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RMS : comments on the flooding in the north of England and on Tropical Storm Debby

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Northern UK Flooding :

A band of heavy rain associated with a low pressure system affected parts of northern England and southwest Scotland on Friday, June 22 through to Saturday, June 23. The counties most notably affected by the heavy rainfall were Cumbria, Lancashire, and West Yorkshire.

The heavy rain, falling on saturated grounds and into swollen rivers from rainfall over the last couple of weeks, and a noticeably wet April and May in the U.K., resulted in widespread flooding. Rivers and streams bursting their banks caused the most severe flooding, while surface water flooding, caused by heavy rain on saturated ground and congested drainage systems caused more localized areas of flooding.

According to the U.K. Met Office, 24 hour rain accumulations from Friday, include; 3.6 inches in Blencathra, Cumbira; 3.4 inches in Keswick, Cumbira; 2.9 inches in Stonyhurst, Lancashire; 2.3 inches in Levens Hall, Cumbira and 2.2 in Morecambe, Lancashire. For some places this is in excess of the June monthly average – the average rainfall across the north of England (1971 – 2000) for June is just 2.7 inches.

The heavy rains lead to record levels in some rivers resulting in many rivers bursting their banks. The peak flow of the River Calder at Hebden Bridge was a record 3.2 m on Friday – it burst its banks in Hebden Bridge, Mytholmroyd, and Todmorden. In Cumbira the River Yarrow burst its banks in Croston, as did the Rivers Caldew and Petteril in the north of the county.

At the height of the flooding there were 73 flood warnings in place, 44 across the northwest, 8 in the northeast, 18 in the southwest and 1 each in the Midlands, Southeast and Wales.

As of Monday, only 3 flood warnings remain are in place across the U.K., one in the northwest and two in the northeast. No further rainfall is forecast in the next few days, however further heavy showers may affect northern regions on Thursday.

Tropical Storm Debby :

Tropical storm winds and heavy rain are affecting the northeast Gulf States as Tropical Storm Debby lingers off the northwest coast of Florida.  As of Monday morning, Tropical storm Debby was located in the northeast Gulf of Mexico approximately 90 miles south-southwest of Apalachicola, Florida with maximum sustained winds of 60 mph.

Debby is a large storm with tropical storm force winds extending outward up to 200 miles, mainly from the north and east of the center (affected the coast of Florida). Debby is almost stationary and rainfall associated with the storm is already starting to affect the mid-west coast of Florida in the Tampa region and areas just to the northeast of Tallahassee.

There is still considerable uncertainty in the track and intensity guidance associated with Debby.  Most forecast track models take Debby in a northeasterly direction over the course of the next few days, making landfall in Florida between 48-96 hours.  The National Hurricane Center has opted for a more northerly track that brings Debby over northwest Florida sometime on Wednesday or Thursday.

As Debby is already fairly close to land and as the storm moves progressively closer, the land will have an impact on storm intensity.  Although some further intensification is forecast by the NHC and a couple of the global models, there is good consensus that Debby will not reach hurricane status due to the expected interaction with land.  However as Debby is located over sea surface temperatures around 27C-28C and upper level air conditions are likely to become more conducive to intensification, the longer Debby stays over these warm waters, the higher the chance there is of further intensification.

“The main risks associated with Debby are tropical storm strength winds that will continue to affect portions of the northeast Gulf Coast over the next few days, coastal flooding related to storm surge, flooding as a result of heavy rainfall,  and the risk of a few tornadoes across the eastern Florida Panhandle,” said Neena Saith, director of catastrophe response at RMS.

The rainfall may prove to be Debby’s greatest hazard, particularly as the storm is very slow moving and the regions due to be impacted already have saturated soils from previous rain events.  Rainfall accumulations of 10 to 15 inches are forecast over the Florida Panhandle and the northern part of the state, with isolated maximum amounts of 25 inches possible.

Debby formed on Saturday, becoming the earliest fourth named storm on record and two months earlier than when the average fourth  named storm is expected to form (i.e. on August 23). The previous record for the earliest fourth named storm was set by Dennis in 2005 which formed on July 5.