Katrina's Surge, Introduction

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

The makings of the nation's largest natural disaster had a quiet beginning.

The last week of August, 2005, conditions in the Gulf of Mexico were unusually conducive for strengthening of hurricanes. Wind shear over the entire area was light and forecast to remain so for a number of days, and the entire Gulf was under a large anticyclone. A ridge of high pressure was in place over the southeastern US, where its clockwise flow could steer disturbances into the Gulf. Surface water temperatures in the GOM were one to two degrees above normal, and even higher in the shallow continental shelf off of the northern Gulf Coast:

Image courtesy of NRL

A Loop Current eddy was south of the Louisiana / Mississippi coastlines. The position of an eddy that far north (27N, and just east of 90W), was not unusual, but it was infrequent. An eddy was usually in that position only one to two weeks during the summer hurricane season. For instance, this year, in 2006, the only time such an eddy was in place was around August 1st (seen below), which happened to be the same timeframe that Tropical Storm Chris, had it not been sheared by upper level lows and subsequently dissipated after interaction with eastern Cuba, could have entered the Gulf of Mexico:

Image courtesy of NOAA AOML

Here is the way the 26-degree isotherm depth (which shows the warm deep water of the Loop Current and eddies) looked prior to Katrina's entry into the Gulf of Mexico:

Image courtesy of NOAA AOML

Into this environment a disturbance formed on Tuesday, August 23rd, in the Atlantic off the Florida coast, and was upgraded to a tropical storm, named Katrina, the next day. Katrina was to have a lifespan of exactly one week -- a week that would both move too quickly and at the same time crawl by, for those watching or in it's path, culminating in a terrifying last-minute monstrous intensification over the weekend prior to landfall, and cementing a certainty of extreme surge for the northern Gulf Coast.

That Wednesday, the newly-formed Katrina headed west for the Atlantic coastline of the Florida peninsula, developing into a weak hurricane. Although forecast to move directly west across the mid-point of the Florida peninsula, Katrina crossed the southern tip of Florida in a southwest direction, remaining over the moist Everglades for much of the journey, enabling the storm to survive intact into the Gulf of Mexico, and further south than anticipated. She left behind minimal damage and a lot of rain south of Miami, and the eye notably moved right over the National Hurricane Center. Katrina was then forecasted to strengthen somewhat and curve to the north, hitting the eastern Florida panhandle.

Although initially forecast to curve north and strike the Florida panhandle, Katrina's continued southwest movement, due to a ridge of high pressure that did not weaken as forecast, resulted in a dramatic change to the track forecast, targeting the Louisiana and Mississippi coastlines, placing a major city and an extremely large section of coastline vulnerable to surge at high risk.

To provide a perspective for the following series, which takes us along the coastline that was affected by Katrina's surge, we first look at the meteorological details, through the eyes of the NHC forecasters, as documented in the advisory discussions, during those seven days. We'll end by reviewing information on Katrina's winds and surge taken from the NHC post Tropical Cyclone Report on Katrina.

Wednesday through Friday: Coalescence

Examining those first three days in detail, starting with the NHC discussion on Wednesday morning:

SATELLITE IMAGERY...DOPPLER RADAR DATA FROM THE BAHAMAS AND MIAMI...AND RECONNAISSANCE WIND DATA INDICATE TD-12 HAS BECOME MUCH BETTER ORGANIZED THIS MORNING AND HAS STRENGTHENED INTO TROPICAL STORM KATRINA.

The 5pm discussion noted:

KATRINA HAS CONTINUED TO BECOME BETTER ORGANIZED THIS AFTERNOON. BANDING FEATURES HAVE INCREASED IN ALL QUADRANTS AND THE UPPER-LEVEL OUTFLOW HAS EXPANDED AND REMAINS QUITE SYMMETRICAL.

Late Wednesday night the discussion forecast anticipated that once Katrina entered the GOM, the track would turn north and threaten the Florida GOM coastline:

THE OFFICIAL FORECAST TURNS KATRINA NORTHWARD OVER THE EASTERN GULF OF MEXICO

On Thursday morning the NHC discussion observed there was little consensus in the path predicted by the models, but still focused on a landfall along the Florida panhandle:

