1. NATS 101-06 Lecture 23 Weather (and Climate) Forecasting

2. Review: ET Cyclones Ingredients for Intensification
Strong Temperature Contrast
Jet Stream Overhead
S/W Trough to West
UL Divergence over Surface Low
If UL Divergence exceeds LL Inflow, Cyclone Deepens
Similar Life Cycles
deepening
filling
Ahrens, Meteorology Today, 5th Ed.

3. Reasons to Forecast Weather & Climate
Should I bring my umbrella to work today?
Should Miami be evacuated for a hurricane?
How much heating oil should a refinery process for the upcoming winter?
Will the average temperature change if CO2 levels double during the next 100 years?
How much to charge for flood insurance?
These questions require weather-climate forecasts for today, a few days, months, years, decades

4. Forecasting Questions
How are weather forecasts made?
How accurate are current weather forecasts?
How accurate can weather forecasts be?
We will emphasize mid-latitude forecasts out to 15 days where most progress has been made.
PLUS comments about climate models where relevant

5. Types of Forecasts
Persistence - forecast the future atmospheric state to be the same as current state
-Raining today, so forecast rain tomorrow
-Useful for few hours to couple days

6. Types of Forecasts
Trend - add past change to current condition to obtain forecast for the future state
-Useful for few hours to couple days
10 am
11 am
12 pm
59 F
63 F
67 F
Past
Now
Future

7. Types of Forecasts
Analog - find past state that is most similar to current state, then forecast same evolution
-Difficulty is that no two states exactly alike
-Useful for forecasts up to one or two days
Can be useful for seasonal forecasts

8. Types of Forecasts
Climatology - forecast future state to be same as climatology or average of past weather for date
-Forecast July 4th MAX for Tucson to be 100 F
-Most accurate for long forecast projections, forecasts longer that 30 days

9. Types of Forecasts
Numerical Weather Prediction (NWP) - use mathematical models of physics principles to forecast future state from current conditions.
Process involves three major phases
Analysis Phase (most expensive piece)
Prediction Phase (modeling, computing)
Post-Processing Phase (use of products)
To justify NWP cost, it must beat forecasts of persistence, trend, analog and climatology

10. Analysis Phase
PURPOSE: to generate the best estimate of the state of the atmosphere (e.g. density, press, temp, winds, humidity, clouds), and the state of the ocean and land which are needed to start the next weather forecast cycle
The analysis state estimate is a combination of the latest observations and the most recent weather forecast

11. Analysis Phase
Current weather conditions are observed around the globe (surface data, radar, weather balloons, satellites, aircraft).
Millions of observations are transmitted via the Global Telecommunication System (GTS) to the various weather centers.
U.S. center is in D.C. and is named National Centers for Environmental Prediction (NCEP)

12. Analysis Phase
The operational weather centers sort, archive, and quality control the observations.
Computers then combine the latest observations and most recent weather forecast to generate the weather analysis and draw maps to help us interpret weather patterns.
Procedure is called Objective Analysis.
Final chart is referred to as an Analysis.
Computer models at weather centers make global or national weather forecast maps

13. Sparse data over oceans and Southern Hemisphere
Surface Data
Sparse data over oceans and Southern Hemisphere
Courtesy ECMWF

14. Some buoy data over Southern Hemisphere
Surface Buoy Reports
Some buoy data over Southern Hemisphere
Courtesy ECMWF

15. Little data over oceans and Southern Hemisphere
Radiosonde Coverage
Little data over oceans and Southern Hemisphere
Courtesy ECMWF

16. Little data over oceans and Southern Hemisphere
Aircraft Reports
Little data over oceans and Southern Hemisphere
Courtesy ECMWF

17. Weather Satellites Satellite observations fill data void regions
Geostationary Satellites
High temporal sampling
Lower horizontal resolution
Limited vertical information
Can’t penetrate clouds (yet)
Polar Orbiting Satellites
Low temporal sampling
Higher horizontal resolution
Geostationary
Polar Orbit
Ahrens, Figs. 9.5 & 9.6

