1. NATS 101 Lecture 25 Weather Forecasting I

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?
How much water will be available for agriculture & population in 30 years
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.

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 10 am 11 am 12 pm 59 F 63 F 67 F Past Now Future
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

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 (estimate present conditions)
Prediction Phase (computer modeling)
Post-Processing Phase (use of products)
To justify NWP cost, it must beat forecasts of persistence, trend, analog and climatology

10. Analysis Phase
Purpose: Estimate the current weather conditions to use to initialize the weather forecast
Implementation: Because observations are always incomplete, the Analysis is accomplished by combining observations and the most recent forecast

11. Analysis Phase
Current weather conditions are observed around the global (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 analyze the data 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
Polar Orbit
Satellite observations fill data void regions
Geostationary Satellites
High temporal sampling
Low spatial resolution
Polar Orbiting Satellites
Low temporal sampling
High spatial resolution
Ahrens, Figs. 9.5 & 9.6

18. Obs from Geostationary Satellites
Courtesy ECMWF

19. Temperature from Polar Satellites
Courtesy ECMWF

20. Atmospheric Models
Weather models are based on mathematical equations that retain 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 used to start the model equations running forward in time
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 2 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. 40 km Model Grid and Terrain

26. Post-Processing Phase
Computer 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

27. Suite of Official NWS Forecasts
CPC Predictions Page

28. NWP Forecasts
Next lecture, we will show more analyses and forecasts from the current suite of NCEP forecast products

29. 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

30. 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

31. Assignment for Next Lecture
Topic - Weather Forecasting Part II
Reading - Ahrens pg
Problems , 9.15, 9.18