28 November 2022

It doesn't take an active hurricane season to be a devastating one

This post marks the end of my 27th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1380 posts spanning 460 tropical cyclones, 205 hurricanes, 94 major hurricanes, and 51 retired storm names. I know some of you reading this have been following along the entire time, but whether you've been reading these posts for 27 days or 27 years, I truly appreciate your continued interest!

The 2022 Atlantic Hurricane Season ends on Wednesday the 30th, and although it was much less active than initially expected, it definitely left its mark.

Track map of all sixteen tropical cyclones that formed during the season.  The storm's peak intensity and lowest central pressure are listed next to the names.

The season included 14 named storms, 8 of which became hurricanes, and 2 of those became major hurricanes.  Climatologically, those numbers are 14, 7, and 3.  But of course there's more to a hurricane season than the number of storms.

A commonly-used metric is the Accumulated Cyclone Energy, or ACE, and that is better at describing the overall duration and intensity of storms rather than the number of storms.  This season ended up with just 80% of an average season's ACE... the lowest since 2015.  

The distribution of activity was very unusual.  July and August were dead (with the exception of Colin which was around for a few hours on July 2 over land), while November ended up as the second-most-active month of the season.  On average, 22% of the season's ACE is accrued during August; this year it was 0%.  And 47% of the season's ACE historically occurs during September; this year it was 80%.

Of the fourteen named storms, four of them accounted for 74% of the total ACE and the other ten made up the remaining 26%.  The top four were Fiona (28%), Ian (18%), Earl (15%), and Danielle (13%).

This November was only the fourth on record to feature three hurricanes, the others were 2001, 1887, and 1870.  But two of the three were active simultaneously and the only other times that happened were 2001 and 1932.

On the flip side, this was only the fifth time since records began in 1851 that August went by with no activity; the last time was 1997.  But 1997 was during a strong El Niño -- this was the first August with no tropical cyclone activity during La Niña on record. Truly a bizarre season.

All other things being equal, La Niña acts to enhance Atlantic tropical cyclone activity... and La Niña has effectively been in place since the middle of 2020.  So what caused August to be so inactive even when it had that favorable background state?  There were three factors that all played a role, though their origins are not yet clear to me.  The mid-levels of the tropical and subtropical atmosphere were extremely dry, there was enhanced vertical wind shear over the tropical Atlantic, and the sea surface temperatures were anomalously cool over the tropical Atlantic.  They combined to overwhelm and suppress anything that tried to form.  Conditions quickly returned to normal once September began and then Danielle, Earl, Fiona, Gaston, Hermine, and Ian all formed during September.

For the first time since 2014, the first named storm formed after June 1, the official beginning of the Atlantic hurricane season.  Even so, it formed a couple weeks earlier than average.  The chart below shows the date of first named storm formation over the past fifty years in blue... the cyan line is the beginning of hurricane season (June 1), the magenta line is the median formation date (June 19), and the dashed gray line is the linear trend through the dates.  There's a lot of interannual variability, but for a variety of reasons, this trend is decidedly getting earlier.

The most intense storm of the season as measured by peak wind speed was Ian, but Fiona had the lowest central pressure, the highest ACE, and the longest duration as a named storm.  Ian also tops the list as the season's most deadly and most destructive.  In a few months, the World Meteorological Organization's tropical cyclone committee will convene and debate which names should be retired from this list.  Factors generally include how deadly and how destructive a storm was.  Using preliminary numbers, the top five deadliest storms this season were Ian, Julia, Hermine, Fiona, and Nicole... the top five costliest storms were Ian, Fiona, Nicole, Julia, and Hermine.  The same five storms made both lists but are just ranked differently.

Six countries experienced a hurricane landfall this season: Nicaragua (Julia), Belize (Lisa), Cuba (Ian), Dominican Republic (Fiona), United States (Ian and Nicole), and Canada (Fiona).  Fiona was technically not a hurricane when it reached Nova Scotia, but was a Category-2-equivalent extratropical cyclone.  Fiona also didn't technically make landfall in Puerto Rico, but passed close enough to deliver a crippling deluge of rain that was extremely destructive and killed over two dozen people.  Although the the final counts are not yet known everywhere, there were approximately 350 people killed by tropical cyclones across the Atlantic during the 2022 season.

Finally, here is a preliminary look at how the National Hurricane Center's forecasts of track and intensity matched up against their own average errors over the previous five seasons.  The track forecasts were very good this season, beating their average error at every forecast lead time.  Out at five days (120 hours), the storm that made the greatest contribution to the errors was Gaston.

Moving to intensity, the forecasts were better than average out to three days (72 hours), then slightly worse at four days (96 hours) and noticeably worse at five days (120 hours).  Those 4-and-5-day errors were dominated by Fiona.