18. Obs from Geostationary Satellites
Courtesy ECMWF

19. Temperature from Polar Satellites
Courtesy ECMWF

20. Prediction Phase: Atmospheric Models
Weather models are based on mathematical equations that represent the most important aspects of atmospheric behavior
- Newton's 2nd Law (density, press, wind)
- Conservation of mass (density, wind)
- Conservation of energy (temp, wind)
- Equation of state (density, press, temp)
Governing equations relate time changes of fields to spatial distributions of the fields
e.g. warm to south + southerly winds  warming

21. Prediction Phase
Analysis of the current atmospheric state (wind, temp, press, moisture) are fed into the model equations
Equations are solved for a short time period (~5 minutes) over a large number (107 to 108) of discrete locations called grid points
Grid spacing is 5 km to 50 km horizontally and 100 m to 500 m vertically

22. Model Grid Boxes m
10-20 km

23. “A Lot Happens Inside a Grid Box” (Tom Hamill, CDC/NOAA)
Rocky Mountains
Approximate Size of One Grid Box for NCEP Global Ensemble Model
Note Variability in Elevation, Ground Cover, Land Use
Denver
50 km
Source:

24. 13 km Model Terrain
Big mountain ranges, like the Sierra Nevada Range, are resolved.
But isolated peaks, like the Catalinas, are not evident!
100 m contour

25. Post-Processing Phase
Computer then draws maps of projected state to help humans interpret weather forecast
Observations, analyses and forecasts are disseminated to private and public agencies, such as the local NWS Forecast Office and UA
Forecasters use the computer maps, along with knowledge of local weather phenomena and model performance, to issue regional forecasts
News media broadcast these forecasts to public

26. Suite of Official NWS Forecasts
CPC Predictions Page

27. SST Forecast Example (from January 2006)
Forecasts of El Nino and La Nina generally did not forecast present El Nino but some ensemble members did come close

28. 3-Month SST Forecast (Issued October 23, 2006)
SST forecasts for the El Nino region of tropical Pacific are a crucial component of seasonal and yearly forecasts.
Forecasts of El Nino and La Nina show skill out to around 12 months.
Current El Nino

29. Winter 2004-2005 Outlook (Issued 20 October 2005)

30. Winter 2004-2005 Outlook (Issued 18 March 2004)

31. Winter 2004-2005 Outlook (Issued 18 March 2004)

32. NCEP GFS Forecasts ATMO GFS Link NCEP global forecast; 4 times per day
Run on 50 km grid (approximately)
GFS gives the best 2-10 day forecasts

33. NCEP GFS Forecasts ATMO NAM Link NCEP CONUS forecast; 4 times per day
Run on 12 km grid (approximately)
NAM gives the best 24 h precip forecasts

34. Different Forecast Models
Ahrens 2nd Ed. Akin to Fig 9.1
Different, but equally defensible models produce different forecast evolutions for the same event.
Although details of the evolutions differ, the large-waves usually evolve very similarly out to 2 days.
AVN-ETA-NGM Comparison

35. Forecast Evaluation: Accuracy and Skill
Accuracy measures the closeness of a forecast value to a verifying observation
Accuracy can be measured by many metrics
Skill compares the accuracy of a forecast against the accuracy of a competing forecast
A forecast must beat simple competitors:
Persistence, Climatology, Random, etc.
If forecasts consistently beat these competitors, then the forecasts are said to be “skillful”

36. How Humans Improve Forecasts
Local geography in models is smoothed out.
Model forecasts contain small, regional biases.
Model surface temperatures must be adjusted, and local rainfall probabilities must be forecast based on experience and statistical models.
Small-scale features, such as thunderstorms, must be inferred from long-time experience.
If model forecast appears systematically off, human corrects it using current information.

