Bertha now a Hurricane

Aug 4 14 1515UTC Bertha
Hurricane Bertha near (27.6N, 73.6W) with 80mph winds. A small eye-like feature can be seen near those coordinates – but this could be a result of dry air, or a shadow from overshooting tops of deep convection in the vicinity, not necessarily an eye.

Hurricane hunter aircraft investigating Bertha found a small area of hurricane force winds of near 70kts to the east of the center in a partial eye-wall feature. For that reason, the National Hurricane Center has classified Bertha as the second hurricane of the 2014 Atlantic Hurricane Season. Otherwise, Bertha had maintained deep convection near its center all night and this morning, but convection was lacking on the north and west side of the center where dry mid-level air and subsidence are both hindering convective development. As of the 11am advisory, Bertha has maximum 1-minute sustained winds of 70kts (80mph), and a minimum central pressure of 998mb. Bertha is moving due north at 17mph.

A turn in Bertha’s track to the northeast is expected in the next day or so as it is steered in the southerly to southwesterly flow around the deep layer Bermuda-Azores high centered to the east of Bermuda. This track takes Bertha almost evenly between Bermuda and the Outer Banks of North Carolina.  Bertha is expected to continue to accelerate northeastward and be near southern Newfoundland Thursday morning, where it then gets caught in mid-latitude westerly flow and heads eastward towards the central north Atlantic.

Aug 4 14 1515UTC Bertha SST Wu
Sea-Surface Temperatures as displayed on Wunderground’s Wundermap with Bertha’s current location and forecast track from the Hurricane layer.

Bertha is over very warm open ocean waters now, with sea-surface temperatures near 30°C (86°F). Further, while vertical wind shear isn’t ideal, it is much lighter than it has been and equatorward upper level outflow has become well established. But, again, the dry and subsident air near to the northwest of Bertha is hindering the spread of convection to that quadrant of the storm. Overall, Bertha’s environment has improved quite a bit and is supportive of a hurricane, perhaps with some additional strengthening possible in the next 24hours. After that period, wind shear over Bertha will increase and Bertha will move over progressively cooler seas. These combined will lead to the gradual weakening of Bertha as it accelerated northeastward. Bertha should begin to transition to a post-tropical cyclone Wednesday night and pass to the south of Newfoundland as a strong post-tropical cyclone Thursday morning, then continue eastward into the central north Atlantic, weakening. For the latest official hurricane information see: The National Hurricane Center.

As Bertha becomes post-tropical Wednesday night, it will bring a trough over/near Bermuda.  The Bermuda-Azores high shifts more towards the Azores, with a ridge extending to the south of Bermuda allowing this trough to linger and bring unsettled weather to Bermuda. This will introduce shower and possibly thunderstorm activity in the area Wednesday night through Saturday with moderate to strong southwesterly winds that veer more westerly on Friday. Swells from Hurricane Bertha are expected to reach Bermuda Tuesday night, building to 6-9ft on Wednesday and then decreasing Wednesday night, this may lead to hazardous sea states for a time on Wednesday. For the latest official forecast for Bermuda see: Bermuda Weather Service.

Aug 4 14 1530UTC Iselle
Hurricane Iselle at sunrise in the East Pacific (about to cross into the central Pacific area of responsibility) with 120kt (140mph) 1-minute sustained winds, making this a category four hurricane.

Meanwhile, Hurricane Iselle is a potential threat to Hawaii as it crosses into the central Pacific. Iselle is a powerful category 4 hurricane with (120kt) 140mph 1-minute sustained winds. Iselle has few banding features and a large eye  – these are some of the structural characteristics of Annular hurricanes. Annular hurricanes tend to not undergo eye-wall replacement cycles because of this structure. Eye-wall replacements are cyclic structural changes within the core of tropical cyclones that lead to weakening, and makes cyclones vulnerable to dry air and wind shear which can lead to further weakening. However, Iselle is moving into an environment with gradually decreasing sea surface temperatures and that is expected to lead to gradual weakening of this hurricane into a tropical storm before it reaches Hawaii in about four days. Hurricane Hunter reconnaissance is scheduled for Iselle tomorrow afternoon.




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