Tropical Development Likely in the Gulf of Mexico Memorial Day Weekend; Major Flash Flood Threat in the Southeast, Florida Into Next Week

The first tropical or subtropical depression of the 2018 Atlantic hurricane season is now likely in the Gulf of Mexico this Memorial Day weekend, bringing the threat of heavy rain and flash flooding to a large swath of the Southeast and Florida, lasting into next week.

(MORE: Memorial Day Weekend Forecast)

Currently, a broad area of low pressure is located near Mexico’s eastern Yucatán Peninsula, producing clusters of thundershowers near the coast from near Cozumel southward to Belize and eastward to the western tip of Cuba.

The National Hurricane Center (NHC) has dubbed this low-pressure system Invest 90L, which is a naming convention used by the NHC to identify features it is monitoring for potential future development into a tropical depression or storm.

(MET 101: What is an Invest?)

Current Satellite and Area to Watch

For the next day, the system’s proximity to land and strong wind shear (change in wind speed and direction with height) won’t allow Invest 90L to develop.

This weekend, however, the overall environment is expected to become more favorable for development.

According to the NHC’s tropical weather outlook, "a subtropical or tropical depression is likely to form by late Saturday over the southeastern Gulf of Mexico."

The Air Force Hurricane Hunters may fly out to Invest 90L to collect data on the system Friday afternoon or early Saturday.

(MORE: What is a Subtropical Depression or Storm?)

Flooding the Main Concern, Regardless

We expect the system to drift northward this weekend, eventually moving ashore somewhere along the northern or eastern Gulf Coast later this weekend. Whether it is a "depression" or is able to gain enough steam to become Tropical or Subtropical Storm Alberto remains uncertain.

(MORE: Tropics Have Been Active on Memorial Day Weekend in Recent Years)

Regardless of what this is called by meteorologists, the main threat from this system will be heavy rain and flash flooding in the Southeast that could last well into next week.

(MORE: Why Intensity Doesn’t Matter Much for Tropical Rainfall Potential)

Developing upper-level low pressure over the Gulf of Mexico and high pressure aloft east of the Bahamas will channel a plume of deep, tropical moisture from the southwestern Caribbean Sea and Central America into the Southeast and Florida.

Deep tropical moisture will be channeled northward into the southeastern U.S. through the Memorial Day holiday weekend, fueling the threat of heavy rain and flash flooding. Higher values of atmospheric moisture are denoted by the darker orange, red and purple contours on this model forecast.

This system may become trapped over the South, maintaining the tropical moisture fetch into the Southeast well into next week.

This means heavy rainfall, to the tune of 3 to 5 inches, is possible over a broad stretch of the Southeast from the northern Gulf Coast to Florida and the Carolinas through the middle of next week. Some communities may receive much higher amounts.

Rainfall Outlook: Next 7 Days

Slow-moving rainbands or clusters of thunderstorms may produce heavier rainfall over a shorter time period – on the order of a few hours – in localized areas this weekend into next week, triggering flash flooding, particularly in urban areas, foothills and mountains, and in areas where the ground has been saturated by recent heavy rain over the past one to two weeks.

In nine days since May 13, parts of St. Lucie and Martin counties, Florida, picked up 12 to 17 inches of rain, according to the National Weather Service.

Most locations in the Florida Peninsula have had one of their top-10 wettest Mays-to-date through May 22, according to the Southeast Regional Climate Center (SERCC).

The SERCC said both Richmond, Virginia, and Stuart, Florida, already set their record-wettest entire month of May, nine days before month’s end.

If that wasn’t enough, this Memorial Day weekend system will raise the threat of rip currents in some locations, both in the Gulf of Mexico and on the East Coast of Florida and Georgia.

(MORE: Rip Currents Are an Underrated Danger)

If it does indeed develop into a tropical or subtropical storm, there may also be some coastal flooding, which may eventually pose problems backing up rain-swollen rivers trying to drain.

In addition to the heavy rain in the United States over the next week, the Servicio Meteorológico Nacional (National Meteorological Service) in Mexico warns of heavy rain, gusty winds and waterspouts around the Yucatán Peninsula into this weekend.

Check back with weather.com for the latest on this holiday weekend threat.

May Storm Origins

Tropical or subtropical storms can occasionally develop in the Atlantic Basin during May.

The Caribbean, Gulf of Mexico and the southwestern Atlantic Ocean are the most likely areas for development in May. This is illustrated on the map below, which shows the origin points for tropical storms that have formed in May since 1851, including a few that became hurricanes.

Each dot represents the origin point of tropical storms that have developed in the Atlantic Basin since 1851.

Since 1950, 20 Atlantic Basin storms have developed before the start of June, according to NOAA’s historical hurricane database. That’s an average of roughly one storm developing before June 1 every three to four years.

(MORE: Why the Projected Path For Hurricanes and Tropical Storms Doesn’t Always Tell the Full Story)

Since 2012, preseason storm activity has been an outlier from the norm.

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.

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