Lorenzo Muscles Its Way to Category 5 Strength

In a remarkable burst of rapid intensification, Hurricane Lorenzo vaulted to Category 5 status late Saturday, becoming the Atlantic’s second Cat 5 storm of the year, the strongest hurricane ever observed so far east in the Atlantic, and one of the northernmost Cat 5s on record. As of 11 pm EDT Saturday, Lorenzo was packing top sustained winds of 160 mph. Update: Lorenzo's expected weakening trend is now under way. As of 11 am EDT Sunday, Lorenzo's top sustained winds are down to 145 mph, making it a Category 4 storm. Lorenzo will likely be a Cat 3 by Sunday night or Monday.

With Lorenzo and Dorian, this year is now one of only seven years in Atlantic hurricane records going back to 1851 that have produced at least two Category 5 storms. None of those years produced more than two Cat 5s except for 2005, which yielded four (Emily, Katrina, Rita, and Wilma).

The last 20 years have seen a striking clustering of Category 5 storms in the Atlantic, as noted by Sam Lillo (University of Oklahoma):

1999-2002 (four years):  no Cat 5s
2003-2007 (five years):  eight Cat 5s
2008-2015 (eight years):  no Cat 5s
2016-2019 (four years):  six Cat 5s and counting

The past 24 hours of #Lorenzo, which is now a category 5 hurricane, the farthest east category 5 Atlantic hurricane that we know of. pic.twitter.com/j33A5RTRM9

— Kim Wood (@DrKimWood) September 29, 2019

Lorenzo’s sudden strengthening took even veteran hurricane forecasters by surprise. Predictive tools such as the Saturday-morning run of the SHIPS model showed virtually no chance of rapid intensification. In its forecast issued at 11 am EDT Saturday—at which point Lorenzo was a low-end Category 3 storm with 115-mph top winds—the National Hurricane Center predicted that the hurricane would be slowly weakening from that point onward.

Instead, Lorenzo took advantage of well-above-average sea-surface temperatures (up to 1°C or 1.8°F warmer than usual for this time of year), as well as a window for strengthening made possible by the completion of an eyewall replacement cycle and the “new” eye that resulted.

Location of where all Atlantic Category 5 #hurricanes first reached Category 5 strength. #Lorenzo is highlighted in blue. pic.twitter.com/dpQfnEXPty

— Philip Klotzbach (@philklotzbach) September 29, 2019

Lorenzo’s peak winds increased by 45 mph in just 12 hours on Saturday, well beyond the benchmark for rapid intensification of 30 knots (35 mph) in 24 hours. This feat was made even more impressive by Lorenzo’s vast size, which typically makes it harder for a hurricane to strengthen or weaken quickly.

The early phase of Lorenzo’s Saturday strengthening, which took it to Category 4 strength, was confirmed by NOAA Hurricane Hunters who were probing the storm for a research project. Those reconnaissance reports helped lend confidence to subsequent satellite-based estimates that were emphatic on Lorenzo’s having reached Category 5 strength. Lorenzo and Isabel (2003) are the only two hurricanes in Atlantic history whose Category 5 strength was not confirmed by reconnaissance data or surface observations.

#Lorenzo is a category 5 hurricane, over 600 miles east-northeast of the previously furthest east category 5s on record - Isabel and Hugo.

The forecast track takes Lorenzo well east of all cat 4/5 hurricanes. pic.twitter.com/I6cN95oywm

— Sam Lillo (@splillo) September 29, 2019

Lorenzo an increasing threat to the Azores

Were it tracking a few hundred miles further west—more in line with climatology—Lorenzo would be safely avoiding land areas. As it happens, Lorenzo may end up being among the strongest storms on record to threaten the Azores. Hurricanes and tropical storms affect the islands every few years, but these are typically weakening under the influence of strong midlatitude winds and relatively cool waters (sea surface temperatures are typically around 23°C or 73°F in late September). As usual, these factors will cause Lorenzo to weaken considerably before it reaches the Azores, but because of the unusually warm SSTs between Lorenzo and the Azores, and a favorable jet stream configuration—as well as Lorenzo’s current size and strength—this weakening process will take time.

Figure 1. Probability of sustained winds of tropical-storm strength (39 mph) along the track of Lorenzo as predicted at 0Z Sunday, September 29, 2019 (8 pm EDT Saturday). The northwesternmost Azores islands of Corvo and Flores lie closest to the predicted path of Lorenzo. Image credit: NHC.

NHC predicts that Lorenzo will approach the Azores on Wednesday as a strong Cat 1 or weak Cat 2 storm. The hurricane’s wind field will gradually broaden as the very strongest winds decrease. By Tuesday, tropical-storm-force winds are predicted to span about 450 miles from northwest to southeast.

Track models late Saturday indicated that the most likely part of the archipelago to experience hurricane-force winds would be the northwesternmost Azores islands, Corvo and Flores. Together, these islands hold only about 4500 of the Azores’ 250,000 residents. NHC predicts that these islands have about a 30-40% chance of getting peak winds of hurricane strength and a greater-than-90% chance of tropical-storm-force winds. However, ensemble model runs show that there is enough east-to-west uncertainty that all of the Azores need to monitor Lorenzo. The hurricane’s vast size means that many parts of the Azores could experience potentially damaging tropical-storm-force winds and significant disruption.

Tropical cyclones affect the Azores about every three years on average, according to hurricanecity.com. Hurricanes that make it this far northeast are typically no stronger than Category 1, although a 1926 hurricane made landfall on São Miguel island near Ponta Delgada while at peak Category 2 strength (105-mph winds). On January 15, 2016, Hurricane Alex—the Atlantic’s first January hurricane in more than 60 years—struck Terceira island as a high-end tropical storm. In October 2017, Ophelia passed well south of the Azores as a Category 3 hurricane, producing tropical storm-force winds in the islands, according to the NHC's final report. For more on Azores tropical cyclone climatology, see Jon Erdman’s weather.com writeup.

By late next week, Lorenzo will be an exceptionally large and strong post-tropical storm. Track uncertanties at this point are large, but a post-tropical Lorenzo could approach Ireland and the United Kingdom around Thursday with very heavy rains and winds well into the tropical-storm range. Update: See this Sunday-morning Twitter thread from Sam Lillo for a detailed discussion of the track uncertainties. In a nutshell, a stronger, faster Lorenzo would tend to track well northwest of Ireland and the UK, while a weaker, slower Lorenzo may reach Ireland/UK.

Latest ECMWF showing potential impacts by #Lorenzo for Ireland and the U.K. This particular ECMWF run does not fully absorb Lorenzo into the mid-latitudes, allowing the storm to maintain some of its strong winds. This is something to watch. https://t.co/K8AM7wTwYy

— Michael Ventrice (@MJVentrice) September 28, 2019