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Above: GeoColor satellite image of Tropical Storm Karen at 1500Z (1 pm EDT) Wednesday, September 25, 2019. Image credit: RAMMB/CIRA/CSU.

The broad center of Tropical Storm Karen moved across the eastern Puerto Rico islands of Vieques and Culebra between 5 and 6 pm EDT Tuesday, when the storm had top sustained winds of 45 mph. In the U.S. Virgin Islands, Karen brought wind gusts as high as 43 mph to St. Thomas and 37 mph to St. Croix; Puerto Rico recorded a 29 mph wind gust at Vieques and 24 mph at the San Juan Airport.



Tropical storm warnings have been dropped for all of the islands, though NHC advises that Karen could bring another 1 – 2” of rain to Puerto Rico and the Virgin Islands before the storm pulls away. No major damage has been reported from Karen’s rains thus far, and the rains may end up being beneficial for Puerto Rico, since southern portions of the island were in moderate to severe drought.

Karen dumped heavy rains in excess of four inches to portions of Puerto Rico and the U.S. Virgin Islands. As of 10 am EDT Wednesday, personal weather stations IDORADO3 and KTOABAJA7 on the west side of San Juan recorded 2-day rainfall amounts of 5.15” and  5.46”, respectively; KCHRIST17 on St. Croix had received 4.36”.




Long-term outlook for Karen

Karen remains a highly disorganized tropical storm—minimal at best—with fragmented showers and thunderstorms (convection) pulsing and fading near its center.

The forecast for Karen is straightforward for a day or two, then gets increasingly murky into the weekend and beyond. Karen will continue cruising north to north-northeast at a steady clip (15 mph at 11 am EDT Wednesday) until late Thursday, when the complications begin. The steering currents driving Karen—an upper low near the Bahamas and an upper high in the central Atlantic—will take a back seat to a strong upper high developing around Karen toward the weekend. Models agree that Karen’s forward motion will stall as the steering currents collapse. The latest NHC forecast depicts Karen making a small clockwise loop before embarking on a west-southwest track Saturday and Sunday as the upper high builds near and to the north of Karen, blocking recurvature. On such a course, Karen would approach the Bahamas about a week from now and could theoretically continue onward into Cuba or Florida later next week. Such a major left-hand turn leading to a U.S. landfall would be unusual, but not entirely implausible; other tropical cyclones have taken large loops off the Southeast U.S. coast, including Jeanne (2004) and Betsy (1965).

Karen may not even exist by next week, though. The 0Z Wednesday runs of our top global track models—the GFS, European, and UKMET—keep Karen weak or dissipate the storm entirely as it heads westward next week. Most of the GFS ensemble runs keep Karen moving slowly across the Northwest Atlantic before dissipating, and less than 20% of the European ensemble runs maintain Karen as a tropical cyclone long enough to reach Florida or Cuba more than a week from now. In contrast to the global track models, the statistical tools used by NHC—such as the SHIPS model—support the idea of Karen becoming a strong tropical storm or minimal hurricane and staying organized through at least five days.

“It’s difficult to ignore what’s being shown by the global models, since there must be something in the environment that they’re deeming to be negative for continued strengthening,” said hurricane specialist Robbie Berg in the 11 am EDT Wednesday NHC discussion.

Levi Cowan (Florida State University) notes that the upper-level high forming over Karen will induce sinking air that will tend to inhibit thunderstorm activity. If Karen’s structure improves, it might be able to overcome this broad-scale negative, according to Cowan: “If a robust, deep vortex manages to form for any reason, it can self-sustain from its own surface fluxes, and suddenly flow deformation and large-scale sinking start mattering less.”

The multi-part tweet below from Cowan has some excellent insight on the large-scale environment evolving around Karen and the implications for the storm over the next few days.



Post-Tropical Cyclone Jerry headed towards a brush with Bermuda

A Tropical Storm Warning is up for Bermuda as Post-Tropical Cyclone Jerry heads east-northeast at 10 mph towards the island. At 11 am EDT Wednesday, Jerry was located 120 miles west of Bermuda, with top winds of 40 mph. A few modest rain showers from Jerry were affecting the island, as seen on Bermuda radar.


Visible image of Jerry at 1530Z 9/25/19
Figure 1. Post-Tropical Cyclone Jerry approaching Bermuda at 1530Z (9:30 am EDT) September 25, 2019. Image credit: NOAA/RAMMB.


Satellite loops early Wednesday afternoon showed that Jerry was no longer a tropical cyclone, since it lacked the characteristic heavy thunderstorm activity. Thanks to moderate to high wind shear and cool sea surface temperatures near 26.5°C (80°F), Jerry is expected to continue to weaken and spin down as it moves past Bermuda Wednesday evening and into the open Atlantic.

On Jerry’s current track, the center is likely to pass less than 70 miles to the northwest of Bermuda on Wednesday evening. Since tropical storm-force winds are predicted to extend out about 70 miles to the northeast of Jerry’s center at that time, Bermuda could see tropical storm-force winds, in addition to about 1” of rain. The 11 am EDT Wednesday wind probability forecast from NHC gave Bermuda a 67% chance of experiencing tropical storm-force winds.

Jerry will be the second named storm to affect Bermuda in the past week; on September 18, Hurricane Humberto passed just to the northwest of the island as a Category 3 hurricane with 120 mph winds. Humberto’s powerful right-front eyewall passed over Bermuda, bringing sustained hurricane-force winds and knocking out power to about 80% of the island. The Bermuda airport recorded sustained winds of 82 mph, gusting to 116 mph, at the height of the storm, and the Bermuda Maritime Operations Center (at an elevated location) recorded a wind gust of 144 mph. No injuries or major damage from Humberto were reported in Bermuda.


Infrared image of Lorenzo at 1705 9/25/19
Figure 2. Infrared image of Hurricane Lorenzo at 1705Z (1:05 pm EDT) Wednesday, September 25, 2019. Image credit: tropicaltidbits.com.


The latest Atlantic hurricane is a biggie

Entraining a massive swath of tropical moisture on its east and south sides, Tropical Storm Lorenzo became the Atlantic’s fifth hurricane of the year at 5 am EDT Wednesday. Hurricane Lorenzo gives the Atlantic 12 named storms, 5 hurricanes, 2 intense hurricanes, and an ACE index of 87 so far in 2019. An average season typically has 9 named storms, 5 hurricanes, 2 intense hurricanes, and an ACE index of 76 by September 25.

With light wind shear (only around 5 knots) and warm sea-surface temperatures (27-28°C or 81-82°F) into Thursday night, Lorenzo has a near-ideal environment for a burst of strengthening. The SHIPS Rapid Intensification Index gives Lorenzo a 47% chance of becoming a Category 3 storm (100-knot or 115-mph sustained wind) by Thursday and a 28% chance of becoming a Category 4 storm (120-knot or 140-mph sustained wind) by Friday. Lorenzo has already missed its chance to overtake Ophelia (2017) as the easternmost Category 3 storm on record in the Atlantic, but it does have a chance of becoming the easternmost Cat 4. That record is held by Carrie (1957), which hit Cat 4 strength at 42°W.

Models agree that Lorenzo will carry out a classic recurving path over the central and eastern Atlantic, most likely between 40° and 50°W, without any threat to land areas. Lorenzo will be a massive wave generator, though, due to its anticipated intensity, longevity, and size. Already, Lorenzo’s tropical-storm-force winds extend close to 350 miles from northwest to southeast. That breadth may approach 370 miles by Thursday.

Elsewhere in the tropics, NHC gives a cluster of convection moving off Mexico’s Yucatan Peninsula a 10% chance of development as it moves across the Bay of Campeche over the next 2-3 days. The Eastern Pacific is free of named storms for the first time in a long time, and it may be several days before the next one develops. There are no reports of major damage from Cyclone Hikaa (below), an unusual hurricane-strength cyclone that made landfall in the Arabian Peninsula. The cyclone struck the eastern coast of Oman as a Category 1 storm on Wednesday night local time, according to the India Meteorological Department.


Cyclone Hikka at 0645Z 9/24/19
Figure 3. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this image at 10:45 a.m. Gulf Standard Time (0645Z) on Wednesday, September 24, 2019, as Cyclone Hikaa’s outer bands moved over Oman. Image credit: Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview.



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