A new tropical storm in the Atlantic Ocean is going to get a lot of play in the news over the next couple of days as official forecasts expect it to become Hurricane Danny by the end of this weekend. The system has plenty of obstacles along its path and it’s a long way from land, but we’re nearing the peak of hurricane season, so it’s worth watching closely.
The National Hurricane Center upgraded Tropical Depression Four to Tropical Storm Danny at the 5:00 PM EDT advisory, making it the fourth named storm of this Atlantic hurricane season. The system is pretty minimal at this point with a minimum central pressure of 1008 millibars (average sea level pressure is 1013 mb) and 40 MPH winds. It’s little but a rainy breeze at this point, but that should change soon.
As with most tropical cyclones that form in this region of the world, the system began life as a small cluster of thunderstorms that formed over western Africa. This tropical wave looked impressive when it entered the Atlantic last week. I mentioned the complex in last Friday’s “Here’s Your World Today, Explained,” a post explaining the various things you could see that afternoon on awesome satellite images from around the world:
The wave just coming off the coast looks pretty healthy, and the latest run of the GFS model has it maintaining some composure as it heads west toward the Caribbean. The National Hurricane Center doesn’t mention it in their outlooks, and environmental conditions aren’t really favorable for development, but it’s a good reminder that we’re creeping toward the peak of hurricane season, and that even though it’s a slow year, it only takes one storm to make a mess.
At the time, it looked like it was going to struggle against the dry, dusty air blowing off of Africa, but here we are. This is the tropical storm today, looking positively swirly:
The National Hurricane Center’s latest forecast shows the system eventually growing into category two Hurricane Danny by the end of the weekend. The storm’s future is highly uncertain, though, because Danny face some pretty steep obstacles as it treks west toward the Caribbean.
Dry Air/Wind Shear
Even though the region is situated deep in the tropics—where there’s typically plenty of moisture to work with—the mid-levels of the atmosphere often turn up dry as a bone relative to what hurricanes are used to dealing with. Dry, dusty air blowing off the Sahara Desert in northern Africa is a death sentence for tropical cyclones, and when prevailing winds allow this mass of floating desert to linger over the ocean, it keeps things pretty quiet.
With the exception of its southern flank, the depression is completely surrounded by dry air and dust from the Sahara. That kind of dry air can kill a cyclone in its tracks, and it’s probably the biggest challenge this system will face as it moves westward and tries to strengthen over the next few days.
Despite this dry air, the National Hurricane Center still expects the storm to thrive in the otherwise-conducive environment. The agency’s forecast discussion notes the layer of dry air to the storm’s north, saying that Danny’s deep convection (thunderstorms) and low wind shear in the lower levels should be enough to keep the dry air at bay, allowing the system to strengthen.
That’s not always a sure thing, however, and any intrusion of dry air will throw the system’s future development into doubt.
There’s also the factor of wind shear, which acts like a guillotine to tropical cyclones, shearing off the tops of the thunderstorms and preventing the storm from maintaining strength and structure. This morning’s run of the GFS model—which admittedly doesn’t strengthen the storm much beyond its current status—shows the storm moving toward wind shear through the atmosphere on the order of 45-55 knots, which could seriously affect the system as it draws closer to the Caribbean.
This storm signals that hurricane season is climbing toward its peak of September 10, and this is the time of year when we typically begin to see storms form near the Cape Verde Islands off the western coast of Africa. We’re in a very slow time of the year for weather, and next week is the ten-year anniversary of Hurricane Katrina, so the temptation will be great for certain people and outlets to draw comparisons between this and other historically-significant storms that started life in this region of the world. Every storm and situation is different. Danny is in a climatologically favorable location for strengthening, and its track could take it anywhere from the Windward Islands to Greenland, or it could fall apart into nothing.
Watch the storm closely over the next week or two. The best case scenario is that the storm is something interesting to break the monotony of the mid-August doldrums, and at worst, it’ll be something we need to watch as it draws closer to land. Regardless of what happens with this system, use this quiet stretch to review your safety plans and prepare for what to do if a storm threatens your location.