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.

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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.

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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.

 

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