Katrina's Surge, Part 16

A Weather Underground 16 part series about Hurricane Katrina, by Margie Kieper.

It is the end of September, and we have come to the last destination, and the end of this series, as we travel the coastline destroyed by Hurricane Katrina's record storm surge a little over a year ago. This is something that has never been shown on the news or talked about, either in the overall, or in detail. What you'll be seeing here is what people on the Gulf Coast have been calling the “Invisible Coastline” for almost a year now.

Today we look at Mobile County, Alabama, including the coastal communities of Bayou La Batre and Coden, the city of Mobile, situated along Mobile Bay, and the barrier island of Dauphin Island.

From Google Maps, the location of this area along the Gulf of Mexico coastline impacted by Katrina:

Pascagoula, Jackson County, MS

Image courtesy of Google Maps

An interesting historical note: Dauphin Island was the first capital of the French colony of Louisiana (1700). The capital was moved to Mobile in 1702.

The surge along the coast in this area ran from about 15 feet in Bayou La Batre, to almost 12 feet in downtown Mobile, with 6-7 feet on Dauphin Island. In the excellent post-Katrina writeup from the Mobile NWS Forecast Office, the section on surge notes:

"Known for her killer storm surge, Katrina's highest surge values were found in a zone from just east of the eye near Bay St. Louis, MS eastward to the northern reaches of Mobile Bay. The Mobile State Docks measured the highest storm surge value of 11.45 feet, while the lowest was 4.1 feet in Santa Rosa Sound, Florida. It is known that the storm surge was as high as 12-14 feet range in Bayou La Batre, Alabama and was likely closer to 20 feet immediately along the Mississippi-Alabama border. Many homes were completely engulfed by Katrina's surge in Bayou La Batre. The surge in Mobile Bay led to inundation of downtown Mobile causing the imposition of a dusk-to-dawn curfew. The Mobile State Docks surge value of 11.45 feet was extremely close to being the highest value ever recorded (previous record of 11.60 feet that occurred on 5 July 1916). The exact location and degree of accuracy of the 1916 record surge value is unknown."

Here a Mapquest image shows the area in more detail. Dauphin Island is to the south, and Mobile to the north, along the eastern coastline.

Mapquest – Coden and Bayou La Batre

Image courtesy of Mapquest

The small coastal area of Coden and Bayou La Batre is a key center for the shrimping industry in the US, and was completely destroyed. This was poignantly documented in an article in the Associated Baptist Press only days after Katrina landfalled. The article had to say, in part:

"It's just unreal. It's the worst I've seen anywhere," said Joseph Rodriguez, a shrimper and boat builder, who is a native of the area. Katrina's surging waves lifted one of his two shrimp boats, the Integrity, from the bayou and involuntarily dry-docked her at a shipyard, right below the drawbridge in the town's center.

Junior Wilkerson, skipper of the Integrity, rode out Katrina on the boat, along with his wife and children. He fought the 100-plus-mph winds and 15-foot storm surge in a vain effort to keep Integrity from breaking loose from its moorings.

Wilkerson said he was never scared during the ordeal. He's ridden out many hurricanes on his boats, including 1969's Camille. "It's the safest place to be," he said. "But you might not be on the water [when the storm stops]."

A tour of the area Sept. 3 -- five days after the storm's passage -- revealed scores of shrimp boats in situations worse that Rodriguez's.

"I went up the bayou the other day -- I counted 87 boats" that had been tossed from the port, some deposited hundreds of yards inland, he said. "I know for a fact that there's about 30 that are in the woods up here."

Regarding a boat being the safest place to be in a hurricane...hold that thought until you see these images below. The first three images, from a NOAA aerial photo, show the shrimp docks, and the many boats stranded inland.

NOAA aerial Coden NOAA aerial Coden NOAA aerial Coden

Images courtesy of NOAA

And as we have seen, these aerial images can not convey the sense of the damage that can be seen close up:

NWSFO MOB Coden and Bayou La Batre damage NWSFO MOB Coden and Bayou La Batre damage NWSFO MOB Coden and Bayou La Batre damage

Images courtesy of NWSFO Mobile

See the boats that are sideways or on top of each other, or, the ones where the hull is damaged? This really isn't the place you want to be in high winds, high waves, and, just under the water, all kinds of large objects that your boat can be dashed against by the waves...things like piers and cars and houses and trees that have broken off, and other boats that have already sunk. A boat is the safest place to be? Think about it...before the storm arrives, you could get in the car and drive far away from the shoreline, and not have to worry about being in something that may sink at a time when swimming isn't really possible, or worry about being carried out to sea where the waves really will swamp you. A hurricane is no time to be in a boat!

Here are several images of the area taken by a local resident:

Coden and Bayou La Batre damage

These two photos are interesting because they show the force of the surge, many miles from where the eye made landfall, and the height of the surge:

NWSFO MOB Coden and Bayou La Batre damage NWSFO MOB Coden and Bayou La Batre damage

Images courtesy of NWSFO Mobile

Flooding engulfed the downtown area of Mobile, and water covered the I-10 causeway over Mobile Bay. One of the most frightening images I've ever seen was, mid-morning, on the day that Katrina hit, this image of the causeway, completely under water:

Mobile I-10 causeway under water

I already knew how much flooding was going to be occurring in Mississippi, but knowing where the eye made landfall, at the LA / MS border, by watching it first on Slidell radar, then on Mobile radar, and then seeing the extent of the flooding this far east, was simply unreal. The causeway usually sits high above the water:

