Blog Archive

Wednesday, September 15, 2010

Jeff Masters: Karl hits the Yucatan; two simultaneous Cat 4s in the Atlantic for 2nd time in history

Karl hits the Yucatan; two simultaneous Category 4 hurricanes in the Atlantic for 2nd time in history

by Jeff Masters, WunderBlog, September 15, 2010
The Atlantic hurricane season of 2010 kicked into high gear this morning, with the landfall of Tropical Storm Karl in Mexico, and the simultaneous presence of two Category 4 hurricanes in the Atlantic, Igor and Julia. Tropical Storm Karl's formation yesterday marked the fifth earliest date that an eleventh named storm of the season has formed. The only years more active this early in the season were 2005, 1995, 1936 and 1933. This morning's unexpected intensification of Hurricane Julia into a Category 4 storm with 135 mph winds has set a new record -- Julia is now the strongest hurricane on record so far east. When one considers that earlier this year, Hurricane Earl became the fourth strongest hurricane so far north, it appears that this year's record SSTs have significantly expanded the area over which major hurricanes can exist over the Atlantic. This morning is just the second time in recorded history that two simultaneous Category 4 or stronger storms have occurred in the Atlantic. The only other occurrence was on 06 UTC September 16, 1926, when the Great Miami Hurricane and Hurricane Four were both Category 4 storms for a six-hour period. There were also two years, 1999 and 1958, when we missed having two simultaneous Category 4 hurricanes by six hours. Julia's ascension to Category 4 status makes it the 4th Category 4 storm of the year. Only two other seasons have had as many as five Category 4 or stronger storms (2005 and 1999), so 2010 ranks in 3rd place in this statistic. This year is also the earliest a fourth Category 4 or stronger storm has formed (though the fourth Category 4 of 1999, Hurricane Gert, formed just 3 hours later on today's date in 1999). We've also had four Cat 4+ storms in just twenty days, which beats the previous record for shortest time span for four Cat 4+ storms to appear. The previous record was 1999, 24 days (thanks to Phil Klozbach of CSU for this stat).

Figure 1. A rare double feature: two simultaneous Category 4 hurricanes in the Atlantic, for only the second time in recorded history.

Tropical Storm Karl made landfall as a strong tropical storm with 65 mph winds and a central pressure of 991 mb at 8:45 a.m. EDT this morning on Mexico's Yucatan Peninsula, just north of the Belize border. Karl took advantage of nearly ideal conditions for intensification, and in just 15 hours intensified from a tropical disturbance to a strong tropical storm with 65 mph winds. Had Karl managed to get its act together just one day earlier, it could have been a major hurricane at landfall this morning. Fortunately, Karl has a relatively small area of strong winds -- tropical storm force winds extend out just 45 miles from the center of the storm, and wind damage is not the main concern. Heavy rains are the main concern, and Belize radar shows heavy rain bands from Karl spreading ashore over northern Belize near the border with Mexico. Cancun radar shows that heavy rains are relatively limited, though, near the tourist havens of Cancun and Cozumel.

Figure 2. Radar image of Karl at landfall this morning near the northern Belize/Mexican border. Image credit: Belize National Meteorological Service.

Forecast for Karl
Karl will traverse the Yucatan Peninsula today and emerge into the Gulf of Mexico's Bay of Campeche Thursday morning as a much weakened tropical storm, with perhaps 40-45 mph top winds. Once in the Gulf, conditions for intensification are ideal, with wind shear is expected to be low, 5-10 knots, SSTs will be warm, 29-30 °C, and the atmosphere very moist. These conditions, combined with the topography of the surrounding coast which tends to enhance counter-clockwise flow, should allow Karl to intensify into a strong tropical storm or a Category 1 hurricane before making landfall between Tampico and Vercruz, Mexico, on Saturday morning. However, since Karl is a small storm, it is possible that passage over the Yucatan will disrupt the storm enough so that it will be much weaker. The ridge of high pressure steering Karl westwards is quite strong, and it is very unlikely that the storm will turn northwest and hit Texas. NHC is giving Brownsville, Texas, an 10% chance of receiving tropical storm force winds of 39+ mph.

Hurricane Igor put on a burst of intensification last night to put it at its strongest yet, a top-end Category 4 hurricane with 155 mph winds. Igor has weakened slightly this morning, but remains a formidable presence in the Central Atlantic with its 145 mph winds. Igor continues to show the classic appearance of a major hurricane on satellite imagery, with a well-formed eye, symmetrical cloud pattern, plenty of low-level spiral bands, and solid upper-level outflow on all sides.

Figure 3. Hurricane Igor as captured at 18 UTC Tuesday September 14, 2010, from the International Space Station. Image credit: Douglas Wheelock, NASA.

Intensity forecast for Igor
Wind shear is low (5-10 knots) and is expected to remain low for the next 2-3 days. Waters are warm (29 °C) and will remain 29 °C for the next 2-3 days. Igor is well armored against any intrusions of dry air for at least the next three days. These conditions should allow Igor to remain at major hurricane status for the next three 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 10 mph weakening Igor experienced early this morning. Igor may regain its lost intensity over the next 36 hours. By Saturday morning, 36 hours before the core of Igor is expected to pass Bermuda's latitude, the trough of low pressure steering Igor northwestwards should bring moderate wind shear of 10-20 knots to the storm, weakening it. The SHIPS models predicts shear will rise to the high range (20-30 knots) during the final 24 hours of the storm's approach to Bermuda. Igor will also be tracking over cooler 28 °C waters during this period, and substantial weakening by perhaps 20-30 mph can be expected. Igor will still probably be at least a Category 2 hurricane on its closest pass by Bermuda on Sunday. NHC is giving Bermuda a 13% chance of experiencing hurricane force winds from Igor, but this probability is likely too low. The Bermuda Weather Service is calling for Category 1 or 2 hurricane conditions for the island on Sunday, with 20-25 foot waves in the offshore waters.

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. Igor appears likely to be a threat to Bermuda, and that island can expect tropical storm force winds as early as Saturday. Igor will be moving at about 12-15 mph as it approaches Bermuda. Tropical storm force winds of 39+ mph will probably extend out about 250 miles to the north of Igor on Saturday, so Bermuda can expect 18 hours of tropical storm force winds before the core of Igor makes its closest pass. In all, Bermuda is likely to experience a very long pounding of 24-36 hours with winds in excess of tropical storm force.

The models have been in substantial agreement over multiple runs that Igor will miss the U.S. East Coast, and the danger to the U.S. will probably only come in the form of high waves. 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 8-10 foot waves on Saturday and 9-12 foot waves on Sunday.

Igor may pass very close to Newfoundland, Canada, but it is too early too assess the likelihood of this happening.

Hurricane Julia put on a remarkable and unexpected burst of intensification this morning to become the season's fourth Category 4 storm. Julia's 135 mph winds make it the strongest hurricane on record so far east; the previous record was held by the eighth storm of 1926, which was only a 120 mph Category 3 hurricane at Julia's current longitude. Julia's intensification was a surprise, since SSTs in the region are about 27.5 °C, which is just 1 °C above the threshold needed to sustain a Category 1 hurricane. Julia is headed northwest, out to sea, and it is unlikely that this storm will trouble any land areas. SSTs will steadily cool to 26.5 °C today, and further intensification today is unlikely. Shear will be moderate, 10-20 knots, over Julia during the next two days, then 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. This should substantially weaken Julia.


No comments: