ams weather studies introduction to atmospheric science, 5 th edition chapter 13 weather analysis...
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AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 13Weather Analysis and Forecasting
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Driving Question
What is the process involved in making a scientific forecast of the weather?
This chapter covers: How forecasts are made Limits of forecast accuracy Making your own weather forecasts
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Case-in-PointEvolution of Tornado Forecasting
Tornado forecasting 1884: started by John P. Finley, U.S. Army Signal Corps
950 “tornado reporters” were gathering data with criteria Finley established
1886: program discontinued Word “tornado” disallowed in Signal Corps forecasts for fear of
public panic 1940-50s: Air Force meteorologists Fawbush and Miller
developed method for forecasting tornadoes Tornado struck Tinker Air Force Base in Oklahoma City, OK on 20
March 1948; motivated Fawbush and Miller List of six atmospheric conditions that preceded tornado outbreak
Those conditions reappeared 5 days later Issued first tornado forecast; accurate, saved lives and aircraft
Primarily issued for military installations Eventually, U.S. Weather Bureau adopted/allowed tornado
forecasting for public distribution Severe Local Storm Warning Center (now the Storm Prediction
Center) established in Norman, OK 1952: ban on “tornado” with issue of first tornado watch
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International Cooperation
International Meteorological Organization (IMO) founded in 1878 IMO became the World Meteorological Organization (WMO)
Headquartered in Geneva, Switzerland, agency of the United Nations
Coordinates the efforts of 189 nations and territories in global weather-monitoring program called World Weather Watch (WWW) Provides meteorological information available internationally
Global Observing System Data from 6 geostationary and 3 polar orbiting satellites, 11,000
land stations, 4000 ships at sea, 3000 reconnaissance and commercial aircraft, radar, 1300 radiosonde stations, 1200 drifting and 1300 moored buoys
Data transmitted to 3 WMO Centers where maps and charts are created, forecasts prepared
Maps/forecasts sent to Regional Specialized Meteorological Centers National Centers for Environmental Prediction (NCEP)
responsible for U.S. forecasts
International Cooperation
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International Cooperation
Weather forecasting entails Acquisition of present weather data Graphical depiction of the state of the
atmosphere Analysis of data and maps Prediction of the future state of the
atmosphere Dissemination of weather information and
forecasts to the public
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Acquisition of Weather Data
Surface Weather Observations Over 2,000 stations across the U.S.
operated by National Weather Service (NWS) personnel Staff of other government agencies,
including the Federal Aviation Administration (FAA)
Private citizens or businesses Automated stations also located in
unmanned locations National Data Buoy Center
Data gathered for preparation of weather maps and forecasts, exchange with other nations, and use by aviation
Observations taken simultaneously Use Coordinated Universal Time (UTC)
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Acquisition of Weather Data
Surface Weather Observations, cont. Automated Surface Observing System (ASOS)
Result of 1990s NWS modernization 947 ASOS units in continual operation Reports temperature (ambient/dewpoint),
pressure (sea-level, altimeter setting), wind (direction/speed), precipitation accumulation, visibility, obstruction to vision, present weather, sky condition
Automated Weather Observation System (AWOS) Similar to ASOS 163 FAA-owned and 1149 non-Federal
NWS Cooperative Observer Network Volunteers Provide daily precipitation, and temperature
readings
Acquisition of Weather Data
Upper-Air Weather Observations Radiosondes
Radio-equipped instrument package Transmits upper air information to a
ground station (rawinsonde observation)
92 radiosonde observation stations
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Acquisition of Weather Data
Locations of radiosonde observation stations.© AMS10
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Weather Data Assimilation, Depiction and Analysis
Weather reported by each observation station is depicted on a map by a station. © AMS11
Weather Data Assimilation, Depiction and Analysis
Surface weather maps Isobars:
Connect points of equal air pressure
Isobaric analysis reveals locations of anticyclones and cyclones, troughs and ridges, horizontal pressure gradients
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Surface Weather Maps, cont. Cyclone centers are indicated by the symbol L
(low) Closely spaced isobars around cyclone indicate steep
pressure gradient and strong winds Fronts originate at storm centers
Anticyclone centers are mapped as an H (high) Usually a relatively weak horizontal pressure gradient,
shown by widely spaced isobars, resulting in weak or calm winds
Synoptic surface maps are drawn every 3 hours for North America, and every 6 hours for the Northern Hemisphere
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Weather Data Assimilation, Depiction and Analysis
Upper-Air Weather Maps Plotted on constant-
pressure surfaces Height contours labeled in
meters above sea level Drawn at 60 m intervals
Altitude of pressure surface varies because of mean temperature differences Air pressure drops more
rapidly in cold air than in warm, due to density differences
Isotherms plotted as dashed lines Drawn at 5 C deg intervals
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Weather Data Assimilation, Depiction and Analysis
Sample 500-mb analysis (NOAA)
Upper-Air Weather Maps, cont. Cyclonic and anticyclonic curvature shown in contours by
troughs and ridges in the prevailing westerlies Center of a ridge is relatively warm with high height
contours, labeled with an H. Often linked to a warm-core anticyclone at the surface.
