Subtropical Storm May Still Develop Off Southeast Coast This Week

Models are still suggesting that a moisture-laden disturbance off the southeast U.S. coast could develop into a subtropical cyclone later this week, although there is large uncertainty about how strong it might be and where it might go. A band of rich moisture now extends from the northwest Caribbean across Cuba into the southeast Bahama Islands. This air mass will shift north over the next several days, where it will intercept a decaying frontal boundary. Heavy rains will develop over the Bahamas over the next couple of days, possibly extending to the southeast Florida coast. If a closed low does form, it will likely be blocked from moving north as upper-level high pressure builds over the Northeast. The GFS model has been bullish on the prospects of a closed surface low forming along the boundary by midweek, then moving very slowly into the weekend. The 00Z and 06Z Monday GFS runs develop the this and bring it into the Georgia/South Carolina coast on Friday, with winds near the threshold for a tropical/subtropical storm, then keep it in that vicinity through the weekend. Such a scenario could bring several days of high surf and potentially heavy rain to parts of the southeast U.S. coast. The European model had been more tepid than the GFS, calling for a much weaker system located further offshore; the 00Z Monday run of the Euro is closer to the GFS in strength, but still keeps the system well offshore, moving away from the coast by the weekend. The 12Z Monday NAM model is even stronger than the GFS with a closed low projected to move close to the South Carolina coast by Thursday evening; however, the NAM is generally less reliable than the GFS or Euro in predicting tropical storm evolution.


Figure 1. The 90-hour surface forecast from the 06Z Monday GFS model run, valid at 8:00 pm EDT Thursday night, May 7. Image credit: WunderMap.


The low should initially develop as a result of the juicy low-level air mass being overtopped by a strong impulse in the powerful subtropical jet stream fed by the current El Niño conditions. Models show the high-altitude flow remaining cyclonic over the region, rather than the anticyclonic flow that would be associated with a typical tropical cyclone. However, winds at the center of the upper low will be quite light over the region of interest, which could allow for a compact warm-core circulation to develop. Sea-surface temperatures are also unusually warm--more than 2°C (3.6°F) above average over a large area just north of the Bahamas and east of Florida, close to where the system would be taking shape. Even so, these SSTs are still marginal for tropical-storm development, running at or above the commonly used 26°C threshold in a limited area from the Bahamas through a narrow band of Gulf Stream waters extending north toward the latitude of the Carolinas. The most recent phase-space diagrams from Florida State University, based on the 06Z Monday run of the GFS, show the system evolving as a very weak tropical cyclone (symmetric warm-core system), bringing it onshore late this week over South Carolina. In a special tropical statement issued at 10 am EDT Monday, the National Hurricane Center pegged the odds of at least subtropical development at 30% over the next five days, but near 0% through Wednesday. An Air Force hurricane-hunter aircraft is on call to investigate the system on Tuesday morning if necessary. If the low becomes a subtropical or tropical storm, it will be named Ana (pronounced "AH-na"]. Regardless of how the disturbance evolves, we can expect an increasing risk of high surf and rip currents from Florida to the Carolinas over the next several days.

Update: Wunderground blogger Steve Gregory weighs in on the potential subtropical/tropical system in his latest post. Readers may also enjoy the video update provided by Levi Cowan this afternoon at his Tropical Tidbits website.

On the scene at the NOAA Hazardous Weather Testbed
I’ll be blogging this week from the NOAA Hazardous Weather Testbed, located at the National Weather Center in Norman, Oklahoma. Each year the HWT and its Experimental Warning Program carry out a multi-week Spring Experiment that brings forecasters and researchers together to test-drive the latest in severe-weather warning technology. This year’s experiment begins today and runs through June 5. There should be at least a seasonable amount of severe weather across the southern Plains through this week, as ample moisture interacts with energy from an upper low gradually taking shape over the Southwest. The most active days appear to be Wednesday and Saturday right now. With widespread nonsevere showers and thunderstorms also expected, the trick will be anticipating where pockets of instability might develop after each night’s round of storms, and where small-scale ripples in the jet stream could set off a new batch of severe weather. For a project designed to push the boundaries of predictability, these challenges could be just the ticket.

Bob Henson