© 2013 Pearson Education, Inc.Chapter 13Weather Fore.docx

© 2013 Pearson Education, Inc.
Chapter 13
Weather
Forecasting
http://www.weather.com/news/weather/video/step-into-the-
future-of-weather-forecasting
http://www.weather.com/news/weather/video/step-into-the-
future-of-weather-forecasting
http://www.accuweather.com/en/weather-
video/channels/bloopers
Weather of the dayyyyyy!http://www.weather.govNOAA
Outlookhttp://www.hpc.ncep.noaa.gov/basicwx/day0-
7loop.htmlhttp://www.spc.noaa.gov/http://www.hpc.ncep.noaa.g
ov/sfc/90fwbg.gifhttp://www.weather.com/
Thanks Alaina!
Derecho

(Putnam county, OH) 6/29/2012
On June 29, 2012, a derecho (deh REYcho) moved east-
southeast at 60 miles per hour (mph) from Indiana in the early
afternoon to the Mid-Atlantic region around midnight. The
states most significantly impacted were Indiana, Ohio,
Kentucky, Pennsylvania, West Virginia, Maryland, Virginia,
Delaware, New Jersey, and North Carolina, as well as
Washington, D.C. Nearly every county impacted by this
convective system suffered damages and power outages.
Winds were commonly above 60 mph with numerous reports of
winds exceeding 80 mph. Some areas reported isolated pockets
of winds greater than 100 mph. The storm resulted in 13 deaths,
mainly a result of falling trees. One major impact from the
derecho was widespread power outages. More than 4 million
customers were without power, some for more than a week after
the storms moved through. To make matters worse, the area
affected was in the midst of a prolonged heat wave. There were
34 heat related fatalities in areas without power because of the
derecho.
http://www.nws.noaa.gov/os/assessments/pdfs/derecho12.pdf
https://www.youtube.com/watch?v=gT8vYDwidao
https://www.youtube.com/watch?v=O15KFbS5bD8
500 mb heights showed no obvious disturbances to indicate
derecho development, high press ridge in NW, 45mph at height

Check out the dew points, the temp ahead of the front, ELR
High CAPE. CAPE is directly related to the maximum potential
vertical speed within an updraft Circular shape indicates what
type of storm?
The storms organized into a large bow echo in eastern
Indiana and raced east-southeast at around 60 mph
Doppler Reflectivity and Base Velocity
Convective Update
Progression

Forecasters undershot warning areas
So, it was a missed forecast with severe consequences. What to
do differently?Establishing a 24-hour warning point and
emergency operations centerHaving more than one method of
receiving severe weather forecasts and alerting the
publicCreating a system that monitors local weather
conditionsPromoting the significance of public readiness
through community seminarsDeveloping a formal hazardous
weather plan that includes training severe weather spotters and
holding exercises
The public perception was that even with the warnings, stronger
words were needed to make them take heed
Weather Forecasting—Both Art and Science
As you can see with the derecho, forecasting is difficult and
encompasses many disciplines, technologies, and tools. Think
about all of the measures and tools we have examined so far in
class. Temperature, humidity, winds in upper and lower
atmosphere, storm behavior, precipitation, sub-solar point,
rotation, revolution. Meteorologists take ALL of these into
account. Mathematics, physics, chemistry, social sciences.
Public perception is half the battle!

Why Is Weather Forecasting Imperfect?Public perception is that
forecasts should be nearly perfect, and people get ANGRY when
they are wrong!Weather forecasting is
extraordinarily difficult, as many
variables are involved that are
constantly changing (temperature,
water content, energy, stability, etc.).So lets think, if I ask you
to tell me
the temp for tomorrow, a forecasters,
what do you need to consider?
ForecastingStart from beginning of the book: temp structure
based of the absorption and scattering of radiation, which
depends on the horizontal and vertical distribution of
atmospheric gases, clouds, vapor, etc. With water changing
between three states constantly, atmospheric composition is
hardly constant. As water is changing state, there is constant
energy exchange. Latent heat! These phase changes are
influenced by vertical up and down drafts, gotta keep an eye on
those too. The season, the breezes, wind direction…
Ayayeyaye!Weather forecasting involves a set of interlocking
problems, each difficult to solve in isolation, let alone in
combination. It should be surprising that they DO get it right,
rather than don’t!
Weather Forecasting—Both Art and ScienceNational Weather