Mobile I-10 causeway

Here are some images of downtown Mobile during the flooding from Katrina, thanks to Wundergrounder Mobal:

Mobile flooding Mobile flooding Mobile flooding Mobile flooding Mobile flooding

Just south of Mobile is the Dog River watershed:

Dog River watershed

Even this far north on the bay, the force of the surge did notable damage:

NWSFO MOB - Dog River watershed damage NWSFO MOB - Dog River watershed damage

Images courtesy of NWSFO Mobile

All of the barrier islands of LA, MS, and AL were heavily impacted in recent years by strong hurricanes in the GOM, including Ivan, Dennis, and Katrina. The USGS has a remarkable set of impact studies, including one on Dauphin Island, showing the dramatic eroding of the western end of the island, which is no more than a sand spit now. When I was a teenager, we used to go out to the western end and walk on the dunes, which are no more.

Here are some damage photos of the western end of Dauphin Island:

NWSFO MOB - Dauphin Island damage NWSFO MOB - Dauphin Island damage NWSFO MOB - Dauphin Island damage

Images courtesy of NWSFO Mobile

"Ancient storm field of study:" That would be paleotempestology, a droll term created by Kerry Emanuel, and it refers to the study of historical tropical cyclone activity, using methods such as examining overwash deposits in lakes, tree rings, or other geological proxies. Examining overwash deposits was originally used to study tsunami history. There's a blog entry coming up on this in future. For now I just want to pass on a very interesting piece of information that I heard secondhand, which is that a recent study of overwash deposits indicated that Katrina was on the order of a 500-year storm. We know already that structures that had been standing all along the northern Gulf Coast since the early 1800s, for around 200 years, were destroyed by the surge. Someday soon this and other studies will be published, and more people will become aware of just how uncommon was the extent of Katrina's surge.

More amazing facts; what Katrina did to buoys: The National Data Buoy Center keeps a record of significant tropical cyclones and the recorded buoy information on wind and waves, and the report on Katrina contained some information, that while simply and factually stated, was nevertheless extraordinary. Below is the map of buoys that were within 300 nautical miles of Katrina's path through the North Atlantic and Gulf of Mexico:

NDBC Katrina buoy map

Image courtesy of NDBC

First, as maximum significant wave heights started approaching 11 meters at buoy 42003, it was capsized. The report notes:

"Data was lost after 08/28/05Z because the buoy capsized. This is this first capsizing of a 10-meter buoy in the Gulf of Mexico in NDBC's 30-year history of operation."

That's a buoy that is about 30 feet wide, that is very bottom-heavy and built to be very stable. Here's an image of one:

NDBC 10m discus buoy

Image courtesy of NDBC

And the highest wave that can usually occur for a given maximum significant wave height, is about 1.9 times higher (theoretically, twice as high). So, for the given max sig wave height before the buoy capsized, which was actually 10.58m, or almost 35 feet, the highest possible wave would have been around 66 feet. For a good explanation of significant wave height, written in layman's terms, check out this link to the Mariner's Weather Log. The NDBC site has a more technical explanation.

Next, BURL1, located at the tip of Southwest Pass, LA, at the entrance to the Mississippi River, failed due to storm surge, when Katrina was still 61 nautical miles (70 mi) away. Not surprising, you think? Well, this station's equipment isn't exactly sitting at sea level:

NDBC BURL1

Image courtesy of NDBC

The station at Grand Isle, GDIL1, failed at about the same windspeed, although that failure could also have been due to surge (remember, we started this series with Part 1 on Grand Isle?). It recorded a very low pressure reading of 944.3 mbar, and Katrina's center at this time was 20 miles to the east, and the minimum pressure in the eye recorded just prior to that time was 918 mbar.

But the most remarkable reading came from reliable buoy 42040, which is south of Dauphin Island and east of the mouth of the Mississippi. Here is the section of the NDBC report on buoy 42040:

"Station 42040, located at 29°11'03"N 88°12'48"W approximately 64 nautical miles south of Dauphin Island Alabama, reported a significant wave height of 16.91 meters (55.5 feet) at 1100 UTC, August 29, 2005. Station 42040 is a 3-meter diameter discus hull buoy deployed and operated by National Oceanographic and Atmospheric Administration's National Data Buoy Center (NDBC). Although 42040 does not measure maximum wave heights, the maximum wave height may be statistically approximated by 1.9 times the significant wave height (World Meteorological Organization, 1998), which would be 32.1 meters (105 feet). At the time of the report, Hurricane Katrina was approximately 73 nautical miles to the west of 42040 with maximum sustained winds of 145 miles per hour (Public Advisory 26A issued by the National Hurricane Center). In addition to the 55-foot report, 42040 reported seas 12 feet or greater for 47 consecutive hours.

"The 55-foot report surpasses the previous highest significant wave height reported by an NDBC buoy in the Gulf of Mexico of 15.96 meters (52 feet), also reported by 42040 during Hurricane Ivan in September 2004, and matches the previous highest significant wave height reported by an NDBC buoy of 16.91 meters reported by station 46003 (in the Northeast Pacific Ocean south of the Aleutian Islands) in January 1991."

We can only speculate about the highest wave heights under the radius of maximum winds (RMW) in the eastern eyewall, some 50 miles or so to the west. Ivan, by contrast, passed almost over the buoy. Although specific data is not available, oil rigs to the west, in Mississippi Canyon, that were close to the RMW, were severely damaged or destroyed by the huge waves.