37. Humans Improve Model Forecasts
Forecasters perform better than automated model and statistical forecasts for 24 and 48 h.
Human forecasters play an important role in the forecasting process, especially during severe weather situations that impact public safety.
Max Temp Accuracy
Aguado and Burt
Rainfall Skill

38. Current Skill 0-12 hrs: Can track individual severe storms
12-48 hrs: Can predict daily weather changes well, including regions threatened by severe weather.
3-5 days: Can predict major winter storms, excessive heat and cold snaps. Rainfall forecasts are less accurate.
6-15 days: Can predict average temp and rain over 5 day period well, but daily changes are not forecast well.
30-90 days: Slight skill for average temp and rainfall over period. Forecasts use combination of model forecasts and statistical relationships (e.g. El Nino).
days: “Slight” skill for SST anomalies.

39. Why NWP Forecasts Go Awry
There are inherent flaws in all NWP models that limit the accuracy and skill of forecasts
Computer models idealize the atmosphere
Assumptions can be on target for some situations and way off target for others

40. Why NWP Forecasts Go Awry
All analyses contain errors
Regions with sparse or low quality observations
- Oceans have “poorer” data than continents
Instruments contain measurement error
- A 20oC reading does not exactly equal 20oC
Even a precise measurement at a point location might not accurately represent the big picture
- Radiosonde ascent through isolated cumulus

41. Why NWP Forecasts Go Awry
Insufficient resolution
Weather features smaller than the grid point spacing do not exist in computer forecasts
Interactions between the resolved larger scales and the excluded smaller scales are absent
Inadequate representations of physical processes such as friction and heating
Energy and moisture transfer at the earth's surface are not precisely known

42. Chaos: Limits to Forecasting
We now know that even if our models were perfect, it would still be impossible to predict precisely winter storms beyond days
There are countless, undetected small errors in our initial analyses of the atmosphere
These small disturbances grow with time as the computer projects farther into the future
Lorenz posed, “Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?”

43. Chaos: Limits to Forecasting
After a few days, these initial imperfections dominate forecasts, rendering it useless.
Chaotic physical systems are characterized by unpredictable behavior due to their sensitivity to small changes in initial state.
Evolutions of chaotic systems in nature might appear random, but they are bounded.
Although bounded, they are unpredictable.

44. Chaos: Kleenex Example
Drop a Kleenex to the floor
Drop a 2nd Kleenex, releasing it from the same spot
Drop a 3rd Kleenex, releasing it from the same spot, etc.
Repeat procedure…1,000,000 times if you like, even try moving closer to the floor
Does a Kleenex ever land in the same place as a prior drop?
Kleenex exhibits chaotic behavior!

45. Atmospheric Predictability
The atmosphere is like a falling Kleenex!
The uncertainty in the initial conditions grow during the evolution of a weather forecast.
So a point forecast made for a long time will ultimately be worthless, no better than a guess!
There is a limited amount of predictability, but only for a short period of time.
Loss of predictability is an attribute of nature. It is not an artifact of computer models.

46. Limits of Predictability
What determines the limits of predictability for the atmosphere?
Limits dependent on many factors such as:
Flow regime
Geographic location
Spatial scale of disturbance
Weather element

47. Sensitivity to Initial Conditions
DAY 3 FORECAST POSITIVE
DAY 3 FORECAST NEGATIVE
DAY 3 FORECAST UNPERTURBED
VERIFYING ANALYSIS

48. Summary: Key Concepts Forecasts are needed by many users
There are several types of forecasts
Numerical Weather Prediction (NWP)
Use computer models to forecast weather
-Analysis Phase
-Prediction Phase
-Post-Processing Phase
Humans modify computer forecasts

49. Summary: Key Concepts
National Center for Environment Prediction (NCEP) issues operational forecasts for
El Nino tropical SST anomalies
Seasonal outlooks
10 to 15 day weather forecasts
2 to 3 day fine scale forecasts

50. Summary: Key Concepts NCEP issues forecasts out to a season.
Human forecasters improve NWP forecasts.
NWP forecast go awry for several reasons:
measurement and analysis errors
insufficient model resolution
incomplete understanding of physics
chaotic behavior and predictability
Chaos always limits forecast skill.

51. Assignment for Next Lecture
Topic - Thunderstorms
Reading - Ahrens pg
Problems , 3, 4, 5, 6, 7, 16