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Weather and Climate Lecture 14 28 April 2008 Forecasting 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 It moves to your north or west Winds will shift to the south

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weather and climate

Weather and Climate

Lecture 14

28 April 2008


if you know where the low or the high is now
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
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

A Low is Approaching
  • Low may change directions, or slow down, especially when it’s 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



Why are forecasts important?

Weather forecasts play a role in the everyday decisions that an average person makes.

who employs weather forecasters
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


the current weather
The Current Weather
  • Before a weather forecast can be made, the forecaster has to know the current state of the atmosphere.
    • You can’t predict if you don’t know your starting point!!
  • How do we know the current state of the atmosphere?
distribution of observations15
Distribution of Observations

The individual satellite paths are obvious!

comments on previous slides
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 there’s more ocean there
    • Also fewer aircraft flights

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)

  • Knowledge of the present structure won’t 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
numerical weather prediction
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

lewis fry richardson
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
why use computers
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!
air flow is governed by equations
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
two models valid at the same time
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

how does forecast change with time
How does forecast change with time?

If sequential model runs show similar results, you should forecast that event with more confidence

What if there are big differences between two different models or between sequential runs of the same model?

Don’t forecast with a lot of confidence in that case:

Emphasize what you know, and mention the possibilities

the human element
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
many reasons
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

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?


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?

persistence forecast
Persistence Forecast

Persistence assumes tomorrow is a carbon copy of today

steady state or trend method
Steady State or Trend Method

Things are changing at a fixed rate that doesn’t change with time

meteogram meteorogram
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’


trend forecasting
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
analogue method
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.

Similar paths, but

Different strengths

Still -- use ‘old’ path to

forecast weather during ‘new’ storm

ensemble forecasting
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 atmosphere…You might be in a region where no matter how the fields are perturbed, the forecast remains the same…or you may be somewhere where forecasts change dramatically

This is the focus of most current advances in NWP

ensemble forecasts
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?

If you’re forecasting for someplace that’s not predictable,

you change the definitiveness of your wording!

ensemble forecasting44
Ensemble Forecasting

You can also use different models, with the same initial fields, and see how each model describes the evolution of the atmosphere

climatological forecasting
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.

probability forecasting
Probability Forecasting
  • Relies heavily on climatological data.

Chance of a White Christmas

forecast for today
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?
how accurate are model forecasts
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
rules of thumb
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”?

meteorological rules of thumb
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
meteorological rules of thumb51
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.



determining movement of weather systems
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.
making a forecast for temperature
Making a forecast for temperature

Clouds affect the overnight low – cloudy nights are usually warmer

making a forecast for temperature54
Making a forecast for temperature

Cloudy days are usually cooler than clear days

making a forecast for temperature55
Making a forecast for temperature

High to your west? Cooler

High to your east? Warmer

making a forecast for temperature56
Making a forecast for temperature?

Are south winds moving in warmer air?

making a forecast for temperature57
Making a forecast for temperature

Snow cover leads to cooler high temperatures


Weather Forecasting

  • What is involved and what do you need to know?
  • Forecasting
    • Minimum Temperature
    • Maximum Temperature
    • Sky condition/Weather Type

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


  • 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?


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

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 today’s 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

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 tomorrow’s 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

Sky Condition/Weather Type


  • 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 Snow
      • >32 F Rain
    • Clouds - Might not have weather associated with them
    • Non-cloud related weather (Fog, blowing snow, etc.)
the forecaster s excuse
The Forecaster’s 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,
the forecaster s excuse66
The Forecaster’s Excuse



What you see

What the meteorologist sees!

how do we get weather information from around the world
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..
who makes forecasts in the us
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! That’s why many forecasts sound the same!

watch vs warning
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.
long range forecasting
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
extended outlook for this past winter
Extended Outlook for this past winter
  • Precipitation:
    • above normal
  • Temperature
    • warmer than normal

Did it verify?

forecasts for several cities
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
forecasts for several cities74
Forecasts for Several Cities
  • Dallas, TX
  • Will get a cold wave
  • Possible showers associated with the front
  • Winds will switch from sw to nw
forecasts for several cities75
Forecasts for Several Cities
  • Denver, CO
  • Cold but clear
  • Will be dominated by high pressure
  • Rising pressure
forecasts for several cities76
Forecasts for Several Cities
  • Chicago, IL
  • Will continue to be in the cold air
  • Could experience quite the snow storm
forecasts for several cities77
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.