Weather Forecasting: Know your lows

By Stan Honey and Ken Campbell

There are three major types of lows: tropical lows, mid-latitude lows, and cut-off lows. It is important to understand the differences between them.

Tropical lows live in the belt of tradewinds, and are also known as tropical depressions, tropical cyclones, hurricanes, and typhoons. They are seasonal and travel east to west in the trades. The GFS weather model dramatically underestimates the strength of tropical lows, but predicts their location and movement reasonably well. It is critical to receive hurricane advisory forecasts for accurate prediction of their strength and movement.

Tropical lows derive energy from the warmth of the oceans and the release of latent heat energy from the formation of the clouds. Cold, dry air can weaken a tropical low. A concentration of thunderstorms over water temperatures of 27° C or warmer is the first sign of a tropical low forming. It is mandatory that there is no jet stream or strong winds aloft for tropical low pressure formation because thunderstorms need to develop vertically and not be torn apart by jet stream winds or wind shear.

Since the jet stream is frequently not present near a tropical low, the low’s movement can be erratic and less predictable than a mid-latitude low. A tropical low is much smaller in size than a mid-latitude low, frequently only 400-500 miles across. A big tropical low might be 800-900 miles across. The strongest winds will be found within 25-50 miles of the center. Barometric pressure gradient is also much less than with a mid-latitude low.

The most dangerous side of a tropical low is the “right side” in the northern hemisphere. If the tropical low is moving east to west, this would be the north side; if it is moving from south to north, this would be the east side. Winds are strongest over a much larger area on the “right side” and the seas are also largest on the “right side.” Even though sailors know that the right side of a tropical low is the dangerous side, never try to cross in front of a tropical low.

North Atlantic and Northeast Pacific hurricane season is generally May to November. It peaks from late August through early October. The low is designated a tropical depression when a defined low pressure circulation exists – rather than just a cluster of thunderstorms – and when sustained wind speeds are under 35 knots. The low becomes a tropical storm, and is named, when sustained winds are 35 knots or higher over any part of the low. The low becomes a hurricane when sustained winds are 64 knots or higher over any part of the low. Tropical storms are called tropical cyclones in the western North Pacific and are given names.

Mid-latitude lows are very different from tropical lows. Mid-latitude lows are the low pressure areas we most frequently experience in mid-latitudes, from 30° to 60° north and south. They are the traditional low pressure areas with attached fronts that move west to east in the mid-latitudes with the variable westerlies. Newly formed lows have small and intense centers. Old lows can have broad centers with light wind. Lows travel in a direction that is parallel to the isobars in the cold sector, the area between the cold and warm fronts on the equator side of the low. They move at about half the speed of the 500mb upper level wind in their vicinity. They are reasonably well forecasted by the GFS weather model.

Energy for the mid-latitude low comes from a mixing of cold and warm air, such as when east coast storms move from land to over the ocean. The greater the temperature contrast, the stronger the low can become. This is why the strongest lows in the northern hemisphere occur during late October through December, and again February through April. Warm currents like the Gulf Stream, Kurishio, East Australian (EAC), and the Agulhas can increase the temperature contrast, which makes these areas breeding grounds for strong lows. These storms can be very large – 3000-5000 miles across – which makes them very difficult to avoid completely. In the northern hemisphere, they will have a warm front to the east of the low, where east winds shift to south as the front passes. They will have a following cold front where south winds shift to west and northwest as the front passes.

As the cold and warm air mix within the low and the air mass becomes more homogeneous, the low will weaken and the winds will diminish. This is the occlusion phase of the storm’s life cycle. However, even though winds diminish, the leftover seas can still be large, leaving rough conditions in spite of the diminished winds. This is the “washing machine” phase with little wind, but agitated seas.

Cut-off lows are critical for a sailor to understand. This will be on the final exam, so memorize it. Cut-off lows occur when a mid-latitude low is removed from the jet stream. They have unpredictable movement as they are “cut off” from both the easterly tradewinds and the westerlies. Many times, but not always, the low is weak and the wind field near the low is also weak. Their movement can be erratic. They sometimes move quickly, but can stay stationary for days.

Cut-off lows can be extremely dangerous and should be avoided. They have their origins in the lower latitudes – south of 30N and north of 30S – and can have some tropical low pressure characteristics. They can transition into a tropical low if they remain over warm water and the jet stream is non-existent. Examples of deadly cut-offs include the Halloween Storm of 1991 (famous as “The Perfect Storm” in book and movie) with 8 mariner fatalities, the Fastnet race in 1979 with 18 fatalities, the Sydney-Hobart race in 1998 with 6 sailor fatalities, and Hurricane Sandy. These were all strong cut-off lows that mixed tropical characteristics with a midFrom Mark Fisher, latitude weather system. When two extreme weather systems merge, they can result in extraordinary weather. (For an excellent review of the 1979 Fastnet Race, see John Rousmaniere’s Fastnet, Force 10: The Deadliest Storm in the History of Modern Sailing. For an excellent book on the 1998 Sidney-Hobart race, see G. Bruce Knecht’s The Proving Ground.)

These are some danger signs to watch: tight core at the center of the low, rapid pressure drop, significant temperature gradient on the polar side of the low, a comma shape, and fast-moving jet stream over the top which can be seen in the 500mb charts.

This resource is provided by the US Sailing Safety at Sea Committee. Read the entire chapter on Weather Forecasting and Waves by Stan Honey and Ken Campbell.

Learn more about US Sailing Safety at Sea Seminars in your area.