Chances for tropical cyclone development are high again this morning for the area of thunderstorms in the Atlantic Ocean that has been keeping forecasters guessing over the past few days. The National Hurricane Center is giving the disturbance (Invest 93L) a 70% chance of reaching tropical depression status over the next two days, possibly as early as Thursday morning.
The disturbance’s nemesis this week has been extremely dry air. While the dry air across the Atlantic ocean has steadily decreased over the past couple of days, there is still a considerable amount of to its north. As Brian McNoldy writes, this situation tends to wreak havoc on a forecast.
One thing that human forecasters and computer models admittedly have a hard time evaluating is dry air and how much it will impact a developing system. … If the circulation were isolated and self-contained throughout the depth of the vortex, that dry air would not play a big role and the storm would probably already be a hurricane. But, if there’s a route for that dry air to get entrained into the circulation, development is choked off and the result is what we’ve seen this week so far.
This dry air mass has its origins in the Sahara Desert. Strong, easterly winds blow dust, sand, and dry air off the continent and over the Atlantic Ocean. The Saharan air layer here has the tendency to hinder tropical cyclone development, since these storms need a very moist atmosphere to grow in.
In addition to the dry air, the disturbance has been battling a moderate amount of wind shear. Over the past 24 hours, the thunderstorm activity has been displaced to the south, exposing the center of circulation on satellite. Over the next 48 hours, wind shear is expected to decrease slightly, though forecasts still estimate 10-20 knots of unfavorable wind shear in the path of the disturbance.
The models seem to have some consensus on the potential track of this disturbance. All of the trusted models suggest that it will continue west-northwest over the next four days, sliding north of Puerto Rico and the Dominican Republic in three days, and arriving just north of the southeast Bahamas by day four. From there, models are expecting the potential storm or hurricane to curve to the north and east, away from land.
In terms of intensity, most models are not expecting this system to become a hurricane by the time it reaches Caribbean populations, though the general forecast is for a steady increase in wind speed. However, there is a lot of uncertainty in trying to forecast how strong a storm will become even before it’s developed. While intensity forecasts remain modest now, the possibility remains that these forecasts will intensify over the next few days.
NOAA Hurricane Hunters have been deployed and are expected to investigate this system today.