Above: Infrared image of Tropical Storm Bertha at 1612Z (10:12 am EDT) Wednesday, May 27, 2020, shortly before Bertha was downgraded to a tropical depression. (tropicaltidbits.com).
In less time than it takes to watch a movie, a disturbance labeled Invest 91L intensified into Tropical Storm Bertha and then moved onto the South Carolina coast, becoming the Atlantic’s first landfalling tropical cyclone of 2020 with startling speed. Already a tropical depression just hours later, Bertha will continue to make weather well inland, as an unusually rich feed of moisture will push toward the Appalachians and lead to heavy rains and potential flash flooding.
Moving northward over the Gulf Stream early Wednesday, 91L featured two bursts of showers and thunderstorms (convection) that evolved into a tight center of rotation as the center moved within range of coastal NEXRAD radar.
Bertha was upgraded by the NOAA/NWS National Hurricane Center directly from invest to named-storm status at 8:30 am EDT Wednesday, and it made landfall around 9:30 am EDT near Mount Pleasant, NC, just northeast of Charleston. Maximum sustained winds were estimated to be 50 mph at landfall and were kept at that range with the 11 am EDT advisory, when Bertha’s small center was located about 15 miles inland, just west of Georgetown.
At 2 pm EDT, NHC downgraded Bertha to a tropical depression with top sustained winds of 35 mph, centered near Manning or about 65 miles north-northwest of Charleston and heading north-northwest at 15 mph.
The time span of just one hour between Bertha's designation by NHC as a tropical cyclone and its landfall is one of the shortest on record. Just last year, Tropical Depression 11 formed near the Texas coast 45 minutes before it was upgraded to Tropical Storm Imelda and 90 minutes before it made landfall near Freeport.
Two named storms before the start of hurricane season
In the Atlantic record of tropical cyclones going back to 1851, only five other years have had two systems recorded to have reached named-storm strength (sustained winds of 39 mph) prior to the June 1 start date of hurricane season. It’s a good bet that at least a few pre-season storms were missed in the pre-satellite era. Here are the known years with two pre-season storms, as compiled by James Kossin (University of Wisconsin–Madison):
1887: Two tropical storms (May)
1908: Hurricane One (March), Hurricane Two (May)
1951: Tropical Storm One (January), Hurricane Able (May)
2012: Tropical Storm Alberto and Tropical Storm Beryl (May)
2016: Hurricane Alex (January), Tropical Storm Bonnie (May) (note that Alex is arguably a late storm from the 2015 season rather than a super-early 2016 storm)
In a weather.com article, Jon Erdman delves into the question of whether getting two named systems before June 1 tells us anything about the season to come. “Admittedly, five years out of a 169-year database isn't a very large sample size to make definitive conclusions,” Erdman says, again noting the possibility that some early-season storms were missed prior to the 1960s. Three of the five years above ended up with an above-average total of both named storms and hurricanes, Erdman noted, but only two of the five years had more major hurricanes than the long-term average. Accumulated cyclone energy was above the norm in four out of the five years, though.
More than 20 groups that carry out seasonal hurricane prediction are unanimous in calling for higher-than-usual odds of a busier-than-usual Atlantic season in 2020. We’ll have more on the outlook as the season kicks off.
Flooding from Bertha a threat to the Carolinas and Virginia
Even before it was designated Invest 91L on Tuesday, the elongated disturbance that gave rise to Bertha was a high-impact system. South Florida was drenched with multiple days of heavy rain from Sunday through Tuesday. Miami-Dade International Airport picked up 14.67”, the heaviest three-day total on record for any May and the 12th largest such total in Miami history. Tuesday alone was the city’s third-wettest May day on record, with 7.40”, and the month had racked up 18.88”, a new monthly record, beating out 18.66” from 1925. Another round of storms was moving into the Miami-Dade area early Wednesday afternoon.
At 8 am CDT Monday, the atmospheric sounding over Charleston, SC, measured 2.28” of precipitable water (the amount of moisture in a column above the surface). This is the highest total on record for the months from November to May at Charleston, where regular soundings have been taken since 1948.
As Bertha and its remnants push northward across the Carolinas into western Virginia, this slug of moisture will lead to additional rain over ground that’s already saturated in many areas from weeks of May rainfall. Widespread 2" - 4" rainfall totals are possible, and localized amounts could top 8".
Flash flood watches are in effect for much of western North Carolina and Virginia. The NOAA/NWS Weather Prediction Center is calling for a moderate risk of excessive rain leading to flash flooding across parts of southwest Virginia that have been hit especially hard by rainfall in recent weeks.