Subtropical or Tropical Development is Possible in Gulf of Mexico, But a Heavy Rain Threat to Florida and the Southeast Regardless

May 15, 2018

An area of low pressure in the eastern Gulf of Mexico is being monitored for subtropical or tropical development this week as it spreads heavy rainfall across Florida and other parts of the Southeast U.S., potentially giving the 2018 Atlantic hurricane season an early start.

(MORE: Hurricane Central)

The National Hurricane Center is currently giving the system a low chance of development into a subtropical or tropical depression or storm over the next five days. Heavy rainfall will be the primary concern regardless, especially in the Sunshine State.


Current Satellite

(MET 101: What is a Subtropical Storm?)

For now, the disturbance consists of a well-defined upper-level swirl of low pressure but a disorganized, elongated area of low pressure at the surface. This system has not become any better organized since Monday, and conditions are becoming less favorable for development.

If surface low pressure becomes a well-defined circulation with enough convection nearby, this would be the first subtropical or tropical depression or storm of the 2018 Atlantic hurricane season, which doesn't officially begin until June 1. If it reached tropical storm strength, it would earn the name Alberto.

(MORE: Here's When the 'A' Storm Typically Gets Named in Hurricane Season)

Four of the past six years have featured named storms before June 1 in the Atlantic, including 2012, 2015, 2016 and 2017. Two of those years – 2012 and 2016 – featured the genesis of two named storms before June 1.

(MORE: When Hurricane Season Starts Early)

In this case, regardless of what meteorologists call it – a disturbance, a subtropical or tropical depression or storm – the main impact will be locally heavy rain with the risk of flash flooding due to the deep tropical moisture associated with this system.

This rainfall is much-needed in central and southern Florida, as well as parts of Georgia and South Carolina, where drought conditions have recently developed.


Current Radar, Watches and Warnings

Much of Florida can expect 1 to 3 inches of rainfall through the week. Locally higher amounts are expected in heavier thunderstorms, and totals of 3 to 7 inches are likely in southeastern and eastern Florida.

This tropical moisture pipeline will also fuel downpours that result in rainfall totals of 1 to 3 inches across other parts of the Southeast – and eventually into the Northeast – this week.

(MORE: We're Entering the Prime Time for Flash Flooding)


Rainfall Forecast

Although this rain is beneficial overall, there could be pockets of localized flash flooding.

As of May 8, more than 26 percent of the Sunshine State was in moderate drought, with a portion of South Florida in severe drought, according to the U.S. Drought Monitor.

Thirty percent of Georgia and roughly one-quarter of South Carolina, mainly in the southern parts of each state, were in drought. 

Drought conditions as of May 8, 2018.
(U.S. Drought Monitor)

Miami only measured 0.37 inches of rainfall in February, 1.88 inches below average for the month. This dry trend was exacerbated in March, when Miami only picked up 0.19 inches, compared to the average rainfall for March – 3 inches.

(MORE: What to Watch Out For and Look Forward to in May)

This dry pattern began to shift in late April in parts of Florida, but locations such as Fort Lauderdale and Naples remained more than 6 inches below average year-to-date as of last Friday. But there appears to be hope for a wetter pattern as the wet season commences.

Wet Season Begins This Month in Parts of Florida

The May-to-October time period when most of the annual precipitation occurs in Florida is known as the wet season.

For example, Miami receives about 45 inches of rain – almost 75 percent of its average annual rainfall – from May to October. By late May, the rainy season is usually ramping up across South Florida. 

Farther north, Tampa records about 70 percent of its average annual rainfall of 46.3 inches during the wet season. The beginning of the rainy season is a bit later, usually in mid-June, across central and northern Florida.

The purple dashed lines depict typical sea-breeze boundaries that help to trigger storm formation from late spring into early fall.

The reason for the increase in rainfall is that cold fronts do not track into Florida during this time, which allows for warmer temperatures and humid conditions to dominate.

(MORE: Rip Currents Have Been More Deadly Than Tornadoes and Lightning This Year)

As humidity increases, thunderstorm activity increases. Thunderstorms develop along sea-breeze fronts as cooler air slides inland from the Gulf of Mexico and the Atlantic Ocean, as shown above.

In addition, hurricane season officially runs from June 1 through Nov. 30 and brings an increased chance for rainfall. It's important to remember that a strong hurricane is not needed to bring excessive rainfall; slow-moving tropical storms or even tropical depressions can result in heavy rainfall and flooding in the region.

Although the beginning of the wet season is good news regarding the needed rainfall, one problem this brings is the possibility of dry lightning. This occurs when lightning strikes hit the ground outside of rainfall, and these strikes can create grass fires, especially where the ground is dry following the dry season. 


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