We're getting to the time of year when medium-range weather forecasting becomes part guesswork at best. Case in point: the on-again, off-again predictions regarding the duration of the current East Coast rain event.

Several days ago, forecasts predicted rain for mid-week followed by clearing. (This rain has now arrived in NYC.) Then, on Sunday, a couple of the computer models began hinting that a "cutoff low" would develop off the Carolina coast and stick around, producing an extended period of clouds and showers.

Cutoff lows are every meteorologist's nightmare. Typically, storms are steered by upper air jet stream currents, so by looking at the upper air winds you can predict how fast a storm will move and where it will travel. Cutoff lows have a mind of their own. They seem to self-generate out of nowhere, and either sit or meander for days.

As late as last evening, most forecasts came into agreement that this cutoff low would keep rain in the area through most of the weekend. But then, all of a sudden, updated computer model runs took this storm out to sea. Meteorologist Nick Gregory was first on the scene, updating his prediction for Friday and Saturday to a mix of sun and clouds with no rain.

Now everyone seems to agree that the rain will last only through Thursday. It's a nice reminder that whatever any computer model shows beyond two or three days should be considered with caution.