Slow-moving Tropical Storm Isaac continues to hammer coastal Louisiana, Mississippi, Alabama, and Florida’s Panhandle with tornadoes, torrential rains, high winds, and a damaging storm surge. Over the past 24 hours, destructive tornadoes have touched down in Biloxi and Pascagoula, Mississippi, and one person was killed by a tree falling on a car in Pearl River County, Mississippi. A major flood event is occurring in Slidell, Louisiana, where Isaac’s storm surge filled Bayou Bonfouca and the W-14 Canal, inundating portions of the city with 1 – 5 feet of water. While Isaac is now a weakening minimal-strength tropical storm, it is still a potent rainmaker, and will cause damaging floods all along its path for the next three days. Major river flooding is occurring or is about to occur on a number of rivers in the landfall area. In north central Tangipahoa Parish in southeast Louisiana and southwestern Pike County in southern Mississippi, a mandatory evacuation has been ordered for all low-lying areas and along the Tangipahoa River, due to the potential failure of the Lake Tangipahoa dam. Audubon Park in New Orleans, recorded 19.30″ of rain as of 7 pm Wednesday night, and 18.7″ of rain in a 24-hour period. This is the greatest 24-hour rainfall event at any official New Orleans site, with weather records extending back to 1871, according to wunderground’s weather historian, Christopher C. Burt. A few other rainfall totals from Isaac, through 11 am EDT on Thursday:

15.02″ Marion, MS
10.09″ Hattiesburg, MS
10.15″ Gulfport, MS
9.80″ Slidell, LA
9.74″ Biloxi, MS
8.52″ Mobile, AL
5.57″ Baton Rouge, LA

Figure 1. Isaac’s winds and storm surge overcomes the seawall and floods South Beach Boulevard in Waveland, Miss., Wednesday, Aug. 29, 2012, the seventh anniversary of Hurricane Katrina hitting the Gulf Coast. (AP Photo/Rogelio V. Solis). Waveland experienced a storm surge in excess of 5′ for 36 hours.

Isaac’s storm surge winds down
Storm surge levels along the coast of Mississippi and surrounding areas are gradually receding, and the surge has finally fallen below 5′ at Waveland, which experienced a storm surge in excess of 5′ for 36 hours. Isaac’s storm surge levels were characteristic of a Category 2 hurricane, and lasted for an exceptionally long period of time. Waveland, Mississippi experienced a peak surge of 8′ and peak storm tide of 9′ (surge plus the natural high tide), which beat the levels that occurred during Category 2 Hurricane Gustav of 2008 (7′ of storm tide.) The peak 11.06′ storm surge at Shell Beach, which is in Lake Borgne, 30 miles southeast of New Orleans, exceeded the 9.5′ surge recorded there during Gustav. According to an article in nola.com, Isaac pushed a storm surge of 13.6′ into Lake Borgne, on the east side of New Orleans. This is not far from the 15.5′ storm surge Hurricane Katrina brought to the location. It is quite possible that Isaac’s storm surge might have breached levees of the east side of New Orleans, flooding areas inhabited by tens of thousands of people, had the Army Corps of Engineers not completed their $14.5 billion upgrade to the New Orleans flood defenses this year. I estimate that storm surge damage from Isaac will exceed $2 billion. Isaac has likely caused $2.5 billion in insured damage not related to flooding, insurance firm Eqecat estimated yesterday. Here were some of the peak storm surge values that were recorded at NOAA tide gauges during Isaac:

11.1′ Shell Beach, LA
8.0′ Waveland, MS
3.5′ Pensacola, FL
4.6′ Pascagoula, MS
3.8′ Mobile, AL

Figure 2. A TRMM satellite 3-D view of rainfall on Aug. 28 showed a few very powerful thunderstorms near Isaac’s eye were reaching heights of almost 17 km (10.6 miles.) Intense bands of rain around Isaac were occasionally dropping rain at a rate of over 2.75 inches per hour. Image credit: SSAI/NASA, Hal Pierce.

Isaac’s storm surge on the Mississippi River
A storm surge estimated at 12′ moved up the Mississippi in Plaquemines Parish near Port Sulphur, LA, near 8:30 pm EDT Tuesday, causing overtopping of the levees and flooding of homes in the mandatory evacuation areas behind the levees. These levees were not part of the $14.5 billion levee upgrade New Orleans got after Hurricane Katrina, and were not rated to Category 3 hurricane strength, like the levees protecting New Orleans are. Since salt water is more dense than fresh water, the surge travelled along the bottom of the river, with the fresh water flow of the river lying on top. The surge continued upriver, and before reaching New Orleans, encountered an underwater barrier in Plaquemines Parish. This barrier was constructed by the Army Corps of Engineers beginning on August 15, in order to keep salt water from moving upstream and contaminating drinking water for Plaquemines Parish and New Orleans. Salt water had made it 90 miles upriver to the outskirts of New Orleans, due to the low flow rate of the river (which had dropped 7′ below average in height due to the drought of 2012.) According to a spokesperson for the National Weather Service River Forecast Office, this barrier was probably able to completely block the flow of salt water upriver due to Isaac’s storm surge, and no salt water made it as far as New Orleans. However, the massive intrusion of ocean water into the river channel caused the mighty Mississippi’s fresh water flow to back up for hundreds of miles. Water levels were elevated by 10′ in New Orleans (103 miles upstream from the mouth of the Mississippi), 8′ in Baton Rouge (228 miles upstream), and 1.4′ at Knox Landing, an amazing 314 miles upstream.

Hurricane Kirk in the Central Atlantic
Hurricane Kirk intensified into a 75 mph Category 1 hurricane this morning, becoming the busy 2012 Atlantic hurricane season’s fifth hurricane. With the season’s mid-point of September 10 still almost 2 weeks away, we’ve already had 12 named storms and 5 hurricanes, which is close to what an entire season experiences in an average year (11 named storms and 6 hurricanes.) Kirk should stay well out to sea and not trouble any land areas.

Figure 3. Morning satellite image of Hurricane Kirk.

Tropical Depression Twelve forms in the Central Atlantic
Tropical Depression Twelve has formed in the Central Atlantic, and appears destined to become a hurricane before the week is out. Fortunately, Hurricane Kirk is weakening the ridge of high pressure to the north of TD 12, and the storm is expected to turn to the northwest and miss the Lesser Antilles Islands. In the long term, it remains unclear if TD 12 will follow Kirk and fully recurve out to sea. The latest run of the GFS model predicts TD 12 will recurve out to sea and not threaten any land areas, but the latest run of the ECMWF model predicts that the trough of low pressure pulling Kirk to the norhteast will not be strong enough to recurve TD 12 out to sea. Instead, the ECMWF predicts that a ridge of high pressure will build in early next week, forcing TD 12 more to the northwest, making the storm a potential threat to Bermuda, the Northeast U.S., and Canada.

from:    http://www.wunderground.com/blog/JeffMasters/article.html