The Mysteries and Challenges of Forecasting Weather

By Mark Bloomer

The morning is bright and sunny with blue sky stretching from horizon to horizon. Nothing can be seen in the vast expanse of sky except for a few lonely contrails and a couple wisps of high cirrus clouds. The air is calm, and the only sounds that can be heard outdoors are the morning birds and a few passing cars. However, when we pour our first cup of coffee and turn on the morning news we hear of major changes in the wind. Clouds are forecast to increase tonight and snow is expected to overspread the town by morning. The forecast is calling for snow to be heavy at times during the day tomorrow before changing to sleet and freezing rain. Forecasters are also calling for southeast winds up to 40 miles per hour, and are mentioning the possibility of wind damage. Why do forecasters believe that all this is coming? How can such a tranquil and clear day turn into a grey and icy storm overnight?

The next day we wake up to the sound of sleet slating on the windows. The wind is howling, and low clouds are tumbling across a silvery foggy sky. Sure enough the forecasters were right. Well, at least they were right about a big windy and icy storm coming. But they seemed to have missed some of the details. What happened to the heavy snow we had expected? Looking out the window, it appears only an inch or so of icy, slushy snow coated the driveway and trees before the precipitation changed over to sleet.

It is amazing that we are able to foretell changes in the skies as well as we do. And yet we still occasionally find ourselves frustrated at the weather turning out differently than we had expected. Forecasting changes in the atmosphere is a complex task. The atmosphere is basically a churning fluid tumbling and flowing across the earth as clouds melt and gel in the turbulent air currents. Weather forecasting involves observing qualities of the atmosphere including temperature, pressure, wind and humidity, and modeling these to predict how details in the atmosphere will change over periods of time. The atmosphere is too vast to measure in its entirety. We can only observe samples at certain locations where we have our instruments. From these samples, we do the best we can to piece together the current state of the atmosphere, similar to creating an image on a piece of paper or a computer screen from an array of colored pixels. Once we resolve these weather features, we project the movement and evolution of those features into the future to forecast how they will effect our changes in weather.

Weather systems propagate as waves along thermal boundaries. The challenges of forecasting often lie in the complexity of these waves and how they interact with each other. If you have ever been at sea or by the ocean watching ocean waves, you may be familiar with the fact that sometimes the waves move along in a smooth, regular rhythm, and at other times the sea may appear to be choppy and chaotic. The more chaotic the waves in the atmosphere are, the more difficult it is to predict how they will affect the weather. Some examples can be recalled looking back at this past season. The late fall and early winter featured large, well-behaved, and relatively regular atmospheric waves. This allowed for some good forecasting of the storm systems, which brought our high winds, rain and snow to the area. Later in the winter season the waves became a bit weaker and more chaotic. In late February, one of these poorly organized waves approached our area with the prospect of significant snow, but ended up tracking further south at the last minute pulling most of the heavy precipitation out to sea. Actually, this particular wave was a combination of three weaker waves, with the middle of the three being the strongest and the most likely to bring heavy snow. The trailing wave, which was supposed to follow the larger wave and take a more southerly track, ended up accelerating and pulling the entire system further to the south.

When several waves approach together they have the potential to combine into one stronger wave, or to remain as separate waves possibly weakening each other. When two waves of moderate intensity combine into one stronger wave we call this Aphasing@. When a wave races along causing a preceding wave to accelerate out of its way we call this Akicking@. Subtle differences in the speeds of each wave often determine if they will Acombine and phase@, or Acompete and kick@.

Forecasting the arrival of a big storm is relatively easy, but forecasting where the heaviest precipitation will occur, especially with wintry precipitation, is considerably more challenging. While an individual storm system may be over a thousand miles in diameter, the bands of precipitation it produces can be only a few tens of miles thick leaving little room for error in forecasting the track or intensity of the storm. During the winter season a storm center that passes south of Maine brings northeasterly winds keeping most of the precipitation snow. A storm center that passes northwest of Maine through the St. Lawrence valley brings southeasterly winds. This can change snow over to sleet and freezing rain, and eventually all rain. Often a storm system passing inland likes to develop a secondary low along the coast. The formation of this second low can stop the progression of warm air northward. Determining how quickly or strongly this secondary low develops is often important in determining how far north the changeover to sleet or rain will progress.

As late spring and summer approaches, the atmosphere becomes warm enough for all rain to fall at the surface. The challenge of forecasting where rain-snow lines will be is left behind to the winter months. But the summer season has a new challenge. The warming atmosphere brings increased chances for thunderstorms, which can be even more difficult to forecast than larger scale storm systems.

Thunderstorms form when relatively warm and humid air near the earth’s surface rises into cooler and dryer air aloft. Tracking atmospheric waves is very important for thunderstorm forecasting, as it is for forecasting larger storm systems. A wave in the atmosphere often occurs in conjunction with a pocket of cold air aloft which can set off thunderstorms as it approaches a region. Atmospheric waves that help to produce thunderstorms in the summertime can often be small and subtle, and therefore difficult to resolve in the grid of data used to resolve larger systems. Satellite data including visible, infrared and water vapor images are quite useful in determining where waves are propagating. A pool of cold air aloft can often be discerned by locating cyclonic, or counterclockwise rotation in satellite images. This in turn helps determine where thunderstorms have a good chance of developing.

It is a wonder that we can forecast the changes in weather, but the atmosphere can be quite complex. Regardless of how good we get at forecasting there will always be challenges. And no matter how fine our data grid becomes, there will always be secrets and subtleties that escape detection. We proudly take our victories and humbly learn from our misses, but always maintain our enthusiasm for determining what the atmosphere will bring to us next.