Tropical Depression 17 forms

An Air Force Hurricane Hunter plane has arrived in the area of disturbed weather 150 miles southeast of the northeastern tip of Nicaragua and found Tropical Depression 17 has formed. The aircraft found a broad center of low pressure with top winds at flight level (600-1000 feet) of 30 mph. The central pressure was 1004 mb, and the plane did see two squalls of 54-58 mph winds at the surface with their SFMR instrument. Visible satellite images have shown a significant increase in the intensity and areal coverage of the heavy thunderstorm activity this afternoon. An upper-level outflow channel has opened to the north, and the latest microwave image from the TRMM satellite shows that low-level spiral bands are beginning to form, though the center of the storm remains broad.


Figure 1. Current satellite image of TD 17.

The intensity forecast
Wind shear has dropped to a low 5 knots this afternoon, and is expected to remain very low, 0-5 knots, over the next two days. Water temperatures are warm, 29°C, and this warm water extends to great depth. These are very favorable conditions for intensification--if the storm avoids passing too close to land near the northeast coast of Nicaragua. The latest (12Z, 7am EST) runs of both the GFDL and HWRF models predict 93L will not interact will land enough to weaken the storm, and predict TD 17 will pass though the Cayman Islands on Saturday morning as a Category 2 hurricane, and strengthen to a Category 3 or 4 hurricane by landfall Sunday morning in central Cuba. The SHIPS intensity model is less aggressive, predicting a strong tropical storm with 70 mph winds by Saturday. I give TD 17 a 70% chance of becoming a hurricane, and a 40% chance of becoming a major hurricane. The storm has the potential to imitate Hurricane Michelle of 2001. Michelle formed in the same region at the same time of year, and took just three days to intensify from a tropical depression to a Category 4 hurricane. Michelle made landfall in central Cuba as a Category 4 hurricane, then weakened to a Category 1 hurricane as it passed through the Bahamas.

The track forecast
A slow north-northwest motion to a point just offshore the Nicaragua-Honduras border is likely for TD 17 through Thursday. At that time, a strong trough of low pressure will be approaching the U.S. East Coast. There is some uncertainty whether this trough will be strong enough to lift TD 17 northwards across Cuba. Two major models, the NOGAPS and GFS, predict that TD 17 will be too weak to "feel" the influence of this trough, and they keep the storm trapped in the Caribbean near Honduras and Nicaragua. If this solution is correct, a major rain event for northern Nicaragua and Honduras is possible, with heavy rains of 5-10 inches likely to cause life-threatening flash floods and mudslides. The storm would likely intensify to a tropical storm or Category 1 hurricane, depending upon how much interaction with land occurs. On the other hand, the GFDL, HWRF, and ECMWF models predict that TD 17 will be strong enough to feel the trough. These models predict a northward motion through the Cayman Islands on Saturday, followed by a northeast turn and a landfall in central or eastern Cuba on Sunday morning. On Sunday afternoon, TD 17 would then pass through the central Bahamas as it accelerates to the northeast. These models predict a deeper, stronger storm that is more likely to feel the influence of the trough. The UKMET model is between these two solutions, and predicts TD 17 will get stranded in the Western Caribbean near the western tip of Cuba.

Given the current increasing trend in organization, I believe TD 17 will grow deep enough and strong enough to get pulled northward by the trough. The storm should move through the Cayman Islands on Saturday and across central or eastern Cuba on Sunday, as the GFDL, HWRF, and ECMWF models are predicting. There is the potential that the storm could enter a period of rapid intensification on Saturday, bringing it to major hurricane status. Residents of the Cayman Islands, western Jamaica, and central and western Cuba should anticipate the possibility of a hurricane--possibly a major hurricane--affecting them Saturday and Sunday. If TD 17 crosses Cuba as expected and moves into the Bahamas, it will weaken due to the interaction with land and the presence of very high wind shear of 30-50 knots just north of Cuba. These effects should weaken the storm by at least 40 mph before it passes through the Bahamas. The HWRF and GFDL models predict TD 17 could be a Category 2 hurricane in the Bahamas, but I think a Category 1 hurricane is more likely.

Links to follow
Wundermap for 16N 83W

I'll have an update Thursday morning.

Jeff Masters