20 September 2021

Watching far eastern Atlantic for next storm... Sam?


Since my previous post on Friday, we have had Odette, Peter, and Rose form in the Atlantic!  And, we're likely going to see the season's 18th named storm form this week: Sam.

Odette (15L) was a named storm for only a day, and came from the area of interest that was off the North Carolina coast on Friday.  The other two areas that were highlighted in that post became Peter (16L) and Rose (17L), and they are both still active. 

Neither Peter nor Rose will be able to strengthen much, and both will remain far out in the ocean.  Both are facing wind shear and dry air to different degrees.

Peter is located just north of the Leeward Islands and is close to a strong upper-level low pressure system which is forcing the thunderstorm activity east of the circulation center.  Rose is west of Cabo Verde and is close to ingesting lots of dry Saharan air.  Both are forecast to track toward the northwest then north in the coming days.  The only potential concern would be Bermuda this weekend, but impacts from Peter would not be significant.


It's worth pointing out that Rose became the season's 17th named storm on September 19th.  Since 1851, only two other seasons had the 17th named storm so early in the year: 2005 and 2020.  Rose is also still a name from the original 1979 list and has never been used before now! 

Now on to what will almost certainly become Sam: Invest 98L.  This easterly wave left the west coast of Africa on Sunday morning and has been strongly favored by the model guidance to develop and to maintain a track in the deep tropics for a while.



The model guidance is generally bullish on this system, eventually.  It's in a tight spot right now, with Rose immediately to its northwest and Saharan air streaming off the continent to the north. 

The latest deterministic (single higher-resolution run compared to the ensembles) European model run brings this storm very close to or over the northern Leeward Islands next Wednesday as a hurricane.  The latest American model run is two days slower and 275 miles farther north.  Clearly, there is some disagreement to sort out in the coming days, but they both have a hurricane tracking through the deep tropics next week.

European (top) and American (bottom) deterministic runs, both valid when the storm is at the longitude of the Leeward Islands.  For the European run, that's next Wednesday, but next Friday for the American model.

Looking at the updated ACE (Accumulated Cyclone Energy) chart, 2021 is now at about 117% of average for the date.  This time of year, it takes a decent amount of activity just to keep up with climatology, and a couple of low-end tropical storms don't quite cut it, so we're creeping closer to average every day... for now.


17 September 2021

A lull after nonstop action with Ida through Nicholas

Since my previous post on Tuesday morning, nothing has happened!  That in itself is odd to say when there are multiple features of interest and it's mid-September.  Neither the disturbance that was near the Bahamas nor the wave in the far eastern Atlantic have developed yet, though both are still expected to.


The western system, Invest 96L, is now centered just 150 miles off the North Carolina coast and appears quite close to becoming the season's next tropical or subtropical cyclone.  In the satellite animation below, it's easy to see the clouds rotating around a low-level center, but all of the thunderstorm activity is displaced to the north and east. It is expected to head off toward the northeast, intensifying as it does so... transitioning to a strong extratropical cyclone by the end of Saturday.

If it develops into a tropical or subtropical storm in the short window remaining, the next name on the list is Odette. Reaching 15 named storms is above average for an entire season, let alone by mid-September... the only seasons with 15+ named storms before September 19th are 2020, 2011, and 2005!


The wave that we've been watching since it left the African coast four days ago, Invest 95L, is now centered about 1200 miles east of the Lesser Antilles and is zipping westward at 20 mph.  It has struggled to develop, but NHC still gives it a 70% probability of formation within the next five days, somewhere in the shaded area in the map at the top. 


If it does form, the model guidance generally keeps it pretty weak and just north of the Caribbean on Monday-Tuesday, then some start showing intensification as it turns north.  There's not a lot of confidence in anything longer-range than Monday-Tuesday because it's so weak and disorganized now.  Certainly the northeast Caribbean should be watching it closely, but there's a lot of time to wait and watch for any potential US east coast or Bermuda encounters.



Finally, a new easterly wave has existed the African coast, but it's likely to turn to the northwest and find itself over cool water in less than a week.

The ACE (Accumulated Cyclone Energy) is now at about 124% of average for the date... it would actually slip below average for the first time this year on September 28th if nothing else is named by then, which is unlikely.


14 September 2021

Hurricane Nicholas hits Texas, and keeping a close eye on Africa

Since my previous update two days ago, Tropical Storm Nicholas formed on September 12th in the Bay of Campeche, then rapidly intensified to a hurricane just as it made landfall near Galveston in the early morning hours of the 14th. Nicholas is the season's 14th named storm and 6th hurricane... the average dates for those to occur: November 18th and October 15th.  This is quite an exceptional level of activity so early in the season.


[By the way, Category 2 Hurricane Ike made landfall in Galveston too, on September 13th, 2008... Nicholas missed that anniversary by a few hours.]

