Goes-16 satellite presentation of Tropical Storm Debby late Monday evening
Extremely intense, very compact storm capable of devastating winds, rains, waves, and storm surges
CATASTROPHIC FLOOD SITUATION COMING TOGETHER OVER THE SOUTHEAST IN THE DAYS AHEAD
By Tom Skilling (Via Facebook)
The RAINFALL SITUATION unfolding in the Southeast–centered from a swath of northern Florida north into Georgia and the Carolinas—is most concerning. The storm will batter the coastal areas of these states with wind, waves, and a storm surge. But, it’s the flooding that may prove devastating.
There is no more prolific rainfall-generating system in nature than a tropical cyclone—i.e. a hurricane or tropical storm. The rate of rainfall is SO extreme, it overwhelms the ability of soils to absorb or rivers to move water out of affected areas fast enough to prevent flooding. Here the latest National Hurricane Center (NHC) public advisory on Debby: https://www.nhc.noaa.gov/…/MIATCPAT4+shtml/051749.shtml?
THE METEOROLOGICAL SET-UP BACKSTOPPING THE DEVELOPING EXTREME RAINFALL SITUATION
Estimated Rainfall totals over the coming days
Here’s one model (the RRFS high resolution, convection-allowing ensemble model which is to become operational in the months to come and is being run in parallel for testing with other NWS models) estimate of rainfall over just the coming 60 hours. Heavy rainfall is to persist beyond that period as the storm’s forward motion slows. “Ensemble” model forecasts average across a whole series of forecast solutions rather than relying on a single forecast and therefore attempt to reconcile variations between individual forecasts. Research has shown the ensemble approach to forecasting leads to more accurate predictions.
With jet stream winds, which might otherwise whisk tropical storm Debby across the Southeast, displaced far to the north, placing the storm in a region of light upper steering winds, a scenario develops in which Debby slows to a crawl in the coming days. This is to take place with the storm over the warm, energy-rich waters of the western Atlantic which are also the source of a prolific source of moisture.
Check out the Monday morning Environment Canada 500 mb analysis I’m posting which shows steering winds surrounding Debby at the 18,000 ft level of 5 to 10 mph. As a result, forecast models all but stall the system over the region producing local 20″ rainfalls–if not more.
Precipitable water–that’s the atmospheric moisture available in a column of air extending past 30,000 ft into the atmosphere to any precipitation-generating system–is off the charts HIGH–Monday morning approaching 3″. The towering cumulonimbus clouds which populate and, indeed, serve as the core of tropical storms, sweep this moisture into concentrated downpours that produce rainfalls many times the precipitable water values.
Our most humid weather system here in Chicago produced precipitable water values topping 2″—what’s available to Debby surpasses that. Yet 16.91″ of rain fell on the Greater Chicago area—a statewide record—in the historic flooding of July, 1996. That was a single 24 hour period! Imagine what days of rainfall from a stall system may end up producing in the eastern Carolinas!
THE BENCHMARK FLOOD SITUATION IN SOUTH CAROLINA
Here’s a summary from South Carolina’s most severe flood episode from the South Carolina State Climatology office:
“1908 – Flood: The flood of August 26-30, 1908, was the most extensive flood of record; all major rivers in the state rose from 9 to 22 feet above flood stage. A low pressure center formed in the Gulf of Mexico and moved northeastward across South Carolina causing unprecedented statewide flooding. There was heavy damage to crops and property along the rivers.”
State residents were also confronted by monumental flooding in 1928 and when Hurricane Hugo swept ashore in Sept 1989.
A MORE COMPLETE summary of some of the state’s historic weather events there can be seen here: https://www.dnr.sc.gov/…/Publications/storms_of_centry.php