Torrential rains and flash flooding affected South Florida on Thursday and Friday beneath the disturbance that’s likely to soon become Arthur. Meanwhile, parts of Texas and Louisiana received upward of eight inches of rain from an upper-level weather system that’s racing east to meet its counterpart in the Sunshine State.
It all comes more than two weeks before the official start of hurricane season, which commences on June 1 and lasts through November. But preseason tropical cyclones are becoming increasingly common, and some atmospheric scientists believe this may be linked to warming ocean waters related to human-caused global warming.
Ingredient one: Florida downpours
All eyes were on a cluster of downpours over South Florida early Friday, where a reservoir of moisture lingers along a decaying front. Heavy rains and gusty squalls dropped nearly half a foot of rain on Lower Matecumbe Key on Thursday. More than five inches fell farther southwest on Marathon Key at the airport, its fifth-wettest day on record in the past 20 years.
More intermittent showers and downpours were moving across the island on Friday, with the National Weather Service warning that “visibility of 1 mile in blinding downpours [is] expected.’
Those same heavy rains were targeting the Miami area on Friday, where between 3 and 5 inches had fallen in just three hours’ time. A flash flood warning was issued for parts of the city.
The weather system affecting Florida is a key part of the weather disturbance that the National Hurricane Center estimates has an 80 percent chance of development, and “will likely become a tropical or subtropical storm on Saturday.” An Air Force Reserve Hurricane Hunter Aircraft is scheduled to investigate the system on Saturday.
Ingredient two: Upper-level disturbance over Gulf Coast
Nearly 700 miles to the northwest, a second, more subtle component to the imminent subtropical or tropical storm was producing heavy rain and flash flooding over East Texas and parts of Louisiana.
Up to 8.5 inches of rain fell in Orange County, Tex., with 5.5 inches in Vermilion Parish, La., and 7.66 inches in Tangipahoa Parish. Nearby Saint Tammany Parish measured 6.12 inches.
The National Weather Service in New Orleans issued a flash flood warning, emphasizing a “considerable threat” of heavy rain. ”
“Between 8 and 12 inches of rain have fallen,” read the bulletin, with “additional rainfall amounts of 1 to 3 inches [possible.]”
That second batch of excessive rainfall stems from a weak lobe of upper-level support passing overhead that will eventually enhance rising motion over Florida, and contribute to the growth of the developing storm.
Ingredient 3: Upper low slips south
Ingredient three was over the Pacific Northwest on Friday morning, appearing as a rather harmless high-altitude wave of low pressure and cold air that will intensify dramatically as it moves over the Northern Plains on Saturday.
As it strengthens, it will take a turn somewhere over the East or Southeast, where it will get stuck for several days next week while bringing cool, unsettled conditions.
This area of low pressure in the upper atmosphere will help intensify the downstream surface disturbance east of South Florida. That increases the odds of a subtropical storm forming and moving to the northeast.
It will also probably affect the steering of any subtropical system, scooting it out to sea.
Subtropical system will probably head out to sea
The subtropical storm, which would have characteristics of a tropical storm and a traditional mid-latitude storm, is anticipated to head northeast, away from the Florida coast. However, it could bring choppy waves and dangerous rip currents from Florida to the Carolinas between Sunday and late Tuesday. Any showers or gusty winds from the storm would probably only reach as far west as the Outer Banks on Sunday night or Monday.
Many systems that form this early in the year are subtropical, since their internal structure and formative processes aren’t fully tropical in origin. If conditions are right, though, they can transition into purely tropical storms over time.