Finally, we come to buoy 42007. This buoy is usually right outside the Chandeleur Islands and the islands of the Mississippi Sound. However, Katrina set her adrift, and after some hours, she ended up right inside the eye, at the southeastern eyewall (as we can tell by the wind and pressure readings), and close to the Waveland and Bay St. Louis area.

The lowest pressure reported was an amazing 927.4 mbar. But what was more interesting was a maximum significant wave height of about 4.7 m while in the eye. This equates to a highest possible wave of 29 feet (from trough to crest), or a fifteen foot wave height on top of the storm surge.

This wouldn't have been possible without the storm surge. The Mississippi Sound is extremely shallow, except where channels have been dredged for shipping, and would never be able to support wave heights in that range without the additional water depth provided by the surge.

Prior to the arrival of the eyewall, easterly winds would have been able to create waves because of the fetch over the length of the sound, and it is possible that the area of the shoreline comprising the communities of Lakeshore, Clermont Harbor, and Waveland initially received significant waves prior to the surge inundation, unlike the remainder of the south-facing Mississippi coastline (which we know also from the Beau Rivage video, which showed at most five-foot breakers hitting the Biloxi shoreline).

Additional measurements of Katrina's waves: In addition to the buoy measurements, on board the NOAA aircraft, the Scanning Radar Altimeter (SRA) collected data on waves generated by the RMW to the northwest of the storm. An article on Live Science from Tuesday discusses the use of the SRA by NOAA. Here is an image from a very interesting technical report that documents waves and wave spectra found using the SRA:

Katrina SRA

Image courtesy of NASA

The image of waves on the left shows a train of 40 foot waves, which means the maximum wave that could have been encountered in that area would have been around 75 feet (max sig wave height of 12m corresponds to a max possible wave height of 22.8m, or about 75 feet).

Were the warnings appropriate? If you have read this entire series, you are aware that the coastal damage and loss of life from Katrina was primarily from storm surge. And as Katrina moved towards the northern Gulf Coast in the days before landfall, a small group of people understood the extent of the surge that was not only possible but probable. Was everything done to emphasize and weight the warnings to the general public towards this specific danger?

The information in the different types of warnings that are generated for hurricanes from different levels of the NWS are very specifically defined. The National Hurricane Center advisories do not discuss surge specifics for a coastline; they provide general surge warnings based only on the storm's intensity and the general characteristics of the area where the storm will landfall (shallow or steep coastline). This will be clarified in a moment. However, while not in the advisory packages, the NHC does many SLOSH model runs for every six-hour advisory period, which does have the surge specifics for defined SLOSH storm basins (at Katrina landfall, surge was calculated over two SLOSH grids, or storm basins: SE LA, and Biloxi). This information is downloaded and shared via phone conferences with the local NWS forecast offices (NWSFO) and emergency management. Then, the NWSFOs provide storm warnings in a product called the Hurricane Local Statement (HLS), where part of the product provides surge warnings on a county by county basis.

The NWSFO are divided geographically into regional centers that extend over state lines. Local warnings for southeastern Louisiana, all bordering Mississippi counties, and the three Mississippi coastal counties, are all handled by the Slidell LA NWS Office (LIX). Additional non-coastal southeastern Mississippi counties, southwestern Alabama, and the western portion of the Florida panhandle are handled by the Mobile NWS office (MOB). The NWS has a map, not very detailed, that shows the areas covered by each NWSFO:

NWSFO

Image courtesy of NWS

Note that the border that is drawn between the states of MS and LA on the map is not correct: the counties of Hancock and Pearl River are mistakenly drawn inside the line of LA, not MS.

All of the NWSFO now have a confusing “point forecast” map on their home webpage that requires two different links, (“what is this?,” or “what is point forecast?,” and “map help,” which only appears on some NWSFO home pages) with involved explanations, to try to explain it, and which masks the area of responsibility of that NWFSO, but provides the benefit of accurate weather information at points on the edges of the two NWSFOs.

It appears that there used to be maps of the counties covered by each NWSFO, but those were phased out. I have a link for Slidell, but it is no longer referenced from their home page:

NWSFO Slidell (LIX)

Image courtesy of NWS

What can be seen from this map is that Slidell had responsibility for almost all of the coastal areas that were decimated by the surge, for a total of ten different parishes or counties that bordered the Gulf of Mexico or Lake Ponchartrain, covering two different states. And within these ten counties were very different surge concerns and very different geographies.

The southern end of Jefferson Parish contained an island, Grand Isle. Plaquemines was completely populated behind ring levees. St. Bernard and Orleans parishes had complicated levee and canal systems. The three Mississippi coastal counties had a series of large estuaries and bays on the coast, and, because of the inverted-L of the coastline, and shallow continental shelf, very extremely vulnerable (Hancock County in particular), to storm surge.

Because of this, there was a need for surge warnings tailored to that parish's or county's situation. This is provided by the Hurricane Local Statement.

The HLS are what are shown on local TV stations, what is broadcast over NOAA Weather Radio, and what scrolls down TWC in local areas. In other words, this is the county-specific warning information that is available to the public, in any given local area.

The NWSFO have many products they are required to issue, on a specific timeline, and the HLS is just one of them. And LIX also issues river forecasts for a very large area. Finally, the NWSFO do most of their coordinating with emergency management by telephone.