Throughout the day on Monday, Nicholas had some really favorable conditions to work with, and some not-so-favorable. The center reformed in different locations, shear made the storm lopsided, dry air eroded large portions of it away, but at the end of the day (literally), the super-warm Gulf of Mexico provided it with enough fuel to just barely cross the Category 1 hurricane threshold.

http://bmcnoldy.rsmas.miami.edu/tropics/radar/

By far, the greatest threat from Nicholas has been and continues to be the heavy rain.  As you can see in that radar loop above, most sectors of the storm are devoid of rainfall, but where it does exist, it's persistent and slow-moving.  The areas at highest risk for flash flooding are highlighted below:


We've been watching pre-Nicholas for the past 5-6 days, since it was near Nicaragua. Although some of these storms aren't named systems for very long, or it seems like they pop up out of nowhere, they most definitely do not. Some just take a long time to fester and once they finally get their act together they're already near landfall.

As my friend and colleague Bob Henson pointed out in a blog post, Nicholas is now the 19th named storm to make landfall in the U.S. since May 2020... the average is about three per year.

Now shifting our attention to Africa, recall that easterly wave I mention in Sunday's update.  It has wasted no time getting better organized once it left the coast.  Currently tagged as Invest 95L, it is quite close to becoming Tropical Depression 15, and then Tropical Storm Odette would be the next name.


The GFS and ECMWF global model ensembles both favor this for development, but neither are alarmingly bullish on intensifying it too soon.  It's in no rush to head westward, so even if it were to develop and reach the Lesser Antilles, it wouldn't reach the islands until the Sunday-Tuesday timeframe.  There's plenty of time to wait and let this do its thing.

One thing the ensembles have indicated in the past several runs is that a stronger version of the storm is more likely to turn northward well before the Lesser Antilles, while a weaker version could cruise much closer to the Caribbean.  We would be wise to watch the progress of a strong wave in the deep tropics in mid-September very closely.


And finally, that area north of the Bahamas that I mentioned on Sunday now has a more focused low pressure center, and we could see some development of that in the coming days as it drifts north.  As of now, it's nothing to be concerned about, just worth keeping an eye on it.  The National Hurricane Center is giving it a 60% probability of becoming a tropical cyclone in the next five days somewhere in the shaded area on the map.


12 September 2021

No active storms on peak of the season, but lots to watch

From last Thursday's post on the peak of the hurricane season, I mentioned that the peak of the season is generally the second week of September, but if a specific date has to be chosen, I recommended September 12th (today). As of this writing, we happen to have zero active tropical cyclones on September 12th... but a whole lot to talk about.


The National Hurricane Center is highlighting five areas of potential formation within the next five days, two of which I'll focus on because of their eventual potential impacts.  But the five areas are shown above, with yellow indicating an area with a low probability of formation, orange is medium, and red is high.

The one of immediate concern is a disturbance in the Bay of Campeche, which has been a feature of interest for the past few days since it was north of Honduras then drifted across the Yucatan peninsula.  This is identified as Invest 94L, and is quite likely to become at least a tropical depression by later today.


Models are in good agreement on a track toward the north, scraping the Mexico/Texas coastline over the next few days, dropping heavy rain along the way.  They're also in good agreement on it remaining relatively weak -- none have it reaching hurricane intensity (but let's not rule that out completely... after all, it's mid-September!).  As of now, the greatest threat posed by this system is the rain, and significant totals are expected over southeast Texas and southern Louisiana in the next five days, which will inevitably result in flash flooding.


The next feature I'll highlight is an easterly wave that's still over Africa... but models have been consistently bullish on its development and eventual track through the deep tropics.  There's another easterly wave centered over Cabo Verde too (Invest 93L), but that one is less likely to develop and more likely to take a track toward the northwest.  Since there's so much going on, I'll just focus on the one still over land.


The American (GFS, top) and European (ECMWF, bottom) global model ensembles are shown below... these are the trackable low pressure centers out through the next ten days from the most recent run.  When you look at long-range forecasts of activity, it's important to not focus on details.  Blur your eyes, look for trends in the percent of members that develop it and the general placement of the tracks.  From this, we can be quite confident that the easterly wave will develop, that it's likely to become a hurricane, and that it could be a threat to the eastern Caribbean next Sunday-Tuesday.


The area north of the Bahamas that's shaded in orange on the map at the top of the post is not a current disturbance at all, but rather an area where one could form later in the week. *IF* something is able to develop there, it could have impacts from North Carolina up into the northeast U.S. in the Friday-Saturday timeframe, but it does not appear to be a big concern at this point.  Something to keep an eye on if you're in that area though.

The next names on the list are Nicholas, Odette, and Peter.  All three of those names are originals from the 1979 list, but they have only been used once before (2003)!

And finally, an update on the progression of Accumulated Cyclone Energy (ACE) for the season: it's at about 143% of average for the date, using a 1971-2020 climatology.  This season's ACE is 1.4 times higher than 2020's was on this date, but way lower than mega-seasons like 2017, 2008, 2005, 2004, etc. In other words, it's noteably high, but very far from touching any records.



