AMS Weather Studies Introduction to Atmospheric Science, 5 th Edition Chapter 13 Weather Analysis and Forecasting © AMS.

AMS Weather Studies

Introduction to Atmospheric Science, 5th Edition

Chapter 13Weather Analysis and Forecasting

© AMS

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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


© AMSGlobal Observing System

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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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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


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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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


Surface weather maps Isobars:

Connect points of equal air pressure

Isobaric analysis reveals locations of anticyclones and cyclones, troughs and ridges, horizontal pressure gradients

Drawn at 4-mb intervals © AMS12

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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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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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


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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

39 © AMS

AMS Weather Studies Introduction to Atmospheric Science, 5 th Edition Chapter 13 Weather Analysis and Forecasting © AMS.

Documents

weather forecasts

weather analysis

analysis weather

preparation of weather

upperair weather maps

analysis slide

public slide

world weather watch