After dropping back to a Category 3 strength storm Wednesday night, Gonzalo has re-strengthened into a strong Category 4 storm. Maximum sustained winds as of 11am Thursday were at 145mph and the minimum central pressure was down to 940mb. The storm exhibited an impressive-looking eye structure after completing an eyewall replacement cycle Wednesday night. The satellite picture showed a very symmetrical looking storm with a classic convective structure.
Sea surface temperatures are fairly warm under Gonzalo, which has allowed it to maintain intensity (and even strengthen). Those sea surface temperatures will start to drop off a bit as the storm approaches Bermuda, but will still likely remain warm enough to support major hurricane intensity. In addition, the wind shear is favorable for the intensity of the storm. An analysis of the wind shear below (courtesy University of Wisconsin) shows Gonzalo in an area of 15kt northwesterly shear, which is moderate. However, the wind shear at the mid-levels of the atmosphere (second image below) is much lower, closer to 5kts or so and is likely helping to intensify Gonzalo.
In addition to the lower mid-level shear, the shear tendency downstream of Gonzalo is favorable for a storm of this intensity (image via University of Wisconsin). Shear tendency measures the pattern of wind shear over the last 24 hours (lessened or strengthened) and shows Gonzalo moving into an environment that has seen the wind shear decrease by 5kts in the last 24 hours. At the same time though, notice how wind shear has increased over Bermuda, as well as over the Eastern seaboard (thanks to a frontal system). Therefore, while conditions will remain favorable for Gonzalo in the short term, beyond 24-36 hours the shear should increase significantly enough (and the sea surface temperatures should cool down enough) to allow Gonzalo to weaken.
While it is increasingly evident at this point that Gonzalo will be a major hurricane as it passes near Bermuda, the key for the island will be the exact track. The current forecast calls for the storm to pass just 20 nautical miles to the west of Bermuda, putting the island on the dangerous east side of the storm and well within striking distance of the powerful winds in the eyewall. However, just a slight shift west or east in track would make a huge difference.
There are several major effects that a storm track like this would have on Bermuda. First and foremost, winds will be intense. Hurricane force winds extend outward up to 45 miles from the eye of Gonzalo, and tropical storm force winds extend outward up to 150 miles. The latest advisory from the NHC gives Bermuda a 74% chance of seeing hurricane force winds from Gonzalo and this number may go up as we get closer to these potential effects. Tropical storm force winds will likely begin by Friday morning, and will intensify to hurricane force by Friday afternoon/evening as the storm passes by the island.
Another effect from Gonzalo will be wave heights and storm surge. On the eastern side of the storm, Bermuda will experience the worse of the storm surge. Elsewhere, high wave heights will be a big issue. Maximum wave heights from the storm may approach 50 feet in areas, and even the East Coast of the US will see increased swells and a higher rip current risk.
The final impact will be significant rainfall. With Gonzalo moving fairly slowly at the moment, rainfall amounts on Bermuda may be 3″-6″+. The latest run of the 12km RPM model shows upward of 6″ of rain across the entire island by Friday night.
With most of the attention on Gonzalo, we can’t lose sight of another dangerous storm, Ana in the Central Pacific. As of 11am Thursday, Ana was a tropical storm with maximum sustained winds of 60mph. Ana has suffered from higher wind shear over the last couple days. However, the latest enhanced satellite imagery shows a slightly improved structure to Ana, along with some deeper convection near the center.
The official track for Ana takes it to the northwest, eventually curving it just southwest of the Big Island as a Category 1 storm Friday night. It then recurves it back toward the islands, taking it very close to Oahu, Kauai, and Niihau. This would be the 3rd tropical storm this season to come close to or impact the Islands (Iselle and Julio). It is important to note, however, that track errors for storms in the East and Central Pacific is much higher than for those in the Atlantic.