Bertha Moving Through Mona Passage


Tropical Storm Bertha RGB satellite image near Advisory 7a time. Blue arrows are surface wind streamlines, red arcs indicate the outflow boundaries/gust fronts associated with convective collapse (dry air), surface circulation is the blue circle, and an estimated mid-level circulation is the green circle.
Tropical Storm Bertha RGB satellite image near Advisory 7a time. Blue arrows are surface wind streamlines, red arcs indicate the outflow boundaries/gust fronts associated with convective collapse (dry air), surface circulation is the blue circle, and an estimated mid-level circulation is the green circle.

Tropical Storm Bertha has changed little in structure since yesterday. The storm is barely meeting criteria for Tropical Cyclone classification at the moment. This is because Bertha barely has a closed circulation with only a small area of westerly winds assumed to exist to the south of the center. Further, the surface center of Bertha is estimated to be over the Mona Passage, while radar out of San Juan, Puerto Rico suggests a better defined mid-level circulation over southwestern Puerto Rico. Despite the continued disorganization, or even deterioration in organization, Bertha has been able to maintain deep convection closer to the center all day today, whereas Bertha’s center was an exposed low cloud swirl yesterday. The sustained convection suggests Bertha is moving into a more moist environment, but the surface and mid-level circulations being displaced from one another suggests that vertical wind shear is playing a role in hindering organization.

Dry air near Bertha has led to the collapse of showers and thunderstorms near the center of the storm, but that convection was quickly replaced. However, low-level outflow from this collapsing convection may be playing a role in keeping the surface circulation weak/disorganized and displaced from the mid-level circulation. The low-level outflow boundaries are also sparking new convection near the center of Bertha because of convergence along them.

Radar out of Puerto Rico with Bertha's center marked. Rain with embedded heavier showers/squalls over Puerto rico, mainly to the north and east of Bertha's center.
Radar out of Puerto Rico with Bertha’s center marked. Rain with embedded heavier showers/squalls over Puerto rico, mainly to the north and east of Bertha’s center. Image captured from Wunderground’s Wundermap.

Heavy rains with isolated squalls are affecting the British and US Virgin islands, Puerto Rico, and eastern parts of the Dominican republic this afternoon where tropical storm warnings are in effect. These near tropical storm conditions will quickly spread to the Turks and Caicos islands and southeastern Bahamas tonight (also under tropical storm warnings) and Sunday while subsiding in the Leeward islands as Bertha continues a fast forward progression at 22mph. Bertha’s main impact will continue to be heavy rain and the threat for flash flooding and mudslides.

Regardless, Bertha is moving back into the southwestern Atlantic this afternoon where it will continue to be steered west-northwestward around the southern side of the deep layered Bermuda-Azores ridge. Bertha should begin to turn more northwestward as it approaches a weakness/break in the ridge over the western Atlantic tonight. That turn will continue with Bertha turning northward Sunday night, then northeastward  Tuesday – after passing between Bermuda and the US East coast.

As Bertha moves away from the Greater Antilles and the Bahamas, it will move into a more moist, less stable environment with less vertical wind shear. Further, the sea-surface temperatures in this region are warmer than climatology, near 30C, and are well above the typically accepted 26C threshold for genesis and maturation of Tropical Cyclones. In other words, Bertha is moving into a more favorable environment. So despite the current state of disorganization, Bertha has an opportunity to strengthen, potentially to a hurricane, while passing to the distant west of Bermuda on Monday and Tuesday.

For the latest official information on the tropics, see the National Hurricane Center.

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