Tropical Storm Bertha brings heavy winds, potential flooding for Carolinas, Virginia, US
Bertha made landfall as a tropical storm on Wednesday, 27 May 2020, east of Charleston, South Carolina, bringing winds and heavy rain to the region before weakening to a tropical depression. The tropical storm formed at about 8h15am, becoming the second named storm of the current Atlantic hurricane season. It made landfall at 9h30am with sustained winds of 50mph. It was a tropical storm for almost six hours before weakening to a tropical depression. This was the second major storm incident to hit the area in as many weeks. Last week a three-day deluge in Southwest Virginia and the western Carolinas, from Wednesday to Friday, 20 to 22 May 2020, caused serious flooding. Up to a foot of rain has fallen in some areas, with downpours continuing late into Friday beneath a vigorous weather system that has hardly budged. The high water closed roads, downed trees, triggered mudslides and left some motorists stranded, in need of rescue. Flood warnings continued in parts of this high-terrain region on Thursday, 21 May 2020, as rainfall rates approached two inches per hour, exacerbating the flood situation. Residents were told to evacuate a dozen homes in southwest Roanoke amid concerns that the Spring Valley Dam might fail. Virginia Governor Ralph Northam tweeted on Thursday afternoon that the situation at the dam was stable but that residents should stay alert.
Despite weakening quickly through the day, Tropical Storm Bertha still has the potential to cause very heavy rain and “life-threatening” flash flooding across parts of the Carolinas, southern Virginia and over the southern Appalachians, according to the National Hurricane Centre. More than six million people are under Flash Flood Watches including Charleston, Charlotte and Roanoke.
Bertha is expected to produce total rainfall accumulations of two to four inches with isolated totals up to eight inches across eastern and central South Carolina, west central to far south-eastern North Carolina and southwest Virginia. The flash flooding threat is enhanced due to the heavy rain that fell across this region last week, leaving saturated soil and swollen rivers unable to handle much more water.
In addition to flooding, a low end tornado risk exists as well, especially across eastern North Carolina and South Carolina.
The tropical moisture associated with Tropical Storm Bertha is the same moisture that fuelled three days of record-setting rainfall for the Miami area.
Roanoke recorded its wettest two-day period in nearly 14 years on Tuesday and Wednesday last week with about half a foot of rain. By Thursday morning, double-digit rainfall totals were common across the upslope, hilly terrain of western parts of the Carolinas and Southwest Virginia. Between Monday and Wednesday, southern Polk County, North Carolina (NC) saw 11,53 inches. More than eight inches fell within 24 hours, concluding 7h00 on Tuesday, 19 May 2020.
Areas near Asheville, NC, were some of the hardest-hit. Just east of Asheville, “McDowell County had at least one mudslide,” said Doug Outlaw, a meteorologist at the National Weather Service in Greenville, South Carolina (SC) “11,4 inches in the west part of the county that was reported by an emergency manager.”
The Carolina Emerald Mine and the Riverbreeze campgrounds in McDowell County were evacuated and a state of emergency was declared.
A mudslide was reported in Salem, Virginia, on Wednesday. The persistent rain forced the Roanoke River over its banks, submerging several roads in downtown Roanoke. The swollen river also swamped roadways in Salem. The river crested at major flood stage on Thursday. The New River in Radford also was predicted to crest near major flood stage late Thursday.
Outlaw, of the National Weather Service, also mentioned that his Greenville, SC, office had received numerous reports of downed trees, which topple easier in saturated soil. Even leading into this week’s event, rainfall for the year to date was running well above normal. “For the year, we have a surplus of over 20 inches” at Greenville-Spartanburg Airport, Outlaw said. “We’re 20,82 inches above normal. The normal amount for this time of the year would be 18,1 inches and we have 38,92 inches.”
In other words, that’s about twice the normal year-to-date accumulation. That was as of midnight Wednesday night, 20 May 2020; since then, the rain has continued to fall. It’s a trend areas east of the Appalachians have seen for much of the year. “The surplus of rain for the Asheville area … is 16,67 inches,” Outlaw said. For May as a whole, 5,13 inches has been measured there, the bulk of which, 4,80 inches, fell in just 72 hours.
The staggering rainfall totals are due to a weather system known as a ‘cut-off low.’ Ordinary low-pressure systems form within a dip in the jet stream, cradled within the southward meander as it slowly translates east. But cut-off lows, characterised by a swirling depression of cold air at high altitudes, become entirely pinched off from the jet stream, left to linger without any real steering currents to carry them along.
In the case of last week’s episode, the upper-level low was anchored over the Tennessee Valley, its counter clockwise spin sweeping a tongue of moisture-rich air from the Gulf of Mexico north-westward into the Carolinas. The mountainous terrain of the Appalachians, meanwhile, helped focus that humidity, channelling it into relentless downpours and storms.
Source: Washington Post