Center of a trough relatively cold with low height contours, labeled with an L. Often linked to a cold-core extra-tropical cyclone at the surface.
Winds that blow across isotherms produce air advection Cold air advection occurs where winds blow from colder to
warmer locations Warm air advection occurs where winds blow from warmer
to colder locations
Weather Data Assimilation, Depiction and Analysis
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Weather Data Assimilation, Depiction and Analysis
Computerized data management systems Spurred by deluge of real-time weather information AWIPS (Advanced Weather Interactive Processing
System) Used by NWS Offices since 2000, AWIPS II began roll-out in
2011 Receives and organizes ASOS data plus analysis and
guidance products from NCEP Allows meteorologists to display, process, and overlay
images, graphics, and other data
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Hydrometeorological Prediction Center Short range: 12-, 24-, 36-, and 48-hr forecasts Medium range: 3- to 7-day extended outlooks
Climate Prediction Center Long range: 6-10 day, 8-14 day, 30-day (monthly), 90-day
(seasonal), and multiseasonal outlook Numerical Weather Forecasting
Computers programmed with numerical model of the atmosphere
Model of equations relate wind, temperature, pressure, and water vapor concentration
Current data used to predict atmospheric properties only a few minutes into the future, that prediction becomes the starting point for another few minutes into the future, that prediction…
Repeated until 12-, 24-, 36-, and 48-hr forecasts are achieved
Weather Prediction
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Numerical Weather Forecasting Computers programmed with a numerical model of
the atmosphere Model of equations that relate wind, temperature,
pressure, and water vapor concentration Uses present data to predict values of atmospheric
properties for a grid of points on a uniform pressure surface
Millions of computations go into 12, 24, 36, and 48-hr forecasts
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Weather Prediction
Weather Prediction
North American Mesoscale Model (NAM) Contains NOAA Environmental Modeling System (NEMS) Divides troposphere into 60 vertical layers Forecasts every 6 hours out to 84 hours
Nested Window Run (NWR) Contains images from Weather Research and Forecast Model (WRF) Run 4x a day, forecasts at 3-hr increments to 2 days
Rapid Update Cycle (RUC) Features 50 levels with horizontal resolution of 13 km Provides short-range, hourly numerical weather guidance Rapid Refresh (RR) scheduled to replace RUC
Global Forecast System (GFS) 64-level model operating at different resolutions and forecast
periods Runs 4x a day © AMS19
To optimize weather forecasting with numerical models: Ensemble forecasting
Numerical model generates several forecasts based on slightly different initial conditions
If forecasts are consistent, they are considered reliable Model Comparison
Comparison is made among forecasts produced by different models
If they agree, the forecast issued with a high level of confidence
If forecasts are inconsistent using either technique, forecast is considered unreliable
Weather Prediction
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Forecasting Tropical Cyclones U.S. Army Signal Corps initially in charge of observation/
forecasting 1873: gathered reports from Cuba to help detect tropical cyclones
28 September 1874: first plotted hurricane 1890: forecasting moved to civilians (U.S. Weather Bureau)
Little attention paid to tropical cyclones 1898: Spanish-American War increased interest in tropical
cyclone forecasting Fear hurricane destroying U.S. fleet increased weather stations in
the Caribbean Technological advances
Invention of radio allowed ship-to-shore reports 1930s: upper air monitored 1950s: weather radar at coastal stations observed tropical storms 1960s: remote sensing via satellites began Recently, buoys have provided additional information Aircraft can now deploy dropwindsondes (similar to a radiosonde) to
receive sounding from inside storm
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Weather Prediction
Forecasting Tropical Cyclones 1940s: Atlantic hurricane forecasting split between
Weather Bureau offices in Miami, New Orleans, Washington, DC, Boston, and San Juan
1967: designated Miami office as the National Hurricane Center (NHC)
Today, forecasting split between NHC and the Central Pacific Hurricane Center (CPHC) in Honolulu. NHC responsible for issuing statements for tropical
cyclones in Atlantic basin and eastern Pacific basin to 140°W Operates SLOSH model for prediction of storm surges
CPHC activated when tropical cyclone develops in central Pacific
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Weather Prediction
Forecasting Tropical Cyclones Predicting track and intensity
Forecasts issued every 6 hours, up to 72, 96 and 120 hrs Track forecasts based on climatology, numerical models, and
experience of forecaster
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Weather Prediction
Atlantic hurricane track forecast error, and basin intensity forecast errors. With lengthening forecast period, error increases.
