If you have been out on Chesapeake Bay or virtually any of its tributaries more than a few times late in the day between June and September, undoubtedly you have seen the western sky change from blue to murky gray to the deep slate color of impending thunderstorms.
If you have been lucky, the building storm has dissipated or passed your position before fully developing.
If you have been wise, the boat has been turned and run for shelter before lightning cracked the sky and high winds and heavy rain have battered your vessel.
Such is the nature of the weather in the tidelands of our region, said Ken Shaver, a forecaster at the National Weather Service office in Baltimore.
"There really are two scenarios," Shaver said. "One, like we had last Monday, is when a frontal system moves through and is preceded by a squall line. Usually, they come from the north-northwest, and those are the ones that often bring the harshest weather."
When these frontal systems move through, the squall line, a wall of very dark clouds usually about 100 miles ahead of the front itself, causes most of the damage by high winds and heavy rains.
In fact, by the time the front itself reaches our area, there may be little or no wind or rain.
"The other is the air mass scenario, which is more customary," Shaver said. "We get this big mass of moist air and a flow from the southwest, and it usually produces some kind of thunderstorm activity."
The air mass scenario is more troublesome for forecasters -- professional or amateur -- because it is more subject to local weather activity.
Knowing what makes the weather and how to recognize impending changes may help the recreational boater make the right decisions.
In the Northern Hemisphere, frontal systems, the boundaries between cold and warm air, normally move in predictable directions.
Cold fronts align along a northeast-southwest line and move east or southeasterly.
Warm fronts align from north to south, west to east or northwest to southeast and expand until a cold front is encountered. The struggle for dominance between the two systems is what usually produces bad weather.
In the summer months, most of our weather is dominated by the Bermuda High, a massive area of high pressure whose center sits west of Bermuda.
But when a strong cold front moves in from the northwest, its denser air fills in below the warmer air on the edges of the Bermuda High, and the result can be chaos, as winds shift abruptly to the south then southwest and build quickly and barometric readings fall fast.
As the cold front closes on a given location, the wind will shift toward the west and through to the north and northeast. Once the wind has moved north of northwest, the worst of the storm will have passed, skies will clear and the barometer will rise.
While the squalls that rise as an advance guard of a cold front can be dangerous, the thunderstorm activity that can build almost any afternoon from mid-June through mid-September is more problematic for the boater, because its territory is more poorly defined.
"Sometimes, thunderstorms build along the [Appalachian] mountains and just peter out there," Shaver said. "Other times, they build, and the storms start heading east and northeast toward the bay and Baltimore."
A rule of thumb used by many experienced bay boaters is that if dark cloud buildup bears south of southwest or north of northwest in relation to your position, the storm probably will pass you by.
If the relative bearing of the buildup is between northwest and southwest, it is best to prepare for a blow.
Some two to four hours before a thunderstorm is fully formed, cumulus clouds begin to bunch in the southwest, west or northwest, building from the heat of the land and the moisture. The wind will start to shift in a counterclockwise direction. By the time the clouds are fully formed, they will be dark and menacing, and the tops will be flattened into an anvil shape by the wind.
Those two to four hours before a series of thunderstorms sweep across your position are best used to get to whatever shelter is nearest.
"Just having that kind of weather buildup in sight," Shaver said, "should be reason enough to adjust your plans."
Monitoring the weather
The best way to monitor weather changes in our area is to watch the sky and to carry a radio that will provide information on forecasts, watches and warnings.
The sky to the Southwest, West and Northwest is the area to watch, because nearly all our weather develops from those directions.
If you are inshore, marinas, yacht clubs and DNR facilities are among those that post signal flags to warn boaters:
* Conditions dangerous to small craft: red pennant by day, red light over white light at night.
* Gale warnings: two red pennants by day, white light over red light at night.
* Storm warnings: square red flag with black center at day, red light over red light at night.