THE MODELS CONTINUE TO DIVERGE SIGNIFICANTLY ON WHEN AND WHERE THE TURN TO THE NORTH TOWARD THE FLORIDA PANHANDLE OR NORTHWEST FLORIDA IS EXPECTED TO OCCUR. THE UKMET IS THE EASTERNMOST AND FASTEST MODEL AND BRINGS KATRINA ACROSS NORTHEAST FLORIDA...WHEREAS THE GFDL IS THE WESTERNMOST MODEL AND TAKES THE CYCLONE ACROSS THE WESTERN FLORIDA PANHANDLE. THE REST OF THE MODELS ARE EVENLY DISTRIBUTED BETWEEN THESE SOLUTIONS. THE OFFICIAL FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK AND IS CLOSE TO THE NHC MODEL CONSENSUS AFTER 48 HOURS. THE GFDL MODEL HAS BEEN CONSISTENTLY FORECASTING A SOUTHWESTWARD MOTION AROUND THE SOUTHERN END OF FLORIDA FOR THE PAST 24 HOURS. WHILE THIS TRACK IS CERTAINLY POSSIBLE TO SOME DEGREE...MY CURRENT THINKING IS THAT THE GFDL IS TAKING KATRINA TOO FAR SOUTH THROUGH THE FLORIDA KEYS AS A MAJOR HURRICANE. KATRINA WILL PROBABLY CONTINUE TO STRENGTHEN RIGHT UP UNTIL LANDFALL OCCURS IN THE FLORIDA PANHANDLE.

On Thursday afternoon, focus shifted to the impending landfall in southern Florida. The Thursday evening discussion noted the eye passing directly over NHC, and also noted the southwest motion:

THE MIAMI NATIONAL WEATHER SERVICE/NATIONAL HURRICANE CENTER OFFICE MEASURED A MINIMUM PRESSURE OF 984.5 MB DURING THE PASSAGE OF THE EYE AND THE WINDS QUICKLY INCREASED IN THE SOUTHEAST EYEWALL WITH A PEAK GUST OF 76 KNOTS. KATRINA IS MOVING TOWARD THE SOUTHWEST OR 225 DEGREES AT 7 KNOTS...STEERED BY THE NORTHERLY FLOW AROUND A STRONG AND LARGE MID-LEVEL HIGH CENTERED WELL TO THE NORTHWEST OF KATRINA. THIS SOUTHWEST DIP HAS BEEN SUGGESTED BY THE GFDL SINCE YESTERDAY. THE HIGH WHICH IS CONTROLLING THE MOTION OF THE HURRICANE IS FORECAST TO MOVE WESTWARD AND A TROUGH OR WEAKNESS IS EXPECTED TO DEVELOP IN THE CENTRAL GULF OF MEXICO. THIS PATTERN SHOULD FORCE KATRINA ON A MORE NORTHERLY TRACK OVER THE EASTERN GULF OF MEXICO AHEAD OF THE TROUGH. ALL INDICATIONS ARE THAT KATRINA WILL BE A DANGEROUS HURRICANE IN THE NORTHEASTERN GULF OF MEXICO IN ABOUT 3 DAYS.

But the picture was starting to shift. Chatter on all the hurricane internet boards noted the strong potential for the path to shift westward and target Louisiana and Mississippi. Early Friday morning the NHC discussion wording had a subtle but significant change:

ALL INDICATIONS ARE THAT KATRINA WILL BE A DANGEROUS HURRICANE IN THE NORTHEASTERN GULF OF MEXICO WITHIN THE NEXT COUPLE OF DAYS...IT IS CERTAINLY POSSIBLE THAT KATRINA COULD ATTAIN MAJOR HURRICANE STATUS BEFORE MAKING LANDFALL SOMEWHERE ON THE NORTHERN GULF COAST.

Now, the forecasts started to also emphasize the possibility for strengthening:

AS KATRINA MOVES FARTHER NORTH IN THE EASTERN GULF OF MEXICO... ATMOSPHERIC CONDITIONS SHOULD ONLY BECOME MORE CONDUCIVE FOR STRENGTHENING AS A LARGE UPPER LEVEL ANTICYCLONE DOMINATES OVER THE GULF OF MEXICO. THE OFFICIAL FORECAST IS CLOSE TO THE SHIPS GUIDANCE IN BRINGING KATRINA TO 90 KT BY 72 HOURS...BUT THIS COULD BE CONSERVATIVE SINCE THE GFDL AND GFDN FORECAST A MAJOR HURRICANE.

And Katrina did not disappoint. Pressure continued to drop, and Katrina doggedly held onto every single millibar. The very next discussion, at 11am on Friday, headlined this:

RECENT DATA FROM AN AIR FORCE RECON AIRCRAFT INDICATES KATRINA'S CENTRAL PRESSURE IS MUCH LOWER...NOW AT 971 MB.