Like most weather products, the HLS have a standard format that does not vary from NWSFO to NWSFO. As all such products, a written guideline (PDF File) exists identifying in detail what should be included. The section on Purpose / Intended Use is as follows:

“The HLS is an event-driven product and is updated as necessary, but as a minimum every 6 hours, to alert the public, media, and local decision makers of potential or actual storm affects due to tropical cyclones. The product is intended to provide information to assist in the preparation and implementation of necessary precautions for the protection of life and property, as well as to minimize the economic losses as a result of tropical cyclones.”

These HLS are archived after the fact and can be reviewed. In reviewing the detail of the section on surge, of all of the HLS issued from Slidell (LIX) and Mobile (MOB) up until the time of MS landfall, we find that no local specific warning information on surge was ever provided from the LIX office. Most of the information was actually a cut and paste of the generic surge warnings put out by the NHC as part of their public advisories. By contrast, the MOB HLS provided a wealth of specific local detail that was updated about every six hours, which allowed the public an immediate awareness of what areas would be in danger. MOB also updated the HLS with current conditions, as surge inundation was observed.

The detail from the HLS: NHC issues Hurricane Watch and Hurricane Warnings based on the number of hours prior to landfall. When a Hurricane Watch is issued, then the NWSFO is required to issue an HLS.

When the first Hurricane Watch statement was issued at 10am CDT Saturday, for the southern coast of Louisiana, LIX dutifully sent out their first HLS. No preparation work had been done to identify any concerns over surge. The surge portion of the HLS contained simply two sentences:

...STORM SURGE AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL ALONG NORTHERN GULF OF MEXICO COAST AS A MAJOR HURRICANE. WHILE EXACT LOCATION LANDFALL IS UNCERTAIN AT THIS TIME SIGNIFICANT STORM SURGE FLOODING UP TO 18 FEET IS POSSIBLE NEAR AND TO THE RIGHT OF THE LANDFALL AREA.

However at this time there was little uncertainty in where Katrina would be making landfall. Twelve hours earlier, the Friday night 10pm CDT NHC discussion on Katrina detailed what had become increasingly likely over the previous 24 hours:

IT IS WORTH NOTING THAT THE GUIDANCE SPREAD HAS DECREASED AND MOST OF THE RELIABLE NUMERICAL MODEL TRACKS ARE NOW CLUSTERED BETWEEN THE EASTERN COAST OF LOUISIANA AND THE COAST OF MISSISSIPPI. THIS CLUSTERING INCREASES THE CONFIDENCE IN THE FORECAST.

So, at 10am CDT on Saturday, the time arrived for NHC to put out the first hurricane watch for the northern gulf coast – when landfall was estimated to be in about 60 hours. This first landfall would be along the southeastern coast of Louisiana that sticks out into the Gulf. Correspondingly, the watch area for this first Louisiana landfall was designated between Morgan City and the mouth of the Pearl River, which is the border between eastern Louisiana and western Mississippi. At this time it was too early to extend the hurricane watch to the Mississippi and Alabama coasts, because the Mississippi landfall would be some six hours after the initial Louisiana landfall (actual landfall times were approximately 4am Monday for southeastern LA and 10am Monday for the MS coast). However, all NWSFO involved would have known the schedule and the certainty of the forecast track.

In the corresponding Saturday 10am discussion, the NHC discussion noted:

KATRINA IS SOUTH OF A DEEP-LAYER RIDGE OVER THE NORTHERN GULF COAST. THIS RIDGE IS FORECAST TO WEAKEN AS A STRONG DEEP-LAYER TROUGH DEVELOPS OVER THE CENTRAL UNITED STATES...AND A NEW DEEP-LAYER RIDGE FORMS OVER THE FLORIDA PENINSULA AND THE ADJACENT ATLANTIC. THIS PATTERN CHANGE SHOULD CAUSE KATRINA TO TURN NORTHWARD DURING THE NEXT 72 HR AND MAKE LANDFALL OVER THE NORTHERN GULF COAST. ALL TRACK GUIDANCE AGREES ON THIS SCENARIO...

THE OFFICIAL FORECAST REMAINS CLOSE TO THE MODEL CONSENSUS...CALLING FOR LANDFALL IN SOUTHEASTERN LOUISIANA IN 48-60 HR. THE NEW TRACK IS BASICALLY AN UPDATE OF THE PREVIOUS PACKAGE.

In other words, Katrina was going to be steered north towards the Gulf Coast, between the clockwise winds of a high pressure area (a ridge) on the east, and the counterclockwise winds of a low pressure area (a trough) on the west, much like a sheet of dough is pulled between the wheels of a pasta machine; a very straightforward situation as far as forecasting hurricane tracks goes. Hurricanes are steered by the upper atmosphere, and the stronger a hurricane gets, the higher in the atmosphere are the currents that steer it. Katrina was forecast to strengthen, so the certainty that it would be steered by these upper-level features was high.

In the 2:30 pm CDT HLS that followed, LIX made a small change, one that was probably not noticed by the LA and MS residents who watched the two-page warning scroll endlessly along their TV screens:

...STORM SURGE AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL ALONG NORTHERN GULF OF MEXICO COAST AS A MAJOR HURRICANE. WHILE EXACT LOCATION LANDFALL IS UNCERTAIN AT THIS TIME...SIGNIFICANT AND LIFE THREATENING STORM SURGE UP TO 18 FEET ABOVE NORMAL IS POSSIBLE NEAR AND TO THE RIGHT OF THE LANDFALL AREA.

