We continue to track Subtropical Storm Alberto this weekend as it begins to move into the Gulf of Mexico from the northwestern Caribbean Sea. Through Saturday night, the storm will move in a northeast direction, making its closest approach to the western Florida coast while remaining well offshore. From Sunday morning and beyond, the upper-level trough over the Gulf of Mexico will pull the storm to the northwest and into the northern Gulf Coast. Monday night the storm will make landfall between New Orleans and Pensacola as a 65 mph tropical storm.
Currently, the storm remains subtropical. Based on geocolor imagery from the GOES-16 satellite, you can see the low-level center, which we have circled in the image. This is the center of Alberto, but notice how most of its convection, or thunderstorms, are displaced to its east. This is due to strong wind shear from the west. The fact that this storm’s clouds are not symmetrical and do not surround the center of circulation is one component that makes this storm subtropical. By the end of the holiday weekend, however, either on Sunday or Monday, Alberto is expected to transition into a tropical storm. It is slowly making that transition now with convection over the center. This is because wind shear will be much lower over the northern Gulf of Mexico, thus allowing for the thunderstorms to organize. It will also move into an area of warmer water temperatures, ranging from 27 to 29°C. This may allow for stronger thunderstorms and for the cold-core of the storm to erode and be replaced by a warm-core, another difference between a subtropical and tropical cyclone.
This combination of weaker wind shear and warmer sea surface temperatures will allow for Alberto to strengthen from a 40 mph to a 65 mph storm prior to landfall. Because the environmental conditions are expected to become conducive enough for this strengthening, there is the risk that Alberto will become a low-end hurricane. We’ve seen storms unexpectedly ramp up into a hurricane over the Gulf of Mexico in a short period of time, and so this outcome wouldn’t be completely surprising if it does happen. If Alberto does become a hurricane, it will break the record for earliest landfall hurricane on the United States. According to Phillip Klotzbach, “The earliest calendar year continental US hurricane landfall on record (since 1851) was Hurricane Alma in 1966 on June 9. It made landfall in Florida as a Category 2.”
There are a multitude of threats that Alberto will brings, including heavy rains that can cause flooding, storm surge, strong to potentially damaging winds, tornadoes, and rough surf. Below, we’ll break down each of these threats in detail.
Flooding will be the greatest threat with this storm. Often times, it’s these early-season, messy-looking storms that can be the most damaging by bringing significant flooding. The Weather Prediction Center has added a moderate risk for flash flooding for portions of the northern Gulf Coast on Monday. Numerous flood watches are in effect due to the significant risk. In some areas, especially near the Gulf Coast where Alberto will be making landfall, there is the potential for widespread flooding. If you encounter flooding, seek higher ground and do not drive into flood waters.
This storm will bring a widespread three to five inches of rainfall to the entire Southeast. Most of the rain will fall east of Albert’s center, as that’s where all of its thunderstorms are located. There will also be several areas that receive over half a foot of rain. This includes extreme-South Florida, the southern Appalachian Mountains, and the northern Gulf Coast.
The Memorial Day Weekend will not be a complete washout. Some locations, especially near the center of Alberto, will experience several hours of a steady, moderate to heavy rain. Otherwise in most other areas, it will be some rounds of rain and thunderstorms that pivot through. It will be the kind of weekend where there will be on and off showers and storms that everyone hopes their region will dodge.
The threat for flooding due to storm surge is increasing. The National Hurricane Center has place the areas from Crystal River in Florida to the Mouth of the Mississippi River under a Storm Surge Watch. This means there is a possibility of life-threatening inundation from rising water moving inland from the coastline in the indicated locations during the next 48 hours. This watch will likely be upgraded to a warning as the storm approaches.
All of these locations under the watch may experience coastal flooding. Two to four feet of surge is forecast for most of these locations.
As with most tropical cyclones, the threat for wind always exists. At this time, sustained winds are forecast to clock up to 60 mph at landfall. If this storm strengthens further than forecast, then sustained winds may top 75 or 80 mph. In terms of gusts, that will be up to 30 mph stronger than the sustained winds. Significant damage from wind is not expected, and the strongest of winds will only affected an isolated area just east of the center of the low pressure.
A Tropical Storm Warning has been issued for the Dry Tortugas in the Florida Keys. This means that tropical storm conditions (winds greater than or equal to 39 mph) are expected within the next 24 hours opposed to a watch, which means these conditions are possible within 48 hours. A Tropical Storm Watch is in effect from Boca Grande to the Anclote River and from the Aucilla River to Grand Isle. It’s also in effect for Lake Pontchartrain and Lake Maurepas.
Brief tornadoes will be a risk with Alberto, beginning this Saturday in South Florida. As the storm moves to the north, the threat for tornadoes will follow. Our forecasters suggest that the risk for low-end tornadoes will exist across most of the Southeast, especially as this storm makes landfall between Monday and Tuesday.
Most landfalling tropical cyclones pose the threat for tornadoes because they create a change in wind direction by height. It’s in the northeastern quadrant of the storm where there’s that greatest risk because winds will be coming out of the southeast at the surface, but more out of the south or southeast in the mid-levels of the atmosphere. It’s definitely not a perfect setup for tornadoes, but short-lived ones with little warning often accompany storms like Alberto.
The last risk with Alberto is rough surf. Hazardous surf conditions are likely to develop along much of the central and eastern Gulf Coast through the holiday weekend. An onshore flow will bring days of rip currents to the Gulf Coast and even the Atlantic beaches of the Southeast. The strong winds will allow create swells that are likely to cause life-threatening surf and rip current conditions. It is best to stay out of the ocean now through at least early next week.