PTC 9, future Isaias, moves through the Caribbean.

Posted at 8:06 AM, Jul 29, 2020

Tuesday morning the National Hurricane Center began issuing advisories on Potential Tropical Cyclone 9 ( a technical term that will be addressed at the end of the article) meaning that the formation of Isaias seems imminent.

The storm currently has tropical storm forced winds, but an exact center of circulation has remained elusive which is keeping this storm from officially getting a name although at this point it's just a matter of semantics.

There hasn't been many changes to this track since it first popped up yesterday, and the models have looked consistent for the last couple of days which helps build a little more confidence in the forecast.

That being said, however, until we can really nail down where that circulation is the forecast could undergo some pretty big changes particularly when it comes to forecasting intensity.

It doesn't mean that there's no value in the forecast though and we can still highlight some of the features that would help determine the ultimate outcome for this storm.

Before we dive deep into what the models are telling us it's worth pointing out a few different features that will be the likely forces that determine the direction of PTC 9.

The biggest steering feature is the Bermuda Ridge which is sitting in the Atlantic, which is currently pulling moisture up along the east coast, and a developing upper level low coming across the plains.

That low will eventually get caught up in the western edge of the ridge and pulled north through New England, this will create a type of wake that could end up pulling PTC 9 north along the west end of that fringe. (This accounts for the turn you see in the NHC track)

These features show up particularly well in the EURO model run where you can see some of these processes playing out, notice the deep dry air post trough at the end of the loop dipping through Memphis and north Alabama.

If the timing continues to match up that would help keep the storm out of the central Gulf of Mexico and Louisiana would remain out of the impact zone from this storm.

This same dance between the storm, the high, and trough is playing out in other long range models as well, which will help with building at least a little confidence (although there's still a good bit of uncertainty with this system).

The thing to watch for that would indicate a major change in forecast is the placement of the low when the storm finally gets it's official name, it's a broad low right now so the NHC has placed it as best as they can.

Since that's the starting point for all the models now if that shifts around in the next 24-48 hours then you'll start to see the rest of the forecast shift around with it.


There are plenty of questions about the designation "Potential Tropical Cyclone" instead of just going ahead and naming the storm, and while it seems random there is a reason for this being the case.

A lot of this stems from nature of science which requires strict definitions for classifications, in order for a storm to be officially a "tropical storm" it has to meet certain criteria.

The winds have to be at a certain range (for a tropical storm this is sustained at 39+ mph) and it has to have a specific closed low to go along with it (there are other criteria as well but for our purposes these are the two important ones).

The case with PTC 9 is the winds are at 45 mph, which are tropical storm forced winds, and the impacts being felt along the Caribbean are the same that are associated with tropical weather.

There is no closed center of circulation, or one that hasn't quite been discovered yet which in the eyes of science doesn't make this a tropical storm even though the impacts are the same.

This then runs in to the strict rules that the NHC must operate under when it comes to meeting the criteria to offer Tropical Watches and Warnings, which can only be done once there is an official named storm.

While all of this does sound tedious there is something to be said for having a uniform set of rules and actions for getting out information clearly and quickly particularly information as vital as tropical forecasts.

A few years ago this came to a head, it was also an issue with Hurricane Sandy in 2012, when there was for all intents and purposes a tropical system causing tropical conditions, but the storm didn't meet the exact criteria for official "tropical status".

This tied the hands of the NHC in issuing their alerts and warnings which they weren't able to do since it wasn't "official", so to avoid this problem when a named storm is seemingly imminent they created the designation of "potential tropical cyclone" making a little loophole that allows them to warn when tropical weather is coming.

So as mentioned above since this current system doesn't seem to have an exact center of circulation it doesn't quite me the scientific criteria for a tropical storm but the NHC needs to place warnings for those in the path, thus it's called PTC 9 until a low is found.

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