Tropical, subtropical, non-tropical... what's the difference? | Raleigh News & Observer

If you've been paying attention to the ever-changing weather reports for the past few days, you've probably noticed meteorologists and reporters saying "tropical low," "non-tropical low," and "subtropical low" or "cyclone" or any variation in trying to describe the storm now known as Ana. For the casual observer, it may seem like we don't know what to call this storm brewing off our coast. The truth is that for a while there, we didn't.

Forecasting for low pressure systems that develop off the southeastern coast in the Atlantic this time of year can be tricky. A truly tropical storm needs ocean temperatures near 80º or warmer, which usually don't occur until the summer months. This is the reason hurricane season starts in June.

When forecasters first noticed the computer models showing a coastal low pressure system developing this week, it would have been easy to assume that in early May with the sea surface temperatures mostly in the 70s, the storm would struggle to become tropical in nature. Even the warmer waters of the Gulf Stream current are just barely near that threshold. So, initially meteorologists were saying that a coastal low or non-tropical cyclone or, at best, a subtropical storm would develop near Florida in the Atlantic. The path of the storm was also in question since it hadn't developed yet. Farther out to sea and away from the Gulf Stream, it would not have much of an effect on land, but closer to the coastline, it could have a major effect.

A subtropical storm could be thought of as a hybrid of a plain Jane storm that develops over cooler waters that would likely never become a hurricane and a tropical storm. Subtropical storms can begin anywhere between the equator and 50º latitude, and they require enough warm water to create convection (thunderstorms). Remember that with thunderstorms, heat is the source of their power. Subtropical storms tend to produce a little wind and a decent amount of rain.

Tropical storms can strengthen to become hurricanes, and require the above mentioned warm ocean temperatures, an initial disturbance, and time to get organized. Tropical storms need to keep moving in order to gain strength. If they stay in the same place, they begin to bring colder water up from the depths of the ocean to replace the warmer water at the surface that they use to fuel themselves. In essence, they commit suicide by standing still.

Storms with tropical characteristics can be named. When the forecasters at the National Hurricane Center saw that Ana's path would take her over the Gulf Stream long enough to develop thunderstorms and a closed circulation, they named her. Even though it is early May, we have our first named storm of the season. The expectation is for Ana to become a tropical storm with wind speeds near 40 mph and very little strengthening beyond that before making landfall.

Sustained winds of 40 mph could blow yard debris around and cause some issues with high profile vehicles among other things. The major concerns with Ana are the storm surge caused by the wind, rainfall of 2-6 inches, and dangerous surf conditions. The storm surge will combine with high tide and should cause some flooding near the coast with water rising 1-2 feet above where it normally would.

Here in the Triangle, expect a showery Mother's Day weekend as Ana invites herself to the celebration. Make sure to have a backup venue to any outdoor plans you've made, especially on Sunday when the greatest chance for rain arrives. Saturday may not be a total wash-out. In fact, we could see some sun in the early afternoon, but the chance for storms increases as the day goes on. Even with the increased rain probabilites this weekend, we may only see 1/10 to 1/2 inch of rain with a bit more embedded in thunderstorms. The worst of the rain will be restricted to the coast.