23 August 2020

An extraordinary and dangerous event is unfolding with Laura and Marco

Both Laura and Marco are still tropical storms as of early Sunday morning (Eastern Time). Laura passed south of Puerto Rico yesterday and is now crossing over Hispaniola en route to Cuba by tonight. Marco passed through the Yucatan Channel yesterday and is now heading north toward Lousiana.  Unfortunately, Laura is headed for the same place.

In a truly extraordinary coincidence, Marco is forecast to make landfall in Lousiana as a hurricane on Monday afternoon, then, Laura is forecast to make landfall in Louisana as a hurricane on Wednesday night.  As I pointed out in yesterday's post, this results in two rounds of heavy rain, and two rounds of storm surge, both of which could be mingled together rather than two separate events with a break. The map below shows the forecast tracks and the combined probability of tropical storm force winds (from 5-100% contours).

Marco's tropical storm winds should arrive by Monday morning. Laura is a couple days behind... its tropical storm winds will first affect Dominican Republic, Haiti, and Cuba (with the southern tip of Florida and extreme southern Bahamas in the outskirts), then are expected to reach the northern Gulf coast on Wednesday morning.

The double landfalls will undoubtedly create flooding problems, not only from three days of storm surge coming and going, but from excessive rainfall. The 7-day rainfall forecast is shown below. The flooding will definitely not be limited to the coastal areas. I have radar loops (and more coming) at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

There has been plenty of talk about the "Fujiwhara effect" with these two storms... that is the process where two low pressure systems get close enough to influence each other's tracks.  That influence could be as remote as a mutual nudge or slingshot off in different directions, or orbiting around a common imaginary center between them, or even completely merging.  More technically, this process is called "binary vortex interaction".  In the case of Laura and Marco, the closest they ever get prior to landfall is about 700 miles (they're about 1100 miles apart now), which is just marginal for any interation between the two vortices -- not close enough to do anything exciting like orbit around each other or merge.

We should also not be too quick to write off Laura after its landfall on Wednesday night. The European global model keeps it somewhat coherent as it tracks over the southeast US, and re-emerges over the ocean near the Outer Banks of North Carolina next Sunday and then reintensifies it to at least a tropical storm over the ocean. The American global model follows a similar evolution, but as of now, fails to re-organize it into a tropical cyclone.

Elsewhere across the basin, things are pretty quiet. But when the time comes, the next name on the list is Nana, and the current record earliest "N" name is September 5 (Nate in 2005).

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