09 September 2021

Larry weakens, Mindy forms during peak of season


Since my previous post on Monday, Hurricane Larry has weakened only slightly but is still a large and powerful hurricane located just east of Bermuda.  Then, the disturbance I first wrote about on September 1st down near Colombia (later tagged Invest 91L) was upgraded to Tropical Storm Mindy just before landfall on the Florida panhandle on Wednesday afternoon.


Once Larry clears Bermuda today, it's not done with land impacts.  A hurricane watch is in effect for eastern Newfoundland, and Larry will reach the island on Friday night... very likely as a hurricane. It's reminiscent of Igor in 2010, and that storm's encounter with Newfoundland earned Igor's retirement as a name.

From the track map at the top of the post, we can see the 10-day journey that Mindy took from the coast of Colombia to the Florida panhandle... it's been a long time coming but it finally got organized on Wednesday afternoon and quickly wrapped up to become a tropical storm.  This radar loop (latest available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/) shows the quick development and landfall.


Mindy has since weakened to a tropical depression and will re-emerge over the Atlantic later this morning.  Due to increasing vertical wind shear, it is not likely to regain tropical storm status, but could be marginal immediately upon exit, over the Gulf Stream. It's moving quickly so significant rainfall isn't expected, just some heavy rain over eastern Georgia and South Carolina as it passes through.

In terms of named storm count, Mindy is the season's 13th named storm.  The average formation date of the 13th named storm is October 25th, so this is really exceptional.  Only four other years reached this mark by September 8th since 1851: 2020, 2012, 2011, and 2005. Keep in mind that it's only September 9th and we have already used 13 of the 21 names in the regular list! The table below shows the 1991-2020 average formation dates.


In terms of ACE (Accumulated Cyclone Energy), the season is at about 155% of average for the date, using the past fifty years as the baseline.  This season has already accrued more ACE than some entire seasons (2015, 2014, 2013, 2009, 2002, 1997, etc), and Larry is largely to thank for that boost, as that one storm has contributed roughly 40% of the total!


Looking farther east, there's a strong easterly wave just about to exit the African coast, and model guidance favors it for development by the end of the weekend.  Once over the ocean, it should track toward the northwest, no threat to the Caribbean or areas west.  When the time comes, the next name on the list is Nicholas.

06 September 2021

Larry still a major hurricane, also watching Gulf coast

The easterly wave that would become Larry left the African coast on August 30th.  It quickly became the season's 12th named storm on September 1st, then the season's 5th hurricane on September 2nd, and the season's 3rd major hurricane on September 4th.  All of those dates are 1-2 months ahead of where an average season would be, keeping the 2021 season activity well ahead of average too.


Larry has been a large Category 3 hurricane since Saturday morning, and it's expected to remain that way for another 3 days or so.  It's also expected to remain far from land, with the exception of a brush with Bermuda on Thursday and Newfoundland on Saturday. These are the hurricanes we all can enjoy: huge beautiful storms far from creating any havoc on land.


However, even these distant long-lived powerful hurricanes do have a noticeable impact on the U.S. east coast.  This comes in the form of large swells and wave energy.  These elevated water levels will affect the U.S. east coast from Wednesday through the weekend:

Moving on from Larry, we've been keeping an eye on a disturbance for the past week, since it was just off the coast of Colombia.  It has since passed over Nicaragua, Honduras, Belize, and Mexico's Yucatan peninsula and is now centered in the Bay of Campeche... tagged as Invest 91L.  


Although environmental conditions aren't too favorable for its development, there is a chance of it becoming the next tropical cyclone later in the week as it approaches the northeast Gulf coast or after crossing the Florida peninsula.  If it happens to reach tropical storm status, the next name on the list is Mindy.  The average date of the 13th named storm formation is October 25th.

Overall, the 2021 hurricane season is at about 148% of the average ACE (Accumulated Cyclone Energy) for the date, and recent seasons that were more active by now include 2017, 2012, 2010, 2008, 2005, etc.  It's well ahead of the 2020 season.


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.