Service (NWS) and the Meteorological Service of Canada
(MSC) employ forecasters 24/7.The NWS was established by
the US Govt. in the 1870’s, now falls under the direction of
NOAAhttp://preview.weather.gov/edd/https://www.youtube.com
/watch?v=2XsFnh-7E2wEvery meteorologist has a set of
procedures for analyzing the weather, given the situation.
There is no defined routine.Technology has changed things
drasticallyThe Advanced Weather Interactive Processing System
(AWIPS) allows forecasters to display all the pertinent weather
information available (current weather conditions, output of
computer forecast models displayed in map form, satellite and
radar images, forecast advisories and discussions from other
weather facilities, etc.).
Typical DayStarts with verbal briefing from a meteorologist
finishing shiftExamine a large amount of data, focus per
geographic region (tornado look at radar, snow storm satellite)
Multiple screened monitors can display an enormous amount of
information to help with decision makingMajor weather systems
are just the tip of the icebergThe first step would often be to
check cloud cover: Cloud cover can obviously indicate precip,
but also is needed to determine tempYou have determine
whether the clouds will stick around, thin vs thick, clear skies
mean clouds could move inIf its 5 degrees warmer than
yesterday AM, then PM will likely be 5 degrees warmer
Weather Forecasting—Both Art and ScienceForecast maps
created by the computer models often depict the predicted

distribution of weather variables directly as an application of
physical laws.Modeling, predicting behaviorFor example, enter
in temp, press, upper level flows, shear, etc, etc
StatisticalOther models rely on statistical methods to assist the
forecaster.Statistically, physical models could tell temp zones
in NA, statistics can “fill in the blanks”
Other forecastsShort term forecasts- aviation warnings,
hydrological info, etcMaritime forecast
http://forecast.weather.gov/product.php?site=CLE&issuedby=C
LE&product=AFD&format=CI&version=1&glossary=1&highlig
ht=offThe most familiar type of weather statement is the zone
forecast, issues at designated times each day and extending out
to a week into the
futurehttp://www.erh.noaa.gov/displayprod.php?product=CLEZ
FPCLE
Forecasting Methods
There are 4 forecasting methods: They can be used separately or
together
Climatological forecasts: based on long-term averages, but not
current weather conditions, Chicago vs Orlando
Persistence forecasts: based on current conditions, no
climatological reference, you do everyday
Analog approach: based on current conditions and similar well-
studied patterns from the past. Past precedence, rule of thumb,

winter snow about the 5000m contour, high clouds bring precip
in 36 hours
Numerical weather forecasting: based on computer programs
that mimic the behavior of the atmosphere.
Types of Forecasts
There are 3 types of forecasts:
Quantitative forecasts: “An inch of rain is expected.” “The high
is going to be 56 ºF today.”
Qualitative forecasts: categorical, classify, “rain/no rain” or
“above/below normal”, cloudy, partly cloudy, etc
Probability (PoP) forecasts: “The rain chance today is 70%.”
“There is a 60% chance of snow.”
Types of Forecasts
Prob of Precip Map
Thanks Brianna!
Assessing ForecastsMeasures are needed when comparing one

forecast to another, decide which method is best, and
whenAssessment measures are needed by those responsible for
developing and administering forecasting programs to ensure
best practices, justify expenditures, determining ROIThe
forecast quality (how close to true) and the forecast value
(loss/payoff tables, risk/rewards) can be assessed. When
assessing the forecast quality, forecast accuracy and forecast
bias (systematic over or under prediction) can be
examined.Forecast skill can also be a factor, use a baseline of
climatology, predicting no rain in Jerusalem in July or a
hurricane in September
Assessing ForecastsMeasures are needed when comparing one
forecast to another, decide which method is best, and when. You
do this when planning vacation!Assessment measures are
needed by those responsible for developing and administering
forecasting programs to ensure best practices, justify
expenditures, determining ROIThe forecast quality (how close
to true) and the forecast value (loss/payoff tables, risk/rewards)
can be assessed. When assessing the forecast quality, forecast
accuracy and forecast bias (systematic over or under prediction)
can be examined.Forecast skill can also be a factor, use a
baseline of climatology, predicting no rain in Jerusalem of a
hurricane in September
Data Acquisition and Dissemination: How does the NWS get all
that data?Data collection is an international effort, blended
bordersThe World Meteorological Organization (WMO),
sanctioned by UN, responsible for data collection across the
globe. Collected from 179 nations, 10,000 land stations, 7000

ship stations, 300 buoys, as well as weather satellites, and 1000
weather balloons, twice a day. Data is sent to world
meteorological centers in Washington D.C., Moscow, and
Melbourne. These centers disseminate to the rest of the world.
In the U.S., the National Centers for Environmental Prediction
(NCEP) are responsible for processing, and issuing regional and
national forecasts.
Data Acquisition and Dissemination
The FAA and NWS have installed a network of more than 800
automated sensors, called Automated Surface Observing
System, or ASOS, for measuring and recording temperature,
humidity, pressure, cloud conditions, wind direction and speed,
visibility, presence of fog or rain, and accumulated precipitation
readings at ground level.
Radiosondes, thousands a day, are launched to track pressure,
temp, and upper level winds. The balloons ascend for a few
hours and burst at the stratosphere.
Forecast Procedures and Products
Numerical models are at the frontline of forecast data analysis.
They are constantly updated. Right now the NWS uses three
different forecast models. From time to time, new ones are
produced, and the older are retired.
Phases in Numerical ModelingModels differ but they all involve
three phases: analysis, prediction, and postprocessing.
Analysis PhaseObservation data is used to supply a current state

