Jon Ahlquist Chapter 13 Weather Forecasting Forecasting is like driving a car down the road where you are only allowed to look out the back window Source unknown Utility of weather forecasts Weather observations and analysis at surface and aloft Weather forecasting methods Accuracy versus Skill Probability forecasts Numerical i e computer forecasts Ensemble forecasting You can forecast Watches warnings and advisories ASOS Automated Surface Observing System Most surface observations come from ASOS Roughly 1000 ASOS stations in US operated by National Weather Service FAA and military ASOS automatically and electronically measures temperature relative humidity pressure wind speed and direction rainfall visibility height of cloud ceiling but not cloud type Also determines whether precipitation is rain or snow and whether there is freezing rain At airports people are contracted to take hourly observations of cloud type and weather that may be missed by ASOS high clouds weather in area US Radiosonde Stations 11 17 2006 Utility of Weather Forecasts What to wear driving airlines shipping Travel US National Weather Service is in Dept of Commerce Construction Agriculture Forest fires Tourism Good forecasts are based on good observations and a good understanding of the underlying science US Radiosonde Network Roughly 92 stations launching twice daily over US N Hemisphere coverage is good for observing midlatitude cyclones over land but inadequate in space and time to provide good observations of thunderstorms Can reach 100 000 ft during 90 minute ascent Equipment about 100 per launch Few recovered Next generation sondes with GPS will cost about 160 each so desirable to increase recovery rate One possibility glidersonde glider the size of model airplane equipped with weather instruments Takes observations as it is lifted by weather balloon Released from balloon at preset altitude and glides back to launch point Upper air obs info http www ua nws noaa gov Other Observing Tools in US MET1010 Intro to the Atmosphere Meteorologists can t change the weather but forecasts can help people prepare for it 2 geosynchronous satellites for E W of US www goes noaa gov 164 Doppler radars covering US see www nws noaa gov ost 88dtech html 35 profilers most in middle third of US measure wind speed and direction as a function of height reporting hourly www profiler noaa gov 1 Jon Ahlquist Other Observing Tools in US Roughly 500 commercial aircraft carry weather sensors report 150 000 observations per day mostly in the upper troposphere called ACARS MDCRS and AMDAR http aviationweather gov general pubs front docs jul 03 pdf http acweb fsl noaa gov Anchored buoys in Caribbean Atlantic also in the Pacific Ocean along near the equator to monitor El Nino See http www ndbc noaa gov World Meteorological Org WMO World Meteorological Org WMO of the United Nations over 175 nation members http www wmo ch facilitates data exchange through World Meteorological Centers in Melbourne Moscow and Washington DC and over 25 regional data centers around the world educates meteorologists in developing countries e g workshops at FSU 11 17 2006 Worldwide Weather Network Northern Oceans Forecasting technique depends on how far into the future you want to forecast Short range a few minutes to next day Persistence weather will continue as is with no change If it is dry raining now it will continue dry raining forever Tomorrow s high and low temperatures will be the same as today s A persistence forecast is often good in tropics which change little Trend method also called steady state method in textbook present trends will continue e g fronts lines of thunderstorms hurricanes etc will continue to move bringing in their associated weather Often used for nowcasting forecast for next few minutes to at most a few hours MET1010 Intro to the Atmosphere Hemisphere land covered fairly well South America and Africa not well covered Over 10 000 land based stations reporting surface conditions Hundreds of ships help with weather observations Weather Analysis What s the weather like now branch Weather Forecasting Methods p 368 373 Over 1500 radiosonde observations per day Wait a few hours after each observation time to collect as many observations as possible before starting the analysis for that time Quality control Discard readings if they are too different from climatology or nearby observations Start analysis using a forecast for the analysis time so every place has weather values Blend in the new observations making weighted averages of the forecast with observations to construct a 3 D grid of weather values Adjust values so that they will work smoothly with the equations in the computer forecasting models Forecasting 1 or more seasons into future Example What would the weather be like for an outdoor wedding in Jacksonville at 10 am on a day months from now Seasonal forecasting methods Forecast the seasonal average i e climatology E g the Farmer s Almanac seems to be climatological Simple statistical models involving correlations between past and future anomalies i e deviations from the mean Can be run on a personal computer or fancy pocket calculator Very complex coupled ocean atmosphere models Requires a supercomputer The simple statistical models are currently about as accurate as the supercomputer forecasts Supercomputer forecasts are improving such as advances in ensemble forecasting by FSU s Dr T N Krishnamurti More about this later in the lecture 2 Jon Ahlquist Forecasting 1 day to 2 weeks into the future Historical ordering of techniques Look at the sky In middle latitudes cirrus clouds are often a sign that a front is coming within 12 to 24 hours See table 13 3 p 348 and p 351 Study daily weather maps year after year and gain an intuitive understanding how the weather typically evolves analogue forecasting weather types Write a computer program using the equations that describe the atmosphere s motion dynamics and temperature thermodynamics numerical weather prediction This has become the dominant technique supplemented by human and statistical improvements Model Output Statistics Probability Forecasts pp 345 346 Probability of precipitation chance of measurable precipitation 0 01 inches or more at a randomly chosen point in the forecast area during the period covered by the forecast often 12 hours Footnote on p 345 not right With showers chance that a random point is hit equals fraction of area hit Randomly chosen point may as
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