Slowly but Surely, 94L Organizing in Tropical Atlantic

While Americans celebrated a holiday on Tuesday, the tropical wave known as Invest 94L stayed on task in the central tropical Atlantic, where models indicated it could become a tropical storm as soon as Wednesday. As of 2 pm EDT Tuesday, 94L was located near 10°N, 34°W, where it had moved little since Monday.

Top sustained winds associated with 94L on Tuesday afternoon were 35 mph, just below the tropical storm threshold. However, the system remained embedded in the Intertropical Convergence Zone, the west-to-east band of heavy thunderstorms that predominates just north of the equator this time of year. The ITCZ was lending 94L plenty of broad cyclonic motion extending out more than 400 miles, as revealed in surface winds detected by the ASCAT scatterometer, but the presence of the ITCZ was also making it difficult for a low-level core to establish itself. Northeasterly wind shear of around 20 knots also impeded development late Monday into early Tuesday. Showers and thunderstorms were extensive but not well organized on Tuesday afternoon.

Working in 94L’s favor is its moist environment. The system has successfully avoided entraining large amounts of the dry air lying just to its north within the Saharan Air Layer (SAL). The environment immediately around 94L should remain fairly moist (with midlevel relative humidities of 70-80%) for the next couple of days. Sea surface temperatures along the path of 94L will remain adequate for tropical development—around 27-28°C (81-82°F), or about 0.5°C above average.

Figure 1. Invest 94L in the Central Atlantic as seen on Tuesday morning, July 4, 2017, by the MODIS instrument on NASA's Terra satellite. Image credit: NASA.

The outlook for 94L

Conditions should become more favorable for 94L over the next day or two. Wind shear was already dropping by Tuesday afternoon, and the shear was predicted by the 18Z Tuesday run of the SHIPS model to drop below 10 knots by Wednesday night. The shear should  remain low for several more days as the system begins moving toward the west-northwest, which will pull it away from the ITCZ and help it establish a more distinct center. Model guidance increasingly agrees that 94L will likely become a tropical storm. All 70 members of the 0Z Tuesday GFS and European ensembles bring 94L to tropical depression strength by late Wednesday, and all GFS members—plus about 80% of ECMWF members—produce a tropical storm by late week.  The next name on the Atlantic list is Don.

While the odds of a tropical storm this week are quite high, the chance of a hurricane appears low. Less than 10% of GFS and European ensemble members bring 94L to hurricane strength. Dry air seems to be the main impediment: as 94L angles more toward the west-northwest, it will likely ingest more of the dry air associated with the SAL. If the system can wall itself off from the arid environment and maintain a moist inner core, it will have a better chance at more sustained strengthening.

Figure 2. A pocket of moisture (cool colors) surrounds the center of 94L in this prediction of total precipitable water (the amount of moisture in a column above the surface, in millimeters) and surface pressure (solid lines, in millibars). The forecast, for 8:00 pm EDT Wednesday, July 5, was generated by the 12Z Tuesday run of the GFS model. Image credit: tropicaltidbits.com.

There’s been little change in the longer-range scenario for 94L outlined in Monday’s post.  A strong ridge of high pressure should steer the system mostly west-northwest at 5 - 15 mph for the next five days, which would bring the storm near or north of the Northern Lesser Antilles Islands this weekend. We’ll need to watch this path for potential impacts to the islands, especially if the track falls toward the southern end of the possibilities in the ECMWF ensemble runs.

The main question for next week is whether we’ll see a weak system continuing west-northwest and perhaps dissipating along the way (a solution favored by many members of the 0Z Monday European ensemble) or a potentially stronger system angling more to the northwest, toward the Bahamas and perhaps further north (as depicted by a number of GFS ensemble members).  A broad weakness in the flow across eastern North America will sharpen over the weekend, then flatten early next week before potentially sharpening again. It’s too soon to know how the timing of these features might shape the path of whatever 94L has become by that point.                                                                                                           

Happy Fourth to all of our U.S. readers!

Dr. Jeff Masters contributed to this post.