Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 3:31 PM GMT on September 04, 2011
Tropical Storm Lee made landfall on the southern coast of Louisiana near 4 am CDT this morning, and continues to inch slowly northward and dump very heavy rains. The storm is not a threat to intensify, since its center lies over land, but enough of the storm's circulation is over water that Lee will be slow to weaken. At New Orleans Lakefront Airport, 8.85" inches of rain had fallen from Lee as of 9 am CDT this morning. Top winds this morning hit 46 mph, gusting to 57 mph, at 6:28 am CDT, when a heavy squall blew in. Latest satellite loops show Lee has lost quite a bit of its heavy thunderstorms since yesterday, but the storm remains a formidable rain-maker. Lee brought a storm surge of 4 feet to New Canal Station in Lake Pontchartrain and at Shell Beach east of New Orleans yesterday afternoon. NOAA's Storm Prediction Center reported eight probable tornadoes from Lee yesterday. These tornadoes damaged approximately ten buildings, but caused no deaths or injuries.
Figure 1. Observed rainfall for the seven-day period ending at 8 am EDT Sunday Sep 4, 2011. Tropical Storm Lee had dumped 5 -10 inches of rain over a large swath of the coast, with a few areas in excess of ten inches (pink colors). Image credit: NOAA/AHPS.
Figure 2. Predicted rainfall for the 5-day period 8am EDT Sunday - 8 am EDT Friday, Sep 9, 2011. Lee is expected to bring a large swath of 4+ inches of rain all the way to the mid-Atlantic. Image credit: NOAA/HPC.
Forecast for Lee
Lee's large size and slow movement will make heavy rains the main concern today. Later in the week, the remnants of Lee's may be a problem for the mid-Atlantic and Northeast, where recovery efforts from the devastating flooding due to Hurricane Irene may be hampered by an additional 2 - 3 inches of rain. Tornadoes from Lee are a 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 9 am CDT.
Hurricane Katia finally overcame the wind shear and dry air interfering with it, and intensified into a Category 2 hurricane with 100 mph winds this morning. Latest satellite loops show a well-defined eye for the first time, but the hurricane still has a lopsided appearance, due to he impacts of dry air and moderate wind shear of 10 - 20 knots on its southwest side.
Katia will not pose a danger to any land areas over the next five days, but may be a threat to the U.S. late this week. The computer models disagree considerably on the position and strength of a trough of low pressure that will develop over the Eastern U.S. late this week, and thus how Katia will be steered. The evolution of this trough will be strongly affected by Tropical Storm Lee, and the models are not very skillful at predicting transitions from tropical storm to extratropical storm, which will happen to Lee by Wednesday this week. We have two of our reliable models--the ECMWF and UKMET--predicting that the trough will form farther west, steering Katia very close to North Carolina late this week. The rest of the models predict a more easterly position of the trough, which would force Katia to turn northwards towards Canada well before reaching North Carolina. Either scenario is possible, and it will probably be at least another day before the models converge on a solution for the long-term fate of Katia. It's likely that locations on the U.S. coast south of North Carolina will not receive a direct hit from Katia, but the entire coast from North Carolina northwards to New England and Canada's Maritime Provinces is definitely at risk. Long period swells from Katia will begin affecting the Bahamas tonight, 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. Morning satellite image of Hurricane Katia.
Elsewhere in the tropics
This week and next week typically mark the two most active weeks of the Atlantic hurricane season, and there are two potential threat areas we need to watch. The GFS model predicts that a tropical depression could form in the southern Gulf of Mexico's Bay of Campeche by Friday or Saturday this week. Several other models give support to this idea, and predict such a storm would track northwards or north-northeastwards towards the northern Gulf Coast between Louisiana and the Florida Panhandle. The other region to watch is between the Lesser Antilles and the coast of Africa. The NOGAPS model is predicting that a tropical wave currently near the coast of Africa could develop into a tropical depression late this week, and arrive in the northern Lesser Antilles by Saturday or Sunday.
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