The first tropical or subtropical storm of the 2018 Atlantic hurricane season is now likely in the Gulf of Mexico 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.
Currently, satellite imagery is showing an area of low pressure becoming better organized just off the east coast of Mexico’s eastern Yucatán Peninsula near Cozumel.
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.
Strong upper-level westerly winds are still blowing most of the thunderstorms to the east of the surface area of low pressure, hampering what would be more immediate development of the disturbance.
However, the U.S. Air Force Reserve Hurricane Hunters are scheduled to fly into the disturbance this afternoon, according to the NHC. If they can find a solid, persistent area of surface low pressure with thunderstorms in close enough proximity, it’s conceivable the NHC could initiate advisories on either a subtropical or tropical depression or storm as soon as Friday afternoon.
Regardless, the overall environment is expected to become more favorable for development and strengthening over the Gulf of Mexico this weekend. Some wind shear may remain over the system, but water temperatures are sufficiently warm, in fact warmer than late May average, over the northern Gulf of Mexico.
At this time, it appears likely we’ll eventually have at least the first named storm of the 2018 Atlantic hurricane season, Alberto, move north toward the northern Gulf Coast Sunday into Memorial Day.
Let’s break down the potential impacts below, as best as we can at this early stage.
Flooding the Main Concern, Regardless
Regardless of what this system 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.
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.
A building dome of high pressure over the Upper Midwest responsible for a holiday weekend heat wave will trap this Gulf system, allowing it move only very slowly near the Gulf Coast from late Sunday into Memorial Day.
Even after landfall, the system will move slowly, maintaining the tropical moisture fetch into the Southeast well into next week.
Flood watches are already in effect for parts of the Southeast, lasting from Saturday evening through Tuesday along parts of northern Gulf Coast.
Slow-moving tropical or subtropical storms are notorious heavy rain producers, and that will be the case here.
A broad area of at least 3 inches of rain is likely from Florida to the northern Gulf Coast to parts of the Carolinas through early next week.
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 most areas away from the Gulf Coast in Alabama, Georgia and the Carolinas, 3-hour rainfall of 3 inches or less would trigger flash flooding, according to the National Weather Service.
This heavy rain will eventually trigger river flooding that will last well into next week, as the National Weather Service in Mobile noted.
As the system gains strength, onshore winds will produce increasingly higher surf along the northern and eastern Gulf Coast from southeast Louisiana to western Florida beginning as soon as Saturday.
This will bring a high threat of dangerous rip currents through at least Memorial Day. Stay out of the water this holiday weekend along the eastern and northern Gulf Coasts.
The other impacts depend on the exact track and intensity of the Gulf system.
Rain-soaked ground may make it easier for winds to topple over trees near where the center tracks Sunday and Monday. Some power outages can also be expected near the landfall location.
If the system remains a subtropical storm, it may have a more expansive wind field, and could bring stronger winds to a wider area.
Coastal flooding or some normally dry areas can also be expected from southeast Louisiana to Florida’s Gulf Coast particularly Sunday and Monday, peaking with high tides and near and to the east of the landfall location along the northern Gulf Coast.
This coastal flooding may eventually pose problems backing up rain-swollen rivers trying to drain to the Gulf, as well.
As with any landfalling system, there will also be a threat of tornadoes Sunday and Monday, possibly lingering into early Tuesday near the northern Gulf Coast from embedded cells within rainbands.
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.
What’s more unusual is to have a storm form in the Gulf of Mexico in May. That’s happened only three other times in NOAA’s historical database, last occurring in 1976.
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.
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.