Recon reports were coming in while the discussion was being put together, and even during this time, the windspeed increased. For the first time, the forecast increased the intensity to that of a major hurricane at landfall, 100 kt, and the discussion noted:

STRENGTHENING TO A MAJOR HURRICANE IS EXPECTED. IN FACT...A RECENT DROPSONDE REPORT RECEIVED FROM THE RECONNAISSANCE AIRCRAFT INDICATES MAXIMUM WINDS ARE NOW UP TO 80 KT. SO...A SPECIAL ADVISORY WILL BE ISSUED SHORTLY TO UPDATE THE CURRENT AND FORECAST INTENSITIES.

And only a half hour later, a special advisory package at 11:30am detailed the strengthening, and increased the intensity at landfall from 100 kt to 110 kt:

RECENT DROPSONDE DATA FROM AN AIR FORCE RESERVE UNIT RECONNAISSANCE AIRCRAFT MEASURED A SURFACE WIND OF 83 KT IN THE SOUTHEAST EYEWALL. THE MINIMUM PRESSURE OF 971 MB ALSO CORRESPONDS TO ABOUT 88 KT SURFACE WINDS. THEREFORE...THE INITIAL INTENSITY HAS BEEN INCREASED TO 85 KT...OR 100 MPH.

GIVEN THE RAPID IMPROVEMENT IN THE INNER CORE STRUCTURE AND THE SHARP PRESSURE DROP...RAPID INTENSIFICATION SEEMS LIKELY FOR THE NEXT 12 HOURS OR SO. AFTERWARDS...STEADY INTENSIFICATION TO NEAR CATEGORY FOUR STRENGTH BY 72 HOURS APPEARS TO BE IN ORDER GIVEN THE VERY WARM GULF WATERS BENEATH THE HURRICANE AND THE VERTICAL SHEAR FORECAST TO DECREASE TO LESS THAN 10 KT BY 48 HOURS.

At the 5pm on Friday, intensity continued to increase, and the discussion forecast increased as well, this time to 115 kt before landfall, noting the favorable conditions for intensification, and the more aggressive model forecasts:

MOST RECENT REPORTS FROM AN AIR FORCE HURRICANE HUNTER AIRCRAFT INDICATE THE CENTRAL PRESSURE HAS DROPPED TO 965 MB

KATRINA IS EXPECTED TO BE MOVING OVER THE GULF LOOP CURRENT AFTER 36 HOURS...WHICH WHEN COMBINED WITH DECREASING VERTICAL SHEAR...SHOULD ALLOW THE HURRICANE TO REACH CATEGORY FOUR STATUS BEFORE LANDFALL OCCURS. THIS IS CONSISTENT WITH THE SHIPS AND GFDL MODELS...WHICH BRING KATRINA UP TO 118 KT. THE FSU SUPERENSEMBLE MODEL IS MORE ROBUST AND BRINGS KATRINA UP TO 129 KT JUST BEFORE LANDFALL.

This was an increase in the intensity forecast in the twelve hour period between 5am and 5pm, from a maximum of 90 kt to 115 kt.

But just as important, Katrina's southwest movement had continued; NHC made the speculated-upon track shift official, and it was one large track shift – over 150 nautical miles (over 170 statue miles):

THE MODELS HAVE SHIFTED SIGNIFICANTLY WESTWARD AND ARE NOW IN BETTER AGREEMENT. THIS HAS RESULTED IN THE OFFICIAL FORECAST TRACK BEING SHIFTED ABOUT 150 NMI WEST OF THE PREVIOUS TRACK...ON THE EAST SIDE OF THE GUIDANCE ENVELOPE. HOWEVER... PROJECTED LANDFALL IS STILL ABOUT 72 HOURS AWAY...SO FURTHER MODIFICATIONS IN THE FORECAST TRACK ARE POSSIBLE.

This track placed the landfall right on the Mississippi-Alabama border. Katrina's movement to the southwest continued, and the 11pm Friday advisory shifted the track west a little further, and now moved the anticipated intensification up to a Cat 4 much sooner in the forecast, from three days away, to one day away:

THE HURRICANE IS EXPECTED TO BE UNDER A TYPICAL 200 MB ANTICYLONE...WITH A CYCLONIC CIRCULATION EXTENDING UPWARD TO THAT LEVEL. THIS IS THE TYPICAL PATTERN OBSERVED IN INTENSE HURRICANES. IN ADDITION...KATRINA IS FORECAST TO MOVE DIRECTLY OVER THE WARM LOOP CURRENT OF THE GULF OF MEXICO...WHICH IS LIKE ADDING HIGH OCTANE FUEL TO THE FIRE. THEREFORE...THE OFFICIAL FORECAST BRINGS KATRINA TO 115 KNOTS...OR A CATEGORY FOUR ON THE SAFFIR-SIMPSON HURRICANE SCALE. THE GFDL IS MORE AGGRESSIVE AND CALLS FOR 124 KNOTS AND 922 MB. THE FSU SUPERENSEMBLE IS EVEN MORE AGGRESSIVE BRINGING KATRINA TO 131 KNOTS.

