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Weather and Climate

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Title: Weather and Climate


1
Weather and Climate
  • Lecture 14
  • 28 April 2008
  • Forecasting

2
If you know where the low or the high is now
  • You should be able to guess your future weather
  • You should be able to guess your future
    temperature

3
A Low is Approaching
  • It moves to your north or west
  • Winds will shift to the south
  • Any frozen precipitation will likely turn to rain
    or drizzle
  • By the time the cold air arrives, moisture is gone
  • It moves to your south or east
  • Winds will shift from easterly to northerly to
    northwesterly
  • Frozen precipitation will continue
  • Cold air will arrive

4
Problem Steering Flow will evolve with time,
changing wind
Which way will this storm move?
5
Where will low move?
6
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7
Problems
  • Low may change directions, or slow down,
    especially when its occluding
  • Low may leave, but clouds stay behind
  • Upper levels affect surface (dry slots)
  • Computer simulations can do a great job of
    describing the future evolution of the weather

Solution
8
Why are forecasts important?
  • Weather forecasts play a role in the everyday
    decisions that an average person makes.

9
Who employs Weather Forecasters?
  • Government
  • Military
  • Private Forecasting Companies
  • Energy Companies
  • TV stations
  • Insurance Companies
  • Airlines
  • Recreational Industries
  • Agricultural Companies
  • And many more!

Many entities need accurate weather information
10
How does a weather forecaster make a forecast?
11
The Current Weather
  • Before a weather forecast can be made, the
    forecaster has to know the current state of the
    atmosphere.
  • You cant predict if you dont know your starting
    point!!
  • How do we know the current state of the
    atmosphere?

12
Distribution of Observations
13
Distribution of Observations
14
Distribution of Observations
15
Distribution of Observations
The individual satellite paths are obvious!
16
Distribution of Observations
17
Comments on previous slides
  • Not many observations over the ocean
  • What does this mean if you live just west of an
    ocean? Will you know the structure of the
    feature that is bearing down on you?
  • Fewer obs in southern Hemisphere compared to
    northern hemisphere
  • Partly because theres more ocean there
  • Also fewer aircraft flights

18
Even if the storm is over the ocean, sometimes
you can infer a lot from its structures on a
satellite image (This storm is likely extremely
strong with strong winds)
19
Still
  • Knowledge of the present structure wont help
    make an accurate forecast
  • It does give you an accurate start
  • But the state of the atmosphere will change in
    ways you might not be able to predict given the
    initial conditions
  • Must turn to Numerical Weather Prediction

20
Numerical Weather Prediction
  • Approximate weather (wind, temperature, relative
    humidity) at points on the globe
  • Use mathematics to describe the evolution of the
    weather
  • Solve equations on a computer

Note the grid values are defined at every point
on the grid
21
Lewis Fry Richardson
  • Visionary Meteorologist who devised a scheme to
    create forecasts numerically during WWI (he was
    an ambulance driver)
  • Published a book describing the methods
  • Abandoned meteorology when War Departments became
    too interested in his work

22
Why use computers?
  • Computers excel at repetitive calculations
  • It would take a team of mathematicians or
    meteorologists years to produce the millions of
    calculations that go into a single forecast!

23
Air Flow is governed by equations
  • Equations can be approximated by computations on
    grid points in a model
  • Step the model forward in time -- you produce a
    forecast
  • Different computer models have different
    assumptions and will yield different results

24
Two Models Valid at the Same Time
Different models make different assumptions about
aspects of the weather. Or, they have different
resolutions. Result different forecast
outcomes. Overall agreement is usually okay
25
How does forecast change with time?
If sequential model runs show similar results,
you should forecast that event with more
confidence
26
What if there are big differences between two
different models or between sequential runs of
the same model?
Dont forecast with a lot of confidence in that
case Emphasize what you know, and mention the
possibilities
27
How Can A Person Improve Numerical Model
Forecasts?
28
The Human Element
  • Humans have great pattern recognition skills
  • Forecaster will identify the deterioration of a
    forecast long before the model simulation does
  • Compare model forecast to satellite images and
    current surface observations
  • Adjust for known model biases

29
Why Do Numerical Forecasts Go Awry?
30
MANY reasons
  • Small scale features poorly resolved in models
    and must be parameterized
  • Large scale features are not reproduced with 100
    accuracy
  • Initial fields of model contain significant
    errors in horizontal and vertical

31
What is the high on the east coast is
actually stronger than analyzed? How will that
affect the evolution of errors in the subsequent
forecast?
32
Sometimes, the computer simulation dies (or is
obviously wrong), or it cannot be transmittedIf
there are problems with numerical weather
prediction models, what other methods can a
forecaster use?
33
Persistence Forecast
Persistence assumes tomorrow is a carbon copy of
today
34
Steady State or Trend Method
Things are changing at a fixed rate that doesnt
change with time
35
Meteogram (Meteorogram)
You could use the information on this chart to
predict weather on the 20th or 21st by assuming
that the evolution of the weather will resemble
the previous days evolutions.
36
Trend Forecasting
  • If today is a bit warmer than yesterday, then
    tomorrow will be a bit warmer than today
  • If the pressure is falling now, the pressure will
    continue to fall tomorrow
  • Changes just keep going out in time without
    alteration

37
Analogue Method
  • Used when a weather map depicts features similar
    to an event observed in the past.
  • Also called pattern recognition.
  • While weather systems may look similar they are
    never exactly the same.

