Severe floods continue to threaten South Carolina: AIR Worldwide

High water levels are still posing a threat in South Carolina, primarily in the Midlands and Lowcountry, and the situation is expected to worsen as rivers continue to rise, catastrophe modelling firm AIR Worldwide said on Friday.

The floodwaters continue to rise due to excess runoff, as the saturated ground cannot absorb more water, AIR Worldwide said in a statement. Several dams have failed – one intentionally – affecting water supply and worsening the flood threat to several communities. Additional rain is expected in some areas over the weekend.

According to AIR, the intense and persistent rainfall over the Carolinas resulted from a combination of several weather systems. As an extratropical cyclone reached the Carolina coast, it became detached from the large-scale westerlies that normally transport such weather systems. “Combined with Hurricane Joaquin’s easterly flow, a strong, jet-like region of moisture-laden air developed across the extratropical cyclone’s northern side in the low to middle levels of the atmosphere,” AIR explained.

Related: As dams bulge and rivers rise, South Carolina still on edge; 2 people die driving into water

The air mass moved rapidly inland within the lower levels of the atmosphere across the Carolina coastline and coincided with a rather strong low-level onshore flow attributed to the synoptic location of a high pressure system in Canada. Thunderstorms developed in the region with rainfall rates of three to four inches per hour, which drifted inland and northward before dissipating. AIR said in the statement that new storms developed along the coastal front as the upper-level storm remained nearly stalled for two days.

Some areas in flooded South Carolina facing second deluge: http://t.co/Pl8cGw3NFc pic.twitter.com/lWDoscNSg6

— Reuters Top News (@Reuters) October 9, 2015

The ground had already been saturated from previous storms that affected the area in September, which prevented much of the water from the October rains from being absorbed into the ground. As a result of the record-breaking rainfall for October, a great deal of excess runoff has been produced, causing rivers to rise and submerging areas on and off of floodplains.

According to the National Oceanic and Atmospheric Administration, the one-day rainfall records for October were broken in at least nine cities, with six of them breaking the one-day record for any month. The rainfall total in Summerville, north of Charleston, reached nearly 18 inches by Oct. 6, surpassing its 117-year record of nearly 11 inches for the entire month of October.

Related: South Carolina governor declares state of emergency after record rainfall

Furthermore, the total rainfall from Oct. 2-6 exceeded 20 inches in several areas, including parts of Brunswick, Georgetown, and Horry counties. Columbia received up to 17 inches in some areas in less than 24 hours. Over 17 inches have fallen at Charleston International Airport; on Oct. 3, the totals had exceeded 11 inches, breaking records of nearly 80 years. Over 27 inches fell at Mount Pleasant, while nearly 24 inches fell at Georgetown, AIR reported.

Related: South Carolina flood: Swamped streets, hundreds of rescues, more door to door searches next

The National Weather Service also said that the Congaree River, whose flood stage is 19 feet, crested at nearly 32 feet on Oct. 4. The Santee River was over 18 feet and is expected to reach 23 feet, more than 13 feet above flood stage. In Williamsburg county, the rising Black and Santee rivers have caused severe flooding in several communities, and residents in areas south of these rivers were asked to evacuate. Many other areas have had water levels over the flood stage.

Numerous dam failures have occurred in several counties, including Lexington, Lee, Richland, Fort Jackson and Aiken counties.

Columbia, S.C., mayor estimates damage from floods could be in the billions of dollars. http://t.co/Mzb1ughbuP. pic.twitter.com/vooXIDD4Gd

— CNN Breaking News (@cnnbrk) October 5, 2015

A disaster declaration is in effect for several counties, and many coastal areas, such as Georgetown, are bracing for the situation to worsen, the statement said. Evacuations were under way in several areas threatened by rising rivers or dam breaches. As of Thursday, approximately 250 roads and 100 bridges remained closed. Farmlands have also been severely affected and many crops have incurred damage, particularly soybean, peanut, and cotton.

AIR continues to monitor the situation.