The key feature everyone is talking about is the ridge of high pressure that is guiding the storm. Here is what it looks like, using data from the GFS model, using the 500mb height and winds. 500mb is about 18,000 feet high, and is a key level in the steering of most storms. First, here is the current situation (as usual, click to embiggen):
Dorian is the round bright object in the lower right corner. Above it is a high pressure ridge, and you can see the wind barbs flowing clockwise around it. Note the “ridge” extending from east to west. That is what is guiding the storm right now. What about Saturday morning?
If you look carefully you will see the barbs have more feathers, and the “ridge” extends further towards the US. This is what is forecast to steer the storm towards The Bahamas and Florida over the weekend. That seems pretty certain at this point. But what happens next is in flux. Here is the forecast situation by Tuesday. Note the storm didn’t move all that far in three days, and note the “ridge” isn’t quite so prominent:
That’s the solution from just the main GFS model run. How uncertain is it? Lets look at the whole GFS ensemble family of tracks. The blue line is the main run (that is shown above), the brown line the average of all 20 runs:
Several other models (including the much worshiped ECMWF family) are showing a similar “weakness” developing in the main runs, but the ensembles tend to be more inland. So what does that mean? It means what it always means: follow the NHC official track forecast since it’s their job to sort all that out! Hopefully this gives you a better idea of the challenges of this kind of work, and why you look for trends over time, not individual model runs. I haven’t seen the news reports on this advisory yet, so I don’t know how much of a bit deal the usual suspects are making about this, but here is a comparison of the tracks between the 11am and 5pm forecast advisories:
In a big picture sense, while the models track shift seems significant, the NHC track shift is not really that much, so nothing much has changed since this morning from an action standpoint. People in The Bahamas and Florida need to be getting ready for a major hurricane. Following the advice of your local Emergency Managers is probably the best bet. This could be a very severe, prolonged event. With this shift the modeled damage is still an eyewatering 46 Billion dollars. Here’s the swath of doom:
For the people of Georgia and South Carolina, this may not seem like a great evolution of the models, but certainly not freaking-out worthy yet. If you have a hurricane plan in place, that’s all you need to do at this point. Impacts for the Coastal Georgia area are still about 5 days away (next Wednesday or Thursday), and the potential track and intensity errors are large enough that it is a time to be vigilant, but not scared. The long-range runs I’ve looked at have tropical storm force winds along the coast (and the coastal flooding in usual places), but otherwise not severe damage. Given the slower speed of the storm, we’re still in the “watch and be wary” phase.