Tropical Storm Possible in the Gulf of Mexico by Monday

A concentrated area of heavy thunderstorms has developed over the Western Caribbean, and this system has the potential for development into a tropical depression on Sunday or Monday. Wind shear was a high 30 knots on Friday morning over the region. This high shear will prevent development until Saturday at the earliest as the disturbance heads west-northwest towards Mexico's Yucatan Peninsula. On Saturday and Sunday, shear will drop and an area of low pressure will form over the waters of the Gulf of Mexico and Western Caribbean near the Yucatan Peninsula. This low and its associated moisture will ride up to the north-northeast into the southern Gulf of Mexico on Sunday under the steering influence of a large trough of low pressure that will move over the central Gulf of Mexico. Our three top models for predicting tropical cyclone genesis--the GFS, European, and UKMET models--all showed a tropical depression or tropical storm developing by Monday and making landfall on the west coast of Florida north of Tampa on Tuesday morning. There will be some high wind shear and dry air over the central Gulf of Mexico early next week in association with the upper-level trough of low pressure there, and these conditions will likely interfere with development, making intensification into a hurricane unlikely. Regardless of development, heavy rains will be the main threat from this system, and  2 - 7" of rain can be expected over much of Florida during the period Monday - Tuesday. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 10% and 60%, respectively. Should this system get a name, it would be Colin. Hurricane Hunters are now scheduled to investigate the Northwest Caribbean disturbance on Saturday afternoon.


Figure 1. Predicted precipitation for the 5-day period ending 8 am Wednesday, June 8, 2016. A tropical disturbance is predicted to bring rainfall amounts of 2 - 7" inches to most of Florida, with most of the rain falling Monday and Tuesday. Image credit: National Weather Service.

Eastern Pacific's 91E could become a tropical depression this weekend
In the Eastern Pacific, satellite loops show that an area of heavy thunderstorms located about 975 miles south-southwest of the southern tip of the Baja Peninsula at 8 am EDT Friday has not improved in organization over the past two days. However, this disturbance (91E) is still likely to develop into the Eastern Pacific's first tropical cyclone of the year this weekend, according to recent runs of the GFS and European models. The disturbance is moving west-northwestward at about 10 - 15 mph, and is not a threat to any land areas. In their 8 am EDT Thursday Tropical Weather Outlook, NHC dropped their 2-day and 5-day odds of development to 40% and 60%, respectively. 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.

Bonnie back on the downswing
After re-consolidating east of North Carolina on Thursday and approaching tropical storm strength, Tropical Depression Bonnie is now on its last legs. Satellite loops show that shower and thunderstorm activity has diminished greatly over the last few hours. Upper-level west-to-east flow just to the north of Bonnie will exert an increasing amount of wind shear over the next 24 hours while dragging the system out to sea. In its 11 AM EDT Friday update, the National Hurricane Center predicts that Bonnie will degenerate to a remnant low by Saturday morning before dissipating over the weekend. Residents of eastern North Carolina will no doubt be relieved to have Bonnie out of their hair as the weekend approaches. Cape Hatteras, NC, saw 8.41” of rain during the last four days of May and another 5.74” in the first two days of June. Last month was the wettest May in 124 years of weather records at Cape Hatteras, with a total rainfall for the month of 12.67”.

Severe weather taking aim on the mid-Atlantic for Sunday
A much different threat will be heading toward the mid-Atlantic region on Sunday. NOAA’s Storm Prediction Center has placed the area from northeast North Carolina to far southern Pennsylvania, including the Washington, D.C., area, under an enhanced risk of severe weather (the third of five risk levels) for Sunday. A surrounding slight-risk area extends from eastern Georgia to northern Pennsylvania and New Jersey, including more than 28 million people. Sunday’s set-up will be driven by very unstable air already in place across most of the region, coupled with favorable vertical wind shear and a sharp upper-level trough moving in on Sunday during peak daytime heating. The situation favors supercell thunderstorms in the afternoon and evening, and all modes of severe weather--including tornadoes, large hail, and damaging winds--are possible. In the shorter term, NOAA/SPC is calling for a slight risk of severe weather on Friday across Iowa and southern Minnesota, associated with the same front and upper-level trough that will head for the East Coast this weekend.


Figure 2. WU depiction of the Day 3 severe weather outlook issued by the NOAA/NWS Storm Prediction Center on Friday, June 3, 2016, and valid for Sunday, June 5.

Tragic deaths in Texas as widespread flooding continues
Five U.S. Army soldiers have been confirmed dead, and four others remained missing on Friday morning, after their vehicle (a troop carrier, resembling a flatbed truck) flipped and was swept away while crossing a creek as part of a training exercise on the grounds of Fort Hood in central Texas. Three other soldiers were rescued, and the Army will be investigating the incident. Officials at a press conference on Friday morning said the crossing at Owl Creek was not designated as a low-water crossing. The incident is a sad reminder of the power of flowing water, which pays no respect to any laws other than those of physics. In an excellent Forbes commentary last April, Marshall Shepherd discussed the counterintuitive strength of even a small amount of water on a roadway. Shepherd linked to a very useful physics-based discussion from Steve Waters, a senior hydrologist at the Flood Control District of Maricopa County (AZ), explaining why flowing water is so powerful and dangerous. The risks include not only the sheer weight of water (62.4 pounds per cubic foot) and the loss of roadway friction, but also the buoyancy factor. Every foot of water rising against the cab of a vehicle allows the vehicle to displace 1,500 pounds of water--effectively reducing the car’s weight by 125 pounds for every inch that water rises around its sides. Two feet of water is enough to float most automobiles, and once a vehicle is free-floating, it will tend to shift toward the fastest-flowing currents. High-clearance trucks with big cabs are hardly invulnerable once the water reaches their sides, since these vehicles displace even more water per foot of rise.


Figure 3. The streets of this neighborhood in Spring, TX (near Houston), became waterways on Thursday, June 2, 2016. Image credit: wunderphotographer threeyellowstarfish.


Figure 4. NOAA satellite image at 1515Z (10:15 am CDT) Friday, June 3, 2016, showing the mid-level circulation across north central Texas and the heavy rains located well to the southeast. Image credit: NCAR/RAL Real-Time Weather Data

A massive gyre of rainfall, with embedded torrential cells, swung northeast across central and eastern Texas on Thursday. The heaviest amounts recorded by the CoCoRaHS volunteer network on Friday morning were west of Fort Worth (6.98” reported in far northeast Callahan County) and from Houston northeast (several reports of 4” - 6”). A mesoscale convective vortex was clearly visible just west of the Dallas-Fort Worth area on Friday morning (see Figure 4), while the heaviest rains had pushed into southwest Louisiana and the northwest Gulf of Mexico by Friday morning. Rain-cooled air has helped to stabilize conditions across Texas, but the central third of Texas and southern Louisiana are still covered by flash flood watches: localized heavy rains are possible into the weekend, and it won’t take much additional rain atop the saturated ground to cause flooding problems.

We’ll be keeping tabs on the potential development in the Atlantic and Pacific through the weekend. Our next post will be midday Saturday.

Jeff Masters and Bob Henson