We need wind to sail, but not too much. Thus most of us would like to avoid low-pressure systems with their strong winds and developed seas. Low-pressure systems over the ocean, excluding hurricanes, form when two air masses (one warm and filled with moisture, the other cold and relatively dry) bump into each other and exchange energy.
There is a type of mid-latitude low pressure system, named by meteorologists as a rapidly developing low which can develop to full strength in less than 24 hours. Sailors should be able to recognize the precursors of these systems since the gales and storms they produce are often of surprising proportions.
Rapidly Developing Lows are powerful weather systems defined as a mid-latitude cyclone whose central pressure falls at a rate of at least one mb/hr for a minimum of 12 consecutive hours. (Rapidly developing lows were once called meteorological bombs because of their symmetrical and tight isobar appearance on weather charts, but meteorological bomb is no longer a politically correct term!)
Rapid Developing Lows are observed to occur in the North Pacific and Northwest Atlantic, and their peak occurrence is late fall to early winter, around the time when many sailors are heading south for the winter or north in the spring.
Famous examples of rapidly developing lows are the QEII Storm of 10 September 1978, Presidents' Day storm of 1979, Northeast Pacific Storm of 13 November 1981, and recent March Storm of the Century. These low pressure systems all developed to great strength in a very short time.
There are certain factors, listed below, that contribute to a rapidly developing low. The more of these factors are present, the more likely the low will evolve into a fierce storm.
Factors contributing to formation of a Rapidly Developing Low:
1. Strong upper level (jet stream) divergence
2. Rapid surface movement of disturbance (30 knots)
3. Surge in Jet Stream winds (110 knots)
4. Upper level trough
5. Warm sea surface temperatures
Meteorologists use satellite imagery to locate and identify many of these factors, but often not in sufficient time to produce and broadcast advisories that give mariners substantial warning. Therefore, sailors need to be diligent in watching barometric pressure and cloud cover as these are the most noticeable indicators of an approaching rapidly developing low. A pressure drop of one mb per hour for 12 or more hours together with increasing cloud thickening and lowering are difficult to miss.
For boats able to receive weather chart broadcasts (using either a weatherfax, laptop/singleside band combination, or the Internet) surface and upper air (500mb) charts will reflect conditions necessary for rapid low development.
For example, upper level troughs are shown on 500 mb charts as a dashed line and when winds in the area of a trough show speed near 110 knots then interest should be aroused. If a trough appears over a warm surface feature, such as the Gulf Stream, then another criteria favoring rapid low formation is met (warm sea surface temperature).
Every sailboat crew heading offshore during times when Rapidly Developing Lows are possible (spring and fall) should familiarize themselves with their formation and have on board, as a minimum, a calibrated and reliable barometer! | 
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