of the atmosphere (3 dimensional, some are planetary)Data is
converted into uniform initial values (buoys, ships, planes,
etc)Values for both the horizontal and the vertical conditions
over very large areas plotted (the computer can pick out
implausibility)
Forecast Procedures and ProductsPhases in Numerical Modeling
Prediction PhaseBased on solving basic governing equations
(equations of motion, continuity, energy)Based on atmos
variables (temp, press, wind, density, moist)Equations used to
obtain new values minutes into the futureEquations solved and
advanced through various forecasting periodsBillions of
equations solved at each step
Forecast Procedures and ProductsPhases in Numerical Modeling
Postprocessing PhaseA series of maps of atmospheric conditions
constructed (sea level press, 500mb, heights and temps at all
levels, absolute vorticity values)Each map centers upon one or a
few variablesForecasters use these products as general guidance
Combinations of model output are generally used (one may have
rain in the SE corner of County, etc)Rules of thumb aren’t ruled
out, but added to the forecast, subjective judgment
Forecast Procedures and ProductsABC, 3 separate models,
forecast, and reality

Forecast Procedures and ProductsMedium-Range
ForecastsMedium range forecasts (MRF) are forecasts for 72
hours to a few weeks.Ensemble forecasting is a forecast in
which the predictions from several different computer model
forecasts are combined to form an ensemble average, spaghetti
diagram, heights
Forecast Procedures and ProductsPredicting long term behavior
can lead to Chaos. Chaos is a condition that occurs in physical
systems (in our case, the atmosphere) that makes it impossible
to precisely predict how a system will appear some time in the
futureIf two models are run with different initial variables,
Chaos can occur (butterfly effect)
Forecast Procedures and ProductsLong-Range Forecasts and
Seasonal OutlooksLong-range forecasts are forecasts that cover
several weeks to several months or more
The Climate Prediction Center (CPC) within the NCEP within
the NWS produces these forecasts
Long-range forecasts use climatology, statistics, numerical
modes, and subjective judgment
CPC also produces a seasonal outlook, or a forecast for the
entire season (colder than normal, etc)

Weather Maps and Images
Surface mapsSurface maps depict prevailing conditions,
pressure distributions, location of fronts, etc.Both large-scale
features and station models are included in surface mapsThink
about all the inferences you can make from a large scale surface
map
Surface Maps: Station Models
Weather Maps and ImagesUpper-Level MapsPlotted twice daily
for different pressure levels.
850mb Maps 850mb level typically found about 1.5km (1mi)
above sea level Heights are plotted as solid lines while
isotherms are dashed lines These charts are useful in
determining daily maximum surface temperatures for non-
mountainous regionsSummer - the surface maximum about 27 ºF
greater than the 850mb temperatureWinter - this number reduces
to 16 ºF and 22 ºF for fall and spring
Weather Maps and ImagesUpper-Level Maps
700mb Maps700mb level typically found about 3km (2mi)

above sea level. Very similar to the 850mb map.Useful for
observing short waves and the formation of mid-latitude
cyclones, and tracking air mass thunderstorms.
500mb Maps 500mb level typically found about 5.6km
(18,000ft) above sea level. Used as a representation of the
middle atmosphere and helpful when viewing the omega high.
Weather Maps and ImagesIf you watch the news, you’re likely
to see satellite ImagesVisible images describe atmospheric
features from the perspective of an observer from space.
Sunlight reflected off of cloud tops. Available only during
daylight.
Infrared images display longwave radiation emitted, not
reflected from clouds, the lower atmosphere, or the surface.
Clear skies appear very dark on these images.
Water vapor images are created by detecting radiation around
6.7µm where water vapor is an effective radiator.
Weather Maps and ImagesSatellite Images
Visible

Infrared
Water Vapor
Weather Maps and ImagesRadar ImagesRadar images display
atmospheric conditions by measuring the amount of radiation
backscattered from liquid and solid particles in the atmosphere,
mainly cloudsA receiver records the intensity and return time of
the echoed pulses (indicative of the number and size of droplets
and crystals)The radar unit rotates 360ºA large volume of air is
examined, giving data on the distribution of clouds, height of
cloud tops and bases, and the intensity of precipitation.Each
unit surveys a 400km (200mi) surrounding area
Weather Maps and ImagesRadar Images
Thermodynamic Diagrams
The maps and images we have examined so far concern 2D
imaging. These fail to provide detailed vertical
information.Lifted Index- created in the mid 1950’s, combines
avg humidity in the lowest km of the atmosphere, the predicted
max temp for the day, and the temp at the 500mb into one
valueThunderstorm potential is actually determined through the
lifted index. Negative values indicate there is sufficient water
vapor and stability conditions to create thunderstorms.Potential
is indicated through the magnitude of the negative values
LI 6 or Greater, Very Stable Conditions
LI Between 1 and 6 : Stable Conditions, Thunderstorms Not

Likely
LI Between 0 and -2 : Slightly Unstable, Thunderstorms
Possible, With Lifting Mechanism (i.e., cold front, daytime
heating, ...)
LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some
Severe With Lifting Mechanism
LI Less Than -6: Very Unstable, Severe Thunderstorms Likely
With Lifting Mechanism
End of Chapter 13
http://www.dnr.sc.gov/climate/sco/Education/wxmap/wxmap.ph
p

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