THE OFFICIAL FORECAST BRINGS THE CORE OF THE INTENSE HURRICANE OVER THE NORTH CENTRAL GULF OF MEXICO IN 48 HOURS OR SO. 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.

Reading this discussion, it is clear that the very aggressive intensity forecast hints that the intensity could go even beyond what could be reasonably predicted.

But even these dramatic predictions were eclipsed over the coming weekend, prior to the core of the storm coming ashore just after midnight on Sunday. By the time the official landfall occurred, sometime around 5am, when the geographical center of the storm moved over land at Empire and Buras in Plaquemines Parish, LA, those towns had already been leveled by the strong winds of the northeastern eyewall and the storm surge, several hours before.

Saturday and Sunday: Combustion

Katrina continued to move southwest, and then west, deeper into the GOM, and headed for the Loop Current.

In December of 2001, a comprehensive Hurricane Evacuation Study (HES) was done by the US Army Corps of Engineers (USACE). This study contained maps that showed large areas of the coastline, corresponding in most part to the populated areas, could be significantly impacted by storm surge from a major hurricane, and catastrophically by a Category 4 or 5. Tropical meteorologists and oceanographers were familiar with the potential for surge along the Mississippi Gulf Coast. However, Mississippi emergency management did not seem to be aware of the specifics in the HES, and neither was it well-publicized to the general public, in spite of the fact that 1969's Camille created what had been the largest documented surge on record, peaking in Pass Christian between 22 and 24 feet. The USACE HES surge maps were online. However these maps were not referenced on the MS Emergency Management page (and still aren't), and were not displayed on local TV stations. Here is a composite image showing the surge potential for all three coastal counties, by hurricane category, and especially the danger to Hancock and Jackson counties:

Image courtesy of USACE

These maps would have provided residents with an understanding of, if not of how high the water would get, at least an idea of whether their home would be in an area that would be flooded by surge. Ironically, the Cat 5, and even Cat 4 surge inundations shown on the map encompass over 90% of the populated areas on the coast. Ultimately surge heights would show that the Mississippi coastline received a Cat 4 level surge in the areas around St. Louis Bay that just border-lined on Cat 5, and a solid Cat 3 surge completely along the remainder of the 70 miles of coast. There was not one mile of the Mississippi coastline that would be untouched by the surge.

Early on Saturday morning, the first advisory noted that the central pressure of the hurricane continued to drop, and continued the ominous litany of the potential for intensification, raising the forecast intensity from 115 kt to 120 kt:

AIRCRAFT RECONNAISSANCE INDICATES THE MINIMUM CENTRAL PRESSURE IS FALLING... MOST RECENTLY MEASURED AT 945 MB.

KATRINA IS LOCATED WITHIN AN ATMOSPHERIC ENVIRONMENT THAT SEEMINGLY CANNOT GET MUCH MORE CONDUCIVE FOR STRENGTHENING... PRIMARILY DUE TO A VERY LARGE UPPER-LEVEL ANTICYCLONE OVER THE ENTIRE GULF OF MEXICO. NOT SURPRISINGLY... UPPER-LEVEL OUTFLOW FROM THE HURRICANE CONTINUES TO BECOME MORE ESTABLISHED. THIS WEAK WIND SHEAR ENVIRONMENT IS FORECAST BY THE DYNAMICAL MODELS TO PERSIST FOR ABOUT THE NEXT THREE DAYS UNTIL AN UPPER-LEVEL TROUGH MOVES INTO TEXAS FROM THE WEST. THEREFORE SHEAR COULD BEGIN TO INCREASE OVER THE HURRICANE NEAR THE TIME IT MAKES LANDFALL ON THE NORTHERN GULF COAST... BUT IT SEEMS THAT WILL BE TOO LATE TO PREVENT KATRINA FROM MAINTAINING MAJOR HURRICANE STATUS AT LANDFALL. THE SHIPS GUIDANCE NOW SHOWS STRENGTHENING TO 123 KT...AND THE OFFICIAL FORECAST NOW PEAKS AT 120 KT OVER THE NORTHERN GULF WITHIN THE NEXT COUPLE OF DAYS.

And in the mid-morning advisory, more strengthening is noted, as well as the fact that the hurricane has started an eyewall replacement cycle (ERC):

REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE THAT THE CENTRAL PRESSURE OF KATRINA DROPPED TO 940 MB AT 0932Z.

