How is a Forecast Made?Gather dataSatellites, radar, station data, soundings, etcAnalyze dataWhy is the weather doing what it is doing?Why is it raining?Issue forecast based on analysisHow and why do I expect the weather to change?Sources of DataSurface weather stationsAirports, ships, buoys, etc.HourlyRawinsondes (weather balloons)Twice daily (00Z and 12Z)SatelliteRadarSatellite ImageryVisible Satellite: Detects reflected sunlight (from e.g., clouds). Does not work at nightThick clouds white, thin clouds greyIR Satellite: Detects infrared radiation. Works 24 hours a dayIR radiation emitted depends on temperatureLow clouds appear gray, high clouds appear whiteWater vapor satellite: Detects IR radiation emitted by water vaporSenses water vapor even where there are no cloudsUpper troposphere only. CANNOT sense water vapor near groundVisible Satellite: Detects reflected sunlight from clouds. Does not work at night!Thick clouds white, thin clouds greyCan see texture and shadowsIR Satellite: Detects infrared radiation. Works 24 hours a dayIR radiation emitted depends on temperatureLow clouds appear gray, high clouds appear whiteBlack=hot, White=coldWater vapor satellite: Detects IR radiation emitted by water vaporSenses water vapor even where there are no cloudsUpper troposphere only. CANNOT sense water vapor near groundDark red/orange=dry, White/purple/green=moistThe National Weather Service (see table on p.248)In the United States, the National Weather Service (NWS) is responsible for the collection and dissemination of all weather dataBranch of U.S. governmentIssues official forecasts, watches, warnings, and advisories for threatening weather conditionsWatches: mean that certain type of hazardous weather (severe thunderstorms, tornadoes, heavy snow) are possibleTornado watch means increased chance of tornadoes over next several hoursSevere thunderstorm watchSpecific location (e.g. large part of a state_ and time period (e.g. several hours)Warnings: Hazardous weather is imminent or occurring occurringObserved or detected by radarA severe thunderstorm or tornado warning means to take shelter!Warnings usually last for less than an hourExcept blizzards, etcAdvisories issued for less hazardous conditionsHigh winds, dense fog, etc.Weather ForecastingSimple Method: Move things alongAssume weather features (lows, fronts, rain, etc) continue to move, unchanged in the direction they are movingCan provide a good first guess at weather for next 24 hours or soCan also run the same model several times to produce an ensemble of forecastsBetter captures variability and uncertaintyAtmosphere is chaotic“If a butterfly flaps its wings in China, it will rain in Central Park”Statistical methodsComputer models are NOT and never will be perfectForecasts continue to improveComputer ModelsAtmospheric motions described by complicated set of equationsCan’t solve them by hand even with three semesters of calculusWhat to do? Use a computer!Supercomputer!Thousands of CPUsWeather ModelingWeather observations from all over the world are assimilated by supercomputersThere is inherent uncertainty and error in these observationsThere are regions without many observationsEspecially over the oceanObservations interpolated to a regular 3D grid of pointsSome models only cover United StatesSome cover entire worldComputer generates analysis of current conditions like this oneNumerical Weather PredictionNumerical model solves atmospheric equations at each grid point and produces a forecastDifferent models have different grid spacingsFiner grid spacing (resolution) does not necessarily make models more accurateModel is able to capture smaller-scale featuresComputer analyzes output to produce graphicsCan be interpreted by forecastersWeather Forecasting 10/24/2012How is a Forecast Made?-Gather dataoSatellites, radar, station data, soundings, etc-Analyze dataoWhy is the weather doing what it is doing?oWhy is it raining?-Issue forecast based on analysisoHow and why do I expect the weather to change?Sources of Data-Surface weather stationsoAirports, ships, buoys, etc.oHourly-Rawinsondes (weather balloons)oTwice daily (00Z and 12Z)-Satellite-RadarSatellite Imagery-Visible Satellite: Detects reflected sunlight (from e.g., clouds). Does not work at nightoThick clouds white, thin clouds grey-IR Satellite: Detects infrared radiation. Works 24 hours a dayoIR radiation emitted depends on temperatureoLow clouds appear gray, high clouds appear white-Water vapor satellite: Detects IR radiation emitted by water vaporoSenses water vapor even where there are no cloudsoUpper troposphere only. CANNOT sense water vapor near ground-Visible Satellite: Detects reflected sunlight from clouds. Does not work at night!oThick clouds white, thin clouds greyoCan see texture and shadows-IR Satellite: Detects infrared radiation. Works 24 hours a dayoIR radiation emitted depends on temperatureoLow clouds appear gray, high clouds appear whiteoBlack=hot, White=cold-Water vapor satellite: Detects IR radiation emitted by water vaporoSenses water vapor even where there are no cloudsoUpper troposphere only. CANNOT sense water vapor near groundoDark red/orange=dry, White/purple/green=moistThe National Weather Service (see table on p.248)•In the United States, the National Weather Service (NWS) is responsible for the collection and dissemination of all weather data•Branch of U.S. government•Issues official forecasts, watches, warnings, and advisories for threatening weather conditions-Watches: mean that certain type of hazardous weather (severe thunderstorms, tornadoes, heavy snow) are possibleoTornado watch means increased chance of tornadoes over next several hoursoSevere thunderstorm watchoSpecific location (e.g. large part of a state_ and time period (e.g. several hours)-Warnings: Hazardous weather is imminent or occurring occurringoObserved or detected by radaroA severe thunderstorm or tornado warning means to take shelter!oWarnings usually last for less than an hourExcept blizzards, etc-Advisories issued for less hazardous conditionsoHigh winds, dense fog, etc.Weather Forecasting-Simple Method: Move things alongoAssume weather features (lows, fronts, rain, etc) continue to move, unchanged in the direction they are moving-Can provide a good first guess at weather for next 24 hours or so-Can also run the same model several times to produce an ensemble of forecastsoBetter captures variability