Transcript Slide 1

AS-201
Weather Forecasting
This section discusses:
1. Various weather forecasting methods,
their tools, and forecasting accuracy
and skill
2. Images for the forecasting of 6 cities
3. Applications of weather forecasting
Four forecasting steps:
• 1) Assess the present state, called the analysis, by
plotting 6 hourly surface and 12 hourly sounding
data
• 2) Predict a future state by running a computer
model of weather changes…
• 3) Interpret the model results, called a prognostic
chart, given forecasting experience…apply ROTs
(Table 14.1 page 369)
• 4) Translate forecast into something that is useful
Forecasting methods
• Persistence
– The future conditions here will be like the present conditions here
• Trend
– The future conditions here will be like the weather upstream is now
• Analogue
– The future conditions here will be like weather that historically occurred
when similar conditions were present
• Climatology
– Future conditions here will be near the monthly average for here
• Numerical Weather Prediction
– Future conditions here will be as predicted by supercomputers running
equations of atmospheric motion
• “Reading the Sky”
– Future conditions here can be predicted by using current conditions and
forecasting rules of thumb (ROT)
Persistence Forecast
The persistence method works well when weather patterns
change very little and features on the weather maps move very
slowly. It also works well in places like southern California, where
summertime weather conditions vary little from day to day.
Trend Forecasting
• Example: frontal
movement
Analogue Forecasting Method—
Regime Forecasting
Figure 14.7
Climatology
Climatology--winds
Probability Forecasts
Figure 14.6
Climate records can be used to generate probability forecasts for a given
event.
Florida has less than a 5% chance of snow on December 25th, while
northern Minnesota has had snow on that date for each of the past 30
years.
Numerical Weather Prediction
(NWP)
• At 00Z and
12Z each
day, data is
collected and
NWP models
are run
• NWP is the
2nd largest
user of super
computers in
the world
NWP--Limitations
Local Weather “signs”
• Understanding how the atmosphere works (“red
sky at morning…”, halo around the moon, clear
skies at sunset, etc, etc, etc)
• Local rules of thumb (ROT) (for example-southerly flow in March)
• Table 14.3, page 378; and appendix E (page
A11)
• If you have good “situational awareness” of the
atmosphere, rudimentary wx measurements
(temp, DP, pressure, winds), and good ROT—
you can make a pretty good 0-24 hour forecast.
Forecast Periods
- Nowcasting (less than 6 hours)
- Mainly surface observations, weather satellite
and radar, and persistence or trend forecasts
- Short-range (<3 days)
- Trend and analogue methods, weather charts
and numerical weather forecast products
- Medium range (3-8 days)
- Analogue forecasts, numerical weather
forecast products, climatology
- Long-range (>8 days)
- Climatology
Weekly & Monthly Forecasts
Figure 14.8A
Figure 14.8B
Stationary weather systems
often allow for trend based
extended weather forecasts,
while multiple runs of
numerical weather models,
known as ensemble
forecasts, allow for 30 to 90
day outlooks.
Ensemble Modeling
• Numerical weather
models are run multiple
times with small changes
in the initial conditions
– Gives forecaster a good
estimate of the possible
range of weather
conditions at a future
times
Accuracy and skill
• 12-24 hours: very good, 2-5 days: fairly good, above 7 days:
slightly more accurate than persistence
• Accuracy—what is considered “right”?
• Skill—how complicated/hard is the forecast (Southern
California in summer…)?
• Forecast skill is determined by comparing your forecast
against either persistence or climatology
• Longer-range climatological forecasting improving
• Tornadoes…where they are likely to form: 3 days in
advance, but precise area struck is forecastable minutes-few
hours in advance
Forecast Product Accuracy
Tools of the Trade
Analysis to Prognosis--NWP
Figure 14.1A
Figure 14.1B
Two forecast model 500 mb progs for the same time.
Forecast models lose accuracy over time due to errors in the model,
chaos in the atmosphere, and measurement errors
Data Processing and Display
Figure 14.2
Viewing weather images, overlays, and graphs in multiple windows is
done using the National Weather Service's Advanced Weather Interactive
Processing System (AWIPS), which gathers data from the Automated
Surface Observing System among other sources.