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Weather Prediction
Forecasting Tropical Cyclones SLOSH (Sea, Lake and Overland Surges from
Hurricanes) model predicts location and height of storm surge Probability forecast included in advisory statements
since 1983 Hurricane Watch: winds of at least 119 km (74 mi)
possible within the next 36 hours Hurricane Warning: hurricane conditions expected
in 24 hours or less Watches and warnings also issued for tropical storms © AMS25
Weather Prediction
During the 20th century, tropical cyclone fatalities in the U.S. generally trended downward. Property damage has trended upward since the 1980s
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Weather Prediction
Aviation Weather Center (AWC) Located in Kansas City, MO Supports FAA Forecasts for aviation interests
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Weather Prediction
Storm Prediction Center (SPC) Located in Norman, OK Forecasts severe storms Also monitors fire weather,
blizzards Convective outlooks identifying
areas expected to experience severe and non-severe thunderstorms in 1-3 days. Issued several times a day Specify areas of severe
thunderstorm risk
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Weather Prediction
© AMSSPC Day 1 convective outlook issued at 1259Z on 2 March 2012.
Weather Prediction
Weather Prediction
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River Forecast Centers (RFC) 13 centers located nationwide Develops river, reservoir, and flood forecasts Monitors and forecasts river discharge and stage
Locations of River Forecast
Centers.
Marine Forecasting Ocean Prediction Center located in Camp Springs, MD Issues forecasts, warnings, guidance for mariners, fisheries
recreational boaters
Space Weather Forecasting Space Weather Prediction Center (SWPC) located in
Boulder, CO Monitors phenomena such as the aurora, solar wind, and
solar cycle Scales used that rank severity from 1 to 5
Weather Prediction
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Forecast Skill Declines rapidly after 48 hrs,
minimal beyond 10 days Missing or inaccurate observational data Failure to detect all mesoscale and micro-
scale circulation systems Imprecise equations in numerical models
1-5 day forecasting Slow but steady improvement Better understanding of atmospheric
processes Larger and faster computers More reliable and sophisticated observational
tools Denser weather observational networks
worldwide
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Tracking an early winter cyclone.
Weather Prediction
Computers won’t replace meteorologists The best forecasters rely on knowledge, experience, and
intuition Begin with previous and current observations Must analyze and interpret computerized predictions
Those forecasts are adapted to regional and local circumstances
Weather Prediction
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Long-Range Forecasting Climate Prediction Center
Monthly, seasonal and multi-seasonal generalized climate outlooks
Long range forecasting relies on teleconnections Linkage between changes in atmospheric circulation occurring
in widely separated regions of the globe 30-day (monthly) outlooks
Relies on circulation patterns at 700 mb level Identifies areas of persistent warm and cold air advection
90-day (seasonal) outlooks Relies on long-term trends and recurring events Computer attempts to match past trends with present
conditions 15-month (multi-seasonal) outlooks began in 1995
Each month 13 forecasts are issued, Each covers a 3-month period Subsequent 3-month forecast overlaps previous by 2 months
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Weather Prediction
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Single-Station Forecasting Short-term weather prediction based on
observations at one location Forecasts usually generalized and tentative Fair-weather bias
Fair-weather days outnumber stormy days almost everywhere
Predicting all fair-weather days would be correct more then half the time
Persistence Weather episodes persist for some period of
time (if it has been cold and storm for several days, it may continue that way for awhile)
Climatology Forecast prepared based on previous years
weather
Weather Prediction
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Private Sector Forecasting Television and radio stations, some newspapers,
and private forecast services Some private meteorologists tailor forecasts to
specific needs of their commercial, agricultural, or industrial clients
Supplement the efforts of government forecasters
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Weather Prediction
NCEP maps and charts transmitted to local NWS Forecast Offices to guide meteorologists in preparing local forecasts
Weather information then distributed to the public
When hazardous weather threatens Outlooks: provided for advanced notice Watches: hazardous weather is possible based on
current or anticipated conditions Advisories: anticipated weather hazards; less
serious then those covered by warnings Warnings: hazardous weather is occurring in the
region or imminent
Communication and Dissemination
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Communication and Dissemination
NWS also issues Tornado Warning: detection of a thunderstorm that is known or
likely to produce a tornado Heavy Snow Warning: snowfall of at least 10 to 15 cm (4 to 6
in.) expected in less then 12 hrs Blizzard Warning: blowing or falling snow with sustained winds
of 56 km (35 mi) per hr or higher, reducing visibility to less then 400 m (1300 ft)
Flash Flood Watch: flash flooding possible within watch area Flash Flood Warning: dangerously rapid rise in river level is
imminent or occurring Public receives weather reports and forecasts via
radio, NOAA weather radio, TV, Internet, newspapers
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