Eta, after two days over the mountainous terrain of Central America, no longer has almost any of the tropical organization it once had.
The storm, now a tropical depression, has essentially lost any markers of a center on satellite. As seen from above, the only way to figure out where Eta actually is is to look for the feeder bands. Because tropical systems have lower pressure than their surroundings, air moves towards them, and the Coriolis force/friction lead to an inward spiral of that air towards the low. And because all that air moving towards a single center is coming from a big area into a small area, there is a lot of inherent convergence. As so much air is shoved together from this convergence that some is lifted vertically, these bands are visible in the deep convection that forms from the lift.
On the satellite image of Eta, the only apparently visible feature is one of these convergent bands creating deep convection from the Pacific, over Guatamala and Belize, over the SE Caribbean, and back through Costa Rica and into the Pacific. To the northeast, a 300mb jet arcing above a weak upper level trough means wind is quickly evacuated from the column above the northern edges of the convergence band. This is pumping gas into the convection there, making it more readily apparent on satellite.
Because Eta is still moving pretty slowly, these feeder bands are still dropping persistent heavy rain over parts of Central America, and I expect a flooding disaster is continuing to unfold there. Because these bands provide their own convergence, heavy rain occurs largely regardless of topography- think Hurricane Harvey for a US analog of what feeder bands can do in terms of heavy rain.
Anyway, some in Cuba and the southeast US are keeping a nervous eye on Eta, as models project various scenarios involving re-organization followed by future landfalls there. Now, we still don’t know a whole lot, as there’s a lot of ambiguity over where exactly Eta is now, and because it’ll be really sloppy and disorganized whenever it manages to re-enter the Caribbean. But there are some suggestions as to how the storm may behave.
Many models, including the GFS, Euro, and HWRF, depict Eta over the next ~15 hours being a story of several competing relative low pressure regions, likely at the hands of topographic influences on what’s now a very weak system. This makes finding its center difficult in the near term- you really have to squint. But we have some idea where to look.
As a trough digs into the western Gulf and upper level flow strengthens to the northeast of the storm, the relative equilibrium of flow over Eta will at last cease, and the storm will be steered north, and then northeast.
The center of the broad circulation will probably reach the northwest Honduras coast tonight or early tomorrow under this steering regime, and there’s pretty tight model agreement that Eta will be back over the Caribbean in around 24 hours. After that, the most important thing to figure out is where the storm will go; knowing this will help us determine intensity and impacts.
It seems a mid-upper level low will close off over the north-west Gulf, with a broadly defined ridge over the Bahamas. Between these two influences, flow will be to the northwest, while to the south, flow will be to the east. As a result, Eta will likely move just north of east towards Cuba, with an increasing northerly component with time. At some point, as the upper level low nears, Eta will suddenly be steered northwest, instead of just north of east.
This introduces a major uncertainty into Eta’s track. Will it eject into the Gulf before it reaches Cuba? Will it hit Cuba and then eject into the Gulf? Will it hit Cuba and then South Florida? Will it make it all the way into the Bahamas, approaching Florida from the east?
Hurricane and global models show that any of the scenarios that I listed above are possible. Let’s try to figure out which is the most likely!
Often, the impacts of upper atmospheric features on tropical systems can be influenced to a great deal by the intensity of the storm, as deeper hurricanes are steered by average winds that stretch higher up in the atmosphere. However, with a reasonably vertically stacked mid/upper level low likely to be responsible for much of the northwest motion we expect, there may be pretty similar flow means regardless of altitude, and so regardless of cyclone intensity.
This seems largely to be the case, as members of the euro ensemble have paths that don’t deviate much based on intensity. There also isn’t a very large spread in potential intensity- we expect a somewhat weak system, due to the degree of disorganization, on approach to Cuba. This all shows us that forecasting the position of the cutoff low is probably the most important thing for us to do in order to figure out where Eta is going.
Forecasting the position of the upper level low is difficult specifically, as it won’t be very strongly forced, but it’s pretty easy to figure out more or less where it’ll be- basically every global model has it in a similar position near the Texas/Louisiana coast, and it looks to me like that makes an Eta landfall in Central Cuba most likely, probably in around 72 hours.
How strong will Eta be on landfall, assuming this is the case?
Sea surface temperature are WARM, exceeding 30°C, but shear will be rather high, with flow at 300mb probably exceeding 30 knots. Now, this won’t all be destructive, as some degree of speed divergence will allow venting of flow, supportive of Eta deepening. However, it will also help keep what is now a horribly disorganized system from getting itself vertically stacked. This will be something to watch unfold as it does, but my current expectation is that Eta will not re-organize enough to take advantage of very warm water due to this high shear.
Although I’m more uncertain than normal about where exactly Eta is, and where exactly it will go over the next three days, I’m reasonably confident the storm will be unable to strengthen very quickly. The NHC projects a tropical storm landfall in Cuba, and I am inclined to agree. I’m setting my initial estimate for intensity cap at low-end category one.
After a Cuba landfall, it seems likely Eta will approach Florida early next week. There’s no guarantee the storm will make landfall in Florida: it looks just as likely that is stays between Cuba and Florida while recurving into the Gulf. If it does get close to Florida, the intensity when that happens depends on too many things we don’t know yet, like how strong it is when it hits Cuba, and where exactly landfall takes place on the island. But with lessening shear and warm water, there is a chance at a bout of intensification, and a low-end hurricane is possible on approach to Florida. Some global models show that. I would expect a strong tropical storm at the moment.
There is a good chance that Eta will slow down again as ridging builds in over the storm. This will probably happen after the storm leaves Cuba, and could mean widespread heavy rain, most likely in Cuba and Florida, this weekend or early next week. We won’t know exactly where this heavy rain is expected to set up until probably the second half of the weekend, but it could end up causing some flooding. Nothing as catastrophic as what’s happening over Central America is expected, but the WPC does estimate up to 15″ in areas:
Once again, uncertainties with track mean we don’t know that much about this threat yet. But it should exist somewhere, and we plan on keeping you posted as we learn more.
Now a poorly organized tropical depression, Eta is really only visible from its feeder bands. But it looks like the storm will move back into the Caribbean in about 24 hours, and some re-intensification is likely as the storm takes a track to the northwest. A landfall in Cuba seems probable at the moment, probably as a strong tropical storm or perhaps weak hurricane, before Eta approaches south Florida while slowing down. While there is still a lot we don’t know, it seems likely that the storm will bring a threat for heavy rain to Cuba and Florida this weekend and early next week, respectively.
Stay tuned to this blog and to our twitter for updates as the storm re-enters the Atlantic- in all likelihood, some things will change as we get a better idea where Eta actually is, and as the flow regime becomes more concrete.