Jeff Masters Weather Blog

Norma and Tammy gain strength en route to Mexico, Leeward Islands » Yale Climate Connections


Two potentially impactful tropical cyclones, both of which could reach land areas as hurricanes over the next several days, were roiling the waters on Thursday west of Mexico and east of the Leeward Islands: Hurricane Norma in the eastern Pacific and Tropical Storm Tammy in the Atlantic.

Norma vaults to category 4 strength off Mexico’s Pacific coast

Hurricane Norma rocketed from tropical storm to category 4 strength from Wednesday to Thursday morning as it moved gradually closer to the southernmost Baja Peninsula of Mexico. Norma’s peak winds jumped from 70 to 130 mph between 11 a.m. EDT Wednesday and Thursday, which was almost twice the National Hurricane Center’s criterion for rapid intensification (30 knots or 35 mph in 24 hours). This burst of strengthening was well predicted by the center as well as by the new HAFS intensity model, while the excellent European and UKMET long-range operational models largely missed the boat — a good reminder that track models like the Euro, UKMET, and GFS should not be taken as gospel when it comes to intensity.

Norma is the seventh major hurricane of the 2023 Eastern Pacific season, which compares to four major hurricanes in a typical entire season (1991-2020). All of 2023’s major hurricanes except one (Calvin) attained category 4 strength, and one of them (Jova) reached category 5. Hurricane seasons are often boosted in the eastern tropical Pacific when an El Niño is strengthening, as has been the case this year.

As of 11 a.m. EDT Thursday, Norma was positioned about 410 miles south-southeast of Cabo San Lucas, moving north at 7 mph. A Hurricane Watch was in effect for the southern tip of Baja California from Todos Santos on the west to Los Barriles on the east, including the Cabo San Lucas area. A Hurricane Hunter flight was scheduled to investigate Norma on Thursday afternoon.

Norma will likely reach its peak intensity on Thursday, and in fact that peak may have already occurred. Over the next several days, Norma will be plagued by increasing wind shear (15-25 mph on Thursday to 30-40 mph by Sunday) and increasingly dry air (mid-level relative humidity dropping from around 60% to 30-40%), even as it traverses unusually warm, El Niño-boosted sea surface temperatures of around 30 degrees Celsius (86 degrees Fahrenheit). The National Hurricane Center predicts that Norma will weaken to category 1 strength by Saturday, well before any potential landfall.

Norma’s track forecast continues to be complex, as discussed in our Wednesday post. As long as Norma maintains a strong, coherent circulation, its motion will be driven mainly by upper-level steering flow. This would keep it rolling toward the north and eventually northeastward toward the Mexico’s Pacific north of Mazatlan. As Norma weakens, though, there’s an increasing chance it will be affected by lower-level trade winds that would tend to drive it westward, away from Mexico. Norma will likely reach an inflection point as it nears southernmost Baja California on late Friday into Saturday. From that point, as shown in Figure 1 below, Norma may either accelerate toward the mainland Mexico coast (as depicted by the GFS) or stall and degenerate just south of Baja California (as depicted by the Euro and UKMET). The more quickly Norma moves, the better chance it will have to arrive as a stronger tropical storm or hurricane. On the other hand, a stalling out could increase the risk of flooding nearby.

Figure 1. Tracks of ensemble model output from the European (red), GFS (green), and UKMET (dark blue) model runs for Norma from 0Z Thursday, October 19. The thick black line shows the average track of all three ensembles. (Image credit: Tomer Burg)

If Norma stalls south of Baja California, its main impact will be swells and high surf over the next couple of days. Torrential rains and dangerous flash flooding will be possible in southernmost Baja, especially on east-facing terrain, with the potential for localized amounts of 15 inches of rain or more. There’s high uncertainty in this higher-end rainfall outlook, hinging on how close Norma gets, how slowly it moves near the peninsula, and how much its rain-making potential is eroded by dry air. If Norma accelerates toward mainland Mexico, the rainfall and flood threat would shift to that area, especially near and south of any landfall location. On such a track, Norma would be moving at a faster clip, which would help to limit the flood and mudslide risk.

Lesser Antilles brace for a strengthening Tammy

Tropical Storm Watches and Warnings are up for much of the Lesser Antilles Islands, and a Hurricane Watch is up for Guadeloupe, as Tropical Storm Tammy gathers strength in the record-warm waters of the tropical Atlantic. Tammy is expected to be a strong tropical storm or category 1 hurricane when it passes near or over the Leeward Islands Friday night through Saturday night.