IT IS NOT OUT OF THE QUESTION THAT KATRINA COULD REACH CATEGORY 5 STATUS AT SOME POINT BEFORE LANDFALL.

It would not be out of the question. The ERC happened to be perfectly timed, such that it would complete at the same time Katrina would be going over the Loop Current, and thereafter would travel over an eddy located very far north in the GOM and close to the eventual landfall in southeast Louisiana. In the map below, the dates are in Universal Time, and the map notations where each new day begins occurred at 7pm Central Daylight Time of the evening before, five hours earlier.

Katrina's path through the Gulf of Mexico and over the Loop Current; image courtesy of LSU Earth Scan Lab

The forecast discussions also start to note at this time a most extraordinary occurrence, and this would continue right up until the Mississippi landfall: Katrina would not only intensify beyond all but the most extreme expectations, but, at the same time, she would expand at a tremendous rate, until she filled the entire eastern 2/3 of the Gulf of Mexico. During this weekend, the area affected by Katrina's tropical storm force winds would expand to over eightand a half times its original size.

Saturday afternoon the first hurricane warnings were issued (these are timed on a specific schedule), replacing the hurricane watches, for the closest landfall point, southeast Louisiana, followed by hurricane warnings six hours later for the Mississippi and Alabama coastlines. These are the last step in a series of warnings, and Saturday marked the culmination of the evacuation warnings along the coastline, although many stayed, only to change their minds and quickly leave at the last minute on Sunday morning, after waking up to find Katrina had grown to an enormous Category 5 hurricane.

For those who had been watching Katrina online, getting little sleep, the overnight hours of the satellite eclipse provided an opportunity to take a much-needed nap. Just before the eclipse, another special advisory was issued, to note that Katrina had reached 125 kt intensity much quicker than forecast:

THIS SPECIAL ADVISORY IS BEING ISSUED TO UPDATE THE INITIAL AND FORECAST INTENSITY OF HURRICANE KATRINA. AN AIR FORCE RECONNAISSANCE AIRCRAFT REPORTED 700 MB FLIGHT LEVEL WINDS OF 137 KT IN THE NORTHWESTERN EYEWALL... CORRESPONDING TO ABOUT 125 KT AT THE SURFACE. THE LATEST MINIMUM CENTRAL PRESSURE MEASURED BY THE AIRCRAFT WAS 935 MB. ALTHOUGH THE HURRICANE HAS REACHED 125 KT MORE QUICKLY THAN PREVIOUSLY EXPECTED...THE INTENSITY FORECAST UP UNTIL LANDFALL HAS ONLY BEEN NUDGED UPWARD TO 130 KT. IT IS POSSIBLE THAT KATRINA COULD GET STRONGER THAN FORECAST AND PERHAPS EVEN REACH CATEGORY FIVE STATUS SOMETIME DURING THE NEXT 36 HOURS.

But this surprise was nothing compared to the first satellite image that came after the eclipse, in the early morning hours on Sunday August 28th. In those hours, Katrina had grown into a very organized and powerful hurricane with a large eye, and a very well-defined outflow pattern, providing the means for the rapid intensification. The Sunday 5am forecast advisory began with this simple observation:

KATRINA CONTINUES TO INTENSIFY AND GROW LARGER.

Katrina had gone beyond hyperbole, to another level where understatement provided more drama than could be generated by a boatload of adjectives. The factual details added to the shock:

AN AIR FORCE RECONNAISSANCE AIRCRAFT MOST RECENTLY MEASURED A MINIMUM CENTRAL PRESSURE OF 935 MB... AND THE WINDS HAVE RESPONDED SIGNIFICANTLY...WITH THE MAXIMUM FLIGHT LEVEL WIND OF 137 KT EARLIER THIS MORNING. THIS OBSERVATION ALONG WITH RECENT DVORAK INTENSITY ESTIMATES SUPPORT THE ADVISORY INTENSITY OF 125 KT. ADDITIONALLY...THE AIRCRAFT DATA AND SHIP OBSERVATIONS INDICATE THAT THE WIND FIELD CONTINUES TO EXPAND. THE INITIAL AND FORECAST WIND RADII HAVE AGAIN BEEN EXPANDED.

THE INTENSITY FORECAST ANTICIPATES THAT KATRINA COULD APPROACH CATEGORY FIVE STATUS PRIOR TO LANDFALL. THE SHIPS GUIDANCE ACTUALLY DOES FORECAST 140 KT AT 24 HOURS.

