Hurricane Dorian Will Make A Turn Northward, Eventually
On one hand, it seems unbelievable that Hurricane Dorian has managed to achieve such a great strength in a relatively short amount of time. On the other hand, the surrounding environmental conditions allowed Dorian to reach its full potential. Since maximum sustained winds sit at 185 mph, as of Sunday, 2pm ET, Dorian can now be put in the same league as some of the most powerful hurricanes ever recorded in the Atlantic Ocean. Furthermore, Dorian is the northernmost category 5 hurricane ever recorded in the Atlantic, not including the Gulf of Mexico. Typically, water contains more heat throughout a greater depth within the Gulf of Mexico and Caribbean; whereas, ocean heat content tends to not be as high elsewhere in the Atlantic. All things considered, Dorian is already one for the record books.
Understandably, many are concerned that Dorian will not make the northern turn at all. Well, it will, eventually. Here’s the problem though, which has inevitably led to much uncertainty over these last few days. Dorian is expected to run parallel to the Florida coastline, either onshore or offshore, once it makes the northward turn. Thus, we’re left trying to predict when the turn will occur, which depends on how quickly the ridge to Dorian’s north weakens. One small error in timing, and Dorian ends up making a landfall in Florida versus staying offshore until approaching the Carolina coast.
Can I say with high confidence that Dorian won’t make landfall in Florida? Honestly, I can’t. Can I say with high confidence that Dorian will make the northward turn quickly enough before impacting Texas, Louisiana, Mississippi, or Alabama? Absolutely, I can. It might seem that once a storm reaches a certain strength that it would be able to call its own shots without worrying about other nearby features. Actually, the opposite is true. A strong hurricane can potentially modify surrounding environmental conditions, but atmospheric steering currents will ultimately guide where Dorian will go.
But why did a storm like Katrina cut across the Gulf of Mexico, yet I can confidently say Dorian won’t do the same thing. Because, the overall pattern was entirely different. Taking a look at reanalysis data, a big mid to upper-level high was centered over Louisiana and Arkansas, which placed Katrina on the southeastern periphery of that high. Flow is clockwise around high pressure systems in the Northern Hemisphere; thus, it is was only natural for Katrina to continue on a westward path into the Gulf of Mexico until the ridge broke down. Once the ridge was out of the way, Katrina moved northward towards Mississippi and Louisiana.
For Dorian, the environment is entirely different. Broad troughing over the northeast quadrant of the U.S. is sandwiched between a ridge over the Four Corners region and Bermuda ridge. Similar to how Katrina moved underneath a ridge positioned over the mid-South, Dorian has been steered by Bermuda ridging that has stretched to its north and northwest. Take away the ridge, and the storm starts pulling northward, given the troughing across the eastern U.S. In fact, Dorian has already begun to slow its forward speed.
With all of this said, the concern that Florida will suffer a landfall are valid. The ridge could stay a hair stronger, delay the northward turn by a little while, and the outcome would be completely different for Florida. But let’s take a look at the National Hurricane Center’s latest projected path. Even though their official track keeps Dorian barely offshore, the cone of uncertainty includes most of the Florida east coast through the Carolinas and even into the Mid-Atlantic. The cone of uncertainty IS NOT meant to forecast impacts. The cone is based on previous forecast track error. In fact, below is NHC’s definition of what the cone means.
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. Based on forecasts over the previous 5 years, the entire track of a tropical cyclone can be expected to remain within the cone roughly 60-70% of the time. It is important to note that the area affected by a tropical cyclone can extend well beyond the confines of the cone enclosing the most likely track area of the center.
NHC’s Explanation of the Cone of Uncertainty
Okay, so let’s break this down further. Based on the last 5 years of actual tracks versus forecasted tracks, 60-70% of the tropical cyclone forecasts (tropical depressions, tropical storms, hurricanes, etc.) remained within the cone. That means that 20-30% of the forecasted tracks didn’t even stay in the cone for a given forecast time. If you are within the cone, the storm’s eye and eyewall could go over your location. Again, the cone of uncertainty isn’t a forecast for impacts. It’s a forecast for where the center could go. Essentially, the National Hurricane Center is providing a realistic margin of error in their forecast. In other words, these forecasts are never perfect or go as predicted.
Around a day ago, I stated that I felt Dorian’s landfall could occur somewhere between Myrtle Beach, SC and the Outer Banks, NC. Although that’s still a completely viable scenario, I strongly encourage those in Florida to prepare for a landfall, just in case. We are dealing with a very powerful storm, and we have very little room to make a mistake in track. Thus, continue to prepare and consider evacuating, especially if it’s mandated. Most of the model guidance brings Dorian dangerously close to Florida or even projects a landfall. This has prompted the National Hurricane Center to begin issuing hurricane warnings for parts of the Florida coast.
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Given Dorian’s current strength of 185 mph, the storm will likely begin to weaken some, especially since it’s forward speed will continue to slow with time. It takes a lot to maintain such strength. Over time, upwelling and vertical mixing of cooler waters, plus Dorian’s interaction with the Bahamas, could induce some weakening. Nonetheless, Dorian will remain a major and deadly hurricane as it nears Florida.
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