At 11 a.m. EDT Thursday, Tammy was located about 420 miles east-southeast of Guadeloupe in the Leeward Islands, headed west-northwest at 15 mph with top sustained winds of 60 mph and a central pressure of 1003 mb. The first hurricane hunter mission into Tammy Thursday morning found that the storm’s strongest winds were confined to the east side of the center, thanks to moderate wind shear of 10-15 winds out of the southwest. Satellite images showed that Tammy had a modest amount of heavy thunderstorms and was beginning to develop low-level spiral bands.

Figure 2. Track forecasts out to five days for Tammy from the 6Z Thursday, October 19, run of the GFS ensemble model (left) and European ensemble model (right). Individual forecasts of the ensemble members are the lines color-coded by the wind speed in knots they predict; red colors correspond to a category 1 hurricane. The models generally show that a stronger storm will miss the Leeward Islands, passing just to the northeast, while the predictions of a weaker storm tend to show a track through the islands, and a potential stall to the north of Puerto Rico. The time in hours from the model initialization time is in gray text. (Image credit: weathernerds.org)

Forecast for Tammy

Conditions will be favorable for development of Tammy over the next three days, with light to moderate wind shear of 5-15 knots, a reasonably moist atmosphere, and record-warm waters of 30 degrees Celsius (86°F), about 1-2 degree Celsius (1.8-3.6°F) above average. The system has plenty of support for development from the models, which generally agree that a stronger storm is likely to turn to the northwest and then north, out to sea, before reaching the Lesser Antilles, while a weaker storm is more likely to track farther west, potentially passing over or near the Leeward Islands on Friday night through Saturday night. None of the 6Z Thursday suite of regional hurricane models (HWRF, HAFS-A, HAFS-B, and HMON) showed sustained winds of over 75 mph (minimal Cat 1 hurricane strength) affecting any islands.

Given the continued westerly wind shear expected to affect Tammy during the next few days, most of the significant winds and rains of the storm will be to the east of the center, where heavy rains of 3-6 inches are predicted. Puerto Rico and the Virgin Islands are expected to be on the drier west side of Tammy, and receive lesser rain amounts.

An intensifying eastern U.S. upper low is expected to force Tammy to recurve northward into the open Atlantic after it passes through the Leeward Islands. However, if Tammy is on the weak side, the steering currents may not be strong enough to recurve it. In this case, Tammy might stall out a few hundred miles north of Puerto Rico and the Virgin Islands, subjecting them to a multi-day stretch of periodic heavy rain showers. The track forecast for Tammy is thus unusually high, and residents of the islands can expect potentially large changes to the forecast for Tammy in coming days.

Figure 3. All hurricanes that have passed within 250 miles of the island of Guadeloupe during the months of October and November in NOAA data extending back to 1851. (Image credit: NOAA Historical Hurricane Tracks)

A rare amount of activity for the Atlantic

Tammy’s formation brings the total 2023 activity in the Atlantic to 20 named storms (including an unnamed subtropical storm in January that was belatedly recognized), as well as 6 hurricanes, 3 major hurricanes, and an accumulated cyclone energy index of 131. The 1991-2020 averages for this date are 12.5 named storms, 6.1 hurricanes, 2.8 major hurricanes, and an accumulated cyclone energy index of 109. Only four other seasons since 1851 have had as many as 20 named storms: 2020, 2005, 2021, and 1933, which had 30, 28, 21, and 20 named storms, respectively. On average, 1-2 named storms form after October 19, so 2023 may end up with the third-highest number of named storms on record.

One hint that the record-warm sea surface temperatures have been driving action in the Atlantic this year despite less-than-ideal atmospheric conditions brought on by El Niño: Comparatively few of this year’s named storms have gotten very strong. In contrast to the high number of named storms, there have been six hurricanes and three major hurricanes this year, which are both close to the 1991-2020 averages to date.

It is unusual to get named storms in the tropical Atlantic east of the Lesser Antilles Islands this late in the year — let alone two, as has occurred with Sean and Tammy this year (see Tweet above). Only a few hurricanes have made it from the Main Development Region into the Lesser Antilles this late in any year, as shown in Figure 1. Sea surface temperatures in the region are typically marginal for supporting hurricanes by mid-October, and midlatitude jet-stream winds are often dipping further south, adding wind shear and hauling storms out to sea. While nearly all of the hurricanes shown in Figure 1 moved with an east-to-west component, one of them — Lenny — was famous for its contrary behavior. Nicknamed “Wrong-Way Lenny” for its west-to-east track, Lenny passed just south of St. Croix in the U.S. Virgin Islands on November 15, 1999, as a high-end Category 4 storm (see bright purple line at top left) with top sustained winds of 155 mph.

Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.





Source link