Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:53 PM GMT on September 03, 2011
Tropical Storm Lee is marching steadily northwards towards landfall in Louisiana, and continues to slowly intensify. The storm's central pressure is now down to 993 mb, as measured by an Air Force hurricane hunter aircraft at 7am CDT. However, the center of Lee is now very close to the coast, and the storm doesn't have much time to intensify further before the center moves over land. The main impact from the storm on the coast thus far has been heavy rains. At New Orleans Lakefront Airport, 5.88" inches of rain had fallen from Lee as of 8 am CDT this morning. Top winds were 35 mph, gusting to 55 mph. Though Lee's top winds are rated as being 60 mph, it is difficult to find any land stations that have reported sustained winds of tropical storm force, 39 mph or greater. One station that has is at the tip of the Mississippi River Delta, where Southwest Pass measured sustained winds of 40 mph, gusting to 58 mph, at 7:03 am CDT. Upper-level winds out of the southwest are creating a moderate 10 - 20 knots of wind shear over Lee, keeping the storm's heavy thunderstorms pushed to the east side of the storm. Latest satelllite loops show Lee is becoming increasingly organized.
Figure 1. Radar-estimated rainfall from Lee from the New Orleans radar. Lee has dumped a large region of 4 - 8 inches so far (orange colors.)
Figure 2. Predicted rainfall for the 5-day period ending at 8 am EDT Thursday Sep 8, 2011. Lee is expected to bring a large swath of 4+ inches of rain all the way to New England. Image credit: NOAA/HPC.
Forecast for Lee
Lee's large size, ill-formed circulation center, and the presence of dry air on its west side due to an upper-level trough of low pressure make Lee look a lot like a subtropical storm on satellite imagery, with a broad center and the majority of the heavy thunderstorms in a broad band well removed from the center. Subtropical storms can undergo only relatively modest rates of intensification, and Lee is unlikely to become a hurricane. Also tending to slow intensification will be the fact that much of its circulation is over land. Damages from Lee are likely to be less than $100 - $200 million, with the greatest threats being fresh water flooding from heavy rains. Given that much of the region Lee will traverse over the next few days is under moderate to severe drought, the storm's rains may cause more economic benefit than damage. Since Texas is on the dry side of the storm, that state will see very little rainfall from Lee, except very close to the border with Louisiana. The rains from Lee appear to have mostly ended across extreme southern Louisiana, so the feared 10 - 15 inches of rain does not look like it will materialize there. One possible concern for Lee's rains will be the mid-Atlantic and Northeast, where recovery efforts from the devastating flooding due to Hurricane Irene may be hampered by the additional 2 - 4 inches that may fall from Lee's remnants by the middle of the week.Tornadoes from Lee are potential hazard today, as NOAA's Storm Prediction Center is highlighting the Northern Gulf Coast in their "slight risk" area for severe weather. A tornado watch is posted for the region, but no tornadoes have been reported as of 8 am CDT.
Lee is the 12th named storm this year, and came eight days before the half-way point of the Atlantic hurricane season. Climatologically, September 10 marks the half-way point. A typical hurricane season has just 10 - 11 named storms, so we've already had more than a whole season's worth of storms before reaching the half-way point. At this rate, 2011 will see 24 - 26 named storms, making it the 2nd busiest season on record, behind 2005. Lee's formation date of September 2 puts 2011 in 5th place for earliest date of arrival of the season's 12th storm. Only 2005, 1995, 1936, and 1933 had an earlier 12th storm.
The latest set of model runs show very little change in the outlook for Hurricane Katia. Katia will continue its long trek across the Atlantic Ocean, and will not pose a danger to any land areas over the next five days. Katia is still struggling with dry air and wind shear that has risen to a high 20 - 25 knots. Latest satellite loops show a lopsided hurricane that is suffering from the impacts of dry air and wind shear on its southwest side.
The models now agree that the upper-level trough of low pressure bringing the wind shear to Katia will move away by Sunday, putting Katia in an environment with low to moderate wind shear. At the same time, ocean temperatures will warm to 29°C, a full 0.5°C over what Katia is experiencing today. These effects should allow Katia to intensify to a Category 2 hurricane by Monday. It is still unclear how much of a threat Katia may pose to the U.S., as this depends on the strength and timing of a trough of low pressure predicted to move off the U.S. East Coast 5 - 7 days from now. Our models do not have enough skill to predict how the steering currents will behave that far into the future. As I mentioned in yesterday's post, part of the problem is due to the inability of the computer models to agree on what will happen to Tropical Storm Talas in the Western Pacific. Talas hit Japan early on Saturday as a strong tropical storm, and is racing northwestwards towards Alaska. Talas is expected to transition into a powerful extratropical storm in the waters south of Alaska early next week. This extratropical storm will create a ripple effect downstream in the jet stream, all the way to North America, affecting the trough of low pressure off the U.S. East Coast expected to potentially recurve Katia when it approaches the U.S. The computer models are not very good at handling these sorts of interactions, leading to more than the usual amount of uncertainty in the long-range outlook for Katia. It will probably be another two days before the models will converge on a solution for the long-term fate of Katia. It's a good bet that locations on the U.S. coast south of North Carolina are in the clear, but residents from North Carolina to New England need to watch Katia. Dr. Bob Hart's Historical Tropical Cyclone Probability web page suggests shows that tropical storms in Katia's current position have a 15% chance of hitting North Carolina, a 22% chance of hitting Canada, a 12% chance of hitting New England, and a 58% chance of never hitting land. One almost certain impact of Katia on the U.S. will be large waves. Long period swells from Katia will begin affecting the Bahamas on Sunday night, then reach the Southeast U.S. by Monday morning. By Tuesday morning, the entire U.S. East Coast will see high surf from Katia, and these waves will increase in size and power as the storm grows closer. Given the slow movement of Katia as it approaches the coast, plus its expected Category 1 to 3 strength as it approaches, the storm will probably cause extensive beach erosion and dangerous rip tides for many days.
Figure 3. Satellite image of Tropical Storm Talas taken September 2, 2011, as the storm approached Japan. Image credit: NASA.
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