A large and vigorous tropical wave, designated 97L by NOAA’s National Hurricane Center (NHC) at 2 p.m. EDT Sunday, August 16, was bringing heavy rain showers to the southern two-thirds of the Lesser Antilles Island chain on Monday afternoon.
The wave was headed west at about 20 mph – a speed fast enough to impart a shearing action on the disturbance, making development difficult. Otherwise, conditions for development of 97L were favorable, with sea surface temperatures near 29 degrees Celsius (84°F) and moderate wind shear of 10 – 15 knots. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 60%. Satellite images showed that 97L had a modest amount of poorly organized heavy thunderstorm activity.
Forecast for 97L
As 97L progresses west to west-northwestward over the next few days, it will slow its forward speed, which should allow increased chances of development. By Friday, when 97L is expected to slow its forward speed to 10 – 15 mph and be in the western Caribbean, very favorable conditions for development may exist. The 12Z Monday run of the SHIPS model predicted that the atmosphere surrounding the system would moisten to a relative humidity of 70%, wind shear would be a light 5 – 10 knots, and SSTs would be a very warm 29.5 degrees Celsius (85°F). The waters of the western Caribbean have the highest heat content of any place in the Atlantic, providing ample fuel for any tropical cyclone that may spin up there.
The 12Z Monday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – did not support intensification of 97L into a tropical depression or tropical storm by Friday. The GFS model predicted development might occur by Saturday, though, when 97L would be entering the Gulf of Mexico. In a 2 p.m. EDT Monday Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of formation of 20% and 50%, respectively. No hurricane hunter missions into 97L are currently scheduled.
Hot on 97L’s heels: a potentially more dangerous threat
A bigger risk of developing into a dangerous threat appears to be a large and complex tropical wave in the eastern Atlantic, located about 500 miles southwest of the Cabo Verde Islands at 2 p.m. EDT Monday. NHC on Monday afternoon designated this weave 98L.
The wave will be moving west to west-northwest at 15 – 20 mph this week. Conditions for development of pre-98L were favorable, with sea surface temperatures near 28 degrees Celsius (82°F) and moderate wind shear of 10 – 15 knots. The system was embedded in a moist atmosphere, with the dry air of the Saharan Air Layer well to the north. Satellite images showed that pre-98L had an impressive amount of heavy thunderstorm activity, but the system had two main centers of action separated by about 500 miles, each developing some spin. The system was struggling over which center to consolidate, which is slowing development.
Forecast for 98L
The future track of 98L will heavily depend upon which of the two areas of action the system ends up consolidating around. In either case, a general west to west-northwest motion can be expected over the next several days. The 12Z Monday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – all supported intensification of 98L into a tropical depression or tropical storm by Friday, when it is expected to be nearing the Leeward Islands. In a 2 p.m. EDT Monday Tropical Weather Outlook, NHC gave 98L two-day and five-day odds of formation of 30% and 70%, respectively.
The next two names on the Atlantic list of storms are Laura and Marco. The earliest twelfth storm in Atlantic tropical history was Luis on August 29, 1995; there is a tie for earliest thirteenth storms, with Lee on September 2, 2011, and Maria on September 2, 2005.
MJO is little help to Atlantic storms early this week – but watch out next week
The Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the equator that moves around the globe in 30 – 60 days, can greatly increase the odds of tropical cyclone formation when strong and in the proper location. The active portion of the MJO features rising air, which increases thunderstorm updrafts and helps tropical cyclones grow stronger and more organized. Adjacent to the active portion of the MJO is a suppressed portion featuring sinking air, which favors high pressure, drying air, and reduced thunderstorm updrafts.
The U.S. Climate Variability and Predictability Program summarized the MJO’s impact in the Atlantic as follows:
Gulf of Mexico and Caribbean Sea hurricanes are four times more likely to occur when the MJO is producing enhanced precipitation and divergent upper level winds than when precipitation is suppressed and upper level winds are convergent. The modulation of major hurricanes (Categories 3-5) by the MJO is even more pronounced.
This week, the MJO is in a position that favors hurricane activity in the Northeast Pacific, but one that has little signal, either positive or negative, over the Atlantic (though the Gulf of Mexico and western Caribbean will experience some enhancement of conditions favorable for tropical cyclones as the week progresses). The favorable MJO location over the Northeast Pacific is probably partly responsible for the expected rapid intensification of Hurricane Genevieve (see below).
By next week, the most active portion of the MJO is expected to progress eastwards across the Gulf and Caribbean and into the open tropical Atlantic, areas where it is likely to enhance tropical storm activity through the first week of September. The level of enhancement remains uncertain at this point, as computer models disagree on how strong the MJO will be when it reaches the Atlantic.
Also favoring hurricanes next week will be a large-scale region of ascending air over the Atlantic, caused by passage of an atmospheric disturbance called a Convectively Coupled Kelvin Wave, as explained in a tweet by Michael Ventrice.
Genevieve seen likely as year’s second major hurricane in the Northeast Pacific
Goosed by the Madden-Julian Oscillation, Hurricane Genevieve was rapidly gaining strength on Monday off the Pacific coast of Mexico, about 750 miles southeast of Cabo San Lucas. Genevieve is passing over slightly warmer than average waters – around 30 degrees Celsius (86°F) – and it is in a near-ideal environment for strengthening, with ample upper-level moisture and light wind shear of only around 5 knots.
The odds of rapid intensification, as calculated by the SHIPS statistical model, are stunning. As of Monday morning, SHIPS gave Genevieve a 95% chance of attaining Category 3 strength (100 knots) by Tuesday morning. That’s more than 15 times greater than the climatological odds! The HWRF and HMON intensity models agree that Genevieve likely will be a major hurricane as soon as Tuesday. According to SHIPS, there is a 65% chance that Genevieve will hit a Cat 4 strength of 140 mph by Wednesday morning. Such rapid intensification is not unheard of, but it’s quite unusual for it to be depicted so strongly in the tools used by forecasters.
Genevieve will track mainly northwest, taking it close to the southern tip of Baja California by Thursday as it encounters cooler waters and drier air. Forecast models are leaning toward a track just offshore, but residents will want to monitor Genevieve closely. The storm could draw close enough to bring high winds and heavy rain to the southern peninsula, and huge surf is a safe bet by midweek.
Bob Henson wrote the Genevieve portion of this post.