38
Similar paths, but Different strengths Still --
use old path to forecast weather during new
storm
39
Ensemble Forecasting
  • Use one set of computer code (One model)
  • Start the forecast at the same time, but add
    small perturbations to the initial field, and see
    what impact those perturbations have on the
    subsequent forecast
  • This tells you something about the predictability
    of the atmosphereYou might be in a region where
    no matter how the fields are perturbed, the
    forecast remains the sameor you may be somewhere
    where forecasts change dramatically

This is the focus of most current advances in NWP
40
Ensemble Forecasts
  • Statistical approach to forecasting what
    forecast scenario is most likely?
  • Use one numerical model, perturb initial
    conditions, and see what happens.
  • In the maps on the next page, you see two 500-mb
    height contours close together at 00h (initial
    time), but they diverge some by 3.5 days (84h)
    and diverge even more by 240h (10 days).
  • Still are there regions where even at 10 days
    the different model simulations suggest similar
    weather?

41
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42
Are you forecasting for a place where the
forecast spread is high or low?
43
If youre forecasting for someplace thats not
predictable, you change the definitiveness of
your wording!
44
Ensemble Forecasting
  • You can also use different models, with the
    same initial fields, and see how each model
    describes the evolution of the atmosphere

45
Climatological Forecasting
  • Think about Phoenix, AZ.......if you had to make
    a forecast for tomorrow, would you forecast rain
    or sunshine?
  • A climatological forecast is based solely on what
    the climate records indicate for that location.

46
Probability Forecasting
  • Relies heavily on climatological data.

Chance of a White Christmas
47
Forecast for Today
  • Persistence (A repeat of Sunday)
  • Climatological Cool with a shower, high 62
  • Trend Compare Saturday and Sunday and
    extrapolate to Monday
  • Think about the assumptions in this type of
    forecast are they valid?
  • How often is a day normal at your location?
  • (This means you have to know what normal is!
  • Will be changes continue from day to day?

48
How accurate are model forecasts?
  • 12 to 24 hour forecasts are usually quite
    accurate
  • 1 to 3 days is pretty good
  • 3 to 5 days better than flipping a coin
  • 3.5 days is where 1 day was 30 years ago
  • beyond 7 days relies mostly on climatology
  • For a forecast to have skill it must be better
    than a forecast of persistence or climatology

49
Rules of Thumb
  • Experience is the best tool a forecaster has!
  • Thus many forecasters rely on rules of thumb.
  • What is a rule of thumb?

50
Meteorological Rules of Thumb
  • Will it rain/snow?
  • look at the -5o C isotherm on 850mb chart
  • look at the 540 thickness line
  • Will it be cloudy or clear?
  • On 700mb chart look for relative humidities 70
  • What will the overnight low be?
  • take the 5pm dew point temperature

51
Meteorological Rules of Thumb
  • Where will the low pressure system move?
  • Low pressure systems tend to move in a direction
    parallel to the isobars in the warm sector.

L
L
52
Determining Movement of Weather Systems
  • Surface pressure systems tend to move in the same
    direction as the wind at 500 mb.
  • Surface systems move at a speed half the speed of
    winds at 500 mb.

53
Making a forecast for temperature
Clouds affect the overnight low cloudy nights
are usually warmer
54
Making a forecast for temperature
Cloudy days are usually cooler than clear days
55
Making a forecast for temperature
High to your west? Cooler High to your east?
Warmer
56
Making a forecast for temperature?
Are south winds moving in warmer air?
57
Making a forecast for temperature
Snow cover leads to cooler high temperatures
58
Weather Forecasting
  • What is involved and what do you need to know?
  • Forecasting
  • Minimum Temperature
  • Maximum Temperature
  • Sky condition/Weather Type

59
What do you need to know and what is involved?
  • Current Information
  • Constraints
  • Exceptions to Constraints
  • Rules/Processes
  • Supplemental Information
  • National Weather Service Forecast
  • Numerical Weather Prediction/Computer Model
    Forecast

60
Forecasting
  • Minimum (Low) Temperature
  • Maximum (High) Temperature
  • Sky Condition/Weather Type
  • Cloudy, Partly cloudy, clear, etc.
  • Rain, Snow, etc.

