1. The Forecast Process
ATMO 4300
Spring 2010

2. Scales of Motion Global (Planetary) Scale Synoptic Scale Mesoscale
In the context of wx anlys and fcsts it is important to consider atmospheric processes occurring on different space and time scales. Planetary scale features include those associated with the General Circulation of the Atmosphere. [NEXT SLIDE]

3. Figure from www.atmos.washington.edu/2003Q3/101/notes
Slide46.mp3
The colors in this figure represent satellite measurements of solar radiation absorbed minus infrared radiation emitted. The purple, red and brown colors represent positive values while the green and blue colors represent negative values. This figure shows that there is more incoming solar radiation that outgoing longwave radiation around the equator while there is more outgoing longwave radiation than incoming solar radiation at higher latitudes toward the poles.

4. Largely driven by uneven heating of the earth’s surface
Largely driven by uneven heating of the earth’s surface. Net gain of energy in tropics, net loss in poles.

5. ITCZ

6. Average Pressure in January

7. Average Pressure in July

8. Largely driven by uneven heating of the earth’s surface
Largely driven by uneven heating of the earth’s surface. Net gain of energy in tropics, net loss in poles.

9. The Polar Front

10. Jet Stream Figure from www.crystalinks.com/jetstream.html
Slide39.mp3
Another reason why the tropopause is important is that the jet stream is located near the tropopause. The jet stream is a relatively narrow band of very fast moving air that help to steer weather systems and can also be an important factor in the development of severe thunderstorms.

11. Seasonal Position of Jet Stream Figure from www. earth. rochester
Seasonal Position of Jet Stream Figure from
Slide31.mp3
Just as the ITCZ and the subtropical high pressure zone shift northward during the northern hemisphere summer, so does the polar front and the associated polar jet stream. This figure shows approximate positions of the polar jet during the winter and summer.

12. Scales of Motion Synoptic Scale Global (Planetary) Scale Mesoscale
In the context of wx anlys and fcsts it is important to consider atmospheric processes occurring on different space and time scales. Planetary scale features include those associated with the General Circulation of the Atmosphere. [NEXT SLIDE]

13. Synoptic Scale

14. Synoptic Scale – 500 mb Image from www.rap.ucar.edu/weather

15. Scales of Motion Mesoscale Global (Planetary) Scale Synoptic Scale
In the context of wx anlys and fcsts it is important to consider atmospheric processes occurring on different space and time scales. Planetary scale features include those associated with the General Circulation of the Atmosphere. [NEXT SLIDE]

16. Mesoscale
In the past decade or so more emphasis has been placed on the mesoscale.
Hazardous weather occurs – severe thunderstorms, tornadoes
Banded precipitation
Improvement in computer technology – talk about mesoscale models later

17. Forecast Methods

18. Forecasting Methods

19. Climatology
Using long-term averages to forecast a particular weather element
Example:
What’s the problem with using Climatology?

20. Persistence Current Weather = Future Weather Accuracy depends on:
1. Time scale
2. Progressive vs. stagnant weather
pattern
• Example:

21. Trend
Extrapolating current weather out in time based on steady-state conditions
Best for approx 0-6 hours (Nowcasting)
Example:
What’s the problem with this method?

22. Analogue Also called “Pattern Recognition”
Today’s weather similar to a past event
Forecast based on what happened with the past event
Example:
What’s the problem with this method?

23. Analogue - Example Go to the following web site:
Click on Forecasts
Click first “new” link
Scroll down to No. 5

24. Variations on Analogue Method
Teleconnections – Relationship between weather in one part of the globe to events in another part
Used in long-range forecasting
Example: El Nino – connection between tropics and North America

25. El Nino
What is it?
An event associated with significant warming of sea surface temperatures in the eastern tropical Pacific Ocean.

26. Normal Sea Surface Temps
Ocean temperatures are normally warmer in the west and colder in the east.
Figures from
Slide38.mp3
In the absence of El Nino, normal sea surface temperatures in the equatorial Pacific Ocean are warmest in the west as shown by the red colors and coolest in the east as shown by the blue colors during the year.

27. El Nino Event Figure from www.cpc.ncep.noaa.gov/products

28. Effects on Atmosphere Figure from www.cpc.ncep.noaa.gov

29. Misconceptions about El Nino
El Nino or La Nina does not directly cause any one particular weather event.
Slide44.mp3
It is worth mentioning that El Nino and La Nina do not directly cause any one particular weather event such as a significant snowstorm in Denver or a tornado outbreak in the southern plains. El Nino or La Nina may change the large scale weather pattern that may set the stage for such events.

30. Variations on Analogue Method
Decision Trees / Checklists
Using common parameters associated with many past events
“Rules of thumb”
Example:

31. Example of Decision Trees
Class Home Page
Click link to All NWS Offices
Click Midland on the map
Click Office Information
Click Local Research
Click on 5th article

32. Numerical Weather Prediction
Using computer models of the atmosphere to predict weather variables
Model Output Statistics (MOS)
Ensembles

33. The Forecast Process

34. The Forecast Process
With all the data and the different forecasting methods, how do you decide what to forecast?
Rain or sunny
Snow amount
Graphic from

35. A Systematic Approach Start with the Big Picture
- Look at the hemispheric (or partial hemispheric) upper wind flow pattern
- Look at a large scale water vapor satellite animation
(current & recent history)

36. A Systematic Approach Review Synoptic Scale
- Surface map – cyclones/anticyclones, air mass boundaries
- Upper air maps – troughs/ridges, PVA/NVA, temp advection, meridional vs zonal flow
- Radar/satellite – precip/clouds in and near forecast area

37. The Forecast Funnel

38. A Systematic Approach Associate current weather with causes. Example:
Evaluate moisture and lift for precip.

39. A Systematic Approach What is the time period of your forecast?
Example: 0 – 6 hours (Nowcast)
Radar, satellite, profilers, detailed surface analysis
Example: Days 1 to 7
Numerical Weather Prediction models

40. A Systematic Approach Choose a forecast methodology(s)
What weather elements am I forecasting and what will affect those elements?
When forecasting different weather elements, usually best to forecast temps last.

41. Summary What happened and why? What is happening and why?
What is going to happen and why?
Adapted from Lance Bosart, SUNY, Albany