09 September 2021

When is the peak of hurricane season?


When is the peak of hurricane season?  It turns out there is no single or correct answer.  It depends on what metric you prefer and which time period you use.  But no matter how you slice it, the peak of the Atlantic hurricane season occurs during the second week of September.  For all of the plots and data presented here, I use a 7-day centered average, because there is quite a bit of noise when strictly using daily values. Let's break down what all of those curves are in the chart above.

One common metric would be the daily average of named storm (NS) activity.  Using the new 1991-2020 "climate normal", that peaks on September 12th, with a secondary peak on August 31st.  But the daily average of major hurricanes peaks on September 3rd, and one might argue that those are much less prone to being over/under counted and are definitely more impactful when close to land.


Using the past fifty years, 1971-2020 (which is still entirely during the satellite era), that chart smooths out a bit and the peak is a tie between September 11th and 12th. There's a secondary peak on August 31st-September 1st.


But zooming out and using the full 1851-2020 period of record (with all the nuances and disclaimers about data from the pre-satellite era), it looks like this: a definitive peak on September 11th for named storms, and a September 5th-7th peak for major hurricanes.


So for named storms, one could conclude that the peak falls on September 11-12, but the September 1-14 period encompasses the various peaks that arise depending on the choice of time period.

Another common metric is the Accumulated Cyclone Energy (ACE).  That is not dependent on the number of storms, but rather is an integrated measure of the intensity and duration of all storms.  Weak, short-lived storms barely make a dent in it, while long-track intense hurricanes make large contributions.

Again beginning with the 1991-2020 climate normal, that metric peaks on September 14th. Also notice the much more pronounced and dramatic rise and fall surrounding the peak of the season compared to the named storm counts. This speaks to the point that the real "meat" of hurricane season typically falls between mid-August and mid-October -- those two months account for 75% of the total ACE of the six-month-long season! 


Using the longer 1971-2020 period, the data expectedly smooth out a bit, and there are three dates that are essentially tied: September 14th, September 13th, and September 1st, though September 14th is technically the highest by a hair.


And the full 1851-2020 period of record results in a much smoother time series, with a broad peak centered on September 10-11.  An exaggerated feature of this period is the abrupt increase of activity during early August, then a much more relaxed decline of activity into November.


If we think of the peak of the season as when 50% of the total ACE has occurred, the peak is September 12th for all three of the time periods considered here (1991-2020, 1971-2020, and 1851-2020).

In summary, rather than assigning a specific date to it, we can conclude that the peak of the Atlantic hurricane season is the second week of September...  accounting for different metrics, different averaging periods, evolving observing technology, and the relatively short period of record.  But if you feel the need to assign a specific date to the peak, it would logically be September 12th.

Keep in mind that this is all based on historic storms and the best record we have of their existence and intensity.  Individual seasons will rarely follow the "average", and very significant hurricanes can and have occurred outside of the peak of the season.  Even the six-month hurricane season doesn't always encompass all of the activity, nor was it designed to.

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.


29 August 2021

Ida makes landfall, Julian forms, TD10 forms


As expected, Ida did indeed rapidly intensify in the Gulf of Mexico, and strengthened from a tropical depression on Thursday morning to nearly a Category 5 hurricane early Sunday afternoon.  Peak winds at landfall were 150 mph, and it's too early to know the aftermath because it is still coming inland as I write this.

Ida is among the strongest hurricanes to ever make landfall in Louisiana, and it the eyewall will pass just west of New Orleans and possibly right over Baton Rouge. We know without a doubt that it will be bad... very bad.  Today is the 16th anniversary of Hurricane Katrina's landfall in roughly the same location, and although Ida is smaller, it's stronger.  


Aside from the wind and storm surge damage, Ida is going to dump a LOT of rain, likely much more than Katrina did, in eastern LA.  Then it will leave a swath of heavy rain along its track through the country over the next five days.



On Saturday night, Tropical Depression 11 formed in the north-central Atlantic, and that was upgraded to Tropical Storm Julian just twelve hours later.  Julian is the season's tenth named storm, and formed 25 days ahead of the average date of the 10th named storm formation.  It is forecast to track to the northeast, then turn north into the cold north Atlantic... not a threat to land at all.


Tropical Depression 10 formed on Saturday morning and is expected to become the season's eleventh named storm, Kate, on Monday.  It is subject to the same generally steering patterns as Julian, so it's also going to track north and remain far away from land.


Finally, for now, there's a strong easterly wave still centered over western Africa that has a lot of support in the model guidance for development in the coming days.  If that one becomes a tropical storm, it would be the season's twelfth: Larry. The long-range outlook indicates that it could become a fairly strong storm, but as of now, the odds favor it recuring to the north well before reaching the Lesser Antilles.


In terms of ACE (Accumulated Cyclone Energy), the 2021 season is at about 153% of average for the date, using the past 50 years as the baseline.  Recent seasons that were this high by this point in the season were 2020, 2012, 2008, 2005, 2004, etc.