WSR-88D Doppler Radar
Weather Surveillance Radar - 1988
Doppler, also known as next generation
radar (NEXRAD), detects severe
weather size, movement, and intensity.
Data received by the NEXRAD unit are
processed by algorithms to assist the
forecaster in weather interpretation.
Figure 14.3
Meteogram Display
Predicted trends in
several weather
variables are plotted for
a 60 hour period on a
meteogram.
Patterns in variable
response, such as rising
pressure and a stop in
precipitation, are
readily observed.
Figure 14.4
NWS Probability of
Precipitation
• Probability of Precipitation
(PoP) describes the chance
of an event occurring at any
point in the forecast area,
over a certain period of time
(usually 12 hours).
• PoP is defined as: PoP = Pa
x Pc, where:
– Pa =the probability that
precipitation will occur
somewhere in the forecast
area during the forecast
period and
– Pc =the percent of the area
that will receive measurable
precipitation, if it does occur
Vertical Sounding Profile
Radiosonde instruments
attached to pilot balloons are
launched twice daily to profile
weather variables with height.
Figure 14.5
This example shows winds
veering (clockwise—warm
advection is occurring) from
easterly at the surface to
southwesterly aloft that may
change the freezing rain in the
saturated lower atmosphere to
non-freezing rain.
Vertical soundings are also used for strength and timing of convective wx, cloud
types and heights, turbulence and icing, etc.
Forecast Practice
Scenarios
Surface Chart Predictions
- 3-hour pressure
tendencies plotted
on isallobar maps
help predict the
movement of
highs and lows
-Lows tend to
move toward the
region of greatest
pressure fall,
while highs move
toward the region
of greatest rise.
Figure 14.10
Upper Level Charts
Figure 14.11
Upper level winds guide the
path of surface pressure
systems.
Observed Movement of Fronts
Surface weather
observations
from 6 PM
Tuesday and 6
AM Wednesday
show how the
fronts, pressure
systems, and
precipitation
have moved as
predicted.
Figure 14.12
Forecast
• Using both trend and persistence forecast techniques,
what is your forecast for 24 hours from now in:
• Denver?
Dallas?
Chicago? Memphis?
• Washington?
Augusta?
Tues 6am
Wed 6am
Observed Weather for 6 AM
Wednesday
Figure 14.13
Another Scenario:
• Forecast for central CA coast for tomorrow
• Slowly falling barometer and increasing
high clouds
• Weak low pressure in Pacific.....
– Will it rain, or not??
Surface Weather for 4 PM Sunday
Figure 14.14
Look at 500mb chart for clues:
500 mb Chart for 4 PM Sunday
Figure 14.15
564 line across the area (local ROT); Omega High present…but cold
advection and shortwave may change the stagnant upper pattern;
upper-level divergence??
12, 24, & 36 hour Progs
Forecaster has access
to several forecast
models
-Which one is best?
-- Choice is based
on initialization and
experience….
Surface Weather Map for 4 AM Sunday
Figure 14.17
500 mb Chart for 4 AM Monday
Figure 14.18
Surface Weather Map for 4 AM
Monday
Figure 14.20
Forecast Applications
Weather Impacts on Aviation
Mishaps
• 88% of all weatherrelated aircraft mishaps
occurred under IMC
(Instrument
Meteorological
Conditions)
• 83% of fatal crashes
involved FAR Part 91
(General Aviation) aircraft
Weather Impacts on National Airspace
System
Wx Impacts on Flight Operations
TRACON (Terminal Radar
Approach Control)
< VFR Conditions
Weather and Commercial Aviation
• Direct costs due to weather on airline operations
can be separated into several categories:
diversion, cancellation, delay and insurance. The
cost of a diverted flight can be as high as
$150,000 and a cancellation close to $40,000
(Irrgang and McKinney, 1992).
• A report from the Air Transport Association (ATA)
states that the direct annual costs to sixteen
member airlines of the first two categories listed
above are $47 million and $222 million,
respectively (Air Traffic Management in the
Future Air Navigation System, 1994).
