Right on schedule, the Atlantic hurricane season looks like it may produce the year's first tropical system off the southeastern coast of the United States this week. The National Hurricane Center gives the system a high 60% chance of developing over the next five days.
The system is pictured in the satellite image above taken on Saturday afternoon, showing a disorganized cluster of showers and thunderstorms hanging a few dozen miles off the coast. The area is conducive to the system's development into a tropical cyclone, which will attain the name Arthur if it develops into a tropical storm.
The ultimate questions are "what will it do?" and "where will it go?"
The three main ingredients tropical cyclones need are warm sea-surface temperatures, low winds, and ample moisture, and we have all three, so we know there's at least a chance that it will develop into a tropical depression or even a storm by next week.
Tropical systems (depressions, storms, and hurricanes) all hate wind shear. If the winds are too strong, especially at the upper-levels, the wind will rip the tops off of the thunderstorms and kill the system. Taking a look at the predicted jet stream over the next couple of days will give us a good idea of how much of an impact wind shear will have on the system.
The jet stream is denoted by the shaded colors, with warmer colors indicating faster wind speeds. The southeastern United States is practically untouched by the jet stream on Tuesday morning, pictured above, falling under what's known as a "ridge," or an area of higher pressure conducive to calm weather. If the storm continues to develop, it will run into no problems with wind shear through at least Thursday.
It sounds silly to even have to say it, but tropical systems need ample moisture to form. Dry air spells death for a hurricane. A hurricane in dry air is a fish out of water or one of the Mafia's friends at the bottom of the Hudson.
When we look at water vapor satellite imagery, it's measuring the moisture in the air around 10,000 feet up. That roughly corresponds to the 700 millibar (mb) pressure level in the atmosphere, so looking at moisture on the 700 mb charts will give you a good idea of what kind of moisture a system will have to work with.
Warmer colors equate to higher humidity, and the system will have moisture to work with through next week. The only issue that could hamper its development could be that ribbon of dry air the GFS model is trying to wrap into the system, but we'll see what happens as we get closer.
Sea-surface temperatures (SSTs) are crucial for the development of a tropical system. Generally, the warmer the better, and temperatures above 80°F (26°C) are best for development and strengthening. Water temperatures off the coast of the southeast range from between 80°F to 85°F, so the system will have plenty of energy to work with as it lingers in the area this week. The only problem it could run into is that if it churns up the water too much, it could cause upwelling of cooler water and lower SSTs.
Where Will It Go?
The biggest question the public has when tropical cyclones threaten is "where will it go?" Well, it's not really going to go anywhere. Not in a hurry, anyway. As the system is expected to slowly develop under a ridge with not much in the way of systems to move it along, it's expected to "drift southward over the next few days," as the National Hurricane Center put it. The ultimate direction of the system depends on how much it strengthens, so anyone along the east coast needs to keep an eye on it, especially so for those who live in the southeast.
That's not the uncertain answer anyone wants to hear, but meteorology is the science of playing the "what if" game. It's essentially chaos theory — if one little thing changes tomorrow, it could dramatically alter the future of the system.
The National Hurricane Center issues tropical weather forecasts every six hours, and every three hours if/when the storm forms since it's so close to land. It should be interesting to watch as we go into this week. Pay attention to the forecasts and make preparations now just in case.
[Images via GOES, WeatherBELL, NOAA]