And only a few hours after that, at 7am CDT, another special advisory was issued:

THE PURPOSE OF THIS SPECIAL ADVISORY IS TO REVISE THE INTENSITY OF KATRINA TO CATEGORY FIVE. AN AIR FORCE HURRICANE HUNTER AIRCRAFT REPORTED A PEAK 700 MB FLIGHT-LEVEL WIND OF 153 KNOTS...WHICH CORRESPONDS TO MAXIMUM SURFACE WINDS OF ABOUT 140 KNOTS. OBVIOUSLY...THE BIG QUESTION IS HOW STRONG KATRINA WILL BE AT LANDFALL. WE HAVE VERY LIMITED SKILL IN PREDICTING THIS. FLUCTUATIONS IN INTENSITY...DUE TO EYEWALL REPLACEMENTS...ARE LIKELY DURING THE NEXT 24 HOURS. NEVERTHELESS...KATRINA IS EXPECTED TO BE A DEVASTING CATEGORY FOUR OR FIVE HURRICANE AT LANDFALL.

For the meteorologically-savvy, this was overwhelming. Katrina was going to destroy – not maim, not injure, but completely obliterate. The HES maps for Mississippi now foretold complete inundation and devastation of the coastline of an entire state. Those in Louisiana wondered how much of the levee system would be able to hold up. Those familiar with the levee heights in Plaquemines may have already known that they were all going to be overtopped by record high surge. Unfortunately, that wasn't the message getting out to the general public. The media, who had been excitedly talking about Cat 4 and Cat 5 winds for days, talked incessantly about wind speeds and wind damage.

But the pinnacle of intensity hadn't been reached yet. Sunday morning was one jaw-dropping moment after another. The 11am Sunday forecast discussion, only a few hours after the special advisory, raised it again:

THE AIR FORCE HURRICANE HUNTERS JUST MEASURED A 166 KT FLIGHT LEVEL WIND IN THE NORTHEAST EYEWALL...WHICH REQUIRES AN ADDITIONAL UPWARD ADJUSTMENT OF THE CURRENT INTENSITY TO 150 KT. A DROP IN THE EYE GAVE A CENTRAL PRESSURE OF 907 MB. KATRINA IS COMPARABLE IN INTENSITY TO HURRICANE CAMILLE OF 1969...ONLY LARGER. GPS DROPSONDE DATA FROM THE NOAA G-IV MISSION EARLIER TODAY SHOWED KATRINA'S INTENSE CYCLONIC CIRCULATION EXTENDING THROUGH THE 200 MB LEVEL...WITH THE FLOW SPIRALING ANTICYLONICALLY OUTWARD IN A WELL-DEVELOPED UPPER-LEVEL OUTFLOW PATTERN BEYOND A COUPLE HUNDRED N MI FROM THE CENTER. FLUCTUATIONS IN STRENGTH...DUE TO INTERNAL STRUCTURAL CHANGES...ARE LIKELY PRIOR TO LANDFALL. HURRICANES RARELY SUSTAIN SUCH EXTREME WINDS FOR MUCH TIME. HOWEVER WE SEE NO OBVIOUS LARGE-SCALE EFFECTS TO CAUSE A SUBSTANTIAL WEAKENING THE SYSTEM...AND IT IS EXPECTED THAT THE HURRICANE WILL BE OF CATEGORY 4 OR 5 INTENSITY WHEN IT REACHES THE COAST.

RECALLING THAT THE AVERAGE NHC 24-HOUR TRACK FORECAST ERROR IS ABOUT 80 N MI...THE ACTUAL LANDFALL POINT COULD STILL BE ANYWHERE FROM SOUTHEASTERN LOUISIANA TO THE MISSISSIPPI COAST. ALSO...WE MUST CONTINUE TO STRESS THAT THE HURRICANE IS NOT JUST A POINT ON THE MAP...BECAUSE DESTRUCTIVE WINDS...TORRENTIAL RAINS...STORM SURGE...AND DANGEROUS WAVES EXTEND WELL AWAY FROM THE EYE. IT IS IMPOSSIBLE TO SPECIFY WHICH COUNTY OR PARISH WILL EXPERIENCE THE WORST WEATHER.

THIS ADVISORY SHOWS AN ADDITIONAL EXPANSION OF THE WIND FIELD OVER THE EASTERN SEMICIRCLE.

That entire Sunday afternoon Katrina maintained Category 5 intensity, and the satellite presentation, including the remarkable large eye, was breathtakingly, despairingly, close to perfection. For people with interest in the northern Gulf Coast, it was one of the longest Sunday afternoons ever experienced; yet every minute also rushed by, bringing the terrible moment of landfall closer.

