Saturday, July 7, 2018

Hurricane Chris (2018)

Storm Active: July 6-12

During the first couple days of July, an disturbance formed in the subtropical Atlantic southeast of Bermuda. Over the next few days, it moved slowly northwestward and environmental conditions gradually improved for development. On July 5, the system acquired a weak low-pressure center. This became someone better defined that day, but thunderstorm activity remained quite limited. It increased on July 6, however, and the system was classified Tropical Depression Three well offshore of the Carolinas.

That night, it turned toward the north and became somewhat more organized over the warm waters. Surface pressure were still high, however, and the system's maximum winds increased only slowly. Meanwhile, steering currents collapsed and the depression moved very little on July 7. Most thunderstorm activity was displaced south and southeast of the center, keeping coastal North Carolina, which was not far to the northwest, dry. A reformation of the surface circulation to the south allowed the system to organize further and strengthen into Tropical Storm Chris by early on July 8. The next day saw gradual strengthening as the circulation tightened, but dry air intrusion prevented Chris from closing off an eyewall. By that evening, the system was on the verge of hurricane strength and finally began to move slowly toward the northeast. Though it began to move away from land, high surf continued to pound the coastline.

Since it had been stationary for days, Chris had caused cold waters to upwell underneath it (due to its strong winds). Though this decrease in temperature was mitigated somewhat by the steady flow of warm Gulf stream waters, it slowed the cyclone's intensification. However, once it started moving, the system rapidly intensified. Late on July 10, it reached its peak intensity as a category 2 hurricane with 105 mph winds and a pressure of 970 mb. As it accelerated northeast, it began to encounter cooler water, weaken, and become less symmetric. Soon, it weakened to a tropical storm and was quickly transitioning to an extratropical system. Chris became fully extratropical on July 12 as it raced northeast over cold north Atlantic waters. The post-tropical cyclone made landfall in southeastern Newfoundland that night, bringing wind gusts to near hurricane force and brief but heavy rains.

The above image shows Hurricane Chris strengthening off the U.S. east coast. Chris was the earliest second hurricane to develop in the Atlantic since 2005.

Chris did not affect land directly as a tropical cyclone, but made landfall in Newfoundland as a post-tropical cyclone.

Thursday, July 5, 2018

Hurricane Beryl (2018)

Storm Active: July 5-8, 14-15

At the beginning of July, a tropical wave emerged off of the coast of Africa. This tropical wave was among the first of the season to develop significant convection over the central tropical Atlantic. Despite anomalously cool waters, conditions were still favorable enough in the deep tropics to support development. By July 4, the disturbance was quite well organized, with low shear in its environment and an evident circulation. Nevertheless, the low did not yet appear closed. The next day, organization increased further, and the system was classified as Tropical Depression Two. The depression was quite small and moving fairly quickly toward the west.

It is not uncommon for small cyclones to change rapidly in intensity, and the system strengthened quickly that evening and into July 6, becoming Tropical Storm Beryl just a few hours after formation. A minuscule pinhole eye appeared on satellite imagery and the maximum winds shot up to hurricane strength, making Beryl the first hurricane of the 2018 season by early on the 6th. The system strengthened a little bit more during that day, reaching a peak intensity of 80 mph winds and a pressure of 994 mb. Beryl turned a bit toward the west-northwest during the evening and into July 7. As it did so, it began to encounter increased wind shear. Quickly, convection was displaced toward the southeast and the center was exposed. As a result, Beryl began to quickly weaken and became a tropical storm. Convection flared up near the center from time to time over the following day but the low-level center all but disappeared as the cyclone accelerated west-northwest toward the Windward Islands. As shear continued to increase, Beryl quickly dissipated into a tropical wave.

Nevertheless, Beryl's remnants passed over the easternmost Caribbean islands on July 8, bringing heavy rains and strong winds out of the east. By the 9th, these rains had begun to move over Puerto Rico and the Virgin Islands. Overnight, the heavy rains moved over Hispaniola as the wave proceeded west-northwestward. The wave began to encounter more favorable conditions over the Bahamas, though the main impact to these areas was still locally heavy rain. After that, the disturbance turned northward and then northeastward, passing to the west of Bermuda.

Only on July 13 did the remnants of Beryl begin to show signs of reorganization, with a well-defined circulation developing. Despite marginal sea surface temperatures and atmospheric conditions, thunderstorm activity reappeared near the low-pressure center. On July 14, a full six days after dissipation, Beryl reformed. Though its windfield was concentrated near the center (typical for a tropical cyclone), the system lay under an upper-level trough. Therefore, it was deriving its energy in a manner uncharacteristic of a tropical cyclone. As a result, forecasters classified it as Subtropical Storm Beryl north of Bermuda.

The system slowed its forward motion and meandered somewhat over the next day. Vertical shear was increasing, but Beryl maintained enough convection in the southeastern quadrant to hang on to subtropical storm status. Even this was short-lived, however, and the system finally weakened into a remnant low late on July 15. This low approached Newfoundland before dissipating.

Beryl was one of the smallest hurricanes ever recorded; winds of hurricane force extended only 10 miles from the center. Note also the minuscule eye on satellite imagery.

A majority of Beryl's impacts occurred while the system was a wave moving through the Caribbean (triangle points).