01 September 2021

Ida, Kate, and Larry keep the season active

On September 1, we have three active tropical cyclones: Tropical Depression Ida, Tropical Depression Kate, and Tropical Storm Larry.  Ida is by far the most impactful at the moment, so I'll begin with that one.

As we know, Ida made landfall on Sunday as an upper-end Category 4 hurricane in southeast Louisiana and brought terrific destruction.  Since then, the center has tracked over the southeast US and is about to pass over the northeast US states today and tomorrow.  As expected, heavy rainfall has been occurring along its path, and that threat of severe flooding continues.  This map shows total rainfall estimates over the past three days:

Then, the map below shows the rainfall forecast over the next two days.  Flash flood watches cover parts of North Carolina to Maine, and Ohio to New Jersey... a huge area with the potential for significant rainfall.

Next is Kate.  We've been watching this thing for a loooong time.  The easterly wave that would become Kate left the African coast on August 22nd, then was upgraded to Tropical Depression 10 on the morning of August 28th, then to the season's 11th named storm on the morning of the 30th, but then weakened back to a tropical depression just 12 hours later.  It is currently centered 900 miles southeast of Bermuda and is absolutely not a threat to Bermuda or anywhere else.  It is forecast to dissipate by the end of the week.

That strong easterly wave that I mentioned in Monday's blog post is now Tropical Storm Larry.  It's centered just south of Cabo Verde, and became a tropical storm rather far east (https://twitter.com/BMcNoldy/status/1433015705394315268).  On average, the 12th named storm forms on October 11th (using the 1991-2020 climate normal period), so this is really exceptional.  The only previous years with 12+ named storms by September 1st are 2020, 2012, 2011, 2005, and 1995.

Model guidance has been extremely aggressive with this storm from the beginning, indicating that not only is it very likely to quickly become the next hurricane, but probably the next major hurricane too. 

Based on these ensemble tracks (GFS on the left, ECMWF on the right), Bermuda should be at least monitoring this storm closely, though it's at least a week from any potential encounter there. Beyond that, the spread increases of course, and it's too soon to completely rule out impacts further west.  2018's Florence is still too fresh in our memories to tune a storm like this out.

And finally, there is a feature of interest in the south-central Caribbean, east of Honduras, that the National Hurricane Center is giving a 30% probability of becoming a tropical cyclone.  It's quite disorganzied now, and will run into central America soon.  But if it remains just far enough away from land, it still has a brief window for development in the Bay of Campeche.

It's also a good time to point out that historically, about 70% of the season's ACE (Accumulated Cyclone Energy) is still to come.  We have already had 12 named storms, 4 hurricanes, and 2 major hurricanes, and the full-season average is 14-7-3.  Impressively, this season has accrued more ACE than 2020 did by this date.

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