Six hours after the original hurricane watch was issued, NHC expanded the watch area, as scheduled, in its 4pm public advisory, to include the MS and AL coasts, based on the subsequent landfall that would occur as Katrina skirted the eastern edge of the LA coast, moving northward to the second, MS landfall. NHC had prepared for this by noting in their 1pm advisory, “A HURRICANE WATCH WILL LIKELY BE REQUIRED FOR OTHER PORTIONS OF THE NORTHERN GULF COAST LATER TODAY OR TONIGHT.” However, this extension of the watch area on Saturday evening, for the first time, to the northern Gulf Coast of MS and AL, beyond LA, led many people to believe that somehow the storm had taken a last-minute log to the right. It had not. The media focus on New Orleans had biased the perception of the area at risk.

At 5pm, LIX put out its third HLS. They simply repeated the previous surge warning. No specific surge information was provided for any of the coastal parishes and counties within its purview, which with the three MS counties now numbered eleven in the HLS. And at 7:30pm, the same generic surge warning was repeated, only adding this general statement, “RESIDENTS IN AREAS PRONE TO STORM SURGE FLOODING SHOULD HEED EVACUATION RECOMMENDATION FROM LOCAL EMERGENCY MANAGEMENT OFFICIALS.”

Meanwhile, the media was focusing exclusively on NOLA, leading local residents in places like Hancock County to believe the hurricane would be heading to Louisiana. The idea that Katrina was so large that it would affect an area much wider than the typical hurricane, was also not being emphasized by the media. Over and over, the emphasis was on the increasing windspeed, and as Katrina bombed over Saturday night and into Sunday afternoon, the media had a field day endlessly talking about Cat 4 and Cat 5 windspeeds and wind damage.

Now that AL was included in the watch area, the Mobile NWSFO was required to put out a HLS for their area as well. And they did; their 7:25pm HLS, for storm surge, read:

...STORM SURGE FLOOD AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL SOMEWHERE NEAR SOUTHEAST LOUISIANA AS A MAJOR HURRICANE. WHILE EXACT LOCATION OF LANDFALL IS STILL UNCERTAIN AT THIS TIME...SIGNIFICANT AND LIFE THREATENING STORM SURGE IS EXPECTED TO BE FELT WELL EAST OF THE STORMS CENTER. BASED ON THE LATEST FORECAST TRACK...A STORM SURGE OF 8 TO 12 FEET IS EXPECTED ALONG COASTAL MOBILE COUNTY AND THE WESTERN PORTIONS OF MOBILE BAY. A STORM SURGE OF 7 TO 9 FEET IS EXPECTED ALONG COASTAL BALDWIN COUNTY. THESE SURGE HEIGHT VALUES ARE EXPECTED TO CAUSE SIGNIFICANT INUNDATION ALONG PORTIONS OF DAUPHIN ISLAND AND FORT MORGAN PENINSULA. ALSO...A STORM SURGE OF 4 TO 6 FEET IS EXPECTED ALONG THE EXTREME NORTHWEST FLORIDA COASTLINE.

We see something very different. The statement starts off the same, then goes on to provide specific warnings for every coastal county that is in the watch area, with actual surge numbers that have been calculated by the MOB NWSFO, based on their understanding of the recent SLOSH runs.

This information went out over the TV to AL and FL panhandle residents, who were immediately much better informed, and who were likely to sit up and take notice, as their particular county was mentioned. No such luck in Mississippi, where the HLS had no specific surge information, and the media continued to focus on NOLA. And the majority of Hancock County residents relied on NOLA TV stations, whereas Harrison and Jackson Counties would have been watching WLOX, based in Biloxi.

It's important to emphasize again that emergency managers were on the phone with the NWSFO, and received information from them directly, without having to rely on the HLS, and so were continually informed. The HLS were used by broadcast media to go over TV and radio. Emergency managers conveyed their county-specific warnings and evacuation orders to the media, so that news coverage would have reflected the local EM warnings, but weather coverage would have been based on whatever weather information was available. Not all the information available to the EM was available to the media and public, so the HLS information was a crucial link in what the public would understand about the oncoming hurricane. But it was by no means the only vehicle for getting information to the public. Just what role it played, or failed to play, is a question mark that I cannot answer.

Now, the next set of HLS were due to come out at around 10pm on Saturday evening. Many people along the northern Gulf Coast would have gone to bed, but some would have stayed up to assess whether they needed to evacuate.

At 10:20pm, MOB's HLS was generated and would have started being broadcast on the news media shortly afterward:

...STORM SURGE FLOOD AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL SOMEWHERE NEAR SOUTHEAST LOUISIANA AS A MAJOR HURRICANE. WHILE EXACT LOCATION OF LANDFALL IS STILL UNCERTAIN AT THIS TIME...SIGNIFICANT AND LIFE THREATENING STORM SURGE IS EXPECTED TO BE FELT WELL EAST OF THE STORMS CENTER. BASED ON THE LATEST FORECAST TRACK...A STORM SURGE OF 10 TO 15 FEET IS EXPECTED ALONG COASTAL MOBILE COUNTY AND THE WESTERN PORTIONS OF MOBILE BAY. A STORM SURGE OF 8 TO 10 FEET IS EXPECTED ALONG COASTAL BALDWIN COUNTY. THESE SURGE HEIGHT VALUES ARE EXPECTED TO CAUSE SIGNIFICANT INUNDATION ALONG PORTIONS OF DAUPHIN ISLAND AND FORT MORGAN PENINSULA. ALSO...A STORM SURGE OF 5 TO 7 FEET IS EXPECTED ALONG THE EXTREME NORTHWEST FLORIDA COASTLINE.

