The weather report last weekend offered a 70 percent chance of rain Saturday, 60 percent chance Sunday and probable heavy thunderstorm activity both days. At 10 p.m. Friday, Vince Iatesta called. Of course we were still going on the overnight cruise, he said - the only thing to decide was where.

Such are the thoughts of a man who had just his Tartan 34 painted a deep bottle blue, who had finished all the teak to boat-show perfection and who happens to have a Swedish teenager temporarily attached to his household. Fun must be had.

By noon Saturday both boats were slipping past the mouth of the South River under glorious sunshine, helped by a warm 12-knot southeasterly toward Round Bay, a clean and well-protected anchorage up the Severn River, perhaps 15 sailing miles away.

The first issue was to keep up, in our 25-footer, with the Tartan, already reaching powerfully under a medium genoa. We cut well inshore at Thomas Point Light, sounding with the leadline, to gain a quick mile. Within a half-hour, Vince had closed again from astern. My 10-year-old sensed what was coming next. "Oh, Daddy." she said, "You're not going to put the spinnaker up." To this was added a disembodied opinion from her 7-year-old sister, coloring down below: "Boy, do I hate spinnakers."

A brief lecture followed on the beauty of a well-flown tri-radial: its pleasing, full-bo lied shape, its unquestioned ability to increase downwind speed; its relative ease in setting if set without haste, and its utter uselessness when left in the bag in the forepeak.

The chute went up without a hitch, and we jibbed at Tolly Point, still ahead of the Iatestas, with the wind now at 15 knots and the persmission of the weatherman thoroughly discredited.

Ten minutes later the spinnaker block fiting at the masthead broke, the spinnaker collapsed dramatically directly in front of the boat, and the boat ran over the spinnaker. We had the mess cleaned up in five minutes, but it was another five minutes before the 10-year-old stopped rolling her eyes.

We entered Round Bay about 4 o'clock, the Iatestas already there and waiting, and rafted up at the southeast end in the company of a dozen other boats.

Two lessons of the Chesapeake then were taugh again, for any in the anchorage who had forgotten them: If you get to where you are going by 4, you'll probably avoid sailing through a thunderstor and - All Weather Comes from the West.

While we steamed a hearty pile of soft-shell clams, low clouds began to roll up in the sky like sand in ocean surf. Would they hit us, or miss us, the children wanted to know. They were coming from the west - they would hit us. So passed a mild squall line, typical of the season. By dark the charcoal cooker on the Iatestas' boat had turned a dozen hamburgers, the rain had stopped and the bottle of wine finished.

The two botas rafted together through the night, and breakfast rose at 9 a.m. under a brilliant, cloudness sky.

Ah, the rigors of shipboard life: crabbing, fishing, a morning-long frisbee game in the shallows, the water tepid and clean and devoid of stinging nettles (which are said to already be abundant in the lower Bay and ready to attack Annapolis by next week). Was this wam water not a treat? the Iatesta guest, Lena, was asked..

No, she replied, the water is even farmer in Sweden.

We broke up the raft at noon, motoring off into a light easterly. By the time we had reached the Rte. 50 bridge, Round Bay was three miles behind us and already cloaked in low, dark clouds, its surface swept by a heavy rain. We passed through the swing bridge, and then the bascule bridge, and in Annapolis habor noted that the entire sailing population of Chesapeake Bay seemed to be running for cover. The children put on their foul weather gear.

I looked at the compass, sightinh west along the 270-degree line, and saw that the horizon there was pitch black. Was that enormous mass all one cloud? Was it 10,000 feet high - or 20,000? At the bottom of the towering formation, ugly fingers of vapor reached down, tornado-like, forming and reforming over the Maryland capitol dome. The blackness was full of lightning and urgent thunder.

"This one's going to miss us, right, daddy?"

"Oh, well maybe. And, uh, maybe not."

I had just returned from a three-week trip to Bermuda and back over choppy seas and had kidded a watch companion during a gate: "You boys think this is noisy, wet business? You ought to try a Chesapeake thunderhead just a few miles from home . . ."

The thunderhead caught us off Tolly Point The children went below. The Iatestas, a mile away, disappeared under the curtain of sudden blackness. At the first blast of wind I tied a double reef in the main, and still the boat broached madly in the gusts. The air was so filled with water you had to look down to breathe.I grabbed a couple of sail stops and groped forward to haul down the flogging jib. Even without the headsail she heeled gunwales awash. The dinghy was already half full of rainwater and we were still going four knots, ready to break the towline, for sure.

I got down the rest of the main, and the wind began to change. So back to the bow, and anchor. The Danforth declined to hold (eel grass on the bottom?) and I made the yearly pledge to obtain a plow anhor as well. Now the wind had come around to the east, making Tolly Point a lee shore, on which we would surely ground if the storm kep on for an hour more. CAPTION: Picture, Cathy Hearn paddles to gold medal in women's kayak. Another Washington-area entrant, John Lugbill, won in canoe competition. By james M. Thresher - The Washington Post