Typical winter surface chart showing frontal boundaries and associated high pressure systems

Although the Cayman Islands have a tropical marine climate, we are in a unique position of being far enough north to be affected by cold fronts during the winter months.

Occasional surges of cooler air from continental North America push south into the western to northwestern Caribbean Sea. The leading edge of these surges are what we call a cold front. Cold fronts are normally associated with a high pressure system that lags just behind it.

These systems are the major contributor to rainfall during the dry winter months (late October through early April).

Most cold fronts do not produce a significant amount of rainfall, but when they align with winter jet streams they can produce higher totals, for example, Jan. 18, 2003, when 9.45 inches of rain fell.

Stronger cold fronts along with associated high pressure systems tend to bring fresh to strong north to northwest winds and rough seas, especially along the western and northern coasts of the Islands. Such fronts are locally called “Northwesters.”

On average, around eight to 10 cold fronts, with three to four being Northwesters, pass through the Cayman Islands each winter.

Impacts

The passage of these systems usually produce fresh to strong north to northwest winds and rough seas (which may last up to a week for each system), requiring the National Weather Service to issue either marine advisories or warnings depending upon its strength.

Cross-section of a cold front.

A number of these systems may produce fresh to strong north to northwest winds and rough seas (which may last for a few days), requiring the National Weather Service to issue advisories or warnings, and also alert the Port Authority and marine interests along Seven Mile Beach.

Definitions

Cold front – the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it.

Stationary Front – a boundary between warm and cold air masses that is moving very slowly or not at all.

Frontal trough – An elongated area of relatively low pressure associated with a cold front that is usually associated with a shift in wind direction. These troughs may be pre-frontal, post-frontal or induced depending on their location and how they formed.

Arctic high pressure system – These are cold air masses located behind cold fronts that form and adopt characteristics of their source region in the Arctic. The cold front is the leading edge of this air mass.

This is the first in a series of monthly public education topics produced by the National Weather Service.

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