Title: Weather and Climate
1Weather and Climate
- Lecture 14
- 28 April 2008
- Forecasting
2If 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
3A 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
4Problem Steering Flow will evolve with time,
changing wind
Which way will this storm move?
5Where will low move?
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7Problems
- 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
8Why are forecasts important?
- Weather forecasts play a role in the everyday
decisions that an average person makes.
9Who 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
10How does a weather forecaster make a forecast?
11The 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?
12Distribution of Observations
13Distribution of Observations
14Distribution of Observations
15Distribution of Observations
The individual satellite paths are obvious!
16Distribution of Observations
17Comments 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
18Even 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)
19Still
- 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
20Numerical 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
21Lewis 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
22Why 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!
23Air 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
24Two 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
25How does forecast change with time?
If sequential model runs show similar results,
you should forecast that event with more
confidence
26What 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
27How Can A Person Improve Numerical Model
Forecasts?
28The 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
29Why Do Numerical Forecasts Go Awry?
30MANY 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
31What is the high on the east coast is
actually stronger than analyzed? How will that
affect the evolution of errors in the subsequent
forecast?
32Sometimes, 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?
33Persistence Forecast
Persistence assumes tomorrow is a carbon copy of
today
34Steady State or Trend Method
Things are changing at a fixed rate that doesnt
change with time
35Meteogram (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.
36Trend 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
37Analogue 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.
38Similar paths, but Different strengths Still --
use old path to forecast weather during new
storm
39Ensemble 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
40Ensemble 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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42Are you forecasting for a place where the
forecast spread is high or low?
43If youre forecasting for someplace thats not
predictable, you change the definitiveness of
your wording!
44Ensemble Forecasting
- You can also use different models, with the
same initial fields, and see how each model
describes the evolution of the atmosphere
45Climatological 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.
46Probability Forecasting
- Relies heavily on climatological data.
Chance of a White Christmas
47Forecast 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?
48How 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
49Rules of Thumb
- Experience is the best tool a forecaster has!
- Thus many forecasters rely on rules of thumb.
- What is a rule of thumb?
50Meteorological 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
51Meteorological 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
52Determining 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.
53Making a forecast for temperature
Clouds affect the overnight low cloudy nights
are usually warmer
54Making a forecast for temperature
Cloudy days are usually cooler than clear days
55Making a forecast for temperature
High to your west? Cooler High to your east?
Warmer
56Making a forecast for temperature?
Are south winds moving in warmer air?
57Making a forecast for temperature
Snow cover leads to cooler high temperatures
58Weather Forecasting
- What is involved and what do you need to know?
- Forecasting
- Minimum Temperature
- Maximum Temperature
- Sky condition/Weather Type
59What 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
60Forecasting
- 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?
61Information 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
62Minimum 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
63Maximum 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
64Sky 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.)
65The 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,
66The 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!!
67What you see
What the meteorologist sees!
68How 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..
69Who 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!
70Watch 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.
71Long-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
72Extended Outlook for this past winter
- Precipitation
- above normal
- Temperature
- warmer than normal
Did it verify?
73Forecasts 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
74Forecasts for Several Cities
- Dallas, TX
- Will get a cold wave
- Possible showers associated with the front
- Winds will switch from sw to nw
75Forecasts for Several Cities
- Denver, CO
- Cold but clear
- Will be dominated by high pressure
- Rising pressure
76Forecasts for Several Cities
- Chicago, IL
- Will continue to be in the cold air
- Could experience quite the snow storm
77Forecasts 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.