Notice that the surge numbers have been increased. These numbers would not be changed in the coming hours, and they would be very close to the actual surge that occurred in those areas. At the MOB NWSFO, there was a lot of discussion about exactly what numbers to go with, and whether to go with numbers this high, and it was decided that these were the numbers; a gutsy and, as it turned out, good, decision. So people who lived along the AL or FL coast, that would be impacted by Katrina, knew late Saturday night exactly what surge heights they would have to be dealing with.

Now, by contrast, the 10:30pm Slidell HLS reads almost exactly the same as before:

...STORM SURGE AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL ALONG THE NORTHERN GULF OF MEXICO COAST AS A MAJOR HURRICANE. WHILE EXACT LOCATION LANDFALL IS UNCERTAIN AT THIS TIME...SIGNIFICANT AND LIFE THREATENING STORM SURGE 15 TO 20 FEET ABOVE NORMAL IS POSSIBLE NEAR AND TO THE RIGHT OF THE LANDFALL AREA. RESIDENTS IN AREAS PRONE TO STORM SURGE FLOODING SHOULD HEED EVACUATION RECOMMENDATION FROM LOCAL EMERGENCY MANAGEMENT OFFICIALS.

Only the number of feet of surge is changed. And this is because it is a copy of the NHC surge forecast in their high-level advisory, although they neglect to mention the “high as 25 feet” part of the warning:

COASTAL STORM SURGE FLOODING OF 15 TO 20 FEET ABOVE NORMAL TIDE LEVELS...LOCALLY AS HIGH AS 25 FEET ALONG WITH LARGE AND DANGEROUS BATTERING WAVES...CAN BE EXPECTED NEAR AND TO THE EAST OF WHERE THE CENTER MAKES LANDFALL.

You can see this is basically a cut-and-paste of the generic NHC information, that means nothing to residents in a specific coastal area, except that wherever the hurricane hits, there will be surge in the small area to the right of the eye, which everyone familiar with hurricanes already knows. There is no county-specific warning to Mississippi residents who are being inundated with media coverage about the hurricane that is about to hit New Orleans.

Overnight, Katrina bombs, and, early Sunday morning, a frightening image emerges from the satellite eclipse, of a powerful Cat 4 hurricane. While people sleep, overnight the NHC and NWSFO have been continuing to generate warnings. In the 1am, 4am, and 7am advisories, NHC repeats the previous surge warning. And LIX continues to cut and paste the NHC generic surge warnings, adding the 25 foot statement to the 1am, and this statement to the 4am, “SECONDARY ROADS OUTSIDE LEVEE PROTECTION WILL LIKELY BECOME IMPASSABLE THIS\ EVENING AND TONIGHT.” again bolstering the idea that the hurricane will be impacting Louisiana, which has a levee system, and not Mississippi, which does not. At 8am, they add this ambiguous statement, “RESIDENTS IN AREAS PRONE TO STORM SURGE FLOODING SHOULD LEAVE NOW!”

Meanwhile, in the overnight Mobile HLS, additional new information has been provided with each advisory. With each new HLS report, MOB provides more information, more detail, documents existing surge conditions and tidal gauge information as it happens. On Sunday morning 2:10a they are told:

MINOR STREET FLOODING WAS ALREADY REPORTED ALONG

NAVARRE BEACH EARLY THIS MORNING.

At 5:55am specific surge heights for every county, for both Cat 4 and Cat 5, are identified, and the following information is added:

IN COMPARISON...HURRICANE GEORGE IN 1988 BROUGHT WATER LEVELS UP TO 8.3 FEET TO BAYOU LA BATRE...AND 9 FEET TO DOWNTOWN MOBILE.

TIDAL WATERS WERE REPORTED WASHING OVER SANTA ROSA ISLAND THIS MORNING.

Here is the comparison of the surge portions of the LIX and MOB HLS from, respectively, 8am and 7:54am on Sunday morning:

HURRICANE KATRINA LOCAL STATEMENT

NATIONAL WEATHER SERVICE NEW ORLEANS LA

800 AM CDT SUN AUG 28 2005

...STORM SURGE FLOOD AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL ALONG THE NORTHERN GULF OF MEXICO COAST AS A MAJOR HURRICANE. WHILE EXACT LOCATION LANDFALL IS UNCERTAIN AT THIS TIME...SIGNIFICANT AND LIFE THREATENING STORM SURGE 15 TO 20 FEET ABOVE NORMAL IS POSSIBLE NEAR AND TO THE RIGHT OF THE LANDFALL AREA. A FEW AREAS MAY EXPERIENCE STORM SURGE FLOODING AS HIGH AS 25 FEET ALONG WITH LARGE AND DANGEROUS BATTERING WAVES NEAR AND TO THE EAST OF WHERE THE CENTER MAKES LANDFALL. RESIDENTS IN AREAS PRONE TO STORM SURGE FLOODING SHOULD LEAVE NOW! HEED EVACUATION RECOMMENDATION FROM LOCAL EMERGENCY MANAGEMENT OFFICIALS. SECONDARY ROADS OUTSIDE LEVEE PROTECTION WILL LIKELY BECOME IMPASSABLE THIS EVENING AND TONIGHT.

and,

HURRICANE LOCAL STATEMENT

NATIONAL WEATHER SERVICE MOBILE AL

754 AM CDT SUN AUG 282005

...STORM SURGE FLOOD AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL SOMEWHERE NEAR SOUTHEAST LOUISIANA AS A MAJOR HURRICANE. WHILE THE EXACT LOCATION OF LANDFALL IS STILL UNCERTAIN AT THIS TIME...A SIGNIFICANT AND LIFE THREATENING STORM TIDE IS EXPECTED TO BE FELT WELL EAST OF THE STORMS CENTER.