The late Sunday afternoon advisory discussion could only note Katrina's continued strength, and continued expansion of the wind field:

KATRINA IS MAINTAINING A CLASSIC PRESENTATION ON SATELLITE IMAGES...AND CATEGORY 5 INTENSITY. THE CENTRAL PRESSURE MEASURED BY A NOAA HURRICANE HUNTER PLANE AT 1755Z AND 1923Z WAS 902 MB...WHICH IS THE FOURTH LOWEST ON RECORD IN THE ATLANTIC BASIN BEHIND HURRICANE GILBERT OF 1988...THE LABOR DAY HURRICANE OF 1935...AND HURRICANE ALLEN OF 1980. HAVING SAID THAT...DATA FROM THE STEPPED-FREQUENCY MICROWAVE RADIOMETER INSTRUMENT ON BOARD THE AIRCRAFT SUGGEST THAT THE SURFACE TO 700 MB FLIGHT LEVEL WIND RATIO IS NOT QUITE AS LARGE AS WE TYPICALLY USE...AND THE INITIAL INTENSITY IS ADJUSTED SLIGHTLY TO 145 KT. HURRICANES DO NOT MAINTAIN SUCH GREAT INTENSITY FOR VERY LONG. HOWEVER THERE ARE NO OBVIOUS LARGE-SCALE MECHANISMS...SUCH AS INCREASED VERTICAL SHEAR...TO WEAKEN KATRINA. THE HURRICANE IS LIKELY TO MAKE LANDFALL WITH CATEGORY 4 OR 5 INTENSITY.

ON THE BASIS OF AIRCRAFT FLIGHT LEVEL AND SFMR SURFACE WIND DATA...THE WIND RADII HAVE BEEN EXPANDED EVEN MORE OVER THE NORTHERN SEMICIRCLE.

Time had now run out for evacuation, and the fate of many along the coast who had decided to stay, was now sealed. Weather would get progressively worse during the night.

Monday: Annihilation

Sunday afternoon was the peak of the almost-continuous intensification, during the two-day trip over the Loop Current. In the following hours, as the enormous hurricane approached land, a number of things occurred. The core of the hurricane moved north of the Loop Current eddy, leaving the deep warm water that had fueled the intensification to a category 5 hurricane. The hurricane was so large that part of its circulation moved over land well in advance of the landfall, removing the hurricane's source of energy, warm water, from the northern portion of the storm. Wind shear increased, dry air was pulled in and eroded the western side of the storm. The dry air reached the eyewall, and eventually eroded the southwestern side prior to landfall. Winds on the western side of the storm were reduced in both expanse and intensity, compared to the eastern side of the large storm. The factors in the weakening of Katrina's winds in the hours prior to landfall were detailed in the NHC Tropical Cyclone Report (PDF File):

The new eyewall that formed late on 27 August and contracted early on 28 August began to erode on its southern side very late on 28 August, while another outer ring of convection consolidated. These structural changes likely contributed to the rapid weakening that was observed prior to final landfall…The rapid weakening of Katrina, from its peak intensity of 150 kt to 110 kt during the last 18 h or so leading up to the first Gulf landfall, appears to have been primarily due to internal structural changes, specifically the deterioration of the inner eyewall without the complete formation of a new outer eyewall. However, Katrina remained very large as it weakened, and the extent of tropical storm-force and hurricane-force winds was nearly the same at final landfall on 29 August as it had been late on 28 August. The weakening could have been aided by entrainment of dry air that was seen eroding the deep convection over the western semicircle while Katrina approached the coast. Gradually increasing wind shear, slightly lower ocean temperatures, and (following the first Gulf landfall) interaction with land each could also have played a role. Without extensive investigation, however, it is not possible to assess the relative roles played by these various factors.

…

The structure of Katrina changed dramatically from 28 to 29 August as it approached the northern Gulf coast. TRMM 85 GHz imagery at 2133 UTC 28 August revealed a developing outer eyewall, and subsequent microwave overpasses depicted the inner eyewall steadily eroding, especially on the southern side…Post-storm analysis of numerous dropwindsonde profiles indicates that the structure of Katrina had changed since the previous day when it was at its peak intensity, such that the usual 90% adjustment of flight-level winds would likely provide overestimates of the surface winds on 29 August. Comparison of flight-level winds collocated with dropwindsondes and SFMR data suggest the flight-level to surface reduction factor that morning was closer to 80% or perhaps even less. Additional evidence of this structural transformation comes from airborne Doppler radar-derived wind speed cross sections on 29 August, obtained from the NOAA Hurricane Hunter aircraft. These data reveal an unusual, broad, and elevated wind maximum in the 2-4 km layer (centered near the 700 mb flight level), well above the more typical location of the maximum wind near the top of the boundary layer (~500 m) that had been observed on 28 August.

