Hurricane forecast: Gulf convergence of Laura, Marco an anomaly for the ages | WeatherTiger

In early September of the otherwise pleasant year of 1933, an exceptionally rare event occurred: nearly simultaneous landfalls of two separate tropical systems along the U.S. Gulf Coast. One was a major hurricane in south Texas, the other a strong tropical storm crossing the Florida Big Bend.

Now 2020, no slouch in the 1933 department, may replicate that feat with the added degree of difficulty of cramming two hurricanes into the Gulf.

Starting with the greater threat to Florida: Tropical Storm Laura is making its way into the northern Lesser Antilles, packing low-end tropical storm force winds as it does. Laura’s center has shifted south of expectations by a degree or so in the last 24 hours, making landfalls in Puerto Rico and the north coast of the Dominican Republic more likely — though not certain — on Saturday. This trajectory would also scrape Laura along the north coast of Cuba on Sunday and early Monday, as shown by the NHC forecast track.

Laura has been a bit of a disheveled mess today, and will likely take another day to get its structural house in order by stacking its mid- and low-level circulations on top of each other. The problem is sea surface temperatures, wind shear, and mid-level moisture parameters are all favorable for intensification once unfavorable land interaction ends.

The good news is today’s slight shift south means land interaction may continue through Monday, though just a modest tick back north would mean strengthening could start late Saturday or early Sunday.

Models Laura's path have been erratic thus far

The NHC intensity forecast splits the difference by keeping Laura as a high-end tropical storm as it passes through the Florida Straits on Monday, and projecting minimal hurricane intensity as it crosses the eastern Gulf and arcs northwestward in the direction of the central Gulf Coast through Wednesday.

If Laura is never able to develop a well-organized core due to tracking longways over Puerto Rico, Hispaniola, and Cuba, intensity could remain lower. However, less time over land would set up faster strengthening, and given the supportive environment otherwise, there is a 25-30% chance of Laura becoming a major hurricane in three to five days.

NHC discussions continue to highlight the reality that forecasts for Laura are of much lower than average confidence, with the track more likely than usual to escape the cone. Computer models for Laura have been erratic at best this week, though recently there is somewhat more of a consensus for the Florida Straits to central Gulf Coast scenario.

It is essential to remember that tropical cyclones do not exist independent of their environment. In this case, the track of Laura is tied not only to its own structural evolution and the strength of the western Atlantic high pressure that will steer it west-northwestward into early next week, but to its interaction with Tropical Depression 14, with which it may share the Gulf by late Monday or early Tuesday.

As of Friday afternoon, TD 14 is moving north-northwestward across the western Caribbean, and should become Tropical Storm Marco as it enters the southern Gulf.

TD 14 has moved east of its forecast track on Friday, and the Friday afternoon NHC forecast is calling for a strong tropical storm or minimal hurricane to approach Texas on Tuesday. With shear increasing to 20 knots on approach to the coastline, thankfully there is less upside for intensity with TD 14 than Laura, though the threat to the western and central Gulf should still be taken seriously.

What's a Fujiwara and is it possible?

On these tracks, the NHC is predicting one hurricane and one tropical storm to be in the Gulf of Mexico on Tuesday morning.

For what it’s worth, no, hurricanes cannot merge or combine their powers like Planeteers. If one is much stronger than the other, the weaker system will be sheared out; if they are of roughly equal size and intensity, they may begin to rotate around a common point halfway between the storms, known as the Fujiwara effect.

A West Pacific style full Fujiwara is unlikely, but Laura and Marco may be close enough come early next week to start to influence one another, a type of feedback that models handle exceptionally poorly. This interaction could impart a little more of a northward component to Laura’s steering in the eastern Gulf than expected.

Overall, the risks from Laura to South Florida have decreased somewhat since yesterday, though Miami and especially the Keys are absolutely not out of the woods considering Laura’s loose organization and potential for rapid intensification over the weekend. The Florida Panhandle lies in the eastern half of the NHC cone.

While the southward shift and the stronger modeled intensity of the western Atlantic ridging in the last day are good news here as well, be careful not to overinterpret short-term trends. The forecast could easily shift back east in the next day, particularly if TD 14 is still moving more north than west in the first half of the weekend and blunting the western Atlantic ridge’s predicted expansion westward towards Florida.

The chances of landfall increase farther west you go in the Panhandle. Pensacola and Destin have a higher chance of significant weather impacts than the Big Bend, but the risk of at least some impact to Panama City and Tallahassee are certainly non-zero given the compounding uncertainties.  

Short version of the forecast: it’s a real mess, even for a business that is known for messiness. Hopefully, the next day will bring a little more clarity, at least in terms of how many islands Laura will hop in the Greater Antilles. In the meantime, South Florida, the Keys, and the entire U.S. Gulf Coast should be preparing for potential hurricane impacts because truly, who knows.

I’ll be keeping you apprised of the latest this weekend and beyond as we make sense of it. Keep watching the skies.

Dr. Ryan Truchelut is chief meteorologist at WeatherTiger, a Tallahassee start-up providing advanced weather and climate analytics, forensic meteorology and expert witness consulting, and agricultural and hurricane forecasting subscription services. For more information, visit us at weathertiger.com or get in touch at ryan@weathertiger.com.

SANDBAGS AVAILABLE IN TALLAHASSEE

Beginning Saturday, August 22 at 8 a.m., sandbags will be available to the public at the following Leon County and Tallahassee locations:

• Leon County's Northeast Branch Library, 5513 Thomasville Road
• Apalachee Regional Park (Solid Waste Management Facility), 7550 Apalachee Parkway
• Fred George Park, 3043 Capital Circle NW
• At the intersection of Oak Ridge Road at Ranchero Road
• Messer Park South
• Northwood Center 
• Lawrence-Gregory Community Center at Dade St.
• Jack McLean Community Center.

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