Weather Forecasting

1
Weather Forecasting
2

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

3
From the Arab News

• We are unable to report the weather today. We
  rely on weather reports from the airport, which
  is closed due to the weather. Whether we will be
  able to report the weather tomorrow will depend
  on the weather.

4
Weather Forecasters are Employed by

• Government
• Military
• Private Forecasting Companies
• Energy Companies
• TV stations
• Insurance Companies
• Airlines
• Recreational Industries
• Agricultural Companies
• And many more!

5
How does a weather forecaster make a forecast?
6
The Current Weather

• Before a weather forecast can be made, the
  forecaster needs to know the current state of the
  atmosphere.
• How do we know the current state of the
  atmosphere?

7
What sort of weather observations do we have?
8
Distribution of Observations
9
Distribution of Observations
10
Distribution of Observations
11
Distribution of Observations
12
Distribution of Observations
13
Numerical Weather Prediction
14
Why use computers?

• It would take a team of meteorologists years to
  produce the millions of calculations that go into
  a single forecast!

15
Two Models Valid at the Same Time
16
How Can We Improve Numerical Model Forecasts?
17
Why Do Numerical Forecasts Go Awry?
18
So if there are problems with numerical weather
prediction models, what other methods can a
forecaster use?
19
Persistence Forecast
20
Steady State or Trend Method
21
Meteogram
22
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.

23
Ensemble Forecasting
24
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.

25
Probability Forecasting

• Relies heavily on climatological data.

Chance of a White Christmas
26
How accurate are 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
• beyond 7 days relies mostly on climatology
• For a forecast to have skill it must be better
  than a forecast of persistence or climatology

27
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?

28
Meteorological Rules of Thumb

• Will it rain/snow?
• look at the 23oF 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

29
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
30
Determining Movement of Weather Systems

• Surface pressure systems tend to move in the same
  direction as the wind at the 500mb level. The
  speed at which the surface system moves is half
  the speed of winds at 500mb.

31
Making a forecast for temperature
32
Making a forecast for temperature
33
Making a forecast for temperature
34
Making a forecast for temperature?
35
Making a forecast for temperature
36
Now is your chance to Forecast!
37
Weather Forecasting

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

38
What do you need to know and what is involved?

• Given Information
• Constraints
• Exceptions to Constraints
• Rules/Processes
• Supplemental Information
• National Weather Service Forecast
• Numerical Weather Prediction/Computer Model
  Forecast

39
Forecasting

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

40
Given Information

• Past 24 hours of observations
• Text
• Meteorogram (Time series)
• Climatology
• Satellite Loop
• Weather Map
• National Weather Service Forecast
• Numerical Weather Prediction Model Forecast

41
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 a record low temperature

• 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

42
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

43
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
• 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
• 32F Rain
• Clouds - Might not have weather associated with
  them
• Non-cloud related weather (Fog, blowing snow,
  etc.)

44
Forecasting Exercise Dane County Regional
Airport - Truax field, Madison, WI

• Forecast Low, High, Sky Condition/Weather
• Explain WHY for eachGive reasons...
• Small Group Discussion Data Review
• Whole Class Discussion

45
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,

46
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!!

47
What you see
What the meteorologist sees!
48
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..

49
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, and they get
the data from NOAA/NCEP!
50
Advanced Weather Interactive Processing System
(AWIPS)

• Computerized system used by NWS to view vast
  amounts of weather data.
• Inspired from U. Wisconsin - McIDAS

51
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.

52
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

53
Extended Outlook

• Precipitation
• above normal
• below normal

• Temperature
• warmer than normal
• colder than normal

54
What does a 50 chance of rain mean?
It means that there is a 50 chance that any
location in the forecast area will receive
measurable precipitation.
55
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

56
Forecasts for Several Cities

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

57
Forecasts for Several Cities

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

58
Forecasts for Several Cities

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

59
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.