AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 13
Weather Analysis
and Forecasting
© AMS
Case-in-Point
 Tornado forecasting began in the late 19th century
– Started by John P. Finley, U.S. Army Signal Corps
 1884: about 950 “tornado reporters” were gathering data
 Finley established criteria for a valid tornado sighting
– Finley’s forecasting lasted only 2 years
 U.S. Army Signal Corps discontinued the program in 1886
– The word “tornado” was disallowed in Signal Corps forecasts
because it might cause the public to panic
– Late 1940s and early 1950s, Air Force meteorologists Fawbush and Miller
developed a method for forecasting tornadoes
 Forecasts were primarily issued for military installations
 Interest in tornado forecasting stemmed from tornado that struck
Tinker Air Force Base in Oklahoma City, OK on 20 March 1948
 Developed a list of six atmospheric conditions that preceded tornado
outbreak
– Those conditions reappeared 5 days later
– Issued first tornado forecast
– Eventually, U.S. Weather Bureau adopted/allowed tornado forecasting
for public distribution
– Severe Local Storm Warning Center (now the Storm Prediction Center)
was established in Norman, OK
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Driving Question
 What is the process involved in making a scientific
forecast of the weather?
– Weather effects everyone
– Forecasts quite accurate out to 7 days
– Weather prediction will never be perfect
 Incomplete information on initial state of atmosphere
 Some scientific questions not yet answered
 Short range forecasts still surprisingly accurate
– U.S. National Weather Service (NWS)
 An agency of the National Oceanic and Atmospheric
Administration (NOAA)
 Issues 24-hour forecasts with 85% accuracy
– This chapter covers:
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 How forecasts are made
 Limits of forecast accuracy
 Making your own weather forecasts
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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 weathermonitoring program called World Weather Watch (WWW)
- WWW makes meteorological information available internationally
 Global Observing System
– Data from 6 geostationary and 3 polar orbiting satellites,11,000 land
stations, 4,000 ships at sea, 3,000 reconnaissance and commercial
aircraft, radar, 1,300 radiosonde stations, and 1,200 drifting and 1,300
moored buoys
– Data transmitted to 3 WMO Centers where maps and charts are
created. Forecasts can then be prepared.
– Maps/forecasts are sent to Regional Specialized Meteorological
Centers.
– National Centers for Environmental Prediction (NCEP) responsible for
U.S. forecasts
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International Cooperation
Global Observing System
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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
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 Use Coordinated Universal Time (UTC)
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Acquisition of Weather Data
 Surface Weather Observations, cont.
– Automated Surface Observing System (ASOS)
and communication ports
 Result of 1990s NWS modernization
 884 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,
and sky condition
– Automated Weather Observation System
(AWOS)
 Similar to ASOS
 188 FAA-owned and 1034 non-Federal
– NWS Cooperative Observer Network
 Volunteers
 Provide daily precipitation, and temperature
readings
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Acquisition of Weather Data
 Upper-Air Weather Observations
– Radiosondes
 Radio-equipped instrument package
 Transmits upper air information to a ground station (rawinsonde
observation)
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Weather Data Assimilation,
Depiction and Analysis
 Weather
reported by
each
observation
station is
depicted on a
map by a
station model
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Weather Data Assimilation,
Depiction and Analysis
 Surface weather maps
– Isobars
 Connect points of
equal air pressure
 Isobaric analysis
reveals locations of
anticyclones (highs)
and cyclones (lows),
troughs and ridges,
and horizontal
pressure gradients
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Weather Data Assimilation,
Depiction and Analysis
 Surface Weather Maps, continued
– 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
Sample 500-mb analysis (NOAA)
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Plotted on constantpressure surfaces
Height contours labeled in
meters above sea level,
drawn 60 m apart
Altitude of pressure surface
varies primarily 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
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Weather Data Assimilation,
Depiction, and Analysis
 Upper-Air Weather Maps, continued
– 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
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Weather Data Assimilation,
Depiction, and Analysis
 Deluge of real-time weather information
spurred the development of
computerized data management
systems
– McIDAS (Man-computer Interactive Data
Access System)
 Ingests weather data and organizes into
guidance products for potential users
– AWIPS (Advanced Weather Interactive
Processing System)
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 Used by NWS Offices since 2000
 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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Weather Prediction
 Hydrometeorological Prediction Center
– Issues short range 12, 24, 36, and 48-hr
forecasts
 Climate Prediction Center
– Generates monthly (30-day), seasonal (90-day)
outlooks, and multi-seasonal outlooks
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Weather Prediction
 Numerical Weather Forecasting
– Computers are programmed with a numerical model of
the atmosphere; a model of mathematical 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
 North American Mesoscale Model (NAM)
– Divides troposphere into 60 vertical layers
– Forecasts every 6 hours out to 84 hours
 Rapid Update Cycle (RUC)
– Features 50 levels with horizontal resolution of 13 km
– Provides short-range, hourly numerical weather guidance
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Weather Prediction
 Nested Window Run (NWR)
– Contains images from the WRF
 WRF run 4 times per day, produces graphics at 3-hr
intervals out to 2 days
 Global Forecast System (GFS)
– Has two 64 level models operating at different
resolutions and forecast periods
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Weather Prediction
 Techniques used to optimize the skill of weather
forecasting based on 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 is issued with a high level of
confidence
– If forecasts are inconsistent using either technique,
forecast is considered unreliable
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Weather Prediction
 Forecasting Tropical Cyclones
U.S. Army Signal Corps was in charge of observation and forecasting
early on
 1873: gathered reports from Cuba to help detect tropical cyclones
28 September 1874: first plotting of a hurricane
1890: forecasting moved to civilians (U.S. Weather Bureau)
 Little attention paid to tropical cyclones
War caused increased interest in tropical cyclone forecasting
 1898, Spanish-American War; fear that a hurricane could destroy U.S.
fleet
 Increased number of weather stations in the Caribbean
Technological advances greatly benefited understanding and
monitoring of tropical cyclones
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Invention of radio allowed ship-to-shore reports
1930s: upper air was 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, continued
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 is split between NHC and the Central
Pacific Hurricane Center in Honolulu.
 NHC responsible for issuing statements for tropical cyclones in
Atlantic basin and eastern Pacific basin to 140°W
 CPHC activated when tropical cyclone develops in central Pacific
 Operates SLOSH model for prediction of storm surges
NHC is a branch of the Tropical Prediction Center (TPC)
 TPC, under WMO agreement, has responsibility for prediction of
tropical cyclones for 24 nations
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Weather Prediction
 Forecasting Tropical
Cyclones, continued
– Biggest challenge is
predicting track and
intensity
 Forecasts issued every 6
hours. Covered 72 hr
periods until 2001 when
period was extended to
96 and 120 hrs
 Track forecasts based on
climatology, numerical
models, and experience
of forecaster
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Weather Prediction
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Weather Prediction
 Forecasting Tropical Cyclones, continued
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 with in the next 36 hours
Hurricane Warning: hurricane conditions expected in 24
hours or less
 Watches and warnings also issues for tropical storms
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Weather Prediction
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During the 20th century, tropical cyclone fatalities in
the U.S. trended downward, whereas property
damage trended upward
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Other Forecasting Centers
 Aviation Weather Center (AWC)
– Located in Kansas City, MO
– Supports FAA
– Forecasts for aviation interests
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 Storm Prediction Center (SPC)
– Located in Norman, OK
– Forecasts severe storms
– Also monitors fire weather,
blizzards
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Other Forecasting Centers
The SPC issues
convective outlooks
identifying areas around
the nation expected to
experience severe and
non-severe
thunderstorms over the
following 1-3 days.
Convective outlooks are
issued several times a
day, and specify areas of
severe thunderstorm risk
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Other Forecasting Centers
 River Forecast Centers
(RFC)
– 13 centers located
nationwide
– Develops river, reservoir,
and flood forecasts
– Monitors and forecasts
river discharge and stage
 Marine Forecasting
– Ocean Prediction Center is
located in Camp Springs,
MD
– Issues forecasts, warnings,
and guidance for mariners,
fisheries, and recreational
boaters
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Other Forecasting Centers
 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
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Weather Prediction
 Forecast Skill
Weather forecasting skill declines rapidly for periods
longer then 48 hrs, and is minimal beyond 10 days
 Missing or inaccurate observational data
 Failure to detect all meso-scale and micro-scale
circulation systems
 Imprecise equations in numerical models
1- to 5-day forecasting has shown slow but steady
improvement
 Better understanding of atmospheric processes
 Larger and faster computers
 More reliable and sophisticated observational tools
Including Doppler radar and remote sensing by satellite
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 Denser weather observational networks worldwide
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Weather Prediction
 Computers aren’t likely
to replace
meteorologists
The best forecasters rely on
knowledge, experience, and
intuition
Begin with previous and
current observations
Must analyze and interpret
computerized predictions
 Adapt those forecasts to
regional and local
circumstances
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Weather Prediction
 Long-Range Forecasting
Climate Prediction Center
 Camp Springs, MD
Prepare 30-day (monthly) 90-day (seasonal) and 15-month (multiseasonal) generalized climate outlooks
– Long range forecasting relies on teleconnections: a
linkage between changes in atmospheric circulation
occurring in widely separated regions of the globe
30-day (monthly) outlooks rely on circulation patterns at
700 mb level
 Identifies areas of persistent warm and cold air advection
– 90-day (seasonal) outlooks rely on long-term trends and
recurring events
 Computer attempts to match past trends with present conditions
– 15-month (multi-seasonal) outlooks began in 1995
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 Each month 13 forecasts are issued, each covering a 3-month
period. Subsequent 3-month forecast overlaps the previous by
two months.
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Weather Prediction
 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 (i.e., 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
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Weather Prediction
 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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Communication and Dissemination
 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, NWS issues:
Outlooks: provided for advanced notice
Watches: hazardous weather is possible based on
current or anticipated conditions
Warnings: hazardous weather is occurring in the region
or imminent
Advisories: anticipated weather hazards; less serious
then those covered by warnings
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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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