BASED ON THE LATEST FORECAST TRACK AND SPEED...A CATEGORY FOUR AND-FALLING HURRICANE WOULD BRING TIDE LEVELS AS HIGH AS 10 TO 15 FEET ALONG COASTAL MOBILE COUNTY AND THE WESTERN PORTIONS OF MOBILE BAY...INCLUDING BAYOU LA BATRE AND DOWNTOWN MOBILE. A STORM TIDE OF 8 TO 10 FEET IS EXPECTED ALONG COASTAL BALDWIN COUNTY. THESE TIDE HEIGHT VALUES ARE EXPECTED TO CAUSE SIGNIFICANT INUNDATION ALONG PORTIONS OF DAUPHIN ISLAND AND FORT MORGAN PENINSULA.

IN COMPARISON...HURRICANE GEORGE IN 1988 BROUGHT WATER LEVELS UP TO 8.3 FEET TO BAYOU LA BATRE...AND 9 FEET TO DOWNTOWN MOBILE.

A STORM TIDE OF 6 TO 8 FEET IS EXPECTED ALONG THE EXTREME NORTHWEST FLORIDA COASTLINE. TIDAL WATERS WERE REPORTED WASHING OVER SANTA ROSA ISLAND THIS MORNING.

A CATEGORY FIVE LAND-FALLING HURRICANE WOULD BRING TIDE LEVELS AS HIGH AS 15 TO 20 FEET ALONG COASTAL MOBILE COUNTY AND THE WESTERN PORTIONS OF MOBILE BAY...12 TO 14 FEET ALONG COASTAL BALDWIN COUNTY...AND 10 TO 12 FEET ALONG THE EXTREME NORTHWEST FLORIDA COASTLINE.

This is the information scrolling down the TV screens of residents in Mobile and Florida. People in Mississippi never get to see that the surge will be high all along the MS coast. They miss out on this type of specific info, and only know that the surge will be high right around the eye at landfall, which means nothing to them, as they believe the hurricane will landfall in New Orleans.

With each subsequent HLS report, LIX now only repeats the generic NHC surge warning, in spite of the fact that it has ten different counties in varied geographic locations with different topographies that need to know surge warning information, covering something like 200 miles of coastline in total, plus a major city that may have issues with surge.

As Katrina increases in intensity, the NHC increases the numbers in its advisory for the storm surge warning:

COASTAL STORM SURGE FLOODING OF 18 TO 22 FEET ABOVE NORMAL TIDE LEVELS...LOCALLY AS HIGH AS 28 FEET ALONG WITH LARGE AND DANGEROUS BATTERING WAVES...CAN BE EXPECTED NEAR AND TO THE EAST OF WHERE THE CENTER MAKES LANDFALL.

And at 10:15am, so does LIX, but they provide no new information. Now the window is closing for Mississippi residents to evacuate; within 12 hours, weather would make evacuating impossible. And the window for evacuation from Plaquemines has already almost closed. However no county-specific information for any of the parishes or counties has been generated in the LIX HLS.

During this period, with every phone conference Butch Loper had with Slidell, they increased the surge numbers expected for Jackson County. He said that he finally increased his numbers higher than theirs, and at the next advisory they had caught up with him. By contrast, Mobile never changed their surge number predictions after 10pm Saturday night.

At 11:48am, MOB issues a new HLS with the following surge information. To help local residents visualize the impact of the surge, they compare with previous hurricanes Frederick, Georges, and Ivan -- and they make a significant change, mentioning the Mississippi coastline, which is not in their jurisdiction:

...STORM SURGE FLOOD AND STORM TIDE IMPACTS...

KATRINA IS EXPECTED TO MAKE LANDFALL SOMEWHERE NEAR SOUTHEAST LOUISIANA AS A MAJOR HURRICANE...LIKELY AS AT LEAST A CATEGORY FOUR AND POSSIBLY A CATEGORY FIVE. A SIGNIFICANT AND LIFE THREATENING STORM SURGE IS EXPECTED TO BE FELT WELL EAST OF THE STORMS CENTER...OVER THE MISSISSIPPI AND ALABAMA COASTLINES.

BASED ON THE LATEST FORECAST TRACK AND SPEED...A CATEGORY FIVE LAND-FALLING HURRICANE WOULD BRING TIDE LEVELS AS HIGH AS 10 TO 15 FEET ALONG COASTAL MOBILE COUNTY AND THE WESTERN PORTIONS OF MOBILE BAY...WITH A SURGE AROUND 20 FEET POSSIBLE IN THE NORTHERN PART OF MOBILE BAY...WHICH WOULD SEVERELY AFFECT DOWNTOWN MOBILE WITH WATER SPILLING INTO THE DOWNTOWN AREA AND EXTENDING AS FAR WEST AS BROAD STREET. THIS WOULD BE THE HIGHEST SURGE LEVEL EVER EXPERIENCED IN MOBILE. FOR COMPARISON...THE HIGHEST SURGE LEVELS PREVIOUSLY EXPERIENCED IN THE DOWNTOWN AREA WERE NEAR 9 FEET WITH HURRICANES FREDERIC AND GEORGES. IN ADDITION...DAUPHIN ISLAND WOULD LIKELY BE INUNDATED...EXCEPT FOR HIGHER GROUND ON THE EAST END.

