous weather is imminent or actually hazardous weather is imminent or actually

occurringoccurring

Advisories: Advisories: like warning but for less hazardous like warning but for less hazardous weatherweather

• The internet plays a crucial role in the global The internet plays a crucial role in the global communication of weather information.communication of weather information.

• Go to: http://www.weather.govGo to: http://www.weather.gov

Weather Forecasting Weather Forecasting ToolsTools AWIPS: AWIPS: Advanced Weather Advanced Weather

Interactive Processing SystemInteractive Processing System used by forecasters and can used by forecasters and can

process satellite, radar, surface process satellite, radar, surface observation, radiosonde data observation, radiosonde data and weather forecasting model and weather forecasting model outputoutput

meteogrammeteogram soundingsounding

Fig. 9-1, p. 238

Near Orlando, FLHail size > 3 inch100% probability

Satellites and Weather Satellites and Weather ForecastingForecasting

geostationary satellitesgeostationary satellites polar orbiting satellitespolar orbiting satellites Geostationary Operational Environmental Geostationary Operational Environmental

SatelliteSatellite infrared images: infrared images: estimate height; 3-D imageestimate height; 3-D image visible images: visible images: not useful at nightnot useful at night water vapor images: water vapor images: particularly useful for particularly useful for

clear skyclear sky http://www.goes.noaa.govhttp://www.goes.noaa.gov

Fig. 9-5, p. 240

850kmaltitude

Cannot see polarareas

Fig. 9-7, p. 241

Fig. 9-9a, p. 242

Visible Infrared Enhanced Infrared

Fig. 9-11, p. 243

The Computer and The Computer and Weather Forecasting: Weather Forecasting: Numerical Weather Numerical Weather

PredictionPrediction Analysis:Analysis: final chart using available datafinal chart using available data

numerical weather prediction (NWP)numerical weather prediction (NWP): : based on modelsbased on models

atmospheric models: atmospheric models: fluid dynamics and fluid dynamics and atmospheric physicsatmospheric physics

Progs: Progs: prosnostic chart for weather forecastprosnostic chart for weather forecast

• One of the world’s first computers was built for theOne of the world’s first computers was built for thespecific purpose of performing weather forecasts.specific purpose of performing weather forecasts.

Fig. 9-12a, p. 245

48-hr NWPforecastof 500 mbHeight

12km vs 60km

Resolution refers togrid spacing

Why NWS Forecasts go Why NWS Forecasts go Awry and Steps to Awry and Steps to

Improve ThemImprove Them grid spacinggrid spacing incomplete data incomplete data

coveragecoverage model deficiencies due model deficiencies due

to subgrid processesto subgrid processes chaoschaos ensemble forecasting: ensemble forecasting:

spaghetti plot to indicate spaghetti plot to indicate the robustness of forecast the robustness of forecast • Chaos theory tells us that weather forecasts are highly sensitiveChaos theory tells us that weather forecasts are highly sensitive

to our ability to observe the weather. Since it is impossibleto our ability to observe the weather. Since it is impossibleto observe the weather at all places at all times, weatherto observe the weather at all places at all times, weatherforecasts will never be perfect.forecasts will never be perfect.

Other Forecasting Other Forecasting MethodsMethods

persistence forecast: persistence forecast: not bad for Tucson in Junenot bad for Tucson in June trend forecast: trend forecast: assuming constant change rateassuming constant change rate analogue method: analogue method: search for similar chart in search for similar chart in

historyhistory statistical forecast: statistical forecast: routinely used; Model routinely used; Model

Output Statistics (MOS)Output Statistics (MOS) probability forecast: probability forecast: particularly for particularly for

precipitationprecipitation climatological forecast: climatological forecast: good for Tucson good for Tucson

rainfall in Junerainfall in June

Table 9-1, p. 249

What does it mean by`chance of rain is 50% for one area’?

Choose the right answera. It will rain over 50% of the areab. 50% chance that rainfall will not occur in the whole areac. 50% change that any random location in the area will receive measurable rainfall in the given period

Fig. 9-15, p. 250

Probability for a `white Christmas’ – 1 inch or more of snow

Types of ForecastsTypes of Forecasts

very short range forecast or nowcast: very short range forecast or nowcast: 0-6 0-6 hrhr

short range forecast: 6 hr – 3 daysshort range forecast: 6 hr – 3 days Mediate-range forecast: 3-8.5 daysMediate-range forecast: 3-8.5 days long range forecast: 8.5 days – 2 weekslong range forecast: 8.5 days – 2 weeks Monthly and seasonal outlooksMonthly and seasonal outlooks

• Long-range forecasts are less specific than short range Long-range forecasts are less specific than short range forecasts.forecasts.

Fig. 9-17, p. 251

Accuracy and Skill in Accuracy and Skill in ForecastingForecasting

persistencepersistence climatologyclimatology

• Both persistence and climatology are surprisinglyBoth persistence and climatology are surprisinglyaccurate forecasting methods.accurate forecasting methods.

Predicting the Weather Predicting the Weather from Local Signsfrom Local Signs

Halo: Halo: `a halo around `a halo around the moon portends the moon portends rain’ (foldlore)rain’ (foldlore)

• To see a halo: block out the To see a halo: block out the sun with your hand and sun with your hand and look at the cirrostratus clouds. look at the cirrostratus clouds. Wear polarized sunglasses if Wear polarized sunglasses if possible.possible.

Determining the Determining the Movement of Weather Movement of Weather

SystemsSystems forecasting rules of thumb: forecasting rules of thumb: surface pressure systems tend to move in the surface pressure systems tend to move in the

same direction as the 500 mb wind; the speed at same direction as the 500 mb wind; the speed at which surface systems move is about half the which surface systems move is about half the wind speed at 500 mbwind speed at 500 mb

using the surface chartusing the surface chart• Internet now provides much of the weather information

http://www.weather.govhttp://www.atmo.arizona.eduhttp://www.wrh.noaa.gov/twc

Fig. 9-20, p. 256

Current front and front 6 hr ago 500 mb height

Estimate for the next 24 hr observation for 24 hr later