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 , 8:28 PM GMT on September 01, 2008
Hurricane Gustav continues plowing inland, and is now just a Category 1 hurricane. The storm surge has peaked and is falling in New Orleans and along the Mississippi coast. A storm surge of 12 feet was recorded at Northeast Bay Gardene, and surges of 10 feet were seen at several other locations (Figure 2). Higher storm surges no doubt affected nearby locations, and some levees were overtopped. However, there are no reports as yet that any levees failed.
Top winds I've seen measured at the surface in Gustav were at the mouth of the Mississippi River at Pilot's Station East, which reported sustained winds of 91 mph, gusting to 117, at a height of 24 meters at 4 am CDT. Top waves were 34 feet at 2 am at Buoy 42040, south of Dauphin Island, Alabama. Gustav has spawned seven tornadoes so far. No serious damage has been reported from these twisters yet.
Figure 1. Hurricane Gustav at landfall, 10:40 am EDT 9/1/08. At the time, Gustav was a Category 2 hurricane with 110 mph winds. A strengthening Tropical Storm Hanna is visible at right. Image credit: NASA.
Figure 2. The tide gauge at Shell Beach (top), on the east side of New Orleans in Lake Borgne, recorded a peak storm surge 9.5 feet at about noon today. The storm surge peaked at 10 feet in Waveland, Mississippi (bottom). Image credit: NOAA Tides & Currents.
Now that the storm surge has died down, the main concerns from Gustav are wind damage and fresh water flooding. NOAA is predicting up to 16 inches of rain could fall in the next five days over Louisiana (Figure 3). So far, up to six inches of rain has fallen in coastal Louisiana.
Figure 3. 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 4). 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 4. 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.
Looking at the satellite loops and wind shear images of Hanna, you'd never suspect that this storm was a hurricane. Hanna is under very high wind shear of 25 knots, thanks to strong northerly upper-level winds that are part of the outflow from Hurricane Gustav. These strong winds have distorted Hanna into an amorphous shifting blob of heavy thunderstorms with little resemblance to a hurricane. Nevertheless, it is a hurricane--the Hurricane Hunters found a central pressure of 983 mb at 3:16 pm EDT this afternoon, with surface winds of 75 mph. However, the shear is so strong that Hanna has not been able to form an eyewall. Recent eye reports from the Hurricane Hunters suggest this process is underway, though.
The track forecast for Hanna
The current steering flow driving Hanna to the west-southwest is very weak, and we can expect erratic motion or a loop over the next two days, in the vicinity of the Bahama Islands. Hanna may move far enough south to hit Cuba, which would seriously weaken the storm. However, only the UKMET model forecasts this, and I'm not counting on Cuba helping the U.S. out again. By Wednesday, a rather strong high pressure ridge will build over Hanna, forcing it to a landfall in the Southeast U.S. Due to the storm's expected rather random motion over the next two days, the location of final landfall has a much higher uncertainty than usual. It is cases like this that really expand the size of NHC's cone of uncertainty, when they go to recalculate the size the cone after hurricane season. So, take your pick of landfall locations:
UKMET, South Florida, Thursday night
GFS, GA/SC border, Friday afternoon
NOGAPS, FL/GA border, Friday morning
GFDL, GA/SC border, Friday morning, Category 3
HWRF, GA, Friday morning, Category 2
ECMWF, GA/SC Friday
These landfall locations have been shifting around quite a bit over the past few days, with North Carolina the favored target yesterday. There have yet to be any model runs showing Hanna recurving out to sea without hitting the U.S. It is likely that Hanna will recurve after landfall, dumping copius amounts of rain on the mid-Atlantic states and New England.
The intensity forecast for Hanna
OK, here are my words from yesterday: "Hanna will not be able to intensify significantly over the next two days, due to upper low it is situated under, and the outflow from Hurricane Gustav." Well, the upper low dissipated, which apparently was enough to allow Hanna to intensify, despite 25 knots of wind shear. It's very unusual to see a tropical storm intensify into a hurricane while under that much wind shear. The shear is expected to remain 20-30 knots over the next 1-2 days, then decrease to 10 knots by Friday as Gustav weakens and pulls away, reducing the amount of its upper-level outflow that is currently shearing Hanna so much. All the major intensity models respond by intensifying Hanna into a Category 2 or stronger hurricane. This is a low confidence intensity forecast--as is typical for intensity forecasts. I wouldn't be surprised if Hanna drops back down to tropical storm strength Tuesday or Wednesday, due to the shear. As is the case with the track forecast, we don't have a very good idea how strong Hanna might be on Thursday and Friday.
Here come Ike and Josphine!
OK, this is really getting nuts. We've got two more very impressive storms that came off the coast of Africa that look like they will become hurricanes. Ike has a good chance of becoming a large and dangerous major Cape Verdes-type hurricane, although our skill in predicting such things five days in advance is nil. The GFDL model makes Ike a Category 2 hurricane by Thursday, while the HWRF forecasts a Cat 4. NHC conservatively forecasts a Cat 1. Visible satellite loops show a large and very intimidating circulation, with plenty of heavy thunderstorm activity and decent upper-level outflow beginning. Ike is expected to pass well north of the Lesser Antilles Islands on Friday or Saturday, but will get forced west-southwest towards Hispaniola or the Bahamas late this week. I do not expect Ike to recurve out to sea. Ike's sister, Josephine (AKA 99L), looks like it will form just off the coast of Africa on Tuesday.
Elsewhere in the tropics
There are two other areas of disturbed weather in the Atlantic that currently don't appear to be threats to develop, due to high wind shear. NHC is giving these systems a low (<20% chance) of developing into a tropical depressions over the next two days. Consult the NHC Graphical Tropical Weather Outlook for more details.
There is one other impressive African tropical wave lined up behind 99L that is likely to be a threat to develop once it moves offshore Africa late this week. It's time for a vacation in the ice-free Arctic this September! Yes, the Arctic now has it's second lowest ice extent on record, and may surpass the record set just last year. The Northwest Passage has opened up for the second time in recorded history, 2007 being the other time. I'll blog about this in more detail once the unbelievable onslaught of hurricanes eases up.
My next blog entry will be Tuesday morning.
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