Ensemble forecasting - a better tool for today's farmers

Monday, February 3, 2014

The North American Ensemble Forecast System provides a seamless set of forecast products that can be tailored to specific needs. Farmers should be using this information

by PHIL CHADWICK

There is a better way to get your weather information. As much as meteorologists want to be detailed and accurate with their predictions, the atmosphere is a complicated and chaotic part of the environment. It is too complex to fully understand and explain in the continuum of time and space. The detail that farmers need and desire is typically beyond the science to calculate and talk about, although everyone does like to "talk about the weather." The forecast does not come from God and, although Einstein said that "God does not play dice with the universe," sometimes a roll of the dice is the best way to communicate weather.

Farmers used to rely solely on their senses. They lived in the weather and understood all of the nuances of the sky, the clouds and the environment. Thankfully, this hasn't changed much, but sometimes it is important to know the weather beyond the horizon or several days or even weeks away in the future. Today's farmers are more than expert enough to use this ensemble forecasting information in their modern operations. This is where current meteorological services can really add to the expert folklore of the farmer.

As a young meteorologist, I would strive to fully analyze and diagnose the atmosphere in order to accurately predict the singlemost likely outcome – a deterministic forecast. I had already weighed the various options for the way that the atmosphere was likely to unfold, but I couldn't communicate other plausible options or any uncertainties to the client. Now we can. This approach is called "ensemble forecasting."

Imagine a roomful of meteorologists. Each produces a detailed deterministic forecast based on their knowledge and experience and the different weights they mentally assign to the atmospheric processes of the current weather situation. The real solution is probably going to closely match one of these deterministic forecasts, but which one?

It isn't always the most experienced meteorologist who gets the right solution. Even the best are sometimes wrong. You can roll the weather dice and bet on a single deterministic forecast from those produced or play it safer and smarter that the solution is somewhere among the many forecasts. Use all of the forecasts to make the best business decision. This is ensemble forecasting and the various forecasts are produced by a myriad of computer simulations of the atmosphere instead of meteorologists.

The solution lying in the centre of the pack is typically the single most likely solution. The measure of "centre" can be the "arithmetic mean" (sometimes negatively influenced by extreme outliers), the median (middle solution more insensitive to extremes) or the mode (the most frequent solution). The experts providing the ensemble predictions typically go with the median forecast.

The spread in the solutions gives an estimate of the predictability of the current atmospheric pattern. The measure of spread typically used is the "standard deviation." Some patterns are more predictable than others. If the spread in solutions is small, then the confidence in a specific outcome is high and you can have some confidence in taking action. If the spread is large between the meteorological solutions, be cautious. It is really uncertain just how the atmosphere will evolve.

Sometimes the predicted forecasts diverge into two or more very different outcomes. Expert meteorologists viewing these ensemble predictions can often pick the correct, non-median solution out of the group of forecast stamps based on their added knowledge of the atmospheric processes.

Sharpness is another desirable property of ensemble forecasts. Provided that the ensemble is reliable, the more an ensemble forecast deviates from the climatological frequency of the event and issues zero per cent or 100 per cent forecasts of that event, the more useful the forecast will be. Forecasts at long lead times cannot be particularly sharp since the inevitable errors in the predictions grow with increasing lead time. Eventually these errors are as large as the separation between the different forecast solutions and that yields no obvious solution.

The communication of the results of ensemble prediction can be really interesting. There are various ways of viewing the many possible solutions such as spaghetti plots, time series with whiskers, stamp maps and a host of others – and the list continues to grow. Some displays are best left to the meteorologists while others are tailored for just about anyone.

The North American Ensemble Forecast System (NAEFS) is a joint project involving the Meteorological Service of Canada (MSC), the United States National Weather Service (NWS) and the National Meteorological Service of Mexico (NMSM). NAEFS was officially launched in November 2004 and combines state-of-the-art ensemble forecasts developed both at the MSC and the NWS.

When combined, the grand ensemble can provide weather forecast guidance for the one-to-14 day period that is of higher quality than the currently available operational guidance based on either set of ensembles alone. It allows the generation of a set of forecast products that are seamless across the national boundaries between Canada, the United States and Mexico. The list of products continues to grow to better meet the needs of specific clients. Farmers should be in this client list and using this information.

The North American Ensemble Forecast System (NAEFS) products can be found at http://weather.gc.ca/ensemble/naefs/EPSgrams_e.html. The site contains details on how to best use the information. Currently, ensemble forecasts are prepared every day (at 0 UTC and 12 UTC) for the next 15 days.

Ontario farmers could benefit from this ensemble approach to weather forecasting. Like farming, the weather is always a gamble and it is important to use all of the information at hand before making some of those really important decisions. Good luck! BF
 
Phil-the-forecaster Chadwick has been a professional meteorologist since 1977 specializing in training, severe weather and remote satellite and radar sensing.