|Above: GeoColor satellite image of Tropical Storm Umberto at 1700Z (1 pm EDT) Saturday, September 14, 2019. Image credit: RAMMB/CIRA/CSU.|
Just two weeks after a catastrophic hit from Hurricane Dorian, the Northwest Bahamas are dealing with Tropical Storm Humberto. The slowly intensifying storm was located just 30 miles east-northeast of Great Abaco Island at 11 am EDT Saturday. Humberto was packing top winds of 50 mph. The strongest winds were concentrated north and east of the center, though, which is good news for an island that was decimated by Dorian.
A complex upper-level pattern has prevented Humberto from strengthening quickly despite very warm sea surface temperatures of 29-30°C (84-86°F). Persistent southwesterly winds related to an upper low in the Gulf of Mexico have kept Humberto under high wind shear, tilted the storm, and pushed most of its showers and thunderstorms (convection) well north and east of the center. At the same time, high pressure to Humberto’s northeast has kept the storm from moving with the southwesterlies in that direction. The result is a gawky, slow-moving tropical storm.
Humberto was nearly stationary on Saturday morning—an eerie echo of Dorian’s destructive stall over Grand Bahama Island. In this case, however, impacts should be minimal apart from scattered heavy rains and gusty winds. Additional rains of 1” – 2” will be widespread across the Bahamas, with pockets of up to 6” possible.
|Figure 1. WU depiction of NHC forecast track for Humberto as of 11 am EDT Saturday, September 14, 2019.|
As southwesterly wind shear relaxes into the 10-15 knot range this weekend, Humberto is beginning to take on a more vertically stacked structure that will allow it to strengthen. Intense convection was just starting to wrap around the low-level center at midday Saturday. Ensemble output from the 0Z Saturday runs of the GFS and European models is close to unanimous in taking Humberto slowly northwestward through the weekend, then turning the storm sharply toward the east-northeast early next week as an intensifying hurricane. On this track, Humberto will stay well away from the Southeast U.S. coast, although the storm’s fringes could bring up to an inch of rain to some coastal locations.
The National Hurricane Center’s outlook brings Humberto to hurricane strength by late Sunday and Category 2 strength by midweek. Rather than interfering with Humberto, a strong midlatitude jet stream will be positioned to help ventilate and intensify the storm (see tweet below).
#Humberto is predicted to significantly interact with the extratropical flow next week. Classic tropical cyclone - jet interaction as indicated by the 200mb irrotational wind field (indicating ventilation of the tropical cyclone) building downstream jet streak. pic.twitter.com/0rlheLTarf— Michael Ventrice (@MJVentrice) September 14, 2019
Humberto could pass near Bermuda as a hurricane around Wednesday or Thursday. A handful of the 70 ensemble members even suggest that Humberto might loop around to make another pass at the U.S. East Coast a week or more from now. This possibility appears remote enough not to worry about, unless it becomes a stronger signal in the ensembles.
|Figure 2. Infrared image of thunderstorms associated with an upper low in the Gulf of Mexico. Image credit: tropicaltidbits.com.|
A system to watch in the Gulf of Mexico
NHC is now flagging a large area of convection in the eastern Gulf of Mexico, just east of the same upper low that’s been shearing Humberto. Much like Humberto, this convection is strung out along a northwest-to-southeast band. Because of the shear, this area is unlikely to develop into a tropical cyclone right away. However, as the upper low retreats toward the western Gulf, conditions may become more favorable. Any such development would likely move westward on the heels of the upper low, and the resulting system would have little time to develop before coming onshore. The 0Z UKMET model run takes this system into the Texas coast early next week, but the GFS and European runs did not develop it.
In its 2 pm EDT Tropical Weather Summary NHC gave this system a 10% chance of developing into at least a tropical depression by Monday and a 30% chance by Thursday.
|Figure 3. Infrared image of thunderstorms associated with a tropical wave in the central tropical Atlantic at 1755Z (1:55 pm EDT) Saturday, September 14, 2019. Image credit: tropicaltidbits.com.|
Tropical wave southwest of the Cabo Verde Islands a threat to develop
A tropical wave located early Saturday afternoon about six hundred miles southwest of the Cabo Verde Islands was headed west at about 10 - 15 mph. This wave had not yet been assigned an “Invest” designation by NHC. The system had a modest amount of spin and strong but disorganized convection on Saturday, as seen on satellite imagery. Conditions appeared favorable for slow development, with light wind shear of 5 - 10 knots and warm SSTs near 28°C (82°F). The wave was located far to the south in the Intertropical Convergence Zone (ITCZ), away from the dry air from the Saharan Air Later (SAL), so dry air should be less of an impediment than we’ve seen for other tropical waves this year.
The tropical wave is predicted to take a track to the west, then west-northwest, over the coming week, passing several hundred miles to the northeast of the Lesser Antilles Islands on Friday. The 0Z and 6Z Saturday runs of our three top models for predicting tropical cyclone genesis—the GFS, European and UKMET—all predicted that development into a tropical depression would occur by Wednesday. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 20% and 60%, respectively.
If it does develop, the new system could end up recurving far at sea into the atmospheric opening soon to be carved out by Humberto, as suggested by the 0Z European and GFS model runs.
Former 96L not expected to develop
The tropical wave formerly known as 96L, about 850 miles east of the Lesser Antilles Islands, is no longer being tracked by NHC, having grown disorganized and lost much of its heavy thunderstorm activity since Friday, as seen on satellite imagery. 96L was encountering dry air from the Saharan Air Later (SAL), and the combination of this dry air and the fast forward speed of the system put an end to its development.
Dr. Jeff Masters co-wrote this post.