COMET Module--NAS
• http://www.meted.ucar.edu/nas/index.htm
Weather Impacts on Economic
Sectors
• Oil and gas exploration:
– improved forecasts of tropical weather conditions (wind, waves,
disturbances) can reduce delays in drilling operations at a cost of up to
$250,000 per rig per day (several thousand rigs in the Gulf).
– improved hurricane track predictions could reduce days of production
shutdown, each day of which costs the industry and the U.S. treasury a
combined $15,000,000.
• Vegetable processing:
– improved temperature and precipitation forecasts can lead to greater
efficiency in chemical spraying (e.g., pesticides), which costs $10-$15
per acre per application for hundreds of thousands of acres.
– on a national scale the annual cost of lost production to the vegetable
processing industry, primarily due to weather, is $42,500,000.
• Insurance:
– a single hurricane could lead to more than $50,000,000,000 in
damages.
– weather-related catastrophes have led to more than $48,000,000,000 in
property insurance claims over the period 1989-1993.
Wx Impacts on Economy
• Rail transportation:
– it costs $2,000 per hour to stop a train. A single tornado warning covering
15 miles of track for 15 minutes can lead to seven stopped trains.
– most weather-related derailments cost $1,000,000 to $5,000,000.
• Electric power:
– using improved thunderstorm forecasts could save one utility $200,000
annually in reduced outage time.
– using "good QPF forecasts" could save one utility $2,000,000 over five
years.
– using improved temperature forecasts could save "hundreds of millions
annually nationwide for the utility sector".
• Aviation:
– every avoided cancellation saves $40,000, every avoided diverted flight
saves $150,000.
– for the 16 members of the Air Transport Association, delays and
cancellations cost $269,000,000 annually.
Severe Weather Impacts
“Own the Weather”
• Understand organization’s mission and operations
– Decision cycles
– Risk adversity
– Leadership and their goals
• Understand how weather impacts the organization and
its competition
• Taking the knowledge of weather effects, and
weather—develop a weather product in the right
format, and deliver to the right person, at the exact
right time
• Be right more than you are wrong!
Own the Weather
• USAF adopted this, and applied to weather
support to US Army, USAF, and Special
Operations forces in the mid-90’s
– USAF weather personnel taught to forecast
early in career
– Most experienced technicians and officers
trained in the concepts, and assigned to tactical
level to tailor weather products
Army Division Weather Effects
Matrix
“Garske Chart”
Synchronizes all significant environmental data into an operational
timeline
Employment
• Over past 10 years, there was a large demand
for new meteorologists--currently about 10,000
BS-level or higher graduates employed
–
–
–
–
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Government (National Weather Service; military)
Private companies
Consulting
Broadcast weather
Other related fields (physical and earth science;
environmental fields)
Key Points and Concepts
• Persistence forecast is a prediction that future
wx will be the same as the present.
• The trend method is based on the assumption
that systems continue presently movement
• The analogue method (regimes) makes a wx
prediction by comparing past wx maps and wx
patterns to those of the present.
• Climatological forecasts are based on the
climatology (average weather) of a particular
region
• NWP is a useful tool for forecasting, but has
limitations
Con’t
• Ensemble forecasting is a technique based on running
several forecast models, each beginning with slightly
different weather information to reflect errors in the
measurements. If they agree—forecaster can have a
high confidence in the model forecast.
• For a forecast to show skill, it must be better than a
persistence forecast or climatological forecast
• After a number of days, the atmosphere’s chaotic
behavior, along with flaws in computer models and
small errors in measurements, limits the accuracy of
forecast models
• Surface lows tend to move in direction parallel to the
isobars (in the warm sector), and toward the direction
of greatest pressure fall (isoallobaric)
Con’t
• Weather conditions have a huge effect on
commercial and civil aviation
– Most GA crashes occur in IMC conditions
• The majority of flight delays in the National
Airspace System (NAS) are due to weather
• Weather can have a significant impact on the
economy—resulting in billions of dollars in lost
revenues; and can kill thousands
– Companies are learning to “own the weather” to
minimize losses and increase competitiveness
• The US armed forces introduced “own the
weather” concepts now in use throughout the
meteorological community
COMET Module—NWS Weather
Forecasting
• http://www.meted.ucar.edu/hazwx/index.ht
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