In
meteorology, a cutoff low is a
low pressure system which is cut off from the jet stream. Most storm systems are entrained in the
jet stream, and are in fact made up of curves in the jet stream. Since the jet stream is also a boundary between two very different air masses, it provides storms with energy and momentum. However, sometimes a horseshoe-shaped
curve in the jet stream becomes so steeply curved that the jet stream will 'shortcut' across the curve and leave a pocket of air rotating in a counter-clockwise direction to the south. (this is similar to the effect in which a meandering river cuts off a bend, creating an
oxbow lake).
Without the jet stream to push this storm from west to east, its motion becomes erratic and unpredictable. Cutoff lows may stay still for days or even 'retrogress' westward. Although these lows are disconnected from the jet stream, they can still contain conserable amounts of moisture, especially if they can tap into a tropical moisture stream. Because of their slow, erratic motion, these lows are often the cause of flooding, notably in California. During El Niño conditions, when tropical moisture is abundant, these storms may sit over areas which rarely see prolonged heavy rain, dousing the areas with several inches of rain a day. This was the cause of some of the flooding of 1997-1998 in Southern California.
Because of the erratic movement of these storms, they are almost impossible to predict, thus the expression 'cutoff low, weatherman's woe'. Usually, they eventually spin themselves out, or reconnect to the jet stream. But when and how this happens is different with each storm.