WEEKEND SAILORS of the Chesapeake region spend their weekdays glued to the weather forecasts, hungry to know what the wind will do. Even fishermen and prudent powerboaters listen for clues to the upcoming size of seas or the probability of sun for the beer break on the fantail.

Yet the more time one spends with the weather forecasters, the more maddening the imprecision of their marine broadcasts becomes. Fourth of July Weekend, for example, I was punching through whitecaps and shortening sail frantically (if happily) all day in 20 knots of northerly breeze simultaneously being described by the VHF broadcast below as "light and variable, less than 10 knots."

Now some of my best friends work for the National Oceanic and Atmospheric Administration, and I know there are reasons for the forecast anomalies. NOAA's weather service is asked to predict everything from hay- drying conditions to global temperatures, and rarely has time or opportunity to explain the overall weather picture in the sort of detail a sailor requires. And the time lag between the weather observations on which the forecasts are based and the forecast itself, not to mention the lag between the time the forecast is recorded and the time you hear it, make many broadcasts laughably useless for anything beyond general temperatures.

But the prudent mariner with an experienced weather eye can use those broadcasts plus a little basic seamanship to make his or her own forecasts about what the wind and waves will do. The forecasts won't be perfect, but they'll go a long way toward helping make informed decisions about whether to take the boat out and where to go. Here are a few basics to watch for:

PREVAILING WIND --

In the summer the Bay's weather is dominated by high- pressure systems that sweep in from Canada, move over the area, and then stall offshore for a week or more at a time, pumping from the sea that hot, moist air that has made Washington's summers infamous worldwide. The erratic arrival times of these systems cause most of the puzzlement about weather on the Bay, but once they're here they follow a fairly dependable progression. The front is usually unstable where its cooler, leading edge meets the moist air of the departing front, and thus may be heralded by overcast skies and showers which can last from a few hours to a day or more. But when they clear they usually give way to that treasure of the Chesapeake, the north wind.

NORTH WIND --

It is the north (or northwesterly) wind that brings the true glory days of Washington summers, those scattered days, such as we had last weekend, of sudden, crystalline clarity when the air cools briefly, trees and water sparkle with color and all of us feel briefly reborn. On the Bay, such days are unsurpassed for sailing, often bringing 15 to 20 knots of wind and air as sweet as young love.

But like young love, the north wind can be fickle. A friend of mine insists that the harder the north wind blows, the sooner it will stop. That frequently seems to be true. How long it blows depends primarily on the size of the high-pressure area, but you can rarely count on more than two days of north wind in the summer in any case. If you walk out your front door Wednesday, fall in love with a north-wind day and start making plans for the weekend, go easy. Chances are all that fresh clear breeze will be gone by Saturday.

On the other hand, the north wind almost always stays at least 12 hours. Sunday, July 5 was supposed to be a day of light and variable winds, lousy for sailing. I anchored the night before in Whitehall Bay just north of Annapolis, hoping the predicted light southerly breeze would discourage mosquitoes.

When one buzzed in the night, however, I woke to find the boat afloat on almost mirror-smooth water, with faint stirrings of a pre-dawn wind starting to swing it toward the north. By 5 the north wind was busy clearing out the usual Bay mist at 15 knots. Since I was nursing a sick engine, and didn't want to find myself becalmed far from home, the question was how long it would last. The weather service had forecast a weak high-pressure system to arrive on Monday. I decided it had just arrived early and gambled on 12 hours of good wind. The gamble paid off with a roaring wonder of a sailing day, with 15 to 20 knots and one of my fastest ever passages between Annapolis and Bloody Point. But the wind began weakening around 3:30. By 5 it was gone.

CLOCKING WIND --

The winds around a high-pressure system in the northern hemisphere circulate clockwise, and as the system moves across our region, the wind will clock around to a southerly or southwesterly flow. This shift will rarely take more than 12 hours in the summer. Winds almost never blow from the east or west for much more than half a day. Once in the south, however, they will settle in for several days and not infrequently for a week or two. Immediately after the shift the southerlies will blow 15 knots or more, slowing only slightly at night. Within three days, however, they usually decay into a progression of calm nights and 10- to 15-knot days.

Then you'll be lucky to get 10 knots anytime, and the wind will show the famous Chesapeake pattern of blowing lightly from 10 to noon, taking a long lunch break, and piping up again with the Bay's legendary "4 o'clock puff" that often continues until around 8. There's no quicker way to sour on sailing than to depart around 9:30 a.m. on such days in July or August and plan to be back ashore by 5. The wind will blow just enough to get you out to swelter and slat around on the swells for several hours, then just be getting good again when you have to go in. On the other hand, some of the Bay's most contented sailors are those who venture out only around 4 for a gentle evening sail and supper on the Bay. They are almost never disappointed with either wind or sea, get a sunset in the bargain and experience the Chesapeake in its most beautiful hours.

RAINFALL --

For hardcore sailors, wind is always more important than rain. But since few really seek out a wet day on the water, there are occasional rain factors to remember. The first is that the weather almost always looks worse from Washington than it turns out to be on the Bay. The old rule of "Rain before 7, clear by 11," remains good. Even if it's raining in Annapolis, it's often clear on the Eastern Shore. If a weather system has been stalled a long time and a new one is expected momentarily, you can generally expect some showers. But that's no reason to cancel a weekend on the Bay; in all the years I've sailed there, I have known only one weekend when it rained all day every day. Every other weekend I can remember produced, along with any showers, long, windy overcast periods and frequently a good deal of sun. Even that one wet weekend had variations in the rain, including misty interludes of great beauty.

The mariner who reaps the Bay's rewards is the one who ventures forth to experience the Chesapeake in all its moods rather than waiting for the perfect day. He also realizes that, like many things in life, it's the commitment of time that brings the payoff: the golden sunsets on the Wye River or the birdsong dawns in a Chester cove. You can't schedule or program them. You can only find them by learning to become part of the Bay yourself and meeting it on its own ground.

Staff Writer Ken Ringle has been sailing the Chesapeake for more than 15 years.