Last month we talked about some of the ways to get forecast weather information of specific use to the marine community. As I mentioned then though, if you know your met well enough then you'll be able to supplement the forecast with information on the hoof, interpreting changes to the weather as you go along.

What then are some of the signs that can warn you in advance of the imminent development of stormy weather, that for smaller craft and crew can cause the most problems?

Generally, sea and swell will take their time to rise and you can usually be aware of these changes enough in advance to head for port. However the most rapid changes usually occur when atmospheric conditions cause sudden squalls and thunderstorms. These are often linked to frontal passages and again, a quick check on the forecast weather chart will usually show if you are likely to encounter them on your journey. A cold front in particular will often give squalls and heavy showers as the wind veers a good 90 degrees in direction, often quite quickly. If the barometer stops falling rapidly and steadies the cold front is usually not far away and approaching from the westerly quadrant.

Thunderstorms that develop in-situ though can be the most hazardous feature in summer especially as a warm humid but otherwise fine morning can deteriorate into heavy stormy weather in the matter of hours sometimes. The precursor of such activity can be south-southeasterly winds, fairly high humidity and the development of Ac castellanus clouds; turreted clouds at about 3000m that look like a row of castle turrets, especially from mid May through to September.  This indicates medium level instability that will allow lower cumulus clouds when they form to rapidly ascend towards the troposphere, creating cumulonimbus clouds. These can give a lot of severe and potentially dangerous weather; squalls, Lightning, large hail, downbursts, choppy confused seas; even waterspouts (ie: tornadoes). If clouds rapidly build and tower up on such days, it's a good bet severe weather isn't far away.

When thunderstorms do develop, as they decay 'daughter' cells may also develop as well, usually on the southern or southwest flank of the storm, so generally running north west, if possible, can be a good way to escape these.

One of the more severe events that can occur at sea is a squall line. These types of storms tend to form in long lines with a notably black striated gust front at the leading edge of the line though sometimes there can be gaps or breaks in it. Typically such lines of thunderstorms tend to have the strongest (or 'mature') storms in the south or southwest end of the line. Moderate storms still not fully mature typically are found in the middle of the line and the dying variety is found in the north or northeast section of the line. Squall lines are often downburst producers - an area of very strong gusty wind. This is not something it is advisable to try and experience in a small craft on the high seas. If in doubt do not stray too far away from the coast in such conditions ready to make for safe harbour.