Beryl Significantly Weaker as We Expected

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Above: Visible satellite image of Beryl from 9 am EDT Saturday, July 7, 2018. Beryl’s circulation center was exposed to view, with just one clump of heavy thunderstorms on the southeast side of the center–the classic appearance of a storm undergoing wind shear.

Tiny Hurricane Beryl suffered a significant disruption to its core on Saturday morning, resulting in the collapse of its eyewall and the loss of most of its heavy thunderstorms. Satellite images on Saturday morning showed Beryl’s circulation center was exposed to view, with just one clump of heavy thunderstorms on the southeast side of the center. That’s the classic appearance of a storm undergoing wind shear, and it is likely that strong mid- to upper-level winds out of the northwest have been driving dry air into Beryl’s circulation, disrupting the storm. The 8 am EDT Saturday SHIPS model run diagnosed a modest increase in wind shear to 10 knots, but with a small storm like Beryl, it doesn’t take much wind shear to cause a significant disruption–if the stronger winds tearing at the storm are advecting in dry air. The SHIPS model also diagnosed drier air surrounding the storm–an average mid-level relative humidity of 55%, a signature of the dryer air to Beryl’s north that was able to infiltrate its core. Beryl’s track did wobble more to the west-northwest overnight, bringing the storm closer to the dry air to its north, which may have aided in the dry air intrusion.

Irma and Beryl
Figure 1. A comparison of MODIS images of Hurricane Irma from September 6, 2017, when it was a Category 5 storm with 180 mph winds over the Virgin Islands, with Hurricane Beryl from July 6, 2018 (inset, upper left), when it was a Category 1 hurricane with 75 mph winds. Irma dwarfed Beryl in size by a factor of five. Irma’s hurricane-force winds extended out up to 50 miles from the center (vs. 10 miles for Beryl), and Irma’s tropical storm-force winds extended out up to 185 miles from the center (vs. 35 miles for Beryl). Image credit: NASA.

Beryl is a very small storm, with the 8 am Saturday NHC advisory warning of hurricane-force winds extending out up to 10 miles from the center, and tropical storm-force winds extending out up to 35 miles from the center. This makes Beryl not much bigger than the smallest Atlantic named storm on record, Tropical Storm Marco of 2008, which had tropical storm-force winds extending out just 12 miles from its center on October 7, 2008, when it was at peak intensity (65 mph sustained winds) in the southern Gulf of Mexico. Beryl had an eye just 6 miles in diameter Saturday morning. The Hurricane Hunters are not scheduled to investigate Beryl until Sunday morning, since the storm is too far from any land bases for their aircraft to fly out of.

Beryl forecast
Figure 2. Predicted winds for Beryl at 2 am EDT Monday, July 9, 2018 from the 6Z Saturday Jul 7, 2018 run of the HWRF model. Beryl was predicted to be moving through the Lesser Antilles between Dominica and Guadaloupe with top winds near 45 kt (52 mph, yellow colors). This is about 5 mph weaker than the previous four runs of the model. The HWRF was our best intensity model in 2017. Image credit: Levi Cowan, tropicaltidbits.com.

Forecast for Beryl: 45 – 50 mph winds in the Lesser Antilles

The ridge of high pressure steering Beryl will strengthen this weekend, causing the hurricane to accelerate to a forward speed near 20 mph by Sunday. This motion should bring it into the Lesser Antilles Islands on Sunday night, with Dominica and Guadaloupe at the highest risk of receiving a direct hit. The north side of Beryl will have much stronger winds than the south side, due to the rapid forward motion of the storm. Due to Beryl’s small size, though, only one island is likely to receive significant winds, and the storm could slip between a gap in the islands without bringing its strongest winds to any of the islands.

Beryl’s intensity is very hard to predict over the next day, since tiny storms like Beryl can rapidly strengthen or weaken due to small changes in their environment. It is quite possible Beryl could re-develop an eyewall by Sunday morning and once again build a solid ring of heavy thunderstorms to wall off the dry air to its north, since wind shear, mid-level moisture, and SSTs will be reasonably favorable for development until then.

Beryl will encounter a much more hostile environment on Sunday morning, when the storm will encounter a region of high wind shear associated with westerly winds from the subtropical jet stream. At that time, Beryl will also be moving with a faster forward speed, which will increase the wind shear for the storm. Beryl will also be entering a region of dryer air with a mid-level relative humidity near 50%. Ocean temperatures will warm a bit, to 27.5°C (82°F), but the one-two punch of high wind shear and dry air is likely to significantly disrupt a small storm like Beryl, making it a disorganized tropical storm by the time it reaches the Lesser Antilles Islands on Sunday night. Our top intensity models–the HWRF, LGEM, and DSHIPS models–predicted in their 6Z and 12Z runs from Saturday morning that Beryl would be a tropical storm with 45 – 50 mph winds at 2 am Monday, when its core is expected to be moving through the Lesser Antilles. I expect that this will be a good forecast, and that wind damage will not be the main concern from Beryl. Heavy rains are likely to be the main concern, since even a disorganized tropical storm can bring dangerous rains to the islands. We saw this occur in 2015, when Tropical Storm Erika, with top winds of just 50 mph, brought a $500 million flood disaster to Dominica, killing 31 people. Beryl is not as large and as wet as Erika was, though, so the odds of an Erika-like flooding disaster are likely to be less than 30%.

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