A STORM TIDE OF 10 TO 12 FEET IS EXPECTED ALONG COASTAL BALDWIN COUNTY...COMPARABLE TO HURRICANE IVAN OF LAST YEAR. SIGNIFICANT INUNDATION IS LIKELY ALONG THE BEACHES OF GULF SHORES AND ORANGE BEACH. PARTS OF THE FORT MORGAN PENINSULA WILL LIKELY BE INUNDATED AS WELL.

TIDE LEVELS ALONG THE NORTHWEST FLORIDA COAST WILL RANGE FROM NEAR 8 FEET IN THE PENSACOLA AREA TO AROUND 5 FEET IN THE DESTIN AREA.

Sunday afternoon, Katrina peaks as a catastrophic Cat 5. The extreme surge that she will bring with her has now been set in motion. SLOSH runs show the extreme number of feet of surge that will peak along the Mississippi coastline, giving a clear indication of the devastation to come. The very last possible moment for anyone from Mississippi to evacuate has arrived.

At 2pm and at 5:45pm, and at 8pm, LIX issues HLS where it repeats the same storm surge information it has issued before. Not once have they mentioned a county or parish in any of the HLS, that would have provided specific information to local residents. This is the surge information that scrolls endlessly along the TV screens along with the other information in the HLS.

At 2:46pm and at 6:29pm, MOB issues another HLS repeating the earlier surge information from the 11:48am.

And now the time for warnings has come and gone, and the effects of the storm begin to be felt throughout the warning area.

Where were the HES maps? No one knew about them, so no one was using them; not emergency managers, not TV stations. Butch Loper, emergency manager for Jackson County, said the only way he could determine the number of feet of water for areas of the county, was to overlay an elevation map onto the SLOSH map.

From an earlier blog entry, here is the description of the HES maps:

The results of the thousands of hypothetical SLOSH runs are a set of maps that show the areas of a county that will flood, for a particular category of hurricane. Here is the set of maps for Hancock County:

Hancock County HES Maps

Image courtesy of USACE

Here is part of one of the maps, showing the surge potential for Lakeshore, Clermont Harbor, Bayside park, Waveland, and part of Bay St. Louis, followed by the map legend. Note how little land is above water in a Cat 4 surge, and how all the densely populated sections of the county - south of I-10 - are underwater:

Hancock County HES Map Detail

Image courtesy of USACE

Hancock County HES Map Legend

Image courtesy of USACE

Hancock County HES Map Legend 2

Image courtesy of USACE

Again, whether these maps would have helped, and how much, is a question that cannot be answered in retrospect. Residents confused their insurance FIRMS, A, B, and C, with the evacuation zones, A, B, and C. Residents for the most part either did not know their home's elevation, or thought it was much higher than it was. So would a number have been meaningful? Would a map showing surge inundation, that wasn't able to convey how deep the water would get, have helped? Since most people are unaware of the force of moving water, even only six inches of moving water, would knowing the specifics about the surge inundation or surge depth have helped? Remember that some residents, even as the water rushed up to their doors, didn't try to escape right away, in the little time they had left, but instead rushed around to collect things to save from the water.

I don't have the answers, and I don't think anyone does. I notice things, and what I noticed about the HLS was that there were omissions. The NWS immediately set about making things better. When Rita came, Slidell provided detailed parish surge information.

Global warming makes the possibility of another strong storm on vulnerable coastal areas more likely. And after only a very short time -- ten years or even five years -- people will have forgotten about what surge can do. So I guess the only question is, how can we preserve the lessons we learned from Katrina, so that in the future people will be aware, and will evacuate when told do to so, when surge threatens?

Hurricane Katrina Storm Surge:

Weather Underground Storm Surge Articles

Storm Surge Safety Actions

  • Minimize the distance you must travel to reach a safe location; the further you drive the higher the likelihood of encountering traffic congestion and other problems on the roadways.

  • Select the nearest possible evacuation destination, preferably within your local area, and map out your route. Do not get on the road without a planned route, or a place to go.

  • Choose the home of the closest friend or relative outside a designated evacuation zone and discuss your plan with them before hurricane season.

  • You may also choose a hotel/motel outside of the vulnerable area.

  • If neither of these options is available, consider the closest possible public shelter, preferably within your local area.

  • Use the evacuation routes designated by authorities and, if possible, become familiar with your route by driving it before an evacuation order is issued.

  • Contact your local emergency management office to register or get information regarding anyone in your household whom may require special assistance in order to evacuate.

  • Prepare a separate pet plan, most public shelters do not accept pets.

  • Prepare your home prior to leaving by boarding up doors and windows, securing or moving indoors all yard objects, and turning off all utilities.

  • Before leaving, fill your car with gas and withdraw extra money from the ATM.

  • Take all prescription medicines and special medical items, such as glasses and diapers.

  • If your family evacuation plan includes an RV, boat or trailer, leave early. Do not wait until the evacuation order or exodus is well underway to start your trip.

  • If you live in an evacuation zone and are ordered to evacuate by state or local officials, do so as quickly as possible. Do not wait or delay your departure, to do so will only increase your chances of being stuck in traffic, or even worse, not being able to get out at all.

  • Expect traffic congestion and delays during evacuations. Expect and plan for significantly longer travel times than normal to reach your family's intended destination.

  • Stay tuned to a local radio or television station and listen carefully for any advisories or specific instructions from local officials. Monitor your NOAA Weather Radio.

Source: NOAA