Active Weather Pattern Develops


Aug 19 15 1445UTC
RGB Satellite imagery at 14:45UTC showing the cloudiness associated with shower and thunderstorm activity in a band running from North Carolina to just south of Bermuda.

Tuesday morning saw locally squally thunderstorms as a north-south oriented line of thunderstorms crossed parts of the island dumping up to unofficial amounts of 2″ of rain in highly isolated spots, however some areas, particularly in the western parishes, did not pick up nearly as much rain. Gusts in and around these thunderstorms generally reached 30-40 kts, an increase from the light and variable winds that had preceded the storms overnight. My PWS recorded 1.12″ of rain for the 18th of August, the Bermuda Weather Service had the same amount.

The line of thunderstorms that dominated the weather scene Tuesday morning formed along a weak surface trough and was supported by an upper level low that was sat to the northwest of Bermuda. This upper level low is slowly drifting southeastwards, towards the island and the best upper level divergence to support thunderstorm activity is also drifting southeastwards. Low level convergence associated with the surface trough in combination with the upper level low will support clusters of showers and thunderstorms that could pass over the island producing locally heavy rain and gusty winds at times with clearer periods.

A more organized surface trough is then expected to develop to the west of Bermuda. At this point, the upper low is expected to still be in the vicinity of Bermuda. The developing surface trough is then expected to stall near to the southwest of Bermuda overnight on Thursday. An area of low pressure could form from this surface trough near to the island as it starts to lift out of the area to the north as we approach the weekend. The interaction of the developing surface low and weakening upper low is a recipe for a potential subtropical transition.

8am 19 Aug 2015
National Hurricane Center chart showing the area being monitored for possible tropical or subtropical development – the yellow area indicates a low chance of a cyclone forming. Link in-text below for the latest chart.

Regardless, the formation of a surface low to the southwest of Bermuda will result in winds likely settling out of the southeast and gradually increasing to moderate/strong with gusts near gale force at times starting Thursday night. The National Hurricane Center has given this potential system a low (30%) chance of becoming a tropical or subtropical cyclone in the next 5 days as of 9am BDA time. The Bermuda Weather Service is also monitoring this potential development and expect the forecast to be fine tuned as the weekend approaches and it becomes more certain whether a cyclone will develop.

Elsewhere in the Atlantic, Tropical Storm Danny quickly formed in the east-central tropical Atlantic out of an African Easterly Wave – a kick-off to the notorious Cape-Verde Season. The National Hurricane Center forecast for Danny indicates steady strengthening to a hurricane by Friday morning as the cyclone moves west-northwestward. This track takes Danny just to the east of the Windward Islands by Monday morning.  Danny is therefore not expected to pass near Bermuda in the next five days, but should be monitored for any track changes, especially for the period after Monday.

The Cape-Verde Season typically encompasses the majority of the tropical cyclone activity for the Atlantic in any given year and is loosely defined as mid-August through early-October. Many of the tropical cyclones that form during this period find their origins near the Cape Verde Islands from African Easterly Waves – this is why it is referred to as the “Cape Verde Season”. Tropical cyclones that form near the Cape Verde Islands tend to have over a week over warm tropical water before reaching any land in the Western Atlantic or Caribbean Sea.

Follow the Bermuda Weather Service and National Hurricane Center for the latest official updates.

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