Igor turns west-northwest; Julia a hurricane; 92L growing more organized

Hurricane Igor remains an impressive Category 4 hurricane with 135 mph winds over the Central Atlantic. Though Igor's winds are 15 mph weaker than at its 150 mph peak yesterday, the hurricane continues to maintain the classic appearance of a major hurricane on satellite imagery, with a well-formed eye, plenty of low-level spiral bands, and solid upper-level outflow to the north and south.


Figure 1. Hurricane Igor as seen by the MODIS instrument on NASA's Aqua satellite at 12:40 pm EDT Monday, September 13, 2010. Image credit: NASA.

Intensity forecast for Igor
Wind shear is low, 5 - 10 knots, and is expected to remain low for the next 3 - 4 days. Waters are warm, 28.7°C, and will warm to 29°C by Wednesday. Igor has moistened its environment enough to keep the dry air of the Saharan Air Layer (SAL) at bay. These conditions should allow Igor to remain at major hurricane status for the next 3 - 4 days. The hurricane will probably undergo one of the usual eyewall replacement cycles intense hurricanes commonly have, where the eyewall collapses and a new eyewall forms from an outer spiral band. This will weaken the hurricane by 10 - 20 mph when it occurs, and may be responsible for the 15 mph weakening Igor experienced since yesterday. Igor will probably regain its lost intensity when the cycle completes in the 12 - 36 hours. By Saturday, when Igor should be nearing Bermuda, the trough of low pressure steering Igor northwestwards should bring moderate wind shear of 10 - 20 knots to the storm, weakening it. Igor will also be tracking over cooler 28°C waters then.

Track forecast for Igor
The track forecast for Igor remains unchanged. Igor has made its long-anticipated turn to the west-northwest, in response to the steering influence of a broad trough of low pressure moving across the Western Atlantic. This trough will steer Igor several hundred miles to the northeast of the Lesser Antilles, and high waves should be the only impact of Igor on the islands. In the longer range, Igor appears likely to be a threat to Bermuda, and that island can expect tropical storm force winds as early as Friday. Igor does have a small chance of making landfall on the U.S. East Coast or Canada. Climatology shows that about 20% of all tropical cyclones that have existed at Igor's current position have gone on to hit the U.S. East Coast; these odds are about 5 - 10% for Bermuda and 15% for Canada. The forecast steering current pattern for the period 5 - 10 days from now from the ECMWF and GFS models shows several modest troughs of low pressure moving across the Western Atlantic. These troughs will probably be strong enough to recurve Igor out sea. However, 5 - 10 day forecasts are prone to large errors, and it is too early to be highly confident that Igor will miss hitting the U.S. or Canadian coasts.

Wave forecast for Igor
Large swells from Igor have arrived in the Northern Lesser Antilles Islands, and will spread westwards over the next few days, reaching the U.S. East Coast on Friday. By Saturday, much of the East Coast from northern Florida to Cape Cod Massachusetts can expect waves of 3 - 4 meters (10 - 13 feet), causing dangerous rip currents and significant beach erosion. These waves will continue through Sunday then gradually die down. The latest NOAA marine forecast for Cape Hatteras, North Carolina calls for 8 - 11 foot waves on Saturday.


Figure 3. Forecast wave heights for 2pm EDT Saturday September 18, 2010, as predicted by 00 UTC 9/14/2010 run of NOAA's Wavewatch III model.

Caribbean disturbance 92L
Tropical disturbance 92L over the Western Caribbean, between Jamaica and the Yucatan Peninsula, has become more organized this morning. The storm remains a threat to develop into a tropical depression, but time is running out for it to do so. There is no evidence of a surface circulation on satellite loops this morning, but the cloud pattern of 92L has become more circular, with low-level spiral bands developing on the west and north sides of the storm. 92L has a moderate but increasing area of intense thunderstorms; these are bringing heavy rains to Jamaica, the Cayman Islands, and western Cuba this morning.


Figure 3. Morning satellite image of 92L.

Forecast for 92L
Rains from 92L will spread over Belize, Northern Guatemala, and Mexico's Yucatan Peninsula tonight through Wednesday, bringing peak accumulations in the 4 - 8 inch range. Lesser peak amounts of 2 - 4 inches are possible over northern Honduras. Wind shear over 92L is low, 5 - 10 knots, and is expected to remain low for the next five days. The waters beneath are hot, 29.7°C, and these warm waters extend to great depth. Water vapor satellite loops show a large area of dry air lies to the north and west of 92L; this dry air has been interfering with development of 92L, and will continue to do so. NHC is putting the odds of 92L developing into a tropical depression by Thursday at 40%; I'll give it a 50% chance. Given the current disorganized state of 92L, it would be difficult for it to intensify quickly enough to become any stronger than a tropical storm with 50 mph winds by Wednesday afternoon, when it will move over Belize and Mexico's Yucatan Peninsula. Once 92L crosses the Yucatan, the ridge of high pressure steering it is expected to remain in place, forcing 92L west or west-southwest to a second landfall in Mexico between Veracruz and Poza Rica early Saturday morning. The shape of Mexico's Bay of Campeche and the topography of the mountains surrounding the Bay help air spiral in a counterclockwise fashion, aiding tropical storm development, and 92L has its best chance of development once it crosses into the Bay of Campeche. With the shear there expected to be low and the waters warm, I give a high 70% chance that 92L will be a tropical depression or tropical storm in the Bay of Campeche. The GFDL model is predicting 92L could be a hurricane at landfall near Veracruz on Saturday morning; the other intensity models are much less aggressive. Given the rapid development of Hermine in a similar location last week, residents of the Mexican Gulf Coast should be wary of the possibility that 92L could intensify into at least a strong tropical storm before making landfall Friday night or Saturday morning. A Hurricane Hunter aircraft is scheduled to investigate 92L this afternoon.

Julia
Hurricane Julia is headed northwest, out to sea, and it is unlikely that this storm will trouble any land areas. The intensification of Julia into a hurricane brings our activity tally for the 2010 Atlantic hurricane season to 10 named storms, 5 hurricanes, and 3 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes, so we've already had a full season's worth of activity, with about 45% of the season still to come.

Shear will be low to moderate, 5 - 20 knots, over Julia during the next two days. Shear will rise sharply to 30 knots 3 - 5 days from now, as Julia moves within 1000 miles of Igor and begins to experience strong northwesterly winds from her big brother's upper level outflow. Julia does have a window of opportunity today and Wednesday to intensify into a Category 2 hurricane before the shear rises.

Elsewhere in the tropics
The GFS and ECMWF models are suggesting the possibility of a new tropical depression forming a few hundred miles off the coast of Africa about seven days from now.

"Hurricane Haven" airing this afternoon
Tune in to my live Internet radio show, "Hurricane Haven", airing at 4pm EDT today. If you want to ask a question, the call in number is 415-983-2634, or you can email a question to broadcast@wunderground.com. Be sure to include "Hurricane Haven question" in the subject line.

Today's show will be about 30 minutes, and you can tune in at http://www.wunderground.com/wxradio/wubroadcast.h tml. The show will be recorded and stored as a podcast.

Next post
I'll have a new post on Wednesday morning.

Jeff Masters