Florida West Coast In Sights of Potential Tropical Storm

A landfalling tropical depression or tropical storm appears increasingly likely to affect the west coast of Florida early in the coming week. On Saturday afternoon, a large tropical wave dubbed Invest 93L was gradually organizing as it moved into the Northwest Caribbean, with an extensive area of convection (showers and thunderstorms) evident on the system’s eastern side (see Figure 1 below). A center of low pressure just beginning to form within this tropical wave is expected to move northward along or just inland from the east coast of Mexico’s Yucatan Peninsula, then emerge late Sunday or Monday in the southeastern Gulf of Mexico. In an outlook issued at 2 PM EDT Saturday, the National Hurricane Center gave this system 70 percent odds of developing into a tropical cyclone by Monday evening and an 80 percent chance by Tuesday evening.


Figure 1. Infrared NOAA GOES image of the tropical wave over the Northwest Caribbean as of 1830Z (2:30 pm EDT) Saturday, June 4, 2016. Widespread showers and thunderstorms are concentrated on the east side of the developing center of low pressure. Image credit: NASA/MSFC Earth Science Office.

Track and intensity outlook
The steering flow that will drive this system is fairly straightforward, as an intensifying upper low across the northeast U.S. is joining forces with a weak but persistent upper low over Texas that has fueled heavy rain and deadly flooding over the last week. Given the large channel of southwesterly upper-level flow that will prevail across the Gulf of Mexico, models are in strong agreement that the expected tropical cyclone would move northeast and make landfall along the middle or upper west coast of Florida, most likely on or around Tuesday. If it does become a tropical storm, it will be named Colin—which would be the earliest we have reached the “C” name since Atlantic hurricanes have been named, and the earliest third storm of the calendar year in all recorded storms since 1851, beating an unnamed storm that formed on June 12, 1887.

Although the track of potential Colin-to-be is fairly well defined, there is a bit more uncertainty on how strong it might be. Much will depend on whether the northward movement of the low pressure center ends up off the Yucatan coast, near the coast, or well inland. The greater the interaction with the Yucatan, the more the system will struggle to develop. Even accounting for a good bit of land interaction, conditions will be quite favorable for the system to strengthen after it moves into the southeastern Gulf. Wind shear will be fairly low (the SHIPS model predicts 5 - 10 knots), and above-average sea surface temperatures of around 28°C (82 - 83°F) are more than warm enough to support intensification. However, the system will have only about 36 to 48 hours over the eastern Gulf, which will most likely keep its maximum strength within the tropical storm range. In addition, wind shear will increase to moderate or strong levels as the system moves toward the west coast of Florida ahead of the approaching upper trough.

Hurricane intensity forecasting has improved notably in the 2010s, as Jeff Masters and I discussed in detail in a blog post on May 6. The best-performing intensity model in 2015 was HWRF, and the last several runs of the HWRF through 12Z Saturday morning have brought this system into the 40 - 50 knot range (45 - 55 mph) just before landfall. The GFS model has also been consistent in calling for a weak to moderate tropical storm at landfall. We should have a better handle on this system’s potential strength after it clears the Yucatan. Should it enter the Gulf in a relatively organized state, I would not rule out the possibility of a higher-end tropical storm.


Figure 3. 5-day rainfall projection issued by NOAA/NWS Weather Prediction Center for the period from 8:00 am Saturday, June 4, 2016, through 8:00 am Thursday, June 9. Image credit: NOAA/NWS/WPC.

Even a weak tropical storm could cause significant water issues
Several factors may come together to produce more coastal and inland flood impacts than one might expect. The potential Colin is likely to be a somewhat asymmetric storm, with the strongest winds and heaviest rains on its east side. This would push water toward the central and northern Gulf Coast of the Florida peninsula at the same time that very heavy rain is falling in some areas. A system developing this quickly and remaining near hurricane strength would not pack a major storm surge, especially away from its immediate landfall location. If a Tropical Storm Colin developed and made landfall in Florida’s sparsely populated Big Bend, as consistently projected by the GFS and ECMWF models, this would further reduce the threat of any major surge impacts. However, Tampa will be on the stronger southeast side of the system, and the eventual track could end up closer to Tampa, as suggested by the 12Z Saturday HWRF run. Even a minor storm-enhanced tide could have noticeable impacts in the highly populated Tampa Bay area, which has been designated the nation’s most vulnerable location to higher-end storm surge (not expected with this system). The highest astronomical tides of the month in Tampa—around 3 feet above the average lowest tide (mean low low water)—occur with the new moon, which will be occurring this weekend. Regardless of its strength as a tropical cyclone, the now-developing system is likely to bring very heavy rains across a swath through central and northern Florida (see Figure 3 above), which would only exacerbate the flood potential.


Figure 4. Track of all tropical storms and hurricanes recorded during the month of June (1851-2015) over the Gulf of Mexico and northwest Caribbean. The color code at right shows each system’s strength (blue = tropical depression, green = tropical storm, yellow = Category 1 hurricane, etc.) Image credit: NOAA Historical Hurricane Tracks.

June favors tropical storms in the eastern Gulf of Mexico
The projected track of the developing tropical cyclone is right on target with climatology, which shows a favored track northward through the eastern Gulf of Mexico. Few of these systems make landfall on the Florida peninsula, though. The only hurricane known to strike Florida’s West Coast in June was an unnamed Category 1 hurricane that made landfall on June 24, 1945, between Tampa and Cedar Key, causing relatively minor damage (see yellow track on Figure 4 above). Tampa recorded a 24-hour calendar-day rainfall of 9.88” on June 23, 1945, which remains the area’s second-highest calendar-day total (topped only by 11.45” on May 8, 1979). This hurricane weakened to a tropical storm while crossing the Florida peninsula, then regained hurricane strength after it moved offshore and made a second landfall, crossing the Outer Banks of North Carolina. It is too soon to know how far north a potential Tropical Storm Colin would track if it were to emerge as a tropical cyclone off the East Coast of Florida, although the predicted strong trough over the eastern U.S. would keep it from threatening the Northeast.


Figure 5. Visible satellite image showing a compact swirl of mostly mid-level clouds around Tropical Depression Bonnie at 1838Z (2:38 pm EDT) Saturday, June 4, 2016.

Bye-bye, Bonnie
As it continued moving eastward hundreds of miles from the Carolina coast, pesky Tropical Depression Bonnie put forth an unexpected burst of convection Friday afternoon and evening, helping push it back up to minimal tropical-storm status. The growth spurt was futile, though, as increasing wind shear and a track toward colder water is sealing Bonnie’s fate. NHC demoted Bonnie once again to tropical depression status at 11 AM EDT Saturday, with further degradation to a post-tropical low expected by Sunday. If nothing else, Bonnie plus the potential Colin may give us a surprisingly large number of days with active tropical cyclones in the Atlantic basin for late May and early June.

Eastern Pacific's 91E could still become a tropical cyclone
In the Eastern Pacific, Invest 91E is taking its time organizing. Satellite loops show that an area of heavy thunderstorms located about 1100 miles south-southwest of the southern tip of the Baja Peninsula at 2 PM EDT Saturday has enlarged but not improved significantly in organization since Friday. This disturbance may still develop into the Eastern Pacific's first tropical cyclone of the year over the next couple of days before conditions become less favorable for development. Moving west at about 10 - 15 mph, the system is not a threat to any land areas. NHC now has 50-50 odds that 91E will develop into a tropical cyclone in the next 2 to 3 days. Should 91E become a tropical storm, it would be named Agatha. The first named storm of the year in the Eastern Pacific typically forms on June 10, so we would be very close to the climatological pace we expect. An even weaker system located several hundred miles south-southeast of Acapulco has a slight chance of intensifying over the next couple of days as it brings locally heavy rains to parts of extreme southern Mexico and Guatemala.

I’ll be back with an update by Sunday afternoon, while Jeff Masters is on long-scheduled travel over the coming week.

Bob Henson