These are the 'normal' forecast parameters
But some companies might want specific things
forecast When will the RH exceed 90? What is
the wind direction?
61
Information that forecasters have
  • Past 24 hours of observations
  • Text
  • Meteorogram (Time series)
  • Climatology
  • Satellite Loop
  • Weather Map
  • National Weather Service Forecast
  • Numerical Weather Prediction Model Forecast

62
Minimum Temperature
  • Rules/Processes
  • Trends/rate of change
  • Moving in of warmer or colder air masses
  • Radiation effects of clouds (blanket effect)
  • Constraints
  • Low will be greater than or equal to the dewpoint
    temperature
  • Low will be less than todays high temperature
  • Almost never forecast record temperatures
  • So, you should know the records for the day!
  • Exceptions to Constraints
  • Dewpoint may change
  • over night
  • Temperature may go up
  • over night
  • It may be that a record low
  • is going to occur

63
Maximum Temperature
  • Rules/Processes
  • Trends/rate of change
  • Moving in of warmer or colder air masses
  • Radiation effects of clouds precipitation
  • Constraints
  • High will be no higher than weather balloon
    forecast method
  • High will be more than tomorrows low temperature
  • Almost never forecast a record high temperature
  • Exceptions to Constraints
  • Weather balloon method only
  • works on dry/windless days
  • Temperature may go down
  • during the day
  • It may be that a record high
  • is going to occur

64
Sky Condition/Weather Type
  • Clouds
  • Rules/Processes
  • Existing clouds moving in?
  • Fronts? Other cloud masses?
  • New clouds forming?
  • New front/low pressure system? Other?
  • Consider trends/persistence especially of
    pressure
  • Stability?
  • Constraint/Exception
  • Too dry for clouds?
  • Weather Type
  • Rules/Processes
  • Clouds --Could be weather with them
  • Consider trends/persistence especially of
    pressure
  • Constraint/Exception
  • Temperature
  • 32 F Rain
  • Clouds - Might not have weather associated with
    them
  • Non-cloud related weather (Fog, blowing snow,
    etc.)

65
The Forecasters Excuse
  • Take a large, almost round, rotating sphere 8000
    miles in diameter,
  • Surround it with a murky, viscous atmosphere of
    gases mixed with water vapor,
  • Tilt its axis so it wobbles back and forth with
    respect to a source of heat and light,
  • Freeze it at both ends and roast in the middle,
  • Cover most of its surface with liquid that
    constantly feeds vapor into the atmosphere as the
    sphere tosses billions of gallons up and down to
    the rhythmic pulling of a captive satellite and
    sun,

66
The Forecasters Excuse
  • THEN TRY TO PREDICT THE CONDITIONS OF THAT
    ATMOSPHERE OVER A SMALL AREA WITHIN A 5 MILE
    RADIUS FOR A PERIOD OF ONE TO FIVE DAYS IN
    ADVANCE!!

67
What you see
What the meteorologist sees!
68
How do we get weather information from around the
world?
  • World Meteorological Organization (WMO) Weather
    Weather Watch (WWW) Program
  • United Nations (UN) agency
  • Each country has one representative
  • Global Telecommunications System (GTS)
  • WMO monitors procedures so that data is
    comparable
  • In The US, data from the WMO is sent to the
    National Oceanic and Atmospheric Administration
    (NOAA)
  • National Weather Service
  • National Centers for Environmental Prediction
  • Others..

69
Who makes forecasts in the US?
  • National Weather Service (NWS)
  • Private Companies
  • TV stations
  • Special national laboratories
  • Storm Prediction Center
  • Hurricane Prediction Center

All of the above get the same data from
NOAA/NCEP! Thats why many forecasts sound the
same!
70
Watch vs. Warning
  • A watch means that conditions are right for
    hazardous weather.
  • A warning means hazardous weather is imminent or
    already occurring.
  • Advisories are issued to warn the public of
    hazardous driving (boating) conditions due to
    weather.

71
Long-Range Forecasting
  • Forecasts made a month or more in advance
  • used in recent years in a local pool
    advertisement
  • These forecasts tend to give general information
    rather than specifics
  • Climate Prediction Center
  • 6 to 10 day forecast
  • 30 day outlook
  • 90 day seasonal outlook

72
Extended Outlook for this past winter
  • Precipitation
  • above normal
  • Temperature
  • warmer than normal

Did it verify?
73
Forecasts for Several Cities
  • Augusta, GA
  • Will start with clear skies, and begin to get
    clouds.
  • Temperatures will warm
  • As warm humid air moves over cold ground, fog may
    form

74
Forecasts for Several Cities
  • Dallas, TX
  • Will get a cold wave
  • Possible showers associated with the front
  • Winds will switch from sw to nw

75
Forecasts for Several Cities
  • Denver, CO
  • Cold but clear
  • Will be dominated by high pressure
  • Rising pressure

76
Forecasts for Several Cities
  • Chicago, IL
  • Will continue to be in the cold air
  • Could experience quite the snow storm

77
Forecasts for Several Cities
  • Memphis, TN
  • Within the next 24 hours there will be a passage
    of both a warm and cold front
  • Wind will shift from SE to SW to NW
  • Pressure will fall, level off, and then fall
    again.
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