There are several radar animations of the storms near land from this past season, from Alex to Nicole... all are available at https://bmcnoldy.rsmas.miami.edu/tropics/radar/

The 2023 hurricane season is six months away, and the list of names begins with Arlene, Bret, and Cindy... the list that was first used in 1981 and every six years since then.  However, quite a chunk of the list has been retired and replaced:
1981: no names retired
1987: no names retired
1993: no names retired
1999: Floyd, Lenny
2005: Dennis, Katrina, Rita, Stan, Wilma
2011: Irene
2017: Harvey, Irma, Maria, Nate
2023: TBD

09 November 2022

Nicole set to make Florida landfall on Wednesday night

Tropical Storm Nicole is crossing over the northern Bahamas midday Wednesday, and is headed for the southeast Florida coast late Wednesday night, perhaps as a Category 1 hurricane.  If it does end up being a hurricane at landfall, it would be just the second hurricane to make landfall in south or central Florida during November in historical records... the one and only previous one was in 1935.

I have several updating radar animations that cover the Bahamas, eastern Florida, and there is one that will be longer and covers the US east coast: https://bmcnoldy.rsmas.miami.edu/tropics/radar/.  An example from the Bahamas is shown here.

Nicole is a very large storm, so heavy rain and stronger winds will arrive at locations long before the center is anywhere close, and those conditions will last longer too.  As of Wednesday morning, tropical storm force winds extend as far as 460 miles from the center.  It's forecast to travel northward along the east coast, dropping heavy rain along its path, and causing coastal flooding along the way too.  Although it will lose its tropical classification, it will still be capable of producing significant impacts!

One key point that seems like it can't be made too often is that the track forecast cone (a.k.a. "cone of uncertainty") is not used to show where impacts will be experienced.  Clearly, the vast majority of impacts are outside of the cone.  Never use the cone to determine what hazards you will be exposed to.

Impacts will be far-reaching, primarily in the form of flooding from heavy rain and storm surge-induced coastal flooding... winds will of course be fairly strong, but generally not very destructive.  This set of Hurricane Threats and Impacts (HTI) graphics is valid through midday Friday, and you can see the distribution and threat level of the various hurricane-related hazards: wind, surge, rain, and tornado. 

The rest of the Atlantic is quiet, so once Nicole is no longer tropical in a couple days, maybe that will be it for the season... maybe.  The Accumulated Cyclone Energy (ACE) is at about 79% of average for the date.

07 November 2022

Nicole forms north of Hispaniola, big impacts expected along southeast US coast

The disturbance I mentioned in Saturday's post was upgraded to Subtropical Storm Nicole early Monday morning.  This is the season's 14th named storm, which is the average number during a full season.

As a bit of an introduction, there are three classifications of cyclones based on their structure, energy source, and other factors: tropical, subtropical, and extratropical.  Those approximately correlate to the latitude bands in which they exist, but there's a lot of overlap... and even the classification can be tricky because the boundaries between the classifications are continuous, not discreet.  Anyway, a subtropical storm doesn't look like the classic symmetric, compact thing we associate with a tropical storm, but the impacts can be the same.  In other words, don't downplay a storm just because it's labeled "subtropical".

Nicole is presently centered about 450 miles north of Hispaniola, and 650 miles east of Miami, Florida.  The wind field is very large, but also very lopsided (that's part of the subtropical classification).  It is expected to transition to a tropical storm by Wednesday, and that means it should become more symmetric, but the wind field will still be expansive.

As always, do NOT use the "cone of uncertainty" for anything other than uncertainty in the track forecast. Notice that right now, almost the entirety of tropical storm conditions (the orange blob) is outside of the cone.

And it's that expansive wind field that is the big problem in the coming days.  We are looking at several days of significant coastal flooding from North Carolina to Florida from the storm, but then we also have already-high tides for a few days around Tuesday's full moon.  This is going to be a big deal, so it shouldn't be underestimated because it's "only" a tropical storm.  The animation below is a forecast of significant wave height (shaded contours), surface wind (maroon barbs), and swell direction (green arrows).

This product is updated every six hours; you can find the most recent at http://bmcnoldy.rsmas.miami.edu/wavewatch/

Tropical storm watches are in effect for extreme southeast and northeast Florida, with hurricane watches for the northern Bahamas and central and south Florida (from Broward County north).  Based on the 10am EST forecast, the tropical storm wind speed probabilities and earliest reasonable time of arrival are shown below.  

The ensemble spread in track forecasts spans from the Florida Keys to Cape Canaveral, with the most likely in the West Palm Beach area.  Impacts will be worse north of the center, and not as bad south of the center, so if it does end up making landfall in the West Palm Beach area, Miami and Fort Lauderdale would fare much better than places like Melbourne, Daytona, and Jacksonville.

Finally, in addition to the widespread threat of tidal flooding, Nicole will deliver a long swath of heavy rain from Florida up to New England over the next five days.