A tropical storm may be brewing in the western Caribbean, and is expected to strengthen as it heads toward the northern Gulf coast this weekend. Early next week, some of its moisture may be drawn up the East Coast towards the Mid-Atlantic.

The disturbance is centered just 130 miles east of the Yucatan peninsula, but is slowly getting better organized, and could become Tropical Depression 12 later today or tomorrow (for additional background, see Tuesday’s blog post).  In fact, all available model guidance suggests that this will become a tropical storm by Thursday morning, at which point it would earn the name Karen… the eleventh named storm of the season.

Early morning visible satellite image over the western Caribbean. (NOAA)

Strong thunderstorms have been increasing in coverage and consolidating around a broad surface low pressure, bringing periods of heavy rain to the entire region today, including the Cayman islands, western Cuba, Belize, and the Yucatan peninsula.  An Air Force reconnaissance plane is tasked to fly into the disturbance later today to collect details on its structure and intensity.

By tonight or tomorrow morning, the center will pass over the Yucatan peninsula, then spend several hours over land before re-emerging over water in the southern Gulf of Mexico.

tracks Track model guidance from this morning showing fairly good agreement in both direction and timing.

There is tight model agreement on the near-term storm evolution, and fairly good agreement even at longer time frames.

Looking ahead 3-4 days, the storm will likely get entangled with a mid-latitude trough in the northern Gulf, which would keep it weak, but actually enhance the forecast rainfall totals along the northern Gulf coast.  Over the next week, estimates in excess of the 2 inch range  cover a large area.  Peak totals of over 6 inches are already anticipated near the center.

The large-scale environmental conditions that it is experiencing now are favorable for its development, but that could all change once it enters the Gulf of Mexico.  Although the system is in a protective bubble of rather moist air now, deep dry air in the Gulf of Mexico will erode away at that bubble and eventually get entrained into the circulation, especially when the vertical wind shear ramps up to over 25 mph in a couple days.

Total precipitable water (left) which mostly shows low-level moisture, and water vapor satellite image (right), which mostly shows upper-level moisture. Both are valid this morning, and reveal the moist bubble associated with the disturbance, as well as the dry air in the Gulf. (tropicaltidbits.com and NASA)

A tropical storm landfall looks likely Saturday, somewhere from eastern Louisiana over to the Florida panhandle.  Since it’s expected to remain relatively weak (or maybe not even tropical!), the exact landfall position is not a big concern. Heavy rain will affect a broad area regardless of where it washes ashore and its technical classification.

GFS model simulates some moderate to heavy rain early Monday morning in Virginia (WeatherBell.com)

By Sunday night into Monday night, the system’s remnants will merge with a front marching towards the East Coast.  Some locally heavy rain could move into sections of the Mid-Atlantic at that time – though pinpointing where the highest rainfall totals will concentrate is not possible yet.