Hanna weakens; Louisiana recovers from Gustav; Ike and Josephine strengthen

Tropical Depression Gustav has moved inland over western Louisiana, and is now primarily a heavy rain threat. The rainfall forecast from NOAA's Hydrological Prediction Center (Figure 1) for the next five days foresees up to eight inches of rain over Louisiana, Arkansas, Missouri, and Illinois, which may cause some moderate flooding problems. Tornadoes are also a threat today; Gustav spawned 17 tornadoes yesterday.

Gustav generated a storm surge of 10-13 feet on the east side of New Orleans. This is characteristic of a strong Category 1 hurricane, and is similar to the surge Katrina generated along the New Orleans levee system. New Orleans was spared a full Category 2 storm surge, since Gustav hit too far to the west. According to NHC's (now retired) storm surge expert Dr. Stephen Baig, a full Category 2 storm surge would have been ~6 ft at the Lakefront, ~7.5 ft on the North Shore of Lake Pontchartrain, >9 ft in Plaquemines , >8 ft in St. Bernard, ~13 ft at Waveland, and ~8 ft at Grand Isle. NOAA tide gauges recorded maximum surges of 4.5 ft at Grand Isle and 10 ft at Waveland. AIR Worldwide Corporation is estimating that Gustav's damage will be $1.8-$4.4 billion dollars.


Figure 1. Predicted rainfall from Gustav and Hanna over the next five days. Image credit: NOAA/HPC.

Comparing Gustav to Katrina
We got very lucky with Gustav--it could have been another Katrina. Both Gustav and Katrina had similar diameters (not radii) of tropical storm force winds at landfall--440 miles. However, Katrina affected the coast with a region of hurricane force winds 170 miles across--45% larger than the 115 miles of coast affected by Gustav (Figure 2). Both storms passed over some very high heat content waters in the Gulf of Mexico--Katrina, over a Loop Current eddy, and Gustav, over the Loop Current itself. Why didn't Gustav explode into a Cat 5 monster storm when it crossed the Loop Current yesterday? Well, when a hurricane has a well-formed circular eyewall that is aligned vertically from the surface to the upper atmosphere, it acts as a very efficient heat engine that can take heat out of the ocean and convert it to the kinetic energy of its winds. When Katrina hit its Loop Current eddy, the hurricane was under low wind shear and had an ideal structure like this for taking advantage of the heat energy offered to it. Gustav, on the other hand, had just crossed Cuba when it hit the Loop Current. Gustav was under about 15 knots of wind shear, which it had been able to hold off, thanks to its tight, well-formed eyewall. However, passage over Cuba disrupted the eyewall structure just enough to allow the upper-level winds shearing it to penetrate into the heart of the hurricane. These winds ripped up the eyewall and tilted it, so that the surface eye was no longer underneath the upper-atmosphere eye. A tilted eyewall structure is not able to act as an efficient heat engine until it can get itself lined up more vertically, so Gustav was unable to take advantage of the warm Loop Current waters it was traversing. It's like when your car engine is not firing on all cylinders and you hit the gas pedal--nothing happens. Once Gustav finally did align its eyewall vertically and armored itself against the effects of the wind shear, it had passed beyond the Loop Current and was over cooler waters of much lower heat content. Thus, Gustav was not able to intensify much before landfall. The computer models that predicted a Category 4 hurricane at landfall could easily have been correct, had the shear been a few knots less when Gustav crossed Cuba.


Figure 2. Comparison of the sizes of Hurricane Gustav and Hurricane Katrina shortly before landfall. The outermost black heavy line denotes the 34 knot radius of tropical storm force winds, while the black heavy line marking the beginning of orange colors (64 knots) denotes the region of hurricane force winds. Both Gustav and Katrina had similar diameters (not radii) of tropical storm force winds at landfall--440 miles. However, Katrina affected the coast with a region of hurricane force winds 170 miles across--45% larger than the 115 miles of coast affected by Gustav. Image credit: NOAA/AOML/HRD.

Hanna
Hanna is struggling with some extremely high wind shear of 30 knots, thanks to strong upper-level winds from the north. It's amazing that the storm has held together in the face of this shear, but Hanna is definitely suffering. Satellite loops show the shear has destroyed all of Hanna's heavy thunderstorm activity except on the south side. These heavy thunderstorms are currently pummeling Haiti.

The track forecast for Hanna
The current steering flow driving Hanna to the west-southwest is very weak, and we can expect erratic motion over the next day. By Wednesday, a rather strong high pressure ridge will build over Hanna, forcing it northwest to a landfall in the Southeast U.S. Due to the storm's expected rather random motion over the next day, plus the expected track of Hanna parallel to the Southeast U.S. coast, the location of final landfall has a much higher uncertainty than usual.

The intensity forecast for Hanna
The wind shear of 30 knots is forecast to gradually relax to 10-15 knots by tomorrow morning, and remain in the 10-20 knot range until landfall Friday. This reduced shear should allow Hanna to intensify, as the storm will be over warm 29°C waters with a Tropical Cyclone Heat Potential (TCHP) of 50-70, just below the value of 80 typically associated with rapid intensification. The GFDL and HWRF models respond by intensifying Hanna to a Category 2 hurricane by landfall; the SHIPS model foresees a Category 1 hurricane. However, these models did not anticipate Hanna's current disorganization. Given the current disorganized state of Hanna, Category 1 strength is probably the maximum the storm has time to achieve before landfall.

Ike
Tropical Storm Ike spun up yesterday in the middle Atlantic, and has the potential to become a major Cape Verdes-type hurricane. Visible satellite loops show an expanding circulation, with good upper-level outflow developing in all quadrants. Ike is in a very favorable upper-level wind environment, with an upper-level anticyclone overhead, and wind shear less than 10 knots. There is not much heavy thunderstorm activity yet, probably due to the presence of some dry air and rather cool sea surface temperatures (SSTs) of 27°C. SSTs will gradually warm to 29°C over the next five days, but the shear is forecast by some models to increase above 20 knots by Thursday. The SHIPS model responds by strengthening Ike only to a Category 1 hurricane. However, the HWRF and GFDL models do not depict as much shear 3-5 days from now, and intensify Ike into a major Category 3 or 4 hurricane on Friday, when it is expected to be 100-300 miles north of Puerto Rico. Both of these models predict landfall in the Dominican Republic or Haiti as a major hurricane on Saturday. This kind of intensification seems unlikely at present, due to the increased shear likely Thursday and Friday. The GFS and ECMWF foresee Ike passing through Cuba and into the Gulf of Mexico next week. The first Hurricane Hunter mission into Ike is scheduled for Friday afternoon.

Josephine
Tropical Storm Josphine formed today off the coast of Africa, just like the long-range GFS model has been predicting for the past week. The GFS has done very well forecasting up to a week in advance the recent string of African tropical waves that have developed. Josephine looks like it could be a problem for the Northern Lesser Antilles Islands a week or so from now, but it is too early to be confident of this. The tropics are too busy to spend much time on Josephine. I'll say more on it later this week.

Elsewhere in the tropics
There's a tropical wave over Africa behind Josephine that the GFS model forecasts will develop into a tropical storm next week.

My next blog entry will be this afternoon. There will be a new Hurricane Hunter plane in Hanna to report on.

Jeff Masters