Weather and Climate

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Weather and Climate

• Lecture 14
• 28 April 2008
• Forecasting

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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

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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

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Problem Steering Flow will evolve with time,
changing wind
Which way will this storm move?
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Where will low move?
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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
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Why are forecasts important?

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

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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
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How does a weather forecaster make a forecast?
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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?

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Distribution of Observations
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Distribution of Observations
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Distribution of Observations
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Distribution of Observations
The individual satellite paths are obvious!
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Distribution of Observations
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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

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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)
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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

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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
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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

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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!

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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

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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
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How does forecast change with time?
If sequential model runs show similar results,
you should forecast that event with more
confidence
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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
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How Can A Person Improve Numerical Model
Forecasts?
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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

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Why Do Numerical Forecasts Go Awry?
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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

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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?
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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?
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Persistence Forecast
Persistence assumes tomorrow is a carbon copy of
today
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Steady State or Trend Method
Things are changing at a fixed rate that doesnt
change with time
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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.
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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

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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.

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Similar paths, but Different strengths Still --
use old path to forecast weather during new
storm
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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
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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?

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Are you forecasting for a place where the
forecast spread is high or low?
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If youre forecasting for someplace thats not
predictable, you change the definitiveness of
your wording!
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Ensemble Forecasting

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

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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.

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Probability Forecasting

• Relies heavily on climatological data.

Chance of a White Christmas
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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?

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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

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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?

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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

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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
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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.

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Making a forecast for temperature
Clouds affect the overnight low cloudy nights
are usually warmer
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Making a forecast for temperature
Cloudy days are usually cooler than clear days
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Making a forecast for temperature
High to your west? Cooler High to your east?
Warmer
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Making a forecast for temperature?
Are south winds moving in warmer air?
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Making a forecast for temperature
Snow cover leads to cooler high temperatures
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Weather Forecasting

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

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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

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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?
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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

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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

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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

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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.)

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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,

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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!!

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What you see
What the meteorologist sees!
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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..

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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!
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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.

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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

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Extended Outlook for this past winter

• Precipitation
• above normal

• Temperature
• warmer than normal

Did it verify?
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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

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Forecasts for Several Cities

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

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Forecasts for Several Cities

• Denver, CO
• Cold but clear
• Will be dominated by high pressure
• Rising pressure

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Forecasts for Several Cities

• Chicago, IL
• Will continue to be in the cold air
• Could experience quite the snow storm

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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.