Potential TC #9 (AL092019); Finding the center?

Potential Tropical Cyclone #9 does not have a well defined center, and is still technically a tropical wave (if any terminology like potential tropical cyclone, tropical wave, etc. is confusing look here).   In the post-Sandy revisions to NWS procedures, if was decided that if it was possible the storm could reach watch or warning criteria, NHC would start advisories under the term “Potential Tropical Cyclone.”  It’s not done that often, only when a storm “spins up” close to land, like this thing is doing.  So where is AL09, where is it going, and how strong will it be when it gets there?  Let’s take a look at how NHC and TC specialists try to figure that out ..

First, where is the storm?  With weaker storms this is a big deal – the track models can’t do a good forecast without a closed circulation to lock on to.  Here are all of the position fixes for AL09, and the “track” NHC is using … as always, click to see full size.

Compare that with Hurricane Dorian as it traversed The Bahamas as a much stronger and well defined storm … the icons are for various techniques like satellite position/intensity estimates, airplane fixes, radar, etc.

Here’s the tropical analysis for this morning, with the IR satellite view (sun isn’t up yet!).  The big bad orange blob, the main rain and convection associated with the system, is to the right of the “L” marking the estimated center (the Low pressure marker).

The lack of a solid circulation to lock on means you have to treat the track models a bit skeptically.  Here’s the full spread for AL09, including ensemble members.  A big confusing mess …

The red line is the official NHC track.  They are essentially splitting the difference between the various scenarios given by the “deterministic” model families (colored lines).  Intensity is an even harder thing to figure out.  Conditions are not favorable for the storm to develop over the next two days.  After that, conditions should improve, but by then the storm may be near or over Florida, limiting development.  So without a good track, doing an intensity estimate is a harder than normal task (and intensity is a harder problem than track in most storms).  NHC is conservatively assuming the storm will stay on or near shore, but still be able to take advantage of the favorable upper level winds to strengthen into a moderate tropical storm.

So what does all that mean?  Well, for people who want to fill the airwaves or bandwidth with confusing speculation, it’s a windfall (see what I did there 😛 ).  But for you, the sophisticated reader, you do what you always do since you have a plan, restocked your supplies after Dorian (including, if comments on Facebook are any guide, your extensive wine cellar), and look at the NHC Key Messages product, and check for watches and warnings. Here’s the estimated impact based on the NHC forecast …

Bottom line: much weaker storm than Dorian, might follow a similar track (or not), not likely to become a hurricane in the near term, mostly a rain and blustery wind thing.  More misery for The Bahamas.  Inconvenience in the US, with the potential for hazardous, but not likely dangerous conditions right on the coast.

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