But the winds, and wind damage, were not the main story. The tremendous surge, which formed in part when Katrina was at major hurricane intensity in the deep water of the Gulf of Mexico, and adjacent to the shallow continental shelf, was carried on to the shallows and the northern Gulf Coast. This contribution was so large because the size of the eye, and the corresponding ring of highest winds in the eyewall (the "radius of maximum winds"), covered such a large area while over deep water. Another major factor contributing to the surge were the swells created when Katrina was at peak intensity in Gulf, several days earlier, which radiated north from the hurricane, and traveled along with it towards the shore (this same area would receive significant but minor flooding some weeks later from the same type of large swells generated by Rita, while many miles south in the GOM). Finally, the extensive windfield on the eastern side of the storm, with onshore winds, helped drive the water towards land, and this immediate wind-driven component of the surge extended the flooded areas of the coast as far as areas of the Florida panhandle. The most far-reaching and highest surge ever documented in this country, Katrina was likely a "500-year" storm in terms of its surge, according to recent research of overwash deposits in coastal lakes. Buildings that had stood for two centuries were taken down by the surge, all along the coastline.

Katrina's surge was so extensive that no single map exists documenting the entire area destroyed by the surge, which included the southeast Louisiana parishes of Plaquemines, St. Bernard, and portions of St. Tammany, the entire Mississippi Gulf Coast, and southern portions of Mobile County, Alabama. The FEMA map showing just the saltwater surge inundation for the coastline of Mississippi (freshwater flooding occurred further inland of this), only shows about half of the coastline that was destroyed:

Image courtesy of FEMA

Notice the similarity to the HES map shown earlier identifying the flood potential along the coastline for storm surge in the different hurricane categories.

The NHC TC Report (PDF File) provided extensive detail about Katrina's surge:

…the storm surge was about 24 to 28 ft along the Mississippi coast across a swath about 20 miles wide, centered roughly on St. Louis Bay. This area encompasses the eastern half of Hancock County and the western half of Harrison County and includes the communities of Waveland, Bay St. Louis, Pass Christian, and Long Beach. The maximum high water mark observation of storm surge was 27.8 ft at Pass Christian, on the immediate Gulf coast just east of St. Louis Bay. The data also indicate that the storm surge was 17 to 22 ft along the eastern half of the Mississippi coast, roughly from Gulfport to Pascagoula. The surge appears to have penetrated at least six miles inland in many portions of coastal Mississippi and up to 12 miles inland along bays and rivers. The surge crossed Interstate 10 in many locations. Observations also indicate Katrina produced a lesser but still very significant storm surge of 10 to 15 ft along coastal areas of western Alabama (Mobile County) including Dauphin Island. Katrina also caused flooding several miles inland from the Gulf coast along Mobile Bay where data indicate a storm surge of 8 to 12 ft occurred, in particular along the northern and western shores of the bay. Observations indicate that the storm surge along the Gulf coast of eastern Alabama (Baldwin County) was as high as about 10 ft.”

Although the storm surge was highest to the east of the path of the eye of Katrina, a very significant storm surge also occurred west of the path of the eye. As the level of Lake Pontchartain rose, several feet of water were pushed into communities along its northeastern shore in St. Tammany Parish from Slidell to Mandeville, Louisiana. High water mark data indicate the storm surge was 12 to 16 ft in those areas. The data also indicate a storm surge of 15 to 19 ft occurred in eastern New Orleans, St. Bernard Parish, and Plaquemines Parish, while the surge was 10 to 14 ft in western New Orleans along the southern shores of Lake Pontchartrain. Farther west, observations indicate a storm surge of 5 to 10 ft along the shores of western Lake Pontchartrain.”

The massive storm surge produced by Katrina, even though it had weakened from Category 5 intensity the previous day to Category 3 at landfall in Louisiana, can be generally explained by the huge size of the storm. Katrina had on 29 August a large (about 25-30 n mi) radius of maximum winds and a very wide swath of hurricane force winds that extended at least 75 n mi to the east from the center…Also, Katrina had already generated large northward-propagating swells, leading to substantial wave setup along the northern Gulf coast, when it was at Category 4 and 5 strength during the 24 hours or so before landfall…Overall, Katrina's very high storm surge is attributable mainly to the large horizontal size of the hurricane, with the total water level being further increased by waves, including those generated the previous day when Katrina was at Category 5 strength.

And now, community by community, we'll take a journey through the miles of coastline that were destroyed by Katrina's record storm surge, and discover the processes that create storm surge.

Hurricane Katrina Storm Surge:

Weather Underground Storm Surge Articles: