Tropical/Subtropical Depression Could Form Today in Northwest Atlantic

Formation of a tropical or subtropical depression appears imminent on Friday or Saturday in the waters between the Bahamas and Bermuda as Invest 91L moves west-northwest or northwest towards the Southeast U.S. coast. Should it become a named storm, it would be called Bonnie.

Satellite loops show that 91L has a pronounced spin near the surface that has improved in organization since Thursday--the circulation is less elongated, and more circular. However, there was only a modest amount of heavy thunderstorm activity that was not changing much in areal coverage associated with 91L on Friday morning. The latest pass from the ASCAT satellite showed an area of surface winds near 35 mph in the heaviest thunderstorms to the north of the center. Sea surface temperatures (SSTs) near 91L's center were about 26°C (79°F), which is 1 - 2°C (1.8 - 3.6°F) above average. These waters are only marginally warm enough to support formation of a tropical storm, and 91L has taken on some characteristics of subtropical system instead of a tropical system. As I explain in my Subtropical Storm Tutorial, a subtropical storm typically has a large, cloud free center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain.

Wind shear on Friday morning had fallen to the moderate range, 10 - 20 knots, increasing the odds of development. A large area of dry continental air lies to the west of 91L, and this dry air is interfering with development. A hurricane hunter aircraft is scheduled to investigate 91L on Friday afternoon.


Figure 1. Latest satellite image of 91L.



Figure 2. Predicted wind speeds for the Southeast U.S. at 2 am EDT (06Z) Sunday, May 29, 2016 from the 00Z Friday, May 27 runs of the European model (left) and GFS model (right). Both models had the storm just off the coast of South Carolina. Image constructed using our wundermap with the "Model Data" layer turned on.

A heavy rain threat for the Southeast U.S. coast
The 12Z (8 am EDT) Friday run of the SHIPS model predicted that wind shear would stay generally in the moderate range, 10 - 20 knots, for the next five days, which should allow some slow development of 91L. On Friday and Saturday, 91L will be traversing waters of 25 - 26°C (77 - 79°F), which should be just warm enough to allow development. The storm may get a small boost when it crosses the axis of the warm Gulf Stream current on Saturday afternoon. The system will have trouble with the large area of dry air to its west; if wind shear remains in the moderate range, 91L may have difficulty moistening its inner core enough to wall off this dry air. In their Friday morning (00Z) runs, our three top models for forecasting tropical cyclone genesis--the American GFS model, the European ECMWF model, and the British UKMET model--all showed the potential for Invest 91L to develop into a tropical or subtropical depression on Friday or Saturday. In a special Tropical Weather Outlook issued at 7:45 am EDT Friday, the National Hurricane Center bumped up their development odds in the 2-day and 5-day time ranges to 90%.

The models are not very gung-ho about intensifying 91L, and the system will have too little time over water to become anything stronger than a 55-mph tropical storm. Thus, heavy rain is the main concern from this system. Heavy rains should reach the coasts of South Carolina and southeastern North Carolina on Saturday afternoon or evening--though the heaviest rains from 91L will likely stay out sea to the storm's east, in a large band of heavy rain typical for a subtropical system. The Friday morning runs of our two top models for hurricane tracking, the GFS and European models, showed the center of 91L reaching the central coast of South Carolina near Charleston early Sunday morning or late Sunday afternoon, respectively. Steering currents will shift on Sunday evening, as 91L gets caught in the circulation associated with a trough of low pressure passing to the north, forcing 91L to turn to the northeast. The center of 91L will likely track inland along the coasts of South Carolina and North Carolina on Monday and Tuesday, spreading heavy rains of 2 - 4" along its path. It is uncertain at this time whether or not 91L will be able to spread heavy rains farther north into Virginia later in the week.

We'll have a post later today analyzing the annual NOAA seasonal hurricane forecast, which will be released late this morning.


Figure 3. A large area of rains exceeding 10” fell between Austin and Houston between 7:00 am CDT Thursday, May 26, and Friday, May 27, 2016. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

From tornadoes to torrents: Rain records inundated in Texas
A large chunk of the upper-level low that lingered over the western U.S. most of this week began pushing onto the Southern and Central Plains on Thursday, triggering widespread severe weather. As opposed to the isolated, tornado-generating supercells earlier in the week, Thursday’s severe storms were far more widespread, but competition between the many storms helped cut down the ability of any one cell to become a cyclic tornado-producer. NOAA/SPC logged a preliminary total of 19 tornado reports on Thursday in Texas, Oklahoma, Colorado, Kansas, and Missouri, with no serious injuries or major damage reported. The highest-impact scare was at Kansas City International Airport, where passengers were herded from the three main terminals into tunnels and parking garages during a 20-minute evacuation. Near Dodge City, KS—not far from where multiple tornadoes struck on Tuesday—hail as large as softballs pelted an area south and east of Clark State Lake on Thursday evening.

Rain-cooled air from the Central and Southern Plains finally pushed back an incredibly rich southerly feed of low-level moisture from the Gulf of Mexico that had fueled severe weather all week. Shunted into Texas, the moisture fed several mesoscale convective systems (MCSs) that produced torrential rain on Thursday afternoon into Friday morning, especially between the Austin and Houston areas. The town of Brenham racked up an amazing calendar-day total of 16.62”, obliterating its previous record of 10.38” from October 17, 1994. Most of that rain fell in a 12-hour period from noon to midnight. All by itself, Thursday’s total would rank as the fourth wettest month in Brenham’s history, and it is larger than May's monthly record of 15.09” from May 2015 (the wettest month by far in state history). Weather records in Brenham extend back to 1897. The heavy rains led to several high-water rescues in the Austin and Bryan areas, and two people were missing as of Friday morning, according to weather.com. Flash flood warnings continued through noon CDT Friday just east of Austin, where 6” to 10” of rain had reportedly fallen. More heavy rain is possible across southern Texas with additional MCS development later on Friday and Saturday.

Overall, the severe weather threat will continue dropping through the Memorial Day weekend, as both vertical wind shear and instability will be much less favorable for supercell formation. However, with a still-respectable amount of low-level moisture in place, there will be plenty of showers and thunderstorms across the central states, making for a damp-at-times Memorial Day weekend from Texas to Minnesota and Wisconsin. Long-range models suggest that heavy rain may recur through next week in some areas, Texas in particular.

Jeff Masters (tropical), Bob Henson (severe)


Figure 4. Projected 5-day rainfall totals from 12Z (8:00 am EDT) Friday, May 27, through 12Z Wednesday, June 1, 2016. Image credit: NOAA/NWS Weather Prediction Center