By Will Brantley

Knowing how to read a weather map may be as crucial to your hunting success as decoys and calls

Few things in waterfowling build more anticipation than an approaching cold front. These weather events greatly influence the movements of ducks and geese in the fall and winter, but because of variations in timing and intensity, cold fronts can affect waterfowl in different ways.

Other weather patterns may also stimulate significant duck and goose movements during hunting season. Heavy rain can create backwater opportunities that puddle ducks will quickly exploit, and deep snow can cover grainfields where geese have been feeding, forcing them to travel elsewhere to find food.

Because of the many excellent weather resources available on the Internet today, observing weather trends and their effect on waterfowl has never been easier. "In this day and age, there is no reason for waterfowl hunters to be surprised by the weather," says Dale Humburg, chief biologist for Ducks Unlimited. "You can instantly check the forecast for wind direction and speed, cloud cover, temperature, and precipitation. If the dew point and temperature are the same, you know there will likely be fog. All this information is available online on weather websites. There's no reason to hunt with the wind in your face, or not have a rain coat if you need it."

Weather and Hunting

Sidney Gauthreaux is a professor emeritus at Clemson University. His area of expertise is radar ornithology, or the study of bird movements with radar. "At a Doppler radar station, you can easily gauge the speed of the target on the radar," he says. "Because waterfowl are such fast fliers, they're readily distinguishable on radar." Gauthreaux's studies have shown him many weather scenarios in which waterfowl are likely to move.

Some waterfowl species such as blue-winged teal are "hard-wired" for migration. In general, these species migrate to their wintering areas around the same time each year and stay there until spring. But other species, such as mallards and Canada geese, are more likely to be in flux near the freeze line throughout the season.

"In midwinter, you see big waterfowl migrations if you have a long period of cold weather," Gauthreaux says. "But what's interesting is that as soon as the conditions stabilize and thaw, birds that flew south may travel back north. And as you enter late winter, especially in the South, we often see big waterfowl movements to the north after long periods of warm weather."

Cold Fronts

A cold front is the edge of a cold air mass that is replacing a warmer air mass. Southerly winds and a falling barometer are usually the precursors, often with clouds and precipitation. As the front passes, the winds generally shift to the southwest and then to the north. Once the front has passed, expect to see high pressure with clear skies, cooler air, and northerly winds-good hunting conditions. But not all cold fronts are the same.

"It doesn't take much to move early- season migrants," Humburg says. "In the Mississippi Flyway, you can expect cold fronts during the first half of September to be responsible for blue-winged teal movements, as well as the initial shoveler and pintail movements. Gadwalls and wigeon move sometime around mid-October, and during the last week in October to the first week in November, expect to see initial mallard movements and some diver movement. But the late migrations require a cold front that freezes things tight up north as well as snow cover. Those conditions will often put an end to the hunting in the northern Midwest, with snow cover being the exclamation point on the end of the season. If it's just cold but there is no snow, many waterfowl may delay their movement because they can still reach their food."

Gauthreaux agrees. "Whether or not numbers of late-season birds move with a cold front all depends on whether or not they will be debilitated in getting access to food," he says.

Milder cold fronts can cause behavior changes in waterfowl on the wintering areas, even if they don't stimulate mass movements. "Temperature affects food choices," Humburg says. "When the temperature falls, ducks shift from seeds and moist-soil foods to lsquo;hot foods' such as corn. But if the weather moderates after a front, the birds may shift right back. I've had some great hunting in the late season over natural, moist-soil wetlands. Any time you have a temperature change there may be a diet shift."

Although waterfowl may fly south at any point during a cold front, they often use the northerly winds behind a cold front to migrate more efficiently. These arctic winds create the chilling side effects associated with cold fronts, but they also provide tailwinds for migrating birds.

"Generally, most waterfowl arrive just behind a front," Gauthreaux says. "Powerful fronts are huge wedges of cold air, and they tend to travel fast. Waterfowl are fast fliers so it is possible for them to jump ahead of a front, but the conditions aren't really favorable for them to do that because they would be flying into the wind. We see much larger movements behind a front because the flying conditions are more favorable."

Warm Fronts

Warm fronts are warm air masses that are replacing cooler air masses, but their effect is often more gradual than that of a cold front. Expect to see low-hanging clouds, precipitation, falling pressure, and often fog as a warm front approaches, followed by rising temperatures and stable pressure. Warm-ups and the weather phenomena associated with them can have a surprising impact-and not always a negative one-on your hunting.

Many hunters see a warm forecast and assume the hunting will be slow. But if your area has been frozen for a week, a warm-up can be just the ticket to bring the ducks back.

"If lots of birds that are scattered over a wide geographical area are suddenly pushed south because of hard weather, you can get a sharp population increase in concentrated areas farther south," Gauthreaux says. "As soon as conditions moderate, many of them may go back north. In a way, they act like the mercury in a thermometer, moving down (south) when it is cold and up (north) when it is warm."

Humburg says the effects of a warm-up can be dramatically different depending on the time of year. "Things just may go flat during a long warm period in November," he says. "But you can definitely see some reverse migration later in the year, especially from mid-January on. Many different biological things are going on at that time of year that weren't taking place in November. By that time of year, 90 percent of mallards are paired. This causes changes in habitat preferences and behavior. Pairs become more isolated, and all birds are putting on fat reserves for their return migration."

Rain is often associated with warming periods in the winter, and it can have both immediate and long-term effects on hunting. "As it's falling, heavy precipitation will put birds down," Gauthreaux says. "From an aerodynamic standpoint, raindrops disrupt wind flow over the wings, causing birds to expend too much energy when flying."

But the situation can change quickly once the rain passes through. Increased runoff from large amounts of rain can flood fields and bottomland, providing new food sources for waterfowl. At the same time, the feeding areas that birds had been using may become too deep-puddle ducks can't reach food in much more than 18 inches of water.

The Perfect Storm

So what are the weather conditions that must fall into place to create waterfowling's "perfect storm"? It depends on the region where you are hunting, the time of year, the species hunted, and even the terrain-in short, too many factors to nail down in one answer. A grand passage of waterfowl is almost always triggered by a major blizzard or some other record-setting weather event-the kind that only occurs every decade or so. In the years in between, waterfowlers are left with predicting duck and goose movements according to more typical forecasts.

Wondering what the weather will bring to the marsh the next morning has likely been pondered by more waterfowlers through the ages than any other question about the sport. "Remember, we're discussing how wild, migratory birds will react to a weather forecast that meteorologists only get right part of the time," Humburg says. "A lot of the lsquo;perfect' duck weather boils down to your area and personal preference. For me, I like cool temperatures, clear skies, and a decent amount of wind on the back side of a cold front. I think that would be a pretty common answer across the country."

How to Read a Weather Map

Predicting weather is a complex science, but there are a number of resources that make understanding the forecast a simple task. The National Weather Service (noaa.gov) and other sites, such as weather.com and accuweather.com, provide detailed surface maps, Doppler radar imagery, and hourly forecast updates. The NOAA site provides current and forecast river levels, which can be particularly useful for river hunters.

There are many symbols for various weather patterns on a surface map. The most important for waterfowlers to understand are the symbols for cold and warm fronts, isobars, and high and low pressure. Taken in combination, a general understanding of these weather symbols can allow a waterfowl hunter to predict how severe an approaching front will be-and, in turn, how it may influence waterfowl.

"When trying to predict how severe a cold front will be, there are other things to look for on the map besides the cold front symbol," says Zwemer Ingram, data acquisition program manager for the National Weather Service in Memphis. "Isobars that are close to one another indicate a tight pressure gradient, and a tight pressure gradient indicates strong winds. Another thing to study is where the high pressure behind a front is coming from. In the winter, fronts that originate in Canada will usually be drier and colder. But sometimes the front will originate from the Pacific Coast. These may not be as cold, but they're more likely to bring precipitation."

Weather Map Symbols

  • Cold Front: Cold fronts are cold air masses replacing warm air masses at ground level. Depending on the severity of the front, they can have huge impacts on your waterfowl hunting. Major cold fronts can initiate migrations; minor cold fronts may not cause large-scale movements, but may influence local waterfowl behavior and hunting conditions.
  • Warm Front: Warm fronts are warm air masses that are replacing cold air masses. In general, warm fronts slow the hunting pace, particularly early in the season. However, a warming trend may stimulate reverse migration late in the season.
  • High Pressure: An area of high barometric pressure is where the local air pressure is higher than that of the surrounding area. High pressure often means blue skies and fair weather.
  • Low Pressure: An area of low barometric pressure is where the local air pressure is lower than the surrounding area. Low pressure is often associated with precipitation, wind, and approaching fronts.
  • Isobars: These are lines on a map connecting areas of equal barometric pressure. Tightly grouped isobars indicate a large pressure gradient and high winds.
  • Doppler Radar: Hunters can forecast advancing precipitation by watching animated Doppler radar maps. Different types of precipitation, such as rain, sleet, and snow, are indicated by color keys.