Many of you are aware, perhaps from hype articles/videos shared by unscrupulous (or perhaps just naive) sources on social media, that the Gulf of Mexico will be visited by two tropical systems this week. Typically, I devote space on this blog to weather that has impacts to the Memphis area.
Since it is appearing possible that we could get some tropical remnants later next week, which I will get into in a minute, I'm going to entertain the discussion/bust myths of merging tropical systems, the possibility of a mega-super-duper-storm, and the "new term" Fujiwhara effect.
Forecast tracks of Marco and Laura
As of 4pm Saturday, the National Hurricane Center forecasts for Marco (currently passing between Cuba and the Yucatan peninsula of Mexico) and Laura (sliding west of Puerto Rico towards Hispaniola) are now presenting the very real possibility not only of two hurricanes in the Gulf at the same time, but a double-landfall within 48 hours of each other on the Gulf Coast, perhaps affecting the same area twice.
If two storms of at least tropical storm strength are in the Gulf at the same time, which looks likely for a short time late on Monday, it will be only the third time on record. If two hurricanes are in the Gulf at the same time, also possible, it will be the first time on record. (Of course, it's #2020.) Here are the forecasts as of 4pm CDT Saturday:
There are still a lot of uncertainties on both systems, which I will not delve into here, but it's good to remember that these forecasts can be fairly uncertain multiple days out despite advances in forecast techniques. You'll notice though that Marco now appears to be headed for the central Gulf Coast with landfall projected Monday afternoon in southeast Louisiana as a hurricane (perhaps a very strong tropical storm). Just two days later, Wednesday afternoon, Laura heads for the same approximate area as a hurricane and likely stronger than Marco. Again, lots can change - and has just in the past 24 hours. However, a double-landfall in the same general area could be devastating for said area in terms of flooding, storm surge, and strong wind that could be dealt a powerful blow by a second storm after being weakened by the first. Let's hope and pray not.
Dualing tropical systems and the Fujiwhara Effect
As for the interaction of the two, it appears right now that this is fairly unlikely given that Laura is forecast to be just entering the southern Gulf as Marco makes landfall in the northern Gulf. But let's talk for a second about the Fujiwhara Effect - a term that is indeed NOT new (just like Polar Vortex and bombogenesis), but new to us because it doesn't happen often, and rarely in the Atlantic/Gulf of Mexico basin. It is more common, though still somewhat rare, in the western Pacific where there tend to be more tropical systems.
The AMS Glossary of Meteorology defines it as "the tendency of two nearby tropical cyclones to rotate cyclonically about each other as a result of their circulations' mutual advection." Notice it does not say "merge into a megastorm and result in massive death and destruction." In fact, the forces acting on and around a tropical system that cause it to maintain itself would be destructive to another system that approaches it. I think the best way I have seen it described in a visual fashion is through the animation in this really cool tweet:
A lot needs to happen before recent #Fujiwhara headlines come true, but here's what it looks like in a high-res simulation.
— Jake Carstens (@JakeCarstens) August 21, 2020
Background rotation is turned up beyond real-Earth values to fit a bunch of tiny TCs. No other forcing, so they're allowed to purely interact on their own. pic.twitter.com/rAZMM4LU8A
Mesmerizing isn't it? Go ahead, watch it again.
Notice that as these "tropical storms in a virtual lab" approach each other, they tend to either repel or weaken one another, with the weaker of the two typically falling apart, sometimes with the remnants being absorbed by the larger one. So, while it doesn't appear Laura and Marco will have much influence on each other this week, if they were, it would likely just be a minor deflection of one or the other system or a further weakening of the "weakest link," not a massive COVID-cane named "Laurco."
Effects on the Mid-South
Looking out a bit further, most models that agree with the NHC assessment of the forecast tracks right now bring the remnants of Laura into the Mid-South later next week. We'll definitely be watching this as it could mean a healthy dose of precipitation - perhaps a couple of inches - and some gusty wind. Further forecasts will refine that, but for now, the Weather Prediction Center of the NWS provides the precipitation forecast for the upcoming week below, which presumes some tropical moisture moving across the Mid-South about Thursday or Friday. It's got the Memphis area solidly in the 2"+ range.
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| The Weather Prediction Center forecast for rainfall over the coming week. The influence of two tropical systems on the Gulf coast is easy to pick out with remnants moving north through the lower Mississippi Valley late in the week. (WPC via Pivotal Weather) |
Erik Proseus
MWN Meteorologist
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