June 1 marks the first day of Atlantic Hurricane season and for the sixth year in a row it is already off to an early and active start. Tropical Storms Arthur and Bertha have already been named, with Bertha on May 27th becoming the first named tropical cyclone to make landfall in the US of the 2020 hurricane season – before it had even officially started. If a third tropical cyclone develops in the Gulf of Mexico this week it will become the earliest named tropical system on record, and could possibly become the season’s second US land-falling tropical system.
An active subtropical Pacific jet stream throughout the winter deflected most cold shots from reaching the Gulf of Mexico, Caribbean, and Subtropical Atlantic. The mild winter left Sea Surface Temperatures (SSTs) above normal at the start of spring, giving the Atlantic Basin a head start to reaching the 26°C (79°F) threshold required for tropical cyclone development.
As depicted in recent satellite imagery, that 26°C threshold has already been met throughout the entire Atlantic Basin as far north as the Outer Banks of North Carolina. These SSTs are generally 1-2°C above normal for this time of year. This is despite a cooler and wetter spring than normal across much of the Southeast outside of Florida.
A new tropical storm is expected develop in that increasingly steamy bath over the Bay of Campeche near southeast Mexico out of the remains of Tropical Storm Amanda, which on Sunday made landfall over Guatamela. SSTs here are around 30°C (86°F) which is plenty warm enough for tropical cyclone development. Amanda and her remnants have unleashed destructive flooding and landslides across Central America that has already killed at least 14 people in El Salvador alone. If a new tropical storm is born from Amanda’s remnants, it will be named Cristobal.
Flooding is expected to worsen further in Central America this week as Amanda’s disorganized thunderstorm remains crawl towards the Bay of Campeche and southern Gulf of Mexico in the midst of slow steering winds. The moist, low-wind shear environment over the very warm water make a tropical storm rebirth into Cristobal extremely likely. If Cristobal forms by Thursday, June 4 it will set the record for the earliest third tropical storm. Tropical Storm Colin in 2016 currently holds that record, forming on June 5.
Cristobal’s eventual track is far from set in stone, but strong high pressure over the Southeast US, northern Gulf, and southwest Atlantic is expected to deflect Cristobal back into or towards Central America, where she will unleash devastating flooding and landslides through at least late week. Though her track in Central America is highly uncertain as of Monday afternoon, there is robust model agreement in the potential for up to 2 feet of rainfall to fall somewhere in the region between southeast Mexico, Belize, Guatemala, and El Salvador before possibly reemerging in the Gulf for a second time late this week.
Impacts from Cristobal in the United States will be possible if the storm returns northward from the Bay of Campeche late this week. The entire Gulf Coast should be on alert for possible tropical impacts although at this time areas west of the Mississippi River appear more likely to experience them. If Cristobal – or his successor in the event that he disintegrates over Central America before reemerging in the Gulf – does approach the US Gulf Coast, wind and rain would likely arrive between Sunday afternoon and Monday evening. Because a potential approach or landfall is still a week or more away, there is too wide a range of impacts to decipher specific details, but information will become clearer in the coming days.
Looking beyond Cristobal, additional tropical activity is possible over the southern Gulf and northern Caribbean in the coming weeks. There are signs in the long-range weather pattern that waves of low pressure passing through the Caribbean could get blocked by similarly placed high pressure as this week over the Eastern Gulf and Eastern US late next week. This could result in low pressure being forced to drift over the southern Gulf of Mexico in favorable environmental conditions for tropical development.
In the United States, any potential future tropical development in the next few weeks would likely be confined to the Southeast Atlantic Coast. Since the polar vortex broke down in April the northern jet stream has been very active. The breakdown has allowed unseasonably cool to cold air to periodically seep into the mid-latitudes. Occasionally this spring, the resulting troughs of low pressure containing the cold pools have cut off from the jet stream and have generated several consecutive days of thunderstorms. Persistent winds from the south ahead of these “cutoff” lows notably forced a tropical wave into the Carolina coast last week, which organized into Tropical Storm Bertha just before landfall. Alternatively, these lows can create enough convection by themselves without a separate wave of low pressure to organize into a tropical storm, as happened with Arthur on May 12.
The constant springtime push-and-pull of frontal systems is keeping the weather pattern active, so there will continue to be sporadic opportunities for persistent thunderstorm development off the Southeast Coast. As long as wind shear is low and the thunderstorm clusters are able to to drift enough to limit temporarily sea-